Structural Geology and Tectonics Field Guidebook ― Volume 1 (Springer Geology) 3030601420, 9783030601423

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Springer Geology Field Guides

Soumyajit Mukherjee Editor

Structural Geology and Tectonics Field Guidebook— Volume 1

Springer Geology Springer Geology Field Guides

Series Editor Soumyajit Mukherjee, Department of Earth Sciences, Indian Institute of Technology Bombay, Mumbai, Maharashtra, India

Springer Geology Field Guides is a book series that provides the details of both well-known and little known transects to discover the beauty and knowledge of Geology, worldwide. Springer Geology Field Guides aims to bring geology field trips to professionals, students, and amateurs to find the most interesting geology worldwide. This series includes carefully crafted guidebooks that help generations of geologists explore the terrain with minimum or no guidance. In this series, the audience will also find field methodologies and case studies as examples. This book series will welcome both authored and edited field guides of all geology disciplines, including structural geology, tectonics, sedimentology, stratigraphy, paleontology, economic geology, among others. Photo-atlases are also welcome.

More information about this subseries at http://www.springer.com/series/16656

Soumyajit Mukherjee Editor

Structural Geology and Tectonics Field Guidebook—Volume 1

123

Editor Soumyajit Mukherjee Department of Earth Sciences Indian Institute of Technology Bombay Mumbai, Maharashtra, India

ISSN 2197-9545 ISSN 2197-9553 (electronic) Springer Geology ISSN 2730-7344 ISSN 2730-7352 (electronic) Springer Geology Field Guides ISBN 978-3-030-60142-3 ISBN 978-3-030-60143-0 (eBook) https://doi.org/10.1007/978-3-030-60143-0 © Springer Nature Switzerland AG 2021 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Dedicated to all fieldwork-based mappers, geoscientists and surveyors

Preface

Despite the explosive growth of structural geology and tectonics in the last few decades, structural geological and tectonic fieldwork remain indispensable components of geosciences education in Bachelor’s and Master’s level worldwide. A (new) field instructor might be in search for (new) terrains close to her institute to explore and demonstrate structures to students. This book fills up that requirement. Through 26 main chapters, 84 authors and co-authors from 13 countries, the book presents few well-known and several rather unknown transects where exciting structures exist, and field-programs can be established. Cite individual chapters in the following format: Kaplay RD, Babar Md, Mukherjee S, Wable D, Pisal K. 2021. Granitic rocks underlying Deccan trap along the margin of east Dharwar craton, Muntyal (Maharashtra)—Bhaisa (Telengana), India—general description and deformation. In: Mukherjee S. (Ed) Structural Geology and Tectonics Field Guidebook— Volume 1. Springer Nature Switzerland AG. Cham. pp. 599-620. ISBN: 978-3-03060142-3. Cite this book in the following format: Mukherjee S. (2021) Structural Geology and Tectonics Field Guidebook—Volume 1. Springer Nature Switzerland AG. Cham. pp. 1-723. ISBN: 978-3-030-60142-3. Mumbai, India February 2021

Soumyajit Mukherjee [email protected] [email protected]

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Acknowledgements

Dripta Dutta (IIT Kanpur), Mohamedharoon A. Shaikh (MS University Baroda), Mohit Kumar Puniya (National Geotechnical Facility, Dehradun), and Narayan Bose (IIT Kharagpur) assisted profusely in preparing this book. The Springer (proofreading) team, especially Boopalan Renu, Alexis Vizcaino, Doerthe Mennecke-Buehler, Marion Schneider, Monica Janet Michael and Annett Buttener helped a lot in different stages of this book preparation and publication. I thank the contributing authors and the reviewers for participation. I am especially thankful to the staff members of my Department for always cooperating. They are Nilesh K. Borkar, Dr. Trupti V. Chandeasekhar, Shamit N. Karnekar, Ramu K. Khandagale, Rajesh Y. Manjrakar, Dr. Shilpa V. Netravali, P. S. Sawant, Javed M. Saikh, Srikanth Jonnala, Staphen T., Mary Thomas, N. N. Vengulakar and Premkumar R. Verma.

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This book consists of 26 main chapters. Chapter “Creating Geologic Maps in the Twenty-First Century: A Case Study from Western Ireland”: Swanger and Whitmeyer (2021) discuss the modern techniques of field mapping and the creation of geologic maps using recent software. The authors also elaborate the same using a case study from Western Ireland. Chapter “Strain Softening in a Continental Shear Zone: A Field Guide to the Excursion in the Ferriere-Mollières Shear Zone (Argentera Massif, Western Alps, Italy)”: Simonetti et al. (2021) field trip in the Western Alps shows evidences of strain softening from ten field stops in the central portion of the Ferriere-Mollières shear zone. The authors further constrain the shearing event between 340 and 320 Ma using in situ U-Th-Pb petrochronology on monazite. Chapter “The Geometry and Kinematics of the Southwestern Termination of the Pyrenees: A Field Guide to the Santo Domingo Anticline”: Field trip to the Santo Domingo anticline in the Pyrenees by Pueyo et al. (2021) reveals complex fold kinematics from the fold–thrust belt. Chapter “Miocene-Quaternary Strain Partitioning and Relief Segmentation Along the Arcuate Betic Fold-and-Thrust Belt: A Field Trip Along the Western Gibraltar Arc Northern Branch (Southern Spain)”: Jiménez-Bonilla et al. (2021) discuss the strain partitioning modes from the hinge to the lateral zones of the Western Gibraltar Arc (southern Spain). Two separate itineraries are presented for the same. Chapter “The Southern Iberian Shear Zone (SW Spain): Inclined Transpression Related to Variscan Oblique Convergence in a HT/LP Metamorphic Belt”: Díaz-Azpiroz and Fernández (2021) present the ductile mesostructures from the boundary between the Ossa-Morena and South Portuguese Zones of the Iberian Massif (Huelva Province, SW Spain). This boundary developed during the Upper Paleozoic due to the sinistral oblique collision between Avalonia and Armorica. Chapter “A Field Guide to the Spectacular Salt Mines of the Transylvanian Basin and Romanian Carpathians”: Tămaș et al. (2021) describe field trips in the Romanian Carpathians and the Transylvanian Basin to study the 3D structural

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features of the salt domes and diapirs in the abandoned salt mines. They propose a route with five stops to explain the link between hydrocarbon and salt tectonics. Chapter “Spectacular Sandstone Rock Cities in the Czech Republic”: Novakova and Novak (2021) describe sandstone rock cities from the Czech Republic. The Cretaceous sandstones are broken into blocks during the Alpine orogenesis and subsequently eroded to form these spectacular exposures. Chapter “Field Guide to RODS in the Pireneus Syntaxis, Central Brazil”: Martins-Ferreira and Rodrigues (2021) present a field guide focussing the linear structural features from the Pireneus Range in Central Brazil. They describe the occurrences as observed at the ten field stations from the Three Peaks area (TrêsPicos) to the Mocó Boulders. Chapter “Low Baric Metamorphic Belts in the Northern Tip of the Arabian–Nubian Shield: Selected Examples from the Eastern Desert/Midyan Terranes, Egypt”: Shallaly and Abu Sharib (2021) explore the pelitic metasediments from the LP/HT andalusite-sillimanite-type metamorphic belts of the Arabian–Nubian shield in Egypt. They report multiple phases of deformation in such belts. Chapter “Review of the Geometric Model Parameters of the Main Himalayan Thrust”: Ansari (2021) reviews geometric model parameters of the Main Himalayan Thrust along different portions of the Himalaya. He compiles variation in the dip-slip and strike-slip rates of this thrust along the Himalayan belt. This chapter is not a field guide strictly speaking, but keeping this work in mind will be important for the Himalayan field geologists. Chapter “Traverses Through the Bagalkot Group from North Karnataka State, India: Deformation in the Mesoproterozoic Supracrustal Kaladgi Basin”: Patil Pillai and Kale (2021) conduct fieldwork along four different traverses within the Balakot Group of the Kaladgi Basin. They report several mesoscale structural features, primary sedimentary structures and bedding plane characters. Chapter “Tectonic Framework of Northern Pakistan from Himalaya to Karakoram”: Ali et al. (2021) explore the rocks along the Islamabad–Khunjerab transect of the Pakistan Himalaya. They describe the lithounits encountered over a period of four days that comprise 27 field stops. Chapter “Structures of Lesser/Greater Himalaya in and Around an Out-of-Sequence Thrust in the Chaura-Sarahan Area (Himachal Pradesh, India)”: Ghosh and Mukherjee (2021) present detail field structural features near an out-of-sequence thrust in the Western Himalaya in India. Such thrusts have been studied so far mainly from geochemical perspectives, and field data were so far missing. This chapter presents a good example of ductile and brittle shear sense indicators, so the reader is referred to few recent publications”: Mukherjee (2011, 2013, 2014a, b, 2015, in press) and Mukherjee et al. (2020). Chapter “Structural Geology Along the Nainital-Pangot Road (Kilbari Section), Nainital Lesser Himalaya (Uttarakhand, India): Focus on Back-Structures”: Puniya and Mukherjee (2021) study the structural geology along the Nainital–Pangot road (Kilbari section) in the Nainital Lesser Himalaya, Uttarakhand, India. The authors report several mesoscale back structures. Such structures have increasingly been

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reported from the Himalaya (e.g., Mukherje 2013; 2019; Bose and Mukherjee 2019a, b). Chapter “Geology, Structural, Metamorphic and Mineralization Studies Along the Mandi-Kullu-Manali-Rohtang Section of Himachal Pradesh, NW-India”: Singh et al. (2021) present lithounits and structures along the Mandi-Larji-KulluManali-Rohtang La transect in the NW Indian Himalaya. Chapter “Tectonics and Channel Morpho-Hydrology—A Quantitative Discussion Based on Secondary Data and Field Investigation”: Biswas et al. (2021) compute 30 geomorphic indices to describe the river channel morphologies and their tectonic controls. They choose three study sites from India: the NE foreland basin of North Bengal, the Singbhum Shear Zone (SSZ) and the Janauri–Chandigarh anticline. Chapter “Geological Field Guide: Malvan (Maharashtra, India)”: Pundalik et al. (2021) present detail of fieldworks from Malvan (Maharashtra, India) from the lithologic, geomorphic and structural perspectives. Chapter “A Field Guide to the Champaner Region, Southern Aravalli Mountain Belt (SAMB), Gujarat, Western India”: Joshi and Limaye (2021) discuss the structural features in the Paleoproterozoic basement gneisses to the recent sediments of the Champaner region in Eastern Gujarat (India). They elaborate the lithounits and structures encountered from 15 field stops along four different traverses. Chapter “Importance of Fracturing in Uranium Mineralization in Gulcheru Quartzite Host: A Case from Ambakapalle Area, Cuddapah Basin, Andhra Pradesh, India”: Goswami et al. (2021a) map the fault zone in the Ambakapalle area within the Cuddappah Basin. They focus on fractures and their influence on uranium mineralization. The authors also discuss two phases of the alteration of rocks. Chapter “Granitic Rocks Underlying Deccan Trap Along the Margin of East Dharwar Craton, Mutnyal (Maharashtra)—Bhaisa (Telangana), India—General Description and Deformation”: Kaplay et al. (2021) study the structural features from the contact between the Eastern Dharwar craton and the Deccan Volcanic Province. They detail shear tectonics along the contact. Chapter “Structural Analyses of the Lunavada–Santrampur Area (Gujarat, India) Using Remote Sensing Images”: Chauhan et al. (2021) analyze the folds and lineaments from the Santrampur area (NE Gujarat, India) using remote sensing images. They use Google Earth for identifying various folds geometries, viz. polyclinal folds, second-order folds and superposed folds. This chapter will enable field geologist to get into the detail of the terrain. Chapter “Fundamentals of Lithostructural Mapping: Example from the SW Part of the Proterozoic Bhima Basin, Karnataka, India: A Note on Dharwarian Crustal Evolution”: Goswami et al. (2021b) explore the geodynamic evolution of the Eastern Dharwar craton with the help of GPS-aided lithostructural mapping of the SW part of the Proterozoic Bhima Basin. Chapter “A 3D Photogrammetric Approach in Mapping Meso-Scale Folds and Shears in Structurally Controlled Syngenetic Mn-Mineralised Zones of Shivrajpur Region, Eastern Gujarat, India”: Joshi (2021) describes an innovative technique of mapping mesoscale structures using 3D photogrammetry. The author maps an

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outcrop scale fold from an abandoned mine from the Mn-mineralized zones of the Shivrajpur region (Eastern Gujarat, India). Chapter “Vein Geometry Around Bhuj (Gujarat, India)”: Omid et al. (2021) present diverse vein geometries from Bhuj area, Kutch Basin, Gujarat, India. Detail field-based and geochemical studies can be taken up in this hitherto unknown area of structures. Chapter “Oriented Rock Samples for Detailed Structural Analysis”: Gaidzik and Żaba (2021) discuss how to collect oriented rocks from field for structural geological analyses. This chapter is particularly important to undertake kinematic analyses of shear zone rocks under an optical microscope. Acknowledgements The Springer (proofreading) team, especially Boopalan Renu, Alexis Vizcaino, Doerthe Mennecke-Buehler, Marion Schneider and Annett Buttener, helped a lot in different stages of this book preparation and publication.

Dripta Dutta Soumyajit Mukherjee Department of Earth Sciences Indian Institute of Technology Bombay Powai, Mumbai Maharashtra, India e-mail: [email protected] [email protected]

References Ali, A., Ahmad, S., Khan, M. A., Khan, M. I., Rehman, G. (2021). Tectonic framework of Northern Pakistan from Himalaya to Karakoram. In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook—Volume 1. Springer Nature Switzerland AG. Cham. pp. 367-412. ISBN: 978-3-030-60142-3. Ansari, K. (2021). Review of the geometric model parameters of the Main Himalayan Thrust. In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook—Volume 1. Springer Nature Switzerland AG. Cham. pp. 305-324. ISBN: 978-3-030-60142-3. Bose, N., Mukherjee, S. (2019a). Field documentation and genesis of the back-structures from the Garhwal Lesser Himalaya, Uttarakhand, India. In Sharma, I. M. Villa, S. Kumar (Eds.), Crustal architecture and evolution of the Himalaya-Karakoram-Tibet Orogen. Geological Society of London Special Publications 481, 111–125. Bose, N., Mukherjee, S. (2019b). Field documentation and genesis of back-structures in ductile and brittle regimes from the foreland part of a collisional orogen: Examples from the Darjeeling–Sikkim Lesser Himalaya, India. International Journal of Earth Sciences, 108, 1333–1350. Biswas, M., Pal, A., Jamal, M. (2021). Tectonics and Channel Morpho-Hydrology—A Quantitative Discussion Based on Secondary Data and Field Investigation. In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook—Volume 1, Switzerland AG: Springer Nature. Cham. pp. 461-494. ISBN: 978-3-030-60142-3.

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Chauhan, G. H., Rao, G. S., Mukherjee, S. (2021). Structural analyses of the Lunavada– Santrampur area (Gujarat, India) using remote sensing images. In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook—Volume 1. Springer Nature Switzerland AG. Cham. pp. 621-638. ISBN: 978-3-030-60142-3. Díaz-Azpiroz, M., Fernández, C. (2021). The Southern Iberian Shear Zone (SW Spain): Inclined transpression related to Variscan oblique convergence in a HT/LP metamorphic belt. In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook—Volume 1. Springer Nature Switzerland AG. Cham. pp. 137-166. ISBN: 978-3-030-60142-3. Ghosh, R., Mukherjee, S. (2021). Structures of Lesser/Greater Himalaya in and around an out-of-sequence thrust in the Chaura-Sarahan area (Himachal Pradesh, India). In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook—Volume 1. Springer Nature Switzerland AG. Cham. pp. 413-428. ISBN: 978-3-030-60142-3. Goswami, S., Shrivastava, S., Das, S., Bhattacharjee, P. (2021a). Fundamentals of litho-structural mapping: Example from the SW part of the Proterozoic Bhima basin, Karnataka, India: A note on Dharwarian Crustal evolution. In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook—Volume 1. Springer Nature Switzerland AG. Cham. pp. 639-684. ISBN: 978-3-030-60142-3. Goswami, S., Upadhyay, P. K., Natarajan, V. (2021). Importance of fracturing in uranium mineralization in Gulcheru Quartzite host: A case from Ambakapalle area, Cuddapah Basin, Andhra Pradesh, India. In Mukherjee S. (Ed.), Structural Geology and Tectonics Field Guidebook—Volume 1. Springer Nature Switzerland AG. Cham. pp. 577-598. ISBN: 978-3-030-60142-3. Gaidzik, K., Żaba, J. (2021). Oriented Rock Samples for Detailed Structural Analysis. In: S. Mukherjee (Ed) Structural Geology and Tectonics Field Guidebook—Volume 1. Springer Nature Switzerland AG. pp. 715- 723. Jiménez-Bonilla, A., Díaz-Azpiroz, M., Expósito, I., Balanyá, J. C. (2021). Miocene-Quaternary strain partitioning and relief segmentation along the arcuateBetic fold-and-thrust belt: A field trip along the Western Gibraltar Arc northern branch (southern Spain). In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook—Volume 1. Springer Nature Switzerland AG. Cham. pp. 103-136. ISBN: 978-3-030-60142-3. Joshi, A. U., Limaye, M. A. (2021). A field guide to the Champaner Region, Southern Aravalli Mountain Belt (SAMB), Gujarat, Western-India. In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook—Volume 1. Springer Nature Switzerland AG. Cham. pp. 529-576. ISBN: 978-3-030-60142-3. Joshi, A. U. (2021). A 3D photogrammetric approach in mapping meso-scale folds and shears in structurally controlled syngenetic Mn-mineralized zones of Shivrajpur region, Eastern Gujarat, India. In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook—Volume 1. Springer Nature Switzerland AG. Cham. pp. 685-706. ISBN: 978-3-030-60142-3. Kaplay, R. D., Babar, Md., Mukherjee, S., Wable, D., Pisal, K. (2021). Granitic rocks underlying Deccan trap along the margin of east Dharwar craton, Muntyal (Maharashtra)—Bhaisa (Telengana), India—general description and deformation. In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook—Volume 1. Springer Nature Switzerland AG. Cham. pp. 599-620. ISBN: 978-3-030-60142-3. Martins-Ferreira, M. A. C., Rodrigues, S. W. (2021). Field guide to RODS in the Pireneus Syntaxis, central Brazil. In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook—Volume 1. Springer Nature Switzerland AG. Cham. pp. 221-264. ISBN: 978-3-030-60142-3. Mukherjee, S. (2013). Higher Himalaya in the Bhagirathi section (NW Himalaya, India): Its structures, backthrusts and extrusion mechanism by both channel flow and critical taper mechanisms. International Journal of Earth Sciences, 102, 1851–1870. Novakova, L., Novak, P. (2021). Spectacular sandstone rock cities in the Czech Republic. In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook—Volume 1. Springer Nature Switzerland AG. Cham. pp. 189-220. ISBN: 978-3-030-60142-3.

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Omid, M. W., Mukherjee, S., Dasgupta, S. (2021). Vein geometry (Bhuj, Gujarat, India). In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook—Volume 1. Springer Nature Switzerland AG. Cham. pp. 707-714. ISBN: 978-3-030-60142-3. Patil Pillai S., Kale, V. S. (2021). Traverses through the Bagalkot Group from north Karnataka state, India: Deformation in the Mesoproterozoic supracrustal Kaladgi Basin. In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook—Volume 1. Springer Nature Switzerland AG. Cham. pp. 325-366. ISBN: 978-3-030-60142-3. Puniya, M. K., Mukherjee, S. (2021). Structural geology of the Nainital-Pangot road (Kilbari section), Nainital Lesser Himalaya (Uttarakhand, India): Focus on back-structures. In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook—Volume 1. Springer Nature Switzerland AG. Cham. pp. 429-436. ISBN: 978-3-030-60142-3. Pundalik, A., Nikalje, S., Samant, A., Samant, H. (2021). Geological fieldguide: Malvan (Maharashtra, India). In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook—Volume 1. Springer Nature Switzerland AG. Cham. pp. 495-528. ISBN: 978-3-030-60142-3. Shallaly, N. A., Abu Sharib, A. S. A. A. (2021). Low baric metamorphic belts in the northern tip of the Arabian Nubian Shield: selected examples from the Eastern Desert/Midyan terranes, Egypt. In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook—Volume 1. Springer Nature Switzerland AG. Cham. pp. 265-304. ISBN: 978-3-030-60142-3. Simonetti, M., Carosi, R., Montomoli, C. (2021). Strain softentening in a continental shear zone: A field guide to the excursion in the Ferriere-Mollières shear zone (Argentera Massif, Western Alps, Italy). In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook— Volume 1. Springer Nature Switzerland AG. Cham. pp. 19-48. ISBN: 978-3-030-60142-3. Singh, P., Ao, A., Thakur, S. S., Rana, S., Sharma, R., Singh, A. K., Singhal, S. S. (2021). Geology, structural, metamorphic and mineralization studies 2 along the Mandi-Kullu-Manali-Rohtang section of Himachal 3 Pradesh, NW-India. In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook—Volume 1. Springer Nature Switzerland AG. Cham. pp. 437-460. ISBN: 978-3-030-60142-3. Swanger, W. R., Whitmeyer, S. J. Creating geologic maps in the 21st century: A case study from Western Ireland. In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook— Volume 1. Springer Nature Switzerland AG. Cham. pp. 1-18. ISBN: 978-3-030-60142-3. Tămaș, D. M., Tămaș, A., Jüstel, A. M., Passchier, M., Chudalla, N., Gotzen, J., Wagner, L. A. P., Tașcu-Stavre, T., Schléder, Z., Krézsek, C., Filipescu, S., Urai, J. L. (2021). A fieldguide to the spectacular salt mines of the Transylvanian basin and Romanian Carpathians. In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook—Volume 1. Springer Nature Switzerland AG. Cham. pp. 167-188. ISBN: 978-3-030-60142-3.

Contents

Creating Geologic Maps in the Twenty-First Century: A Case Study from Western Ireland . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . W. R. Swanger and S. J. Whitmeyer Strain Softening in a Continental Shear Zone: A Field Guide to the Excursion in the Ferriere-Mollières Shear Zone (Argentera Massif, Western Alps, Italy) . . . . . . . . . . . . . . . . . . . . . . . . . M. Simonetti, R. Carosi, and C. Montomoli The Geometry and Kinematics of the Southwestern Termination of the Pyrenees: A Field Guide to the Santo Domingo Anticline . . . . . . E. L. Pueyo, B. Oliva-Urcia, E. M. Sánchez-Moreno, C. Arenas, R. Silva-Casal, P. Calvín, P. Santolaria, C. García-Lasanta, C. Oliván, A. Gil-Imaz, F. Compaired, A. M. Casas, and A. Pocoví

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Miocene-Quaternary Strain Partitioning and Relief Segmentation Along the Arcuate Betic Fold-and-Thrust Belt: A Field Trip Along the Western Gibraltar Arc Northern Branch (Southern Spain) . . . . . . . 103 Alejandro Jiménez-Bonilla, Manuel Díaz-Azpiroz, Inmaculada Expósito, and Juan Carlos Balanyá The Southern Iberian Shear Zone (SW Spain): Inclined Transpression Related to Variscan Oblique Convergence in a HT/LP Metamorphic Belt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137 Manuel Díaz-Azpiroz and Carlos Fernández A Field Guide to the Spectacular Salt Mines of the Transylvanian Basin and Romanian Carpathians . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 Dan Mircea Tămaș, Alexandra Tămaș, Alexander Magnus Jüstel, Martijn Passchier, Nils Chudalla, Lina Gotzen, Luis Alberto Pizano Wagner, Teodora Tașcu-Stavre, Zsolt Schléder, Csaba Krézsek, Sorin Filipescu, and Janos L. Urai

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Contents

Spectacular Sandstone Rock Cities in the Czech Republic . . . . . . . . . . . 189 Lucie Novakova and Petr Novak Field Guide to RODS in the Pireneus Syntaxis, Central Brazil . . . . . . . 221 Marco Antonio Caçador Martins-Ferreira and Sérgio Wilians de Oliveira Rodrigues Low Baric Metamorphic Belts in the Northern Tip of the Arabian–Nubian Shield: Selected Examples from the Eastern Desert/Midyan Terranes, Egypt . . . . . . . . . . . . . . . . . 265 N. A. Shallaly and A. S. A. A. Abu Sharib Review of the Geometric Model Parameters of the Main Himalayan Thrust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 305 Kutubuddin Ansari Traverses Through the Bagalkot Group from North Karnataka State, India: Deformation in the Mesoproterozoic Supracrustal Kaladgi Basin . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325 Shilpa Patil Pillai and Vivek S. Kale Tectonic Framework of Northern Pakistan from Himalaya to Karakoram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 367 Asghar Ali, Sajjad Ahmad, Sajjad Ahmad, Mohammad AsifKhan, Muhammad Irfan Khan, and Gohar Rehman Structures of Lesser/Greater Himalaya in and Around an Out-of-Sequence Thrust in the Chaura-Sarahan Area (Himachal Pradesh, India) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 413 Rajkumar Ghosh and Soumyajit Mukherjee Structural Geology Along the Nainital–Pangot Road (Kilbari Section), Nainital Lesser Himalaya (Uttarakhand, India): Focus on Back-Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429 Mohit Kumar Puniya and Soumyajit Mukherjee Geology, Structural, Metamorphic and Mineralization Studies Along the Mandi-Kullu-Manali-Rohtang Section of Himachal Pradesh, NW-India . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 437 Paramjeet Singh, Aliba Ao, S. S. Thakur, Shruti Rana, Rajesh Sharma, A. K. Singh, and Saurabh Singhal Tectonics and Channel Morpho-Hydrology—A Quantitative Discussion Based on Secondary Data and Field Investigation . . . . . . . . 461 Mery Biswas, Ankita Paul, and Mostafa Jamal Geological Field Guide: Malvan (Maharashtra, India) . . . . . . . . . . . . . . 495 Ashwin Pundalik, Shiba Nikalje, Arnav Samant, and Hrishikesh Samant

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A Field Guide to the Champaner Region, Southern Aravalli Mountain Belt (SAMB), Gujarat, Western India . . . . . . . . . . . . . . . . . . . . . . . . . . 529 Aditya U. Joshi and Manoj A. Limaye Importance of Fracturing in Uranium Mineralization in Gulcheru Quartzite Host: A Case from Ambakapalle Area, Cuddapah Basin, Andhra Pradesh, India . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 577 Sukanta Goswami, P. K. Upadhyay, and V. Natarajan Granitic Rocks Underlying Deccan Trap Along the Margin of East Dharwar Craton, Mutnyal (Maharashtra)—Bhaisa (Telangana), India—General Description and Deformation . . . . . . . . . . . . . . . . . . . . 599 R. D. Kaplay, Md. Babar, Soumyajit Mukherjee, Deepak Wable, and Kunal Pisal Structural Analyses of the Lunavada–Santrampur Area (Gujarat, India) Using Remote Sensing Images . . . . . . . . . . . . . . . . . . . 621 Geetika H. Chauhan, G. S. Rao, and Soumyajit Mukherjee Fundamentals of Lithostructural Mapping: Example from the SW Part of the Proterozoic Bhima Basin, Karnataka, India: A Note on Dharwarian Crustal Evolution . . . . . . . . . . . . . . . . . . . . . . . 639 Sukanta Goswami, Shivam Shrivastava, Suman Das, and Purnajit Bhattacharjee A 3D Photogrammetric Approach in Mapping Meso-Scale Folds and Shears in Structurally Controlled Syngenetic Mn-Mineralised Zones of Shivrajpur Region, Eastern Gujarat, India . . . . . . . . . . . . . . . 685 Aditya U. Joshi Vein Geometry Around Bhuj (Gujarat, India) . . . . . . . . . . . . . . . . . . . . 707 Mohammad Walid Omid, Soumyajit Mukherjee, and Sudipta Dasgupta Oriented Rock Samples for Detailed Structural Analysis . . . . . . . . . . . . 715 Krzysztof Gaidzik and Jerzy Żaba

Creating Geologic Maps in the Twenty-First Century: A Case Study from Western Ireland W. R. Swanger and S. J. Whitmeyer

Abstract Techniques of geologic mapping and geologic map creation have changed significantly from traditional paper-based methods. Geologic mapping and data collection in the field is now primarily facilitated by mobile devices and dedicated geologic mapping software. Geologic map production has become a fully integrated process, importing digital data from the field and making use of cartographic software, such as ArcGIS and Adobe Illustrator, to create interactive geologic map products. Dissemination of geologic maps incorporates several types of map products to support the variety of uses that practitioners have for geologic maps and field data. Some of these map products include: 1. layered PDF maps, where layers can be toggled to show different map components; 2. Google Earth KML and KMZ files that can be viewed in the virtual 3D terrain of the geobrowser; and 3. GIS geodatabases that include not only the geologic map interpretation of a field area but also the primary field data. Geologic field data should also be archived in community databases, such as StraboSpot.org, so that future field workers can access and validate the data in their projects. This modern approach to creating geologic maps is highlighted in a case study from the lakes region of western Ireland, where undergraduate geoscience students have used digital mapping techniques in field exercises for several years. A brief discussion of the history of digital field mapping and map creation sets the stage for a discussion of modern techniques. Current best practices are highlighted for field mapping and data collection, geologic map creation, dissemination of map-related products, and archiving of data and products.

Electronic supplementary material The online version of this chapter (https://doi.org/10.1007/978-3-030-60143-0_1) contains supplementary material, which is available to authorized users. W. R. Swanger · S. J. Whitmeyer (B) Geology and Environmental Science, James Madison University, Harrisonburg, USA e-mail: [email protected] W. R. Swanger Department of Earth and Environmental Sciences, University of Kentucky, Lexington, USA S. J. Whitmeyer Division of Earth Sciences, National Science Foundation, Alexandria, USA © Springer Nature Switzerland AG 2021 S. Mukherjee (ed.), Structural Geology and Tectonics Field Guidebook—Volume 1, Springer Geology, https://doi.org/10.1007/978-3-030-60143-0_1

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1 Introduction Field data collection and the creation of geologic maps have been the cornerstone of interpreting continental geology since the birth of the geosciences in the early nineteenth century (e.g., Smith 1815; Cuvier and Brongniart 1822). Even with recent dramatic advances in remote sensing (Dasgupta and Mukherjee 2017, 2019) and laboratory-based analytical techniques, geologic maps remain one of the primary vehicles for the interpretation and illustration of geologic and tectonic features and processes. However, the advent of digital publication and presentation methods, as well as mobile technologies for data collection, have resulted in modern approaches to the creation of geologic maps that substantially differ from traditional paper-based methods. Traditional field data collection with hardback field books and on paper topographic maps in protective map boards was the norm throughout most of the twentieth century, even as the production of final map products was switching to computer-based cartography tools, such as ArcGIS and Adobe Illustrator. Early practitioners explored the benefits of digital equipment for field mapping early in the twenty-first century, especially after selective availability (civilian degrading) of the Global Positioning System (GPS) was turned off in 2000. This resulted in a proliferation of handheld GPS devices that enabled geolocation for cars or for individuals in the field, rendering manual positioning methods, such as triangulation, obsolete. Shortly thereafter, the advent of ruggedized, all-weather computer hardware, such as Trimble handheld PCs, Panasonic Toughbooks, and other tablet PCs (Knoop and van der Pluijm 2006), many with built-in GPS receivers, facilitated the digital revolution in field mapping and data collection (Pavlis et al. 2010; Whitmeyer et al. 2010). The next big advance came with the initial release of the iPhone in 2007, quickly followed by other enhanced cellular communication devices (Novakova and Pavlis 2017). The development of mobile field mapping software, coupled with the release of the iPad tablet in 2010, has ushered in the modern era of mobile tablet-based field mapping, which has made paper-based field mapping obsolete for today’s geoscience professionals. A coincident evolution of software solutions accompanied the changes in field mapping hardware. Mobile field-mapping software is initially centered on commercial GIS platforms, such as ArcPad for handheld PCs and then iGIS for iPads. More recently, geology-focused apps, such as StraboSpot, have become accepted as the standard for both field data collection and for the archiving of field-based data in an open, community database (Walker et al. 2019). Collectively, this steady evolution of advances in equipment has resulted in a new digital workflow that encompasses initial collection of geologic data in the field through the production of professional geologic maps and other data products. In the sections that follow, we present a modern approach to creating geologic maps, from data collection through dissemination and archiving, recognizing that this is still a fast-moving field and that tomorrow’s best practices will certainly differ to some extent from what is presented here.

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2 A Field Area in Western Ireland as an Example Case Study As an illustration of a workflow for modern field data collection and geologic map production, we focus on a field area in western Ireland that has been the site of digital mapping exercises for the James Madison University Geology Field Course in Ireland for several years (De Paor and Whitmeyer 2009; Whitmeyer et al. 2019). The field area incorporates the mountains of Bencorragh and Knock Kilbride, along the Kilbride Peninsula, located in the lakes region north of Galway in western Ireland. This area is along the southern margin of the South Mayo Trough, just north of the suture with the Connemara terrane—a Dalradian terrane that was sutured to the South Mayo Trough during the mid-Paleozoic Caledonian orogeny (Williams and Harper 1991; Williams and Rice 1989; Dewey and Ryan 2016; Fig. 1). In the Kilbride Peninsula region the oldest lithologic units are at higher elevations of the mountains of Bencorragh and Knock Kilbride and are predominantly composed of Ordovician basalt pillows and flows (Williams 1990; Draut and Clift 2001) of the Lough Nafooey Fm. (brown color, Fig. 2). On the southern and eastern shores of Lough Nafooey (labeled on Fig. 2), north of Bencorragh, the basalts are overlain by northwest-dipping Ordovician conglomerates, arkoses, and sandstones of the Rosroe Fm. (gray color, Fig. 2). The southern slopes of the mountains consist of a mostly planar, moderately to steeply southeast-dipping, Silurian transgressive sequence of volcanic rocks (Basal Lough Mask member, purple color), terrestrial red sandstones (Lough Mask Fm., red color), near-shore quartz arenites (Kilbride Fm., blue color),

Fig. 1 Simplified geologic map of the boundary region between the Connemara Terrane and the South Mayo Trough. The field-mapping area is indicated by the red ellipse. After McConnell et al. (2009)

Fig. 2 Authors’ crowd-sourced geologic map of the Kilbride Peninsula and the mountains of Bencorragh and Knock Kilbride in the border regions of Counties Mayo and Galway, western Ireland. The map area is roughly the same as in Figs. 3 and 4; Explanation for geologic map shown in below the image

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siltstones (Tonalee member, green color), and shelf/slope turbidites (Lettergesh Fm., yellow/orange color) that overlie Ordovician basalts (Graham et al. 1989; Chew et al. 2007; Whitmeyer et al. 2010). The Kilbride Peninsula area is broadly folded and cross-cutby Late Devonian (Mohr 2003; Johnson et al. 2011), steeply-dipping normal and transverse faults, with offsets that range from decimeters to decameters (Fig. 2). In the sections below, we make use of this field area to highlight and discuss the three principal steps to creating geologic maps: 1. Collection of geologic data in the field, 2. Creation of a geologic map and related products from field data, and 3. Dissemination of the new geologic map and related products to the community. Below, we cover each of these steps in detail, starting with historical contexts and then highlighting modern methods.

3 Modern Methods of Digital Field Data Collection Published geologic maps that incorporate the Kilbride Peninsula field area include the original geological map of Ireland by Griffith (1838; Fig. 3), as well as more recent regional geologic maps, such as the South Mayo geologic map by Graham et al. (1989) and the 1:100,000 Galway Bay map by the Geological Survey of Ireland (Pracht et al. 2004; Fig. 4). These maps reflect a progression in the tectonic interpretations of the bedrock geology of the region, but do not differ significantly in their depictions of the areal extent of lithologic units or their offsets along faults. Importantly, none of these existing maps are detailed enough to highlight the density of individual outcrops that occur in the field area or the abundance of faults that can be inferred from offsets in bedding contacts given a high density of outcrop data (Whitmeyer et al. 2019; Fig. 2). We suggest that new approaches to crowd-sourcing data collection in the field, facilitated by modern mobile equipment, can enable better geologic map interpretations through a significantly increased density of outcrop data.

Fig. 3 Detail from the first geologic map of Ireland by Griffith (1838). Map is cropped to show the same field area as Fig. 2

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Fig. 4 Portion of the 1:100,000 scale bedrock geology map of Galway Bay (Pracht et al. 2004). Map is cropped to show the same extent as seen in Figs. 2 and 3

Geology-focused field investigations have traditionally been conducted by a solitary geologist, sometimes with a field assistant. As a result, mapping strategies prioritized efficient coverage of a field area with the understanding that not all outcrops could be visited in the time available for fieldwork. Recently, crowdsourcing approaches to fieldwork have been advocated, where multiple field geologists collaborate on mapping a field area (House et al. 2013), or teams of (sometimes novice) geologists map the same field area (Whitmeyer and De Paor 2014; Whitmeyer et al. 2019). These approaches can produce a significantly greater density of field data, which in turn, can lead to a more complete geologic map and better geologic interpretations. For the Kilbride Peninsula field area, field data was collected by multiple teams of James Madison University (JMU) field course students in digital mapping exercises over several years. The inherent redundancy of several teams mapping each section of a field area resulted in differing interpretations for some field locations (e.g., Whitmeyer and De Paor 2014), which necessitated an expert control dataset for validation (see the Discussion section below). Advances in digital methods of geologic mapping and data collection in the field help facilitate crowd-sourcing methods when field datasets are exportable in a common format. For the Kilbride Peninsula field area, a variety of software solutions were used during approximately 15 years of student mapping projects. Software for field data collection included ArcPad and ArcMap in the early years (pre iPad), followed by mobile apps for the iPad, such as iGIS, FieldMove (Midland Valley 2017), and most recently StraboSpot (Walker et al. 2019). Older field-mapping software required users to upload georeferenced basemaps (topographic maps and/or aerial photographs) and a predefined data structure before field data collection

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could occur. Modern mobile apps (e.g. FieldMove, StraboSpot) automatically access basemap imagery (generic, publicly available road maps, shaded topographic maps, or satellite imagery), which can be cached on the mobile device for offline use. However, users can still upload their own specialized datasets, such as high-resolution LiDAR topography or digital elevation models (DEMs). A standard data structure for geologic mapping and data collection is also built into the apps (Fig. 5), although users can often customize these data structures to some extent. Notably, all of these field-mapping software solutions allowed for the export of field data in standard GIS formats (shapefiles). This enabled us to combine field datasets into a master database of all field data, while retaining identifiers for novice (student) versus expert (control) datasets.

Fig. 5 StraboSpot app on an iPad (protectively enclosed in an OtterBox case), showing the data entry menu for a planar outcrop feature

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Recent initiatives in the geoscience community, as well as funding agencies like the National Science Foundation, have championed the storage and archiving of data in curated, often cloud-hosted, community databases. The StraboSpot data system (strabospot.org; Walker et al. 2019) was developed in response to a community workshop that conceptualized a data storage system for geoscience field data (Mookerjee et al. 2015). A mobile StraboSpot app is now available for iOS and Android devices and functions as a field-mapping and data collection system that interfaces with the StraboSpot.org community data system. Users can choose to keep their StraboSpot data private or make their data public to share it with the broader community (Fig. 6). This allows geoscientists to privately store field data during ongoing projects, prior to releasing it to the public upon completion of the project. Student mapping projects can be kept private and only made available to the local class community or research group. A significant advantage to ultimately making field data publicly available is that published geologic maps can reference the supporting data and allow other users to investigate geologic interpretations based on the original data (e.g., the geologic map of Fig. 2 and the publicly available supporting data shown in Fig. 6). The ability to provide original data as supplemental material to geologic maps allows future geologists easy access to resources such as field notes, photographs and other data that previously could not be included in delivered products.

Fig. 6 Screen capture of a map-viewing search page from StraboSpot.org that shows a publicly available dataset. The expert control dataset for the Bencorragh field area (left part of Fig. 2) is displayed. Red box shows the area of Fig. 7

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4 Building a Modern Digital Geologic Map Geologic mapping in the field is now typically undertaken with mapping apps specifically designed for geologists, such as FieldMove (Midland Valley 2017) or StraboSpot (Walker et al. 2019). General GIS apps, like iGIS or Collector for ArcGIS, can also be used but take a bit more work to set up for field mapping, as the apps are not specifically targeted at geoscientists. Other apps designed for specialized geoscience data collection in the field include StratMobile for collecting stratigraphic data and preparing stratigraphic columns and strip logs, and Stereonet Mobile, which builds stereonets from orientation measurements (Allmendinger et al. 2017). All of these apps work on iPads (some of them also work on Android tablets), which we enclose in protective OtterBox Defender cases—necessary for field work in the frequently wet weather of western Ireland. Some of these apps can be used on mobile phones, including a simplified version of FieldMove called FieldMoveClino, but our experience is that the larger screen area available on mobile tablets is important for seeing a sufficiently large expanse of the digital field area and working field map. As almost all modern mobile devices incorporate internal sensors, such as an accelerometer, a gyroscope, and a magnetometer (check out the Sensor Kinetics app to see these sensors in action), several geoscience apps for the field also include a digital geologic compass for taking orientation measurements (e.g., strike and dip of bedding). There has been discussion in the field-mapping community regarding whether measurements obtained from these digital compasses are accurate and precise enough for professional field work. However, recent work by Novakova and Pavlis (2017) and Whitmeyer et al. (2019) suggest that digital compass measurements can produce measurements that are at the same level of precision as analog geologic compasses (e.g., Brunton Pocket Transits), with an accuracy that is better than ± 5 degrees for both azimuth (strike) and vertical (dip) angles (Whitmeyer et al. 2019). The important caveat is that digital compasses have a sensor calibration routine (waving the mobile device in a figure-eight pattern) that should be invoked periodically to keep the compasses measuring surfaces correctly. Finally, in some field situations where two-handed manipulation of an analog compass can be challenging, the ease of use of a digital compass may be preferable and more effective (Whitmeyer et al. in review 2020). A significant advantage of mobile-mapping devices is the capability to take a large amount of background information into the field. This can be in the form of highresolution aerial or satellite imagery or a high-resolution LiDAR or photogrammetrybased topographic DEM. The use of high-resolution shaded relief maps as basemaps for field mapping has been demonstrated to significantly enhance the interpretation of geologic structures (e.g., House et al. 2013). If publicly available, datasets previously collected in a field area can be preloaded onto a mobile device for reference or validation while in the field. We used this approach when we validated the crowdsourced student field data for the Kilbride Peninsula. We uploaded the student dataset on our iPads and then traversed the field area to check the field data and sometimes modify the students’ interpretations. In addition, maps, publications, or references

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that cover a region of interest can be preloaded on a mobile device as distinct files (e.g., PDFs) or within an app such as Flyover Country, which is designed to provide geolocated background information for a field area or field trip (Birlenbach et al. 2019). Much of the initial drafting of a geologic map can be done on location in the field, in the form of outcrop point data and linework (Fig. 7a), such as known contacts and faults that can be walked out or sketched. Geologic maps drafted in the field which include polygons and annotations (Fig. 7b) can be a very useful teaching tool, providing a way to quickly visualize geologically complex areas. However, the drafting of more permanent linework and polygons for the aerial expanses of lithologic units is usually best left for a cartographic program on a laptop or desktop computer. In general, advanced cartographic and design software, such as ArcGIS and Adobe Illustrator, has a suite of tools that enable professional drafting of geologic maps, and the larger screens of desktop computers make it easier to see the broad expanse of field data and thus make more informed geologic interpretations. Modern pen-enabled touch screens, such as those made by Wacom, facilitate easy and precise drafting of lines and polygons. Cartographic programs also provide relative straightforward mechanisms for added auxiliary geologic map components,

Fig. 7 StraboSpot app on an iPad showing part of the Bencorragh field map a, with dashed lines for approximate locations of contacts (black) and faults (red), and outcrop locations indicated by black strike and dip symbols; an individual outcrop data point is highlighted in the pop-up balloon. b shows same field map with polygons colored by lithology (pink = Lough Nafooey Fm., red = Lough Mask Fm., blue = Kilbride Fm., green = Tonalee member)

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such as map scales, north arrows, map explanations with lithologic unit descriptions and symbology, and other map production elements (Fig. 8; see the Supplementary File Layered_Geologic_Map.pdf for an example).

5 Geologic Map Products and Dissemination Though today’s geologic maps include many of the same components as William Smith’s (1815) hand-drawn and colored geologic map of England and Wales (e.g., surface extents of lithologic units that are represented by colored regions, vertical interpretations of geologic structure, and orientation shown in a cross section, an explanation that shows the color coding of the lithologic units), the modern mediums for delivering geologic maps have come a long way from Smith’s rolled paper stock. Paper geologic maps can still be purchased from many U.S. state geologic surveys, but most state surveys have made their geologic and geographic map data available via a GIS-based Web interface (e.g., the Interactive Geologic Map of Virginia; https://www.dmme.virginia.gov/webmaps/DGMR/). Similarly, the U.S. Geological Survey maintains the National Geologic Map Database (NGMDB; https://www. usgs.gov/core-science-systems/national-cooperative-geologic-mapping-program/ science/national-geologic-map?qt-science_center_objects=0#qt-science_center_ objects), where an interactive Web-based GIS portal provides national- and statelevel geologic maps and information, much of which is available for download in a digital format. Country-level international geologic maps and information can be accessed at the OneGeology portal (http://www.onegeology.org/). Historical maps are sometimes included on these Web portals as georectified (geolocated) raster images, but most modern geologic maps are built in a GIS and stored as digital data and thus are easily incorporated into GIS-based Web portals. Most of these portals can serve the digital map data in a variety of formats to satisfy a diverse array of end user requirements. If a user desires the raw field data and/or interpretations of the mapping geologists, the geologic map can be made available as a geodatabase (see the Supplementary File Bencorragh_geodatabase.gdb for an example). Alternatively, if a user desires a professional finished map product to either view on screen or print out, they can download a PDF file of the map. Modern PDF files are layered and somewhat interactive, where users can turn on a selection of available layers to display or print the desired map information (Fig. 8; see the Supplementary File Layered_Geologic_Map.pdf as an example). Other interactive mediums for digital display of geologic maps and information include virtual globes or geobrowsers, such as Google Earth. Virtual globes have the advantages of being rotatable and zoomable to any point on the globe. Most areas in Google Earth display fairly high-resolution satellite or aerial imagery over a moderate resolution (10-30 m) DEM, although some areas on the globe are better resolved. Geologic maps can be draped over the Google Earth terrain to view the maps in virtual 3D. Other location-specific information, like outcrop data and/or outcrop photographs can also be linked to the geologic map and displayed in Google

Fig. 8 Authors’ multi-layered PDF file of the publishable geologic map of the Kilbride Peninsula and Bencorragh, western Ireland. Modern PDF maps can have selectable layers to display different features of a geologic map, such as topographic and high-resolution satellite imagery basemap options. Control data and crowd-sourced data can also be viewed separately to highlight how increased data density enables a more detailed and accurate structural interpretation

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Fig. 9 Oblique view of the crowd-sourced geologic map of Fig. 2 draped over the virtual 3D terrain in google earth, with a pop-up balloon showing an outcrop photograph

Earth (Fig. 9). The maps and other associated data require yet another data format for display on virtual globes, specifically a scripting language called KML (Keyhole Markup Language; https://developers.google.com/kml). Many Web GIS servers can export geologic maps and data in KML or KMZ (zipped packages of KML script and associated imagery) format, although these exported files are typically less user friendly than a KMZ map and data package that is custom developed by an experienced KML programmer (see the Supplementary File Ireland_Map.kmz for an example).

6 Discussion In this manuscript, we advocate for an all-digital workflow for the creation of geologic maps, from field data collection through dissemination and archiving. However, we recognize that there is a learning curve associated with the adoption of these methods for both novice and expert geoscientists that are less familiar with the various aspects of using digital technologies for geologic mapping. Indeed, our experiences with many years of teaching digital methods of field mapping to students in undergraduate field courses suggests that the use of mobile equipment in the field adds an additional cognitive load for students that may still be grappling with the basics of learning how to do geology in the field (De Paor and Whitmeyer 2009; Mogk and Goodwin 2012). The JMU Geology Field Course in Ireland still starts students

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with analog compasses and paper field slips (topographic basemaps) for their initial geologic mapping exercises, in order for the students to focus on basic lithologic identification, measuring orientations, and the mechanics of making a geologic map and interpreting the geology of a field area. Modern digital methods and equipment are not introduced until the third or fourth mapping exercise, after the students know how to strategize field data collection, work with multiple working hypotheses in the field (e.g., Chamberlin 1890), and synthesize a geologic interpretation from their field data. Even after introducing digital methods, some students still preferred to use traditional equipment for their later culminating mapping projects (Whitmeyer and De Paor 2014). The integration of digital methods, specifically the ability to drape geologic maps and highlight field data in Google Earth, is crucial to the learning process of the novice geologist. When students can rotate to oblique views of data draped over a DEM, they can more easily grasp the vertical component of structurally complex areas. Throughout the several years of digital mapping exercises in the Kilbride Peninsula area, student field data was collected in a master GIS file of all outcrop data points. This resulted in a dataset with over 15,000 points, some of which were redundant that yielded a density of field data unprecedented for any previous geologic map of the area. This density of field data facilitated the creation of a geologic map with a greater resolution and better characterization of fault offsets (Fig. 2) than previously published maps (Figs. 3 and 4) and thus arguably a more accurate depiction of the geology of the region. However, the abundance of outcrop data from students that were relative novices at geologic mapping in the field also highlighted local areas where students had differing interpretations of the geology—usually highlighted as contrasting interpretations of the lithology (Whitmeyer and De Paor 2014). We have previously argued that the “good” data significantly dwarfs the “bad” data, but how does one accurately characterize the “good” data? In areas where there was a single clear outlier in a lithologic interpretation, it was fairly easy to cull the spurious data. However, in other locations with unclear interpretations, often in areas where there were gradational changes between lithologic units, a control dataset was used to decide on the most parsimonious interpretations. For the Bencorragh and Kilbride Peninsula field areas, we had a dataset collected by an expert in the geology of that region (one of the field course instructors) that covered much of the map area (e.g., Fig. 6). We used the expert dataset to constrain most of the conflicts in our crowd-sourced dataset. However, we still found it necessary to return to the field area with a preliminary version of the cross-sourced geologic map loaded on our iPads in order to field check a few problematic locations. Though not always possible in remote field areas, returning to the field with a draft geologic map in order to constrain problematic areas or areas with little field data enables a geologist to cover a large area in a short period of time and can greatly improve the accuracy of the final map product.

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7 Conclusion and Best Practices We have discussed modern methods of field mapping and the creation of geologic maps, while providing some historical context of past practices. While we understand that technological advances to hardware and software relevant to geoscience field work are always changing rapidly, we present below a bulleted list of some current best practices that we have identified for efficient creation of geologic maps, ca. 2020: Field mapping and data collection • We suggest using the StraboSpot app for field mapping and data collection, along with its community database storage system. • Explore legacy geologic maps and legacy field datasets (if available) for the field area and upload them to the mobile device as background data. • Assemble basemaps, LiDAR images, and shaded relief maps and upload to the mobile mapping device as desired. • We recommend exploring crowd-sourcing methods for field data collection, if enough personnel are available. Geologic map creation, dissemination, and archiving • If crowd-sourcing methods are used, then control/validation datasets by mapping experts may be needed to constrain areas with problematic interpretations. • High-resolution background imagery can also help constrain interpretations on geologic maps; shaded relief maps from LiDAR and/or photogrammetry data have proven effective. • Produce a variety of geologic maps and map products for today’s disparate audiences, including layered PDF files of maps and map data, GIS geodatabases, and Google Earth KMZ files. • Raw field data should be made available on publicly accessible online databases, such as StraboSpot.org. Keep in mind that the data can be embargoed until the project is completed. • Finished map products can be archived on community map servers, such as those hosted by state geologic surveys, OneGeology.org, and the National Geologic Map Database: ngmdb.usgs.gov. Acknowledgements The authors would like to acknowledge JMU students that have participated in digital mapping projects over the years, as well as field course faculty: Declan De Paor, Martin Feely, John Haynes, Steve Leslie, Beth McMillan, Eric Pyle, and Shelley Whitmeyer. This work was supported, in part, by NSF awards 1323468 and 1841132. Any opinion, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation, and review comments provided by Soumyajit Mukherjee (IIT Bombay). Thanks to Marion Schneider, Annett Buettener, Boopalan Renu, Alexis Vizcaino, Doerthe Mennecke-Buehler, and the proofreading team (Springer). Dutta and Mukherjee (2021) encapsulate this work.

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References Allmendinger, R. W., Siron, C. R., & Scott, C. P. (2017). Structural data collection with mobile devices: Accuracy, redundancy, and best practices. Journal of Structural Geology, 102, 98–112. Birlenbach, D. M., Shinneman, A. C., Loeffler, S., & Myrbo, A. (2019). Flyover country: Creating flexible field experiences using a mobile geoscience app. In The Trenches, 9(3). Chamberlin, T. C. (1890). The method of multiple working hypotheses. Science, 15(366), 92–96. Chew, D. M., Graham, J. R., & Whithouse, M. J. (2007). U-Pb zircon geochronology of plagiogranites from the lough nafooey (= Midland Valley) arc in western Ireland: constraints on the onset of the Grampian orogeny. Journal of the Geological Society of London, 164, 747–750. https:// doi.org/10.1144/0016-76492007-025 Cuvier, G., & Brongniart, A. (1822). Description géologique des environs de Paris (p. 428). Paris: Chez Dufour et D’Ocagne. De Paor, D. G., & Whitmeyer, S. J. (2009). Innovations and redundancies in geoscience field courses: Past experiences and proposals for the future. In S. J. Whitmeyer, D. Mogk, & E. J. Pyle (Eds.), Field geology education: Historical perspectives and modern approaches, GSA Special Paper (Vol. 461, pp. 45–56). https://doi.org/10.1130/2009.2461(05) Dutta, D., & Mukherjee, S. (2021). Introduction to Structural Geology and Tectonics Field Guidebook-Volume 1. In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook—Volume 1. Switzerland AG: Springer Nature. Cham. pp. xi-xvi. ISBN: 978-3-030-60142-3. Dasgupta, S., & Mukherjee, S. (2017). Brittle shear tectonics in a narrow continental rift: Asymmetric non-volcanic Barmer basin (Rajasthan, India). The Journal of Geology, 125, 561–591. Dasgupta, S., Mukherjee, S. (2019). Remote sensing in lineament identification: Examples from western India. In: A. Billi & A. Fagereng (Eds.), Problems and solutions in structural geology and tectonics. Developments in structural geology and tectonics book series. Series Editor: S. Mukherjee Elsevier (Vol. 5, pp. 205–221). ISSN: 2542–9000. ISBN: 9780128140482. Dewey, J. F., & Ryan, P. D. (2016). Connemara: Its position and role in the grampian orogeny. Canadian Journal of Earth Science, 53, 1246–1257. https://doi.org/10.1139/cjes-2015-0125. Draut, A. E., & Clift, P. D. (2001). Geochemical evolution of arc magmatism during arc-continent collision. South Mayo, Ireland, Geology, 29, 543–546. Graham, J. R., Leake, B. E., & Ryan, P. D. (1989). The geology of south mayo (p. 75). Western Ireland, Edinburgh: Scottish Academic Press. Griffith, R. J. (1838). In Outline of the geology of Ireland, Dublin:Railway Commissioners. House, P. K., Clark, R., & Kopera, J. (2013). Overcoming the momentum of anachronism: american geologic mapping in a twenty-first-century world. In V. R. Baker (Ed.), Rethinking the fabric of geology: Geological society of America special paper (Vol. 502, pp. 103–125). https://doi.org/ 10.1130/2013.2502(05) Johnson, E. A., Sutherland, S., Logan, A. V. L., Samson, S. D., & Feely, M. (2011). Emplacement conditions of a porphyritic felsite dyke and timing of motion along the Coolin Fault at Ben Levy, Co galway. Irish Journal of Earth Sciences, 29, 1–13. Knoop, P. A., & van der Pluijm, B. (2006). GeoPad: Tablet PC-enabled field science education. In D. Berque, J. Prey & R. Reed (Eds.), The impact of pen-based technology of education: Vignettes, evalations, and future directions (pp. 103–114). West Lafayette, Indiana: Purdue University Press. McConnell, B. M., Riggs, N., & Crowley, Q. G. (2009). Detrital zircon provenance and Ordovician terrane amalgamation, western Ireland. Journal of the Geological Society, 166, 473–484. https:// doi.org/10.1144/0016-76492008-081. Valley, Midland. (2017). Field move users guide (p. 63). Edinburgh: Petroleum Experts. Mogk, D. W., & Goodwin, C. (2012). Learning in the field: Synthesis of research on thinking and learning in the geosciences. In K. A. Kastens & C. A. Manduca (Eds.), Earth and mind II: A synthesis of research on thinking and learning in the geosciences. Geological society of America special paper (Vol. 486, pp. 131–163). https://doi.org/10.1130/2012.2486(24).

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Mohr, P. (2003). Late magmatism of the Galway Granite Batholith: I Dacite dykes. Irish Journal of Earth Sciences, 21, 71–104. Mookerjee, M., Viera, D., Chan, M., Gil, Y., Goodwin, C., Shipley, T., et al. (2015). We need to talk: Facilitating communication between field-based geoscience and cyber infrastructure communities. GSA Today, 25, 34–35. Novakova, L., & Pavlis, T. L. (2017). Assessment of the precision of smart phones and tablets for measurement of planar orientations: A case study. Journal of Structural Geology, 97, 93–103. Pavlis, T. L., Langford, R., Hurtado, J., & Serpa, L. (2010). Computer-based data acquisition and visualization systems in field geology: Results from 12 years of experimentation and future potential: Geosphere, 6, 275–294. Pracht, M., Lees, A., Leake, B. E., Feely, M., Long, B. C., Morris, J., et al. (2004). Geology of galway bay: A geological description to accompany the bedrock geology 1:100,000 scale map series, sheet 14, Galway Bay (p. 76). Dublin: Geological Survey of Ireland. Smith, W. (1815). A geological map of England and Wales and part of Scotland, London: British Geological Survey, 1 sheet. Walker, J. D., Tikoff, B., Newman, J., Clark, R., Ash, J., & Good, J., et al. (2019). StraboSpot data system for structural geology. Geosphere, 15. https://doi.org/10.1130/GES02039.1 Whitmeyer, S. J., & De Paor, D. G. (2014). Crowdsourcing digital maps using citizen geologists. EOS, 95, 397–399. https://doi.org/10.1002/2014EO440001. Whitmeyer, S. J., Nicoletti, J., & De Paor, D. G. (2010). The digital revolution in geologic mapping. GSA Today, 20(4/5). https://doi.org/10.1130/gsatg70a.1 Whitmeyer, S. J., Pyle, E. J., Pavlis, T. L., Swanger, W., & Roberts, L. (2019). Modern approaches to field data collection and mapping: Digital methods, crowdsourcing, and the future of statistical analyses. Journal of Structural Geology, 125, 29–40. https://doi.org/10.1016/j.jsg.2018.06.023 Whitmeyer, S. J., Atchison, C., & Collins, T. D. (2020). Using mobile technologies to enhance accessibility and inclusion in field-based learning, GSA Today, v. 30. https://doi.org/10.1130/ GSATG462A.1 Williams, D. M. (1990). Evolution of Ordovician terranes in western Ireland and their possible Scottish equivalents. Transactions of the Royal Society of Edinburgh–Earth Sciences, 81, 23–29. Williams, D. M., & Harper, D. A. T. (1991). End-Silurian modifications of Ordovician terranes in western Ireland. Journal of the Geological Society of London, 148, 165–171. https://doi.org/10. 1144/gsjgs.148.1.0165 Williams, D. M., & Rice, A. H. N. (1989). Low-angle extensional faulting and the emplacement of the Connemara Dalradian. Ireland: Tectonics, 8, 417–428. https://doi.org/10.1029/tc008i002 p00417

Strain Softening in a Continental Shear Zone: A Field Guide to the Excursion in the Ferriere-Mollières Shear Zone (Argentera Massif, Western Alps, Italy) M. Simonetti, R. Carosi, and C. Montomoli

Abstract Fieldwork, integrated with several techniques for microstructural and geochronological analyses, plays a fundamental part in the study of shear zones. In this guide, we describe a two-day field trip in the Argentera External Crystalline Massif (Western Alps). The Massif is cross-cut by the NW-SE oriented FerriereMollières Shear Zone, a spectacular example of a nearly 25-km-long regional transpressive zone with a maximum thickness of 2 km and a complex and long-lasting evolution during the Variscan time. Recent detailed geological mapping coupled with structural, microstructural, and petrochronological studies reveal that the FerriereMollières Shear Zone was characterized by strain softening that localized strain in its central part during the syn-shearing exhumation and retro-metamorphism. The aim of the field trip is to visit some key outcrops that allow to recognize such evolution and to observe the main features of the Ferriere-Mollières Shear Zone that can be regarded as a good example of a strain softening shear zone in the continental crust.

1 Introduction Shear zones are portions of rock in which strain is clearly higher than in the wall rocks that separate less strained or unstrained portions of the lithosphere. They have a primary influence on the geometry and evolution of orogenic belts and rifts, on crustal rheology, magma ascent and emplacement, and fluid flow. Assessing the timing of shear zone activity is crucial to reconstruct the tectonometamorphic evolution of the lithosphere. Shear zones can form at all scales and at all structural levels and can record a complex and polyphase deformation history (Ramsay 1980; Fossen and Cavalcante 2017 and references therein). Because of

M. Simonetti (B) · R. Carosi · C. Montomoli Dipartimento Di Scienze Della Terra, Universita‘ Di Torino, Via Valperga Caluso 35, Turin, Italy e-mail: [email protected] C. Montomoli IGG-CNR PISA, Via Moruzzi 1, Pisa, Italy © Springer Nature Switzerland AG 2021 S. Mukherjee (ed.), Structural Geology and Tectonics Field Guidebook—Volume 1, Springer Geology, https://doi.org/10.1007/978-3-030-60143-0_2

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this, they vary a lot in orientation, length, thickness, displacement, strain geometry, coaxiality, and deformation mechanisms. Geometry, orientation, and relative movement of the walls control the deformation within the zone. Several processes may happen temporally leading to different behavior and evolution of shear zones. For example, compaction by pressure solution and related loss of material in the zone or strain localization where margins are left inactive may produce a thinning of the high-strain zone. Inclusion of portions of wall rocks and interaction between adjacent shear zones may produce widening (Fossen 2016; Fossen and Cavalcante 2017). The lengthening and thickening of ductile shear zones are not explored to the same extent as fault growth. Shear zones lengthen generally because they tend to connect with other shear zones to give rise to composite systems or networks (Peacock and Sanderson 1991; Soliva and Benedicto 2004; Ponce et al. 2013; Mukherjee 2014; Fossen and Rotevatn 2016). Shear zone thickness is an issue that is influenced mainly by strain and rheology of the deformed rocks. According to Fossen (2016), four models for the evolution of the thickness of shear zones are possible. In type I shear zones, deformation in the central part slows down as the zone thickens as strain propagates into the walls. In type II shear zones, the deformation localizes in the central part. Thus, the margins become inactive and the active part gets thinner. Type III shear zones develop a fixed thickness and the entire zone keeps deforming without internal localization. Type IV shear zones have the same behavior as type I, but the whole shear zone remains active throughout the deformation history. Shear zones might behave with or without growth in thickness and with or without localization to its central parts due to strain hardening or strain softening processes. Strain hardening and weakening are mainly controlled by changes in grain size, presence of fluids, metamorphic reactions, strain accumulation, dynamic recrystallization process, and efficiency of recovery process(Fossen and Cavalcante 2017 and references therein). The evolution of shear zones over time can be studied regionally where deformation accumulates for several millions of years and could be resolved by combining detailed field work with structural analysis, microstructural analysis, and geochronometers. In the last few years, this approach has led to satisfactory results such as, for example, in the Variscan Belt (northern and central Sardinia Sardinia, Argentera Massif and Maures Massif; Di Vincenzo et al. 2004; Carosi et al. 2012; Simonetti et al. 2018, 2020b; Montomoli et al. 2018), in the Ross Orogen (Antarctica; Di Vincenzo et al. 2007), in the Himalaya (Nepal; Carosi et al. 2018; Cottle et al. 2015; Iaccarino et al. 2015; Montomoli et al. 2013, 2015), in the Northern Cordillera (Canada; Parson et al. 2018), and in the Borborema Province (Brazil; Viegas et al. 2014) were the unrevealed structural evolution of crustal-scale shear zones provided important constraints for new tectonic models. Examples of regional shear zones, whose evolution is well-constrained and described in detail, are of fundamental importance and are increasingly essential in order to test theoretical models and to verify the applicability of the process-oriented studies that, usually, are made on small-scale structures.

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The basement of the Argentera Massif, in the Western Alps, shows elegant folded and sheared Variscan migmatites only locally affected by Alpine tectonics. This Massif is divided in two metamorphic complexes by a 25-km-long transpressive shear zone known as the Ferriere-Mollière shear zone (FMSZ). The FMSZ was first recognized by Faure-Muret (1955), who refers to it as the ’Valletta Unit,’ and first mapped by Malaroda et al. (1970). The FMSZ has been recently studied in detail by Carosi et al. (2016) and Simonetti et al. (2017, 2018) by integrating fieldwork with multiple techniques for structural analysis and geochronology, that constrained a type II evolution during Variscan time. The FMSZ is therefore a good example of a regional zone in continental crust whose evolution, controlled by strain softening phenomena, has clear field evidences that are supported by well-constrained structural and chronological data. The aim of the field guide is to visit some key outcrops that allow to recognize such evolution and the features of both Variscan and Alpine deformation in the Argentera Massif.

2 The Variscan Belt in the Mediterranean Area The Variscan Belt is the result of a continent-continent collision between LaurentiaBaltica and Gondwana between ~ 380–280 Ma (Arthaud and Matte 1977; Burg and Matte 1978; Tollmann 1982; Matte 1986; Di Vincenzo et al. 2004). Major exposures of the Variscan units occur in Central and Western Europe (Fig. 1), parts of

Fig. 1 Distribution of the Variscan units in Europe at the present day. S = Sardinia; C = Corsica; MTM = Maures-Tanneron Massif; MC = Massif Central; IM = Iberian Massif; AM = Armorican Massif; RH = Rheno-Hercynian; BM = Bohemian Massif. Red circle indicate the location of the Argentera Massif

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Morocco and Algeria and north of the West African Craton. The portion of the belt exposed in Central and Western Europe is known in literature as the Variscides. This region is characterized by a composite orocline showing two large-scale arcs (Matte and Ribeiro 1975; Matte 1986): a western branch known as the Ibero-Armorican arc (Matte and Ribeiro 1975; Brun and Burg 1982; Dias and Ribeiro 1995; Dias et al. 2016; Fernández-Lozano et al. 2016) and a smaller eastern branch (Matte 2001; Bellot 2005; Ballevre et al. 2018; Simonetti et al. 2018, 2020a, b). The southeastern sector of the Variscides reworked and fragmented during the Alpine orogeny. Because of this, reconstruction of the southern European Variscan Belt and the correlation between the different fragments in the Mediterranean area is debated (Stampfli et al. 2002; Rosenbaum et al. 2002; Advokaat et al. 2014). Key areas to understand the evolution of the southern European Variscan Belt are the Maures-Tanneron Massif in southern France, the Corsica-Sardinia Block, and the Alpine External Crystalline Massifs (Fig. 1). Some authors proposed that, during the Variscan time, the future Alpine External Crystalline Massifs were in lateral continuity with southern France and Corsica-Sardinia Block, and all these sectors were affected by a transpressive regional-scale shear zone known as East Variscan Shear Zone (Matte 2001; Corsini and Rolland 2009; Carosi et al. 2012; Simonetti et al. 2018, 2020a, b). Evidence of transpressional deformation is well-documented in northern Sardinia (Carosi and Palmeri 2002; Iacopini et al. 2008; Frassi et al. 2009; Carosi et al. 2005, 2012; Cruciani et al. 2015) in the Maures Massif (Schneider et al. 2014; Simonetti et al. 2020b) and in the Alpine External Crystalline Massifs (Carosi et al. 2016; Simonetti et al. 2018, 2020a), in places were the Alpine metamorphic overprint is very weak.

3 Geological Setting of the Argentera Massif The External Crystalline Massifs represent the remnants of the Variscan Orogeny (Fig. 1) occurred between 380-280 Ma as a result of the collision between Laurussia and Gondwana (Matte 1986; Di Vincenzo et al. 2004). They are made up of high-to-medium grade metamorphic basement intruded by the Permo-Carboniferous granitoids. The Argentera Massif is located at the boundary between Italy and France (Fig. 2a) and is the southernmost of the Alpine External Crystalline Massifs. It is composed by two metamorphic complexes made by high-grade migmatitic gneisses (Ferrando et al. 2008; Compagnoni et al. 2010), the southwestern Tinèe Complex and the northeastern Gesso-Stura-Vesubiè Complex (GSV), which are divided by the Ferriere-Mollières Shear Zone (FMSZ, Fig. 2a). The GSV complex is mainly constituted by migmatitic gneiss derived from the Late Ordovician granitoids and migmatitic paragneiss. Field relationships between the two lithotypes suggest an original intrusion of ortho-derived migmatites in the para-derived lithotypes (Compagnoni et al. 2010). Migmatites show paragenesis indicative of amphibolite facies metamorphism (Compagnoni et al. 2010). Associated

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Fig. 2 a Geotectonic map of the Argentera Massif; FMSZ: Ferriere-Mollieres shear zone; FCSZ: Fremamorta-Colle del Sabbione shear zone; BF: Bersezio fault; VLS: Valle Stura Leucogranite; ACG: Argentera Central Granite (Compagnoni et al. 2010); 1 = geological map of the Rifugio Migliorero area in the Ischiator valley (Fig. 3); 2 = geological map of the Prati del Vallone area in the Pontebernardo valley (Fig. 4); b Satellite view (from Google Earth) of the Ischiator and Pontebernardo Valley. Circles indicate the location of the stops of the field trip (red = stops of day 1; green = stops of day 2)

to the ortho- and para-derived migmatites amphibolic migmatites of the BoussetValmasque Complex (Rubatto et al. 2001), bodies of metavulcanites and mafic and ultramafic boudins are present. Ferrando et al. (2008) and Compagnoni et al. (2010), by the study of mafic lithotypes, recognize a metamorphic evolution with four stages:

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(1) HP metamorphic peak; (2) initial decompression stage; (3) amphibolite-facies metamorphism of HT–MP; (4) amphibolite-facies metamorphism of LT–LP. The GSV is intruded by a large granite body at 292 ± 10 Ma (Ferrara and Malaroda 1969), known as Argentera Central Granite. The Tinée complex is divided into three formations (Faure-Muret 1955) deformed under amphibolite-facies conditions: (1) Valerios-Fougieret Formation constituted by biotite and plagioclase-bearing migmatitic paragneisses with graphite and sillimanite; (2) Anelle-Valabres Formation constituted by plagioclase, biotite-bearing migmatitic metagreywakes sometimes with muscovite; (3) Rabuons Formation constituted by migmatitic metapelites, with K-feldspar, biotite, muscovite and sillimanite. In Anelle-Valabres Formation and in Rabuons Formation eclogites relicts have been recognized (Faure-Muret 1955; Malaroda et al. 1970).The Tinée complex underwent a minor degree of melting than the GSV complex, testified by a minor amount of leucosomes (Compagnoni et al. 2010). The FMSZ is a ductile shear zone cross-cutting the Paleozoic basement (Malaroda et al. 1970; Musumeci and Colombo 2002; Compagnoni et al. 2010). It strikes NW– SE and extends from the village of Ferriere (Valle Stura) to the northwest, to the village of Mollières to the southeast (Fig. 2a) showing a dextral movement. Mylonites have been recently mapped by Carosi et al. (2016) according to the percentage of matrix and porphyroclasts both at the meso—and at the microscopic-scale following the classification proposed by Sibson (1977) and Passchier and Trouw (2005). Mylonites of the FMSZ result from the shearing of the migmatites belonging to both the GSV and the Tinée complexes. A deformation gradient marked by a transition from protomylonite, mylonite, and ultramylonite has been detected (Carosi et al. 2016). The main lithotypes in the shear zone are medium-grained dark-green mylonitic schists and biotite and white mica bearing mylonitic gneisses. Embedded within the mylonitic schists amphibole-bearing gneiss, showing variable amounts of deformation, are present (Carosi et al. 2016). Different interpretations have been proposed in the literature about both the tectonic meaning and the deformation age of the FMSZ. Musumeci and Colombo (2002) obtained a cooling age of the Rocco Verde mylonitic leucogranite of 327 ± 3 Ma by Rb/Sr analyses on the whole rock and magmatic muscovite grains. The reduced thickness of this intrusion implies a rapid cooling in the lower grade country rocks, allowing to interpret this age as an emplacement age of the leucogranite. Furthermore, as it shows evidence of mylonitic deformation, these data have been interpreted to mark the lower limit of the FMSZ activity suggesting that it is a late Variscan shear zone. Musumeci and Colombo (2002) interpret the activity of the FMSZ as linked to an extensional tectonic setting. On the contrary Corsini et al. (2004) and Sanchez et al. (2011) propose a deformation age of ~22 and ~20 Ma, respectively, obtained by 40 Ar/39 Ar on phengites from FMSZ mica schists. This suggests that the FMSZ suffered a nearly complete reactivation during Alpine Orogeny with a strike-slip kinematics. Recently, Carosi et al. (2016) and Simonetti et al. (2017, 2018) demonstrated that the FMSZ is a transpressional type II strain softening shear zone active during Variscan time. Detailed investigation of the

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main mineral recrystallization mechanisms and of the syn-kinematic mineral associations, along the gradient, revealed that the activity of the shear zone happened under decreasing temperature conditions. Deformation in the FMSZ started at ~340 Ma under high-temperature amphibolite-facies conditions and evolved during retro-metamorphism up to green schist-facies within at least ~20 Ma. The Alpine greenschist-facies metamorphic overprint is generally weak in the Argentera Massif and limited to shear zones cross-cutting the Variscan structures (Compagnoni et al. 2010). The main alpine structures of the Argentera Massif are the NW-SE striking Bersezio Fault Zone and the E–W striking Fremamorta-Colle del Sabbione Shear Zone. Such brittle and brittle–ductile structures developed during the lower Miocene as a consequence of the transpressional regime that affected the whole Argentera Massif during this time (Baietto et al. 2009).

4 Geological Field Trip The two-days field trip develops entirely in the Valle Stura di Demonte (CN, Italy). The stops are located in the Ischiator (Fig. 3) and Pontebernardo (Fig. 4) valleys (Fig. 2b). Locations of the first day are reachable following the road SS21 that starts from Borgo San Dalmazzo (CN) village up to Bagni di Vinadio and from here, traveling along the road for the locality of Besmorello where it is possible to park. From the parking location, all the stops can be reached by walking along the P26 marked trekking path. The itinerary develops in a high mountain environment and the overall altitude difference is about 750 m. Locations of the second day are reachable following the road SS21 up to the Pontebernardo village. The first stop is located along the road which leads to the locality of Prati del Vallone. From here the other stops can be reached by walking along the GTA marked trekking path and then following directions to Colle Panieris, along a minor path. The overall altitude difference is about 1000 m. Day 1—stop 1: ultramylonitic schist (coord. 44°17’20.9"N; 07°01’20.6"E) In the first stop (Fig. 3), we can observe the most sheared rocks of the FMSZ that crop out in the lower part of the Ischiator valley (Fig. 2b). They are ultramylonites (Fig. 5a) with > 90% matrix. They show a continuous cleavage (Fig. 5b) made by white mica and chlorite striking NW-SE dipping steeply toward the NE (Fig. 6a). This mineral assemblage indicates a greenschist-facies metamorphism. Mineral lineation plunges at low angle toward the NW. In agreement with the syn-kinematic mineral assemblage, quartz is recrystallized by subgrain rotation (Fig. 5c) (Piazolo and Passchier 2002; Stipp et al. 2002). This indicates temperature of the deformation to be ~400 °C. An S–C–C’ fabric, associated to a top-to-the SW sense of shear, is welldetectable both at the meso- and microscale (Fig. 5a and d). Flow analysis performed by Simonetti et al. (2018) with the C’ shear band method (Kurtz and Nortrup 2008) highlights a general shear with ~60% simple shear (Fig. 7).

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Fig. 3 Structural–geological map (1:10000 scale) of the Ischiator Valley (Fig. 2). 1: undifferentiated debris (udb); 2: biotite-bearing mylonitic gneiss (BtMGns); 3: leucogranite (lgn); 4: ultramylonitic leucogranite (lgnU); 5: mylonitic schist (MsBW); 6: ultramylonite (Uml); 7: mylonitic gneiss with biotite and white mica (MgBW); 8: protomylonitic gneiss with biotite and sillimanite (PgBS); 9: migmatitic gneiss of the Tinée complex (MgmBS)

Day 1—stop 2: mylonitic schist (coord. 44°17’14.1"N; 07°01’39.7"E) In this stop, it is possible to observe the most classical mylonites of the FMSZ. They are medium-grained dark-green mylonitic schist (Fig. 8a) made of quartz, K-feldspar and plagioclase porphyroclasts in a fine-grained biotite and white mica matrix (Fig. 8b). The amount of matrix is nearly 75%. Foliation strikes NW-SE and dips steeply toward the NE or SW with a mineral lineation that plunges at low angle toward the NW (Fig. 6b). The orientation of the main foliation is perfectly concordant with the one observed in the ultramylonitic schist at stop 1. Foliation is a penetrative disjunctive cleavage with smooth and parallel cleavage domains (Fig. 8b), made by biotite and white mica. Both at the meso- and microscale kinematic indicators are well recognizable. They are mainly represented by rotated porphyroclasts, quarter mats (Fig. 8c), S–C’ fabric (Fig. 8c), mica fish (Fig. 8d), and quartz oblique foliation representing a top-to-the SW or top-to-the SE sense of shear, depending on the dip direction of the foliation. Quartz is characterized by the presence of bigger grains with irregular boundaries and new grains of smaller size. Subgrains with irregular shape also exist (Fig. 8e). These

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Fig. 4 Structural–geological map (1:10000 scale) of the Pontebernardo Valley (fig. 2; modified after Carosi et al. 2016). 1: undifferentiated debris (Udb); 2: fluvial deposit (Fdp); 3: undifferentiated glacial deposit (Ugd); 4: limestone breccia (Lbc); 5: triassic quarzite(QtzTr); 6: alpine mylonitic schist with chlorite and white mica (MsGS); 7: migmatitic gneiss of the GSV complex (MgmB); 8: biotite-bearing mylonitic gneiss (BtMGns); 9: mylonitic leucogranite (Lgn); 10: ultramylonitic leucogranite (LgnU); 11: amphibole-bearing mylonitic gneiss (AmGns); 12: ultramylonitic schist (UmlS); 13: mylonitic schist (MsBW); 14: quarzite (Qtz); 15: phyllonite (Pll); 16: ultramylonite (Uml); 17: mylonitic gneiss with biotite and white mica (MgBW); 18: mylonitic gneiss with biotite and sillimanite (MgBS); 19: protomylonitic gneiss with biotite and sillimanite (PgBS); 20: amphibole-bearing migmatitic gneiss (GnsAmp); 21: migmatitic gneiss of the Tinée complex (MgmBS); 21: amphibolite (Amp). Yellow stars show the stops of the itinerary

features suggest subgrain rotation and recrystallization (Piazolo and Passchier 2002; Stipp et al. 2002) as the main dynamic recrystallization mechanism overprinting a previous grain boundary migration. In some domains, effects of grain boundary migration such as window and pinning structures can still be recognized (Fig. 8f).This indicates a deformation compatible between the transition of the two recrystallization mechanisms at ~500 °C. Kinematic vorticity analysis (Fig. 7) in these rocks was undertaken by Simonetti et al. (2018) and revealed a general flow with a dominant component of pure shear

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Fig. 5 a Ultramylonite at the outcrop scale, an S-C’ fabric is indicative of a top-to-the SW sense of shear; b continuous cleavage made by white mica and chlorite. Amount of matrix is over 90% (crossed nicols); c quartz in the ultramylonite with bimodal grainsize, subgrains and new grains indicative of a subgrain rotation recrystallization (crossed nicols); D) S-C-C’ fabric in the ultramylonite pointing a top-to-the SW sense of shear (crossed nicols)

between ~55–70% (C’ shear band method and stable porphyroclasts method). In situ U-Th–Pb petrochronology on syn-kinematic monazites (Fig. 9) constrain the age of shearing in these rocks at ~320 Ma (Simonetti et al. 2018). Day 1—stop 3: mylonitic gneiss with biotite and white mica (coord. 44°16’58.1"N; 07°01’09.2"E) Following the P26 path, we reach Rifugio Migliorero (Fig. 2b) in the middle of Ischiator valley (Fig. 3). Here elegant outcrops of protomylonitic gneiss exist (Fig. 10a). These rocks are made of K-feldspar, plagioclase, and quartz porphyroclasts in a finegrained biotite and white mica matrix (Fig. 10b). The amount of matrix is ~50%. The foliation is oriented NW-SE and dips at high angle toward the NE (Fig. 6c), similar to previous stops. The foliations are penetrative/disjunctive type with smooth and parallel cleavage domains (Fig. 10b). Quartz grains, variable between micrometric and millimetric size, show lobate grain boundaries (Fig. 10c) indicating that the main dynamic recrystallization mechanism is grain boundary migration. Incipient subgrain rotation recrystallization is recognized locally. The mineral assemblage indicative of amphibolite-facies conditions, and quartz microstructures reveal higher

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Fig. 6 Equal angle, lower hemisphere projections: a Poles to foliation (yellow squares, 60 data) and mineral lineation (blue triangles, 13) in the ultramylonitic schists; b Poles to foliation (green dots, 163 data) and mineral lineation (blue triangles, 27 data) in the mylonitic schists; c poles to foliation (pink diamonds, 110 data) and mineral lineation (blue triangles, 18 data) in the protomylonitic gneiss; d poles to foliation (blue dots, 53 data) and fold axis (red triangles, 13 data) in the Tinèe complex; e poles to foliation (blue dots, 39 data) and fold axis (red triangles, 11 data) in the GessoStura-Vesubie complex; f poles to mylonitic foliation (gray dots, 17 data) and mineral lineation (pink triangles, 7 data) in the Colle Panieris-Mt Peiron alpine thrust

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Fig. 7 a Polar istograms used for calculating kinematic vorticity number (Wk) with the C’ shear band method (Kurz and Northrup 2008) for a protomylonite (Wk = 0.34), a mylonite (Wk = 0.61) and a ultramylonite (Wk = 0.81); b results of kinematic vorticity analysis with stable porphyroclasts method (Passchier, 1987; Wallis et al. 1993; Law et al. 2004) in a protomylonitic and in a mylonitic sample. Data are represented with Rigid Grain Net graphs (Jessup et al. 2007). Wk values are 0.56 and 0.67 for protomylonite and mylonite, respectively. Values indicate a pure shear-dominated deformation and point out also an increase of the simple shear component in the mylonite

deformation temperature, >~500 °C (Piazolo and Passchier 2002; Stipp et al. 2002; Passchier and Trouw 2005), compared to the mylonitic schist at stop 2. At the outcrop scale, asymmetric and stretched leucosomes are recognizable. In thin section, rotated porphyroclasts and porphyroclasts with asymmetric strain shadows (Fig. 10d), mica fish, C and C’ shear band occur. Kinematic indicators point to a top-to-the SW shear. Vorticity estimation (Fig. 7) in these rocks revealed a deformation with a prevalent component of pure shear (~70%; Simonetti et al. 2018). Recent U-Th–Pb in situ petrochronology on syn-kinematic monazites, revealed a deformation age of ~ 328 Ma (Fig. 9; Simonetti et al. 2018), older than the age of shear in the mylonitic schist of stop 2.

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Fig. 8 a Medium-grained dark-green mylonitic schist at the outcrop scale with S-C’ fabric pointing to a top-to-the SW sense of shear is well-detectable b feldspar porphyroclasts in a finer matrix (nearly 75%), foliation is a penetrative disjunctive cleavage with smooth and parallel cleavage domain made by biotite and white mica (crossed nicols); c S-C’ fabric and white mica quarter mats (green arrow) around a feldpsar porphyroclast, both kinematic indicators are consistent with a top-to-the SW sense of shear (crossed nicols); d micafish made of white mica showing a top-to-the SW sense of shear (crossed nicols); e quartz grains with irregular boundaries and new grains of smaller size, subgrains are also present (crossed nicols); f quartz with lobate grain boundaries in a domain where grain boundary migration effcts are still preserved (crossed nicols)

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Fig. 9 a Age of shear deformation along the deformation gradient (U–Th–Pb on monazite; Simonetti et al. 2018); b Examples of textural position and zoning of monazite grains selected for in situ dating in a protomylonite and a mylonite. Compositional map of Y shows the spot location and the corresponding 206 Pb/238 U age (modified from Simonetti et al. 2018)

Day 1—stop 4: protomylonitic gneiss with biotite and sillimanite (coord. 44°16’46.8"N; 07°00’59.1"E) We observe protomylonitic gneiss with biotite and sillimanite located at the boundary of the FMSZ (Fig. 11a). At the outcrop scale, this lithotype is very similar to the mylonitic gneiss of stop 3, the main difference is the presence of sillimanite and the lower abundance of white mica (Carosi et al. 2016). Asymmetric and stretched leucosomes, variable between centimetric and decimetric size, are recognizable (Fig. 11a).The amount of matrix in this lithotype is ~40% which testifies a less intense deformation of these rocks compared to those visited in stops 1, 2, 3 and 4. The main foliation, oriented NW-SE and dipping at high angle toward the NE, is a penetrative disjunctive cleavage (Fig. 11b) with smooth and anastomosed cleavage domains rich in biotites and prismatic or fibrolitic sillimanite (Fig. 11b). The orientation of the main foliation is perfectly concordant with the orientation observed in the more sheared rocks. At the microscale kinematic indicators, such as porphyroclasts with

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Fig. 10 a Protomylonitic gneiss cropping out close to the Migliorero hut, an S–C’ fabric pointing to a top-to-the SW sense of shear; b penetrative disjunctive cleavage with smooth and parallel cleavage domains in the protomylonitic gneiss (crossed nicols); c quartz showing lobate grain boundaries indicative of grain boundary migration (crossed nicols); d feldspar porphyroclasts with asimmetric strain shadows indicative of a top-to-the SW sense of shear (crossed nicols)

asymmetric strain shadows, mica fish, and C–C’ shear bands are present (Fig. 11c). All of them point to a top-to-the SW sense of shear. Quartz is dynamically recrystallized by grain boundary migration (Fig. 11d). Deformation temperature above 500 °C is in agreement with the syn-kinematic biotite + sillimanite assemblage and quartz microstructures. The age of shear deformation, obtained with in situ U–h–Pb petrochronology, on syn-kinematic monazites (Simonetti et al. 2018), is ~ 340 Ma (Fig. 9). Deformation is therefore older compared to the age of shear deformation obtained in the mylonitic schist (stop 2) and in the mylonitic gneiss (stop 3). Day 1—stop 5: migmatitic gneiss of the Tinèe Complex (coord. 44°16’39.2"N; 07°00’29.3"E) In the area near the Laghi dell’Ischiator (Fig. 3), migmatitic gneiss with biotite and sillimanite (‘Rabuons Formation’ in the literature; Faure-Muret 1955; Malaroda et al. 1970), belonging to the Tinèe Complex, crops out (Fig. 12a). Migmatites resulted from partial melting of metasediments (Malaroda et al. 1970; Ferrando et al. 2008; Compagnoni et al. 2010) are not affected by shear of the FMSZ. Leucosomes show variable thickness. These rocks show a disjunctive foliation (Fig. 12b) oriented

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Fig. 11 a Protomylonitic gneiss with biotite and sillimanite at the outcrop scale; b disjunctive cleavage with smooth and anastomosed cleavage domain rich in biotites and sillimanite.Rotated porphyroclasts show a a top-to-the SW sense of shear (parallel nicols); c S–C–C’ fabric indicative of top-to-the SW sense of shear (parallel nicols); d quartz showing lobate grain boundaries indicative of grain boundary migration (crossed nicols)

NW-SE and variably dipping to NE or SW (Fig. 6d). Quartz presents a big grain size and lobated margins in accordance with grain boundary migration deformation mechanism (Fig. 12c). Open to tight symmetric folds with subvertical axial planes, oriented NW-SE and sub-horizontal axes, are present (Fig. 12e). Fold axial plane is parallel to the mylonitic foliation within the FMSZ so that the folding event occurred in a strain regime compatible with the shearing deformation in the FMSZ (Simonetti et al. 2018). Folds contributed to accommodate the nearly horizontal shortening resulted from the pure shear component acting during transpression. Transpressional deformation is generally subjected to strain compatibility problems (Hudleston 1999) because it is not easy to explain how the flattening component of the deformation is accommodated. The presence of syn-shear folds in the unsheared migmatites testifies that the Tinée Complex and the shear zone could have been stretched together during transpression and therefore no strain compatibility problems are present.

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Fig. 12 a Migmatitic gneiss of the Tinée complex at the outcrop scale; b disjunctive foliation of the migmatitic gneiss (crossed nicols); c quartz showing big grainsize and lobated margins indicative of grain boundary migration (crossed nicols); d symmetric folds with subvertical NW-SE striking axial plane in the migmatitic gneiss

Day 2—stop 1: migmatites of the Gesso-Stura-Vesubie Complex (Coord. 44°20’00.8"N; 06°59’54.6"E) From Pontebernardo, we follow the road toward the locality Prati del Vallone (Figs. 2b and 4). Halfway down the road it is possible to leave the car and follow a footpath up to a small abandoned technical building where we can reach the first outcrop of the day. Here migmatites of the Gesso-Stura-Vesubie Complex, formed at the expense of metasedimentary rocks, can be observed (Fig. 13a). These rocks are medium-grained migmatitic gneiss with a quartz-plagioclase (An20–30 )-biotite-fibrolitic sillimanitecordierite assemblage with few K-feldspar and muscovite (Fig. 13b). Relict kyanite and garnet rarely included in plagioclase suggest that these rocks reached P-T conditions of the upper amphibolite facies (650–700 °C; 0.6–0.8 GPa) and subsequently suffered a decompression from the kyanite to the sillimanite stability field (Compagnoni et al. 2010). Meter-thick leucocratic portions with less biotite locally occur. About 10–20 Ma after the Carboniferous HP metamorphism, the Argentera Massif underwent an amphibolite-facies metamorphism with extensive development of migmatites at 323 ± 12 Ma (Rubatto et al. 2001). Older ages for the anatectic event are reported from other Variscan migmatites in different segments of the Variscan chain (Compagnoni

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Fig. 13 a Para-derived migmatites belonging to the Gesso-Stura-Vesubie Complex; b alternating levels of red biotite and quartz + cordierite domains that materialize a gneissic foliation (parallel nicols); c folds affecting the gneissic foliation of the GSV migmatites; d gneissic foliation folded in thin section (parallel nicols)

et al. 2010; Oliot et al. 2015; Schneider et al. 2014), suggesting that several melting events may have succeeded one to each other in a complex anatectic history. Structurally, the migmatites show a disjunctive foliation that is affected by open to tight symmetric folds (Fig. 13c, d), with fold axes gently plunging toward the NW and subvertical NW-SE striking axial planes (Fig. 6e; Carosi et al. 2016). Axial planes are parallel to the mylonitic foliation of the FMSZ (Fig. 14a). Similarly to the structural framework observed in stop 5 of day 1, folding event occurred in a strain regime compatible with shear in the FMSZ (Fig. 14b; Carosi et al. 2016; Simonetti et al. 2018) and contributed to accommodate a nearly horizontal shortening. We come back on the road, and we reach the locality Prati del Vallone where we can leave the car. Day 2—stop 2: sheared amphibole-bearing gneiss (coord. 44°19’06.7"N; 06°59’01.5"E) In this stop, we can observe a decameter-sized lense of amphibole-bearing gneiss (Fig. 4) within the FMSZ (“Embrechiti ed anatessiti anfiboliche” according to Malaroda et al. 1970). These are light-colored gneiss made of amphibole (hornblende), quartz, biotite, and plagioclase (An 50–60%) generally with a massive

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Fig. 14 Geological cross-section of the FMSZ (legend is the same as in Fig. 4; modified after Carosi et al. 2016): a section across the Pontebernardo Valley (section A, Fig. 4); b relationship between the mylonitic foliation in the FMSZ and the folds observed in both the GSV and the Tinée complexes (Carosi et al. 2016); c section across the Mt Peiron-Colle di Stau-Rocco Verde (section B, Fig. 4)

appearance (Fig. 15a) or only weakly foliated (Fig. 15b). When present, the foliation is disjunctive(Fig. 15c). The gneiss belongs to the Tinée complex and extensively crops out in the north-western sector of the crystalline basement of the Argentera Massif (Fig. 4; Carosi et al. 2016). According to Compagnoni et al. (2010), the protolith of the gneiss is an intermediate igneous rock. Several lenses of this lithotype (Fig. 15d), showing different amounts of deformation, exist within the FMSZ (Carosi et al. 2016). Sometimes deformation is very strong so that the original structure of the rock is nearly unrecognizable (Fig. 15e) and the percentage of matrix in higher than 65%. Because of the position of this lense, very close to the central part of the shear zone, it is possible to observe sheared portions with evident S-C-C’ fabric indicative of a top-to-the SE sense of shear (Fig. 15f). Day 2—stop 3: Mt Peiron-Colle Panieris alpine thrust (coord. 44°19’31.6"N; 06°57’24.6"E) From Prati del Vallone, we follow the ‘GTA’ path and then we go toward the Colle Panieris on a minor path. Below the Mt Peiron, we can observe evidence of Alpine deformation in the basement of the Argentera Massif. In the Mt Peiron-Colle Panieris area, a mylonitic belt with metric thickness (Fig. 4) thrusts amphibole-bearing mylonitic gneiss of the FMSZ above Permo-Triassic sedimentary cover of the Argentera Massif (Fig. 16a; 14c), represented by light-colored quartzite. Mylonitic foliation strikes nearly E–W and gently dips toward the N with a NNW plunging mineral lineation (Fig. 6f). Mylonite are made of quartz and feldspar porphyroclasts in a fine-grained chlorite + white mica matrix (Fig. 16b). The

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Fig. 15 a Amphibole-bearing gneiss with massive aspect; b foliated amphibole-bearing gneiss; c amphibole-bearing gneiss in thin section, the disjunctive foliation can be recognized (parallel nicols); d decametric lenses of amphibole-bearing gneiss (highlighted in yellow) within the FMSZ; e low strain domain and strongly sheared domain with S–C’ fabric in the amphibole-bearing gneiss, sense of shear is top-to-the SW; f S–C’ fabric at the outcrop scale, top-to-the SW sense of shear

matrix ranges 65–75%. The mineral assemblage indicates greenschist-facies metamorphism. Foliation varies from a continuos to a disjunctive cleavage with zonal and parallel cleavage domains. An S–C fabric is well recognizable both at the outcrop scale (Fig. 16c) and in thin section is indicative of an inverse top-to-the SSE sense of shear. Quartz is dynamically recrystallized by subgrain rotation recrystallization, and feldspar porphyroclasts are fractured suggesting temperature of deformation in agreement with the syn-kinematic mineral assemblage.

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Fig. 16 a Mylonitic belt with metric thickness thrusting amphibole-bearing mylonitic gneiss of the FMSZ above Permo-Triassic sedimentary cover of the Argentera Massif in the Mt Peiron-Colle Panieris area; b mylonite of the Mt Peiron-Colle Panieris thrust in thin section, quartz and feldspar porphyroclasts in a fine-grained chlorite + white mica matrix can be recognized (crossed nicols); c S-C fabric at the outcrop scale showing a top-to-the S sense of shear

The Argentera Massif was involved in the Alpine orogeny at ~ 22 Ma (Corsini et al. 2004; Sanchez et al. 2011). Most of the deformation is concentrated in the sedimentary covers (Malaroda et al. 1970; Barale et al. 2016; d’Atri et al. 2016) that are detached from the metamorphic basement along evaporites and limestone breccias levels. Alpine overprint in the basement is limited and concentrated along ductile and brittle ductile shear zones (Compagnoni et al. 2010; Carosi et al. 2016; Simonetti et al. 2017, 2018), the Mt Peiron-Colle Panieris thrust can be considered as one of such shear zones. Day 2—stop 4: undeformed Permo-Triassic quartzite (coord. 44°19’18.8"N; 06°57’12.8"E) Not far from stop 2, following a small path toward the SW for nearly 450 m, whitegray quartzites of Permo-Triassic age (Barale et al. 2016) crops out and lay in angular

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unconformity directly above the migmatitic basement (Fig. 17a). They are made of quartz and very few white mica (Fig. 17b), ripple marks are often well recognizable (Fig. 17c). Above the quartzites tectonic carbonatic breccias known as ‘Formazione delle Carniole’ (Malaroda et al. 1970) with both gypsum and dolomitic limestone clasts, occur (Fig. 4). They are interpreted as weak horizons along which the detachment of the sedimentary succession occurs. The presence of undeformed Permo-Triassic sediments directly above the basement is a good constrain for the age of Variscan transpression.

Fig. 17 a Permo-Triassic quarzites in angular unconformity directly above the migmatitic basement in the Colle Panieris area; b fine-grained white mica in the quarzite (crossed nicols); c sedimentary bedding with ripple marks

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5 Discussion The features observed in the outcrops of the field trip highlight a complex evolution of the FMSZ. Kinematic vorticity analysis (Simonetti et al. 2018) highlights a transpression (Fig. 18a) with a variable component of simple shear both spatially and temporally (Fig. 18b), increasing along the deformation gradient from the wall rocks to the high-strain zone in the central part of the FMSZ (Fig. 18c). This matches with the structural asset of the FMSZ in the field where a subvertical mylonitic foliation can be recognized, with constant orientation in all the lithotypes. This is in

Fig. 18 a Diagram showing relationship between the orientation of the maximum Instantaneous Stretching Axis (ISAmax) with respect to the shear zone boundary (angle θ) related to the kinematic vorticity number Wk (modified after Fossen and Tikoff 1993; Fossen et al. 1994). Wk value for which simple shear = pure shear is 0.71 according to Law et al. (2004) and Xypolias (2010). The distribution of the samples shows a variation from a pure shear-dominated transpression to a simple shear-dominated transpression linked to the increase of the vorticity number. Orange dashed line represents the theoretical trend of the angle θ in relation to Wk; b Percentage of pure shear (PS) and simple shear (SS) in relation to the calculated maximum and minimum Wk values; c variation of the vorticity number in relation to the distance of the sample from the center of the shear zone. The distribution of points shows increasing Wk values (i.e., increasing of simple shear component of deformation) toward the central part of the shear zone (trend: blue-dashed line)

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agreement with the model of transpression (Fossen et al. 1994). Folds with subvertical axial plane parallel to the mylonitic foliation in the GSV Complex and Tinèe Complex is another evidence of horizontal shortening attributed to the pure shear component. Because the two walls of the FMSZ (the GSV and the Tinèe Complex) can deform together with the shear zone, no strain compatibility problems are present (Simonetti et al. 2018). In situ petrochronology (combining U-Th–Pb ages with textural observations and monazite chemistry) revealed that the Variscan transpression in the Argentera Massif started at the metamorphic peak of the GSV complex (dated at ~340 Ma; Compagnoni et al. 2010; Rubatto et al. 2010), or shortly after, and triggered its exhumation (Simonetti et al. 2018). This approach revealed a long-lasting activity of the FMSZ that activated for at least 20 Ma under changing metamorphic conditions, from high to low temperatures. The presence of medium- to high-grade metamorphic mylonites associated with lower-grade ones, localized in the central part of the FMSZ, the strong deformation gradient and the changes in the deformation regime and age of deformation along the gradient evidence that the FMSZ evolved as a strain softening type II shear zone (Fig. 19; Fossen 2016). In this type of shear zone, deformation progressively concentrates in the central part because of strain softening (due to the presence of channeled fluids, metamorphic reactions, and grain size reduction) leaving the external parts of the structure progressively inactive (Fig. 19). Because of this the active thickness decreases, whereas the total thickness remains constant (Fig. 19). Strain softening can be enhanced by the presence of fluids joined with metamorphic reactions and grain size reduction, well-detectable both at the meso—and microscale. Evidence of the first two factors is the breakdown of amphibolite-facies minerals such as sillimanite and biotite and consequent growth of white mica and chlorite in the most sheared rocks. Along the deformation gradient a reduction in the amount of quartz and feldspar in the rocks, in favor of the amount of phyllosilicates, is evident. Because of the type II evolution, protomylonites in the external part of the FMSZ preserve features acquired during the early stage of shear (testified by the older age of deformation) during high-temperature conditions, whereas mylonites, in the internal part of the shear zone, record features acquired during younger deformation stages at lower temperature. Finally, the ultramylonites preserve evidence of the final stage of activity of the shear zone. Despite a similar evolution, characterized by strain localization during decreasing temperature, has been recently described in detail in the Yukon River shear zone (Northern Cordillera, Canada; Parsons et al. 2018), other well-described examples of regional-scale strain softening shear zones are not so common in the literature up to now. The FMSZ should be considered as a new example of a strain softening regional-scale shear zone, that can be useful to the community of structural geologists for future process-oriented investigations. Furthermore, the described data are yet another example that, as pointed out by Fossen and Cavalcante (2017) and Oriolo et al. (2018), a multidisciplinary approach is strictly necessary to obtain meaningful results to address unsolved problems in modern structural geology and in the study of shear zones, were complexity arises

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Fig. 19 Sketch summarizing the evolution of the Ferriere-Mollières Shear Zone according to the type II growth model (Fossen 2016). Graphs show the theoretical variation of total and active thickness during time and the amount of strain within the shear zone

from the coupling between long-lasting progressive and/or polyphasic deformation, metamorphism and fluid–rock interaction. Detailed field work and meso-structural analysis should always be combined with microstructural observations, with modern techniques for vorticity analysis, with deformation temperature estimation and with detailed petrochronology in order to unreveal the fundamental steps of the tectono-metamorphic history of shear zones.

6 Conclusive Remarks • A deformation gradient, marked by a transition from protomylonites to mylonites and to ultramylonites, is recognizable from the marginal part of the FMSZ shear zone toward the central part;

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• Foliation of the FMSZ shows a constant trend of orientation in all the different types of mylonites; • Fold axial planes the GSV complex and Tinèe complex are subparallel to the mylonitic foliation; • Vorticity analysis constrained a transpressional deformation with a prevalent component of pure shear acting together with simple shear. Simple shear component increases along the deformation gradient; • Syn-kinematic mineral assemblage, along the deformation gradient, highlights a syn-shear retro-metamorphism from high-temperature amphibolite-facies to greenschist-facies. Quartz microstructures also highlight a temperature decrease during deformation; • Because of strain softening phenomena, deformation in the FMSZ starts to concentrate in its central part leaving the margin inactive according to a type II evolution; • Across the deformation gradient it is possible to observe different stages of formation of the FMSZ; • In situ U-Th–Pb petrochronology on monazite revealed a long-lasting activity of the FMSZ (between ~ 340 Ma and ~ 320 Ma) and constrained the different strain softening evolution stages; • The FMSZ is a very good case study of strain softening shear zone at regional scale, for future process-oriented investigations. Acknowledgements The staff of Rifugio Migliorero and the staff of Rifugio Prati del Vallone are acknowledged for the hospitality during field work. Research supported by funds from Torino University (Ricerca Locale 2017, 2018) and PRIN 2015 (resp. R. Carosi and C. Montomoli). The Springer team (Marion Schneider, Annett Buettener, Boopalan Renu, Alexis Vizcaino, Doerthe Mennecke-Buehler) is thanked for proofreading and other assistance. Soumyajit Mukherjee is thanked for editorial work and constructive review. Dutta and Mukherjee (2021) encapsulate this work.

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Barale, L., Bertok, C., D’atri, A., Martire, L., Piana, F., & Domini, G. (2016). Geology of the Entracque-Colle di Tenda area (Maritime Alps, NW Italy). Journal of Maps, 12, 359–370. Bellot, J.P. (2005). The Palaeozoic evolution of the Maures massif (France) and its potential correlation with other areas of the Variscan Belt: a review. In: R. Carosi, R. Dias, D. Iacopini, G. Rosenbaum (eds) The southern Variscan belt. Journal of the Virtual Explorer, 19, 1441–8142 Brun, J. P., & Burg, J. P. (1982). Combined thrusting and wrenching in the Ibero-Armorican arc: A corner effect during continental collision. Earth and Planetary Science Letters 61, 319–332. Burg, J.-P., & Matte, P. (1978). A cross section through the French Massif central and the scope of its variscan ggeodynamic evolution. Zeitschrift der Deutschen Geologischen Gesellschaft, 129, 429–460. Carosi, R., & Palmeri, R. (2002). Orogen-parallel tectonics transport in the Variscan belt of northeastern Sadinia (Italy): Implications for the exhumation of medium-pressure metamorphic rocks. Geological Magazine, 139, 497–511. Carosi, R., Montomoli, C., Tiepolo, M., & Frassi, C. (2012). Geochronological constraints on post-collisional shear zones in the Variscides of Sardinia (Italy). Terra Nova, 24, 42–51. Carosi, R., D’addario, E., Mammoliti, E., Montomoli, C., & Simonetti, M. (2016). Geological map of the northwestern portion of the Ferriere-Mollieres shear zone, Argentera Massif, Italy. Journal of Maps, 12, 466–475. Carosi, R., Frassi, C., Iacopini, D., & Montomoli, C. (2005). Post collisional transpressive tectonics in northern Sardinia (Italy). Journal Virtual Explorer, 19(3), 1–18. Carosi, R., Montomoli, C., Iaccarino, S., & Visonà, D. (2018). Structural evolution, metamorphism and melting in the greater Himalayan sequence in central-western Nepal. In P. J. Treloar & M. P. Searle (Eds.), 2019 Himalayan tectonics: A modern synthesis (Vol. 483, pp. 305–323). London:Geological Society, Special Publications. Compagnoni, R., Ferrando, S., Lombardo, B., Radulesco, N., & Rubatto, D. (2010). Paleo-European crust of the Italian western alps: Geological history of the argentera massif and comparison with Mont Blanc-Aiguilles Rouges and Maures-Tanneron Massifs. In M. Beltrando, A. Peccerillo, M. Mattei, S. Conticelli & C. Doglioni (Eds.), Journal of the Virtual Explorer, 36, 4. https://doi.org/ 10.3809/jvirtex.2009.00228 Corsini, M., Ruffet, G., & Caby, R. (2004). Alpine and late Hercynian geochronological constraints in the Argentera Massif (Western Alps). Eclogae Geologicae Helvetiae, 97, 3–15. Corsini, M., & Rolland, Y. (2009). Late evolution of the southern European Variscan belt: Exhumation of the lower crust in a context of oblique convergence. Comptes Rendus Geosciences, 341(2–3), 214–223. Cottle, J. M., Searle, M. P., Jessup, M. J., Crowley, J. L., & Law, R. D. (2015). Rongbuk Re-visited: Geochronology of Leucogranites in the footwall of the south tibetan detachment system, Everest region, Southern Tibet. Lithos, 227, 94–106. Cruciani, G., Montomoli, C., Carosi, R., Franceschelli, M., & Puxeddu, M. (2015). Continental collision from two perspectives: A review of Variscan metamorphism and deformation in northern Sardinia. Periodico di mineralogia, 84(3), 657–699. d’Atri, A., Piana, F., Barale, L., Bertok, C., & Martire, L. (2016). Geological setting of the southern termination of Western Alps. International Journal of Earth Sciences (Geol Rundsch), 105, 1831–1858. https://doi.org/10.1007/s00531-015-1277-9. Dias, R., & Ribeiro, A. (1995). The Ibero-Armorican Arc: A collision effect against an irregular continent? Tectonophysics, 246, 113–128. Dias, R., Ribeiro, A., Romão, J., Coke, C., & Moreira, N. (2016). A review of the arcuate structures in the Iberian Variscides; constraints and genetic models. Tectonophysics, 681, 170–194. https:// doi.org/10.1016/j.tecto.2016.04.011. Di Vincenzo, G., Carosi, R., & Palmeri, R. (2004). The relationship between tectono-metamorphic evolution and argon isotope records in white mica: Constraints from in situ 40 Ar–39 Ar laser analysis of the Variscan basement of Sardinia. Journal of Petrology, 45(5), 1013–1043. Di Vincenzo, G., Talarico, F., & Kleinschmidt, G. (2007). An 40Ar–39Ar investigation of the mertz glacier area (George V Land, Antarctica): implications for the ross orogen–east antarctic craton

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The Geometry and Kinematics of the Southwestern Termination of the Pyrenees: A Field Guide to the Santo Domingo Anticline E. L. Pueyo, B. Oliva-Urcia, E. M. Sánchez-Moreno, C. Arenas, R. Silva-Casal, P. Calvín, P. Santolaria, C. García-Lasanta, C. Oliván, A. Gil-Imaz, F. Compaired, A. M. Casas, and A. Pocoví Abstract We introduce a field trip to the southwestern termination of the Pyrenean sole thrust: the Santo Domingo anticline. The field trip is articulated in three main stops with panoramic views. We pursue to emphasize some outstanding characteristics of this structure: (A) the large-scale progressive (laterally angular) unconformity that crops out in its southern flank, fully records its folding kinematics and witnesses for two distinct deformation periods in relation to the basement activity (Gavarnie gentle folding and Guarga paroxism) (stop #1). (B) The remarkable conical geometry E. L. Pueyo (B) Instituto Geológico y Minero de España, Unidad de Zaragoza. C/Manuel, Lasala 44, 9º, 50006 Saragossa, Spain e-mail: [email protected] E. L. Pueyo · C. Arenas · P. Santolaria · A. Gil-Imaz · A. M. Casas · A. Pocoví Unidad Asociada en Ciencias de la Tierra, IGME-Universidad de Zaragoza, Saragossa, Spain B. Oliva-Urcia Dpto. de Geología y Geoquímica, Universidad Autónoma de Madrid, 28049 Madrid, Spain E. M. Sánchez-Moreno · P. Calvín Dpto. de Física, Universidad de Burgos, Av/Cantabria, S/N, 09006 Burgos, Spain C. Arenas · A. Gil-Imaz · A. M. Casas · A. Pocoví Geotransfer (IUCA), Universidad de Zaragoza, Saragossa, Spain C. Arenas · R. Silva-Casal · A. M. Casas · A. Pocoví Instituto Universitario de Investigación en Ciencias Ambientales, Universidad de Zaragoza, Saragossa, Spain P. Santolaria Universitat de Barcelona, Geomodels Research Institute, Barcelona, Spain C. García-Lasanta Geology Department, Western Washington University, Bellingham, WA 98225-9080, USA C. Oliván Freelance Geologist. C/Duquesa de Villahermosa1, 2º, 22001 Huesca, Spain F. Compaired Servicio de Planificación y Gestión Forestal. Dpto. Medio Ambiente. Gobierno de Aragón, Saragossa, Spain © Springer Nature Switzerland AG 2021 S. Mukherjee (ed.), Structural Geology and Tectonics Field Guidebook—Volume 1, Springer Geology, https://doi.org/10.1007/978-3-030-60143-0_3

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of the fold termination (San Marzal area) that is completely documented by structural and palaeomagnetic analyses (stop #2). (C) The rotational kinematics of this structure recorded in its northern flank. Both the folding and the rotational kinematics can be wholly time-bracketed by means of seven long magnetostratigraphic sections and numerous additional palaeomagnetic sites within the syn-tectonic (syn-rotational) sequence (stop #3). We also describe the overall structure of the Western Pyrenees from a panoramic view from the summit of the anticline (1520 m). All these stops are accessible by car (dirt roads). Finally, an additional field stop (#4) is devoted to the sedimentology of the Lutetian-Bartonian carbonates following a hiking path (3 km) from stop #3.

1 Introduction The Southern Pyrenees (Barnolas et al. 2019; Muñoz 2019; Martín-Chivelet et al. 2019), and specifically their western part (Teixell 1992, 1996; Martinez-Peña and Casas 2003; Casas and Pardo 2004; Millán et al. 2006; Meresse 2010; Labaume and Teixell 2018, Labaume et al. 2016), have been the target for numerous geological field trips for academic and industry organizations seeking to understand the sedimentary architecture of potential reservoirs and to establish tectonic models for hydrocarbon traps during the last decades. This is due to several reasons: the outstanding presentday exposure conditions of the rocks favoured by the dry, Mediterranean climatic conditions (precluding a dense forest cover). In addition to that, the exhumation level endured by this mountain range during and after its construction adequately exposes rocks that were buried below ~ 4000–6000 m of sediments. In addition, the endorheic character of its foreland basin (Ebro), at least, until 11–9 Ma (Garcés et al. 2020), considerably slowed down the erosional evolution of the landscape (low exhumation in distal areas but relatively strong in source areas of the mountain belt) (Fitzgerald et al. 1999). Another additional and suggestive factor in the Pyrenees is the occurrence of numerous syntectonic sedimentary records, especially (but not only) during the Cenozoic (Puigdefàbregas and Souquet 1986). Classic examples of growth strata were described in the eastern Pyrenees (so-called progressive unconformities after Riba 1976) but they can be found all along the mountain range. Focusing on the Southwestern Pyrenees, some classic examples are well-known; Pico del Águila anticline (Millán et al. 1994), Balzes anticline (Barnolas and Gil-Peña 2001), Sos del Rey Católico (Turner 1988; Teletzke 2012; Anastasio et al. 2020), etc.… (see overview of structures in Millán et al. 2000). The opportunity given by syntectonic deposits (growth strata, Suppe et al. 1992) to understand the deformation kinematics together with the powerfulness of magnetostratigraphic studies to semicontinuously date sedimentary piles, motivated pioneer studies of these geometries in the 90’s (Burbank et al. 1992a, b; Bentham 1992; Hogan 1993; Holl and Anastasio 1993; Hogan and Burbank 1996; Bentham and Burbank 1992, 1996; Meigs 1997, etc.). Numerous magnetostratigraphic investigations followed those works

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(Beamud et al. 2003; Oliva-Urcia et al. 2019; Anastasio et al. 2020) and were focused on both the South Pyrenean Basin and the Ebro foreland Basin (see overviews by Pérez-Rivarés et al. 2018; Garcés et al. 2020) producing one of the best illustrated chronostratigraphic frames to understand the source to sink evolution of orogens and basins. Therefore, the collision-related geological features and the occurrence of exceptional and well-dated syntectonic sequences, located in easily accessible locations preserved across the Southern Pyrenees, have facilitated increasing interest of the scientific community to provide a detailed understanding of the geological evolution of the mountain range. Only in the last five decades, more than a thousand scientific contributions (see overview by Pocoví 2019) have targeted multiple geological topics in the Pyrenees (e.g. Van der Voo 1969; Van Der Voo and Boessenkool 1973; Burbank et al. 1992; Choukroune 1992; Puigdefàbregas et al. 1992; Muñoz 1992; Coney et al. 1996; Friend et al. 1996; García-Ruiz and Valero-Garcés 1998; Vergés et al. 1998); this scientific production is actively increasing nowadays (e.g. Chevrot et al. 2018; Marcén et al. 2018; Núñez-Lahuerta et al. 2018; Teixell et al. 2016; Sancho et al. 2018; Silva-Casal et al. 2019; Uzel et al. 2020). The Southern Pyrenees are a commonly visited destination for field trips in undergraduate and graduate academic geology programmes from institutions all around the world. Within their academic offer, neighbouring universities, such as Universidad de Zaragoza, Université Paul Sabatier (Toulouse), Université de Pau et des Pays de l’Adour, Euskal Herriko Unibertsitatea, Universitat de Barcelona and Universitat Autònoma de Barcelona, include Geology and Earth Sciences programmes that develop a wide offer of field trips (one-day or two-day visits in most cases) to multiple Pyrenean locations, in order to instruct students in a wide range of geological topics. But education-related geology visits to the Pyrenees are not restricted to the closest high education institutions in Spain and France; in addition, geology field trips in the Pyrenean Range are currently organized by numerous institutions worldwide, such as Imperial College of London, University College of London, Universiteit Utrecht, University of Plymouth, University of Derby, Universitetet i Bergen, University of Missouri, South Dakota School of Mines and Technology, Karlsruher Institut für Technologie or Nasjonal Forskerskole i Petroleumsfag (Petroleum Research School of Norway) among many others. In the past, additional institutions organized periodical Pyrenean field trips as well, including ETH Zurich, University of Leeds and Royal Holloway University of London. In other cases, education-related geology field trips to the Pyrenees were specific events in certain college programmes, such as the ones organized by the University of Wisconsin-Madison in 2001 or the University of Michigan in 2004, as well as part of workshops and periodical training courses developed by private companies (usually from the oil industry), like NEXT-Oil & Gas Training and Competency Development, STATOIL, TOTAL, etc. More than 15 years ago, the geological values of the Southwestern Pyrenees also motivated the development of outreach activities focused on the general public as the TransPyrenean Geological Route (www.rutatranspirenaica.com) between Huesca and Oloron, or the very active Sobrarbe-Pirineos UNESCO Global Geopark (https://www.geoparquepirineos.com/). Recently, other activities as the PrePyrenean

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Geological Train (Riglos sector) as well as several “Geolodays” organized by the Spanish Geological Society since 2005 (https://geolodia.es/) have been devoted to the Southern Pyrenean geology. However, most of these field trips have been focusing on visiting some of the renowned localities, mostly, along the Central Pyrenees, such as the Ordesa Valley (part of the Ordesa-Monte Perdido National Park), the Aínsa and Tremp-Graus Basins and the structural features of the South Pyrenean Frontal Thrust outcropping in Riglos and Pico del Águila areas. The westernmost termination of the Southern Pyrenean front (west of the Riglos pinnacles and the Gállego river transect) dominated by the large and complex Santo Domingo anticline (Puigdefàbregas and Soler 1973; Pocoví et al. 1990) has received very little attention and remains almost unexplored for outreach and academic purposes except for some British and Spanish universities that developed some research activities and academic works in the 90’s (Turner 1990; Turner and Hancock 1990a, b; Nichols 1987 and 1989; McElroy 1990; Arenas (1993); Millán 1996; Millán Garrido 2006). During the last years, many other studies had faced diverse research targets: structural geology (Teixell and García-Sansegundo 1995; Hervouët et al. 2005; Oliva-Urcia et al. 1996 and 2012a; Ramón et al. 2016a), sedimentology (Alegret and Aurell 1999, 2002; Silva-Casal et al. 2019), geophysics (Calvín et al. 2018), magnetostratigraphy (Oliva-Urcia et al. 2019; Silva-Casal et al. 2019; Anastasio et al. 2020) and palaeomagnetism (Pueyo et al. 2020 and references therein). This article introduces a field trip in this portion of the Pyrenees emphasizing some remarkable characteristic of its structure: (A) a large-scale progressive (laterally angular) unconformity (growth strata) in its southern flank that fully recorded the folding kinematics; (B) the remarkable conical closure of the fold termination (early described by Nichols 1984 and Millán et al. 1992); (C) the rotational kinematics of this structure (registered in its northern flank); (D) an overall description of the Western Pyrenees structure from a panoramic view in the summit of the anticline (1520 m). Finally, an additional field stop is focused on the sedimentology of the Lutetian carbonates (Guara Fm) recently studied by Silva-Casal (2017).

2 General Geological Setting We summarize the structure of the SW part of the Range. The Pyrenean Range is an asymmetric double vergent collisional orogen (Roure et al. 1989; Muñoz 1992; Labaume et al. 2016; Teixell et al. 2016). This fold and thrust belt developed from Late Cretaceous to Early Miocene times in relation to the opening of the Atlantic Ocean and the indentation of the Iberian plate pushed by the African plate under the Euroasiatic one (Rosenbaum et al. 2002; Sibuet et al. 2004; Vissers and Meijer 2012). This collision developed five different structural zones depending upon their characteristics (Mattauer and Séguret 1971; Séguret 1972), from north towards south are: the Aquitanian foreland Basin, the North Pyrenean Zone, the Axial Zone, the South Pyrenean Zone and the Ebro foreland Basin. Therefore, the Southern Pyrenean

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Zone comprises imbricated south-vergent fold and thrust system affecting Palaeozoic to Cenozoic rocks (Séguret 1972). They developed in a foreland-breaking sequence in this SW sector (Martínez-Peña and Casas-Sainz 2003; Teixell 1996, 1998). The South Pyrenean Zone, which concentrates most of the shortening (between ~120 and 160 km of shortening, Teixell et al. 2016; Beaumont et al. 2000) of the Pyrenean Range during the alpine convergence, is located south of the Axial Zone. The Axial Zone represents the backbone of the Range, where the Palaeozoic rocks crop out forming an antiformal stack in the central part of the Range, which disappears towards the west, in the area of interest (Teixell 1996). Within the South Pyrenean Zone in this SW sector, and towards south, we come across the Internal Sierras, the turbiditic and the molassic Jaca Basin (the piggy-back basin incorporated to the orogen as deformation migrated towards south, forming the Jaca-Pamplona Basin) and the External Sierras, ramping up over the Ebro foreland Basin (Fig. 1) (Labaume et al. 2016). The Internal Sierras comprises of Late Cretaceous to Palaeogene carbonates with the orogenic WNW-ESE trend. Structurally, they constitute a south-vergent imbricated fold and thrust system branching in the basal Larra thrust that connects towards the north with a basement thrust, the Lakora thrust (Teixell 1996). This Larra thrust system is tilted towards the south by the Gavarnie basement thrust, which affects, together with the Jaca basement thrust, the cover structures towards the south in the turbiditic and the molasse basin (Labaume et al. 1985; Teixell 1996, 1998; Oliva-Urcia and Pueyo 2007; Izquierdo-Llavall et al. 2013). According to recent interpretations of seismic sections and thermochronologic data, < 50 km to the east of Teixell’s (1996) cross section, Broto and Fiscal basement thrust sheets affect the cover in the Jaca-Pamplona piggy-back Basin (Labaume et al. 2016). Finally, the Guarga basement thrust sheet produces the final shortening in the External Sierras (the southernmost unit) and deforms the northern tip of the Ebro foreland Basin, which is the focus of this field trip (Pocoví et al. 1990; Millán 1996; Arenas et al. 2001). The general structure of the External Sierras can be synthetized as the diachronous emplacement (Lutetian to lower Oligocene, Gavarnie, in age) of an imbricate thrust system detached on evaporitic facies of Triassic age under submarine conditions and developing in a foreland-breaking sequence that migrated towards west (Millán et al. 2000). Later on, the incipient mountain chain was affected by cover thrusts in a hinterland breaking sequence migrating towards west (OligoceneMiocene) but in a more isochronous manner (Millán et al. 2000). The distribution of the evaporitic Triassic detachment and contractional salt remobilization might, somehow, influence the evolution of compressional structures in the External Sierras (Anastasio 1992; Anastasio and Holl 2001; Vidal-Royo et al. 2009; Pueyo et al. 2020). The kinematic evolution of the South Pyrenean Zone can be determined through the abundant syntectonic turbiditic and molasse sediments. They seem to point to the Gavarnie Unit (comprising the basement thrusts from Gavarnie to the south except Guarga) evolving from Eocene to Oligocene (Priabonian to Chattian, 33–24.6 Ma) times and producing shortening in the Jaca-Pamplona Basin and the External Sierras. The Guarga basement thrust caused the latest shortening in the External Sierras from Chattian to Aquitanian (Oliva-Urcia et al. 2019 and references therein) times.

Fig. 1 a Geological map, simplified from Choukroune and Séguret’s (1973) and 1:400,000 geological map from Bureau de Recherches Géologiques et Minières/Instituto Geológico y Minero de España (BRGM/IGME) maps. The inset map represents the structural units (simplified after 1:400,000 geological map from BRGM/IGME) Barnolas et al. (2008), in Oliva-Urcia, 2018. b ECORS-Arzacq cross section (Teixell y García-Sansegundo, 1995, Teixell 1996, 1998). The southern part is modified after Millán (1996). SPZ: South Pyrenean Zone. NPZ: North Pyrenean Zone

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The deformation observed in this field trip is related to the shortening accommodated in the External Sierras due to both the Gavarnie and Guarga basement thrusts (Puigdefàbregas 1975; Oliva-Urcia et al. 2019 and references therein). Structure of the Western External Sierras Zooming in, the field location is placed in the westernmost sector of the Pyrenean External Sierras, west of the Gállego river transect and the prominent conglomeratic pinnacles of Mallos de Riglos. Back in the nineteenth century, Mallada (1878, 1881) already mentioned the Santo Domingo anticline and range, but before the 1980s, they draw very little attention except for the geological maps done in the 1950s by the Geological and Mining Institute of Spain (IGME, Almela and Rios 1950a, 1951a, b). The Santo Domingo anticline represents the topographic relief shaped by the South Pyrenean sole thrust in its westernmost termination (Puigdefàbregas 1975; Nichols 1987; Turner 1990; McElroy 1990; Arenas 1993; Millán 1996; Pueyo 2000; OlivaUrcia 2000). It is the southwesternmost expression of the Pyrenean fold and thrust front that can be tracked (around continuously) 250 km along-strike from the Oliana anticline at the east (Sussman et al. 2004) and delineates the boundary between the Pyrenees and the Ebro foreland Basin (Figs. 1 and 2). The Gállego river meridian also represents a clear transition in the structural style of the External Sierras. To the east, a thin-skinned imbricate fold and thrust system developed during the Eocene in relation to the emplacement of the Gavarnie basement thrust sheet, which was also responsible for many outstanding oblique structures (e.g. Pico del Águila anticline). The system was later beheaded by a second thrusting event (Guarga) in a break-back sequence during Oligocene-Miocene times. This second event already affected the upper part of the very thick molasse (Campodarbe Fm) and the Miocene alluvial fans (Uncastillo Fm), conditioned the change in structural style and generated a remarkable thrust ramp (20–30°, dipping to the north) that can be tracked along-strike the geological map (and in seismic sections; Labaume and Teixell 2018) exceeding hundred km, until the Balzes anticline to the east (RodríguezPintó et al. 2013 and 2016) and beyond (Martínez-Peña and Casas 2003; Santolaria et al. 2020). This scenario drastically changes to the west of the Gállego river in our target area. There, the thrust ramp laterally and progressively evolves to a tight, highamplitude (5 km) meso-scale fold, the Santo Domingo detachment anticline, that finally vanishes to the west in the San Marzal periclinal closure, where the Middle Eocene limestones disappear. The latter shows a strongly plunging termination to the west (Nichols 1984; Oliva-Urcia et al. 1996, 2012a; Pueyo et al. 2020), which has been related to a large-scale conical geometry (Millán 1996; Pueyo 2000; Pueyo et al. 2017a). The evaporitic nature of the detachment level (Keuper and M2 facies) has been of key importance to delineate the transition between the ramp anticline and the detachment folding because of a conspicuous negative gravimetric anomaly (Calvín et al. 2018; Pueyo et al. 2020). The structural anisotropy of the Santo Domingo anticline can be followed for many km westwards (Tafalla anticline; Puigdefábregas 1975) affecting an even higher thickness of the Campodarbe Fm in that region (OlivaUrcia et al. 2012a).

Fig. 2 Geological map of the Western External Sierras (Southern Pyrenees). UTM coordinates (30 N datum ETRS89) Source modified from the GEODE harmonized geological map of Spain; Ebro and Pyrenees (Robador et al. 2011, 2019). The location of the cross sections (in red) in Fig. 3 is also shown as well as the magnetostratigraphic profiles (in blue)

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Another remarkable feature in the Western External Sierras is the occurrence of tête plongeantes (synform anticline) structures; the Riglos, Punta Común and San Felices imbricate thrust sheets crop out in the southern limb of the Santo Domingo anticline. They have been linked to the westernmost oblique (and almost ignored) structures of the External Sierras (La Peña, Fachar and Peña Ronquillo anticlines) that crop out in its northern limb (Millán et al. 1995). These têtes plongeantes were formerly part of the westernmost sheets of the imbricate system (coeval to the Gavarnie stage of basement thrusting in the Axial Zone) and were affected by the second folding event (coeval to the Guarga stage) and can be only distinguished in the eastern sector (Salinas Range) due to the overall gentle plunging of these imbricates to the east (Millán 1996; Pueyo et al. 2003a) and the current erosional level. Along-strike shortening differences across the External Sierras front are well-known (McElroy 1990; Millán et al. 2000: Huyghe et al. 2009); from >30 km in the Guara Range to the east to just ~10 km in the San Marzal sector (in between, ~19 km in the Gallego meridian) (Millán 1996; Pueyo et al. 2004). These differences imply the accommodation of moderate vertical axis rotations revealed by numerous palaeomagnetic studies since the late 1990s (Hogan 1993; Pueyo et al. 1997 and 2020; Oliva-Urcia et al. 2012a; Pueyo-Anchuela et al. 2012). Finally, the numerous magnetostratigraphic studies (Hogan and Burbank 1996; Oliva-Urcia et al. 2016, 2019; Teletzke 2012; Pueyo et al. 2016a; Silva-Casal et al. 2019; Anastasio et al. 2020) in the region (13 sections covering more than 14 km of mostly syntectonic sequences) offer an outstanding chronostratigraphic frame and help in accurately dating deformation events and unravelling their kinematics (first and third stops of this field trip). The deformation related to the emplacement of the Gavarnie nappe was accommodated in the Western External Sierras cover structures (Riglos, Punta Común and San Felices imbricate thrust sheets) during Oligocene times (Rupelian-Chattian; 31.3 and 24.55 Ma) as well as the incipient folding of the Santo Domingo anticline (Millán et al. 1995; Oliva-Urcia et al. 2019). The San Felices angular unconformity (Millán et al. 1995, 2000; Arenas et al. 2001) as well as their western continuation as a progressive unconformity (growth strata) in Luesia magnetostratigraphic section and La Peña flexure (Turner 1990 and Anastasio et al. 2020) concurs with the Oligocene ages. The onset of the emplacement of the Guarga nappe, and its effect on the External Sierras, took place right after the emplacement of the Gavarnie nappe (from 24.55 to 21.2 Ma; Chattian-Aquitanian times, Millán et al. 2000; Teletzke 2012; Oliva-Urcia et al. 2019; Anastasio et al. 2020). This second and intense deformational pulse largely developed the anticline and folded the former imbricate thrust sheets (currently visible on the southern flank) (Fig. 3). Stratigraphy The oldest rocks cropping out in the Southern Pyrenees and in the External Sierras are Triassic in age: Muschelkalk facies (the upper M3-dolostomes and the lower M2-gypsum and clays; López-Gómez et al. 2019). These rocks are found in the core of the anticline together with the silty and gypsiferous Keuper facies. These Triassic sequences acted as the regional detachment level. Some remnants of Jurassic rocks (Barbed et al. 1988; Lobato and Meléndez1988) are only found in the recumbent and

Fig. 3 Serial cross sections in the western sector of the External Sierras (see location in Fig. 2). Cross sections by Millán (1996); San Marzal, San Felices and Gállego. Sangüesa, Sos-Undués and Isuerre by Oliva-Urcia (2000) and Oliva-Urcia et al. (2012a). Cross sections San Marzal, San Felices (Millán 1996) are also reinterpreted after the gravity modelling (Calvín et al. 2018) IT WILL BE POSSIBLE TO SPLIT THIS FIGURE IN TWO PAGES???, THEN THEY SHOULD BE VERTICAL

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southernmost unit (San Felices). A non-depositional hiatus presumably precludes the occurrence of Lower Cretaceous rocks all along the External Sierras (Millán et al. 1994; Teixell et al. 2009; García-Sansegundo et al. 2009-MAGNA reports). Above the Triassic and scarce Jurassic rocks very thin transitional and bioclastic Upper Cretaceous ocher calcarenites and siltstones can be found (Adraen-Bona Fm; ≈ 70–85 Ma, 60 unpublished sites evenly distributed around the fold termination allows to observe the vectors, after bedding correction, displaying a progressive CW rotation from south to north with respect to the expected reference (Fig. 13). Similar to the palaeomagnetic information, the data derived from the analysis of the magnetic fabrics (AMS) (Sánchez-Moreno et al. 2013) also show an identical distribution around the fold closure. This coincidence is due to early recording and blocking of AMS of a far-field layer parallel shortening (almost coeval to sedimentation) and its later passive character in the southernmost portions of the

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Fig. 12 (next page): Structural data in the San Marzal pericline (bedding poles, joints, tension gashes). An example of a specific outcrop, SM05, is also shown (see location in Fig. 13) together with structural and AMS data in that outcrop. Finally, a schematic 3D model integrating most observations (including simplified palaeomagnetic vectors) is presented for the entire fold closure

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Fig. 13 Palaeomagnetic data (yellow cones) in the San Marzal fold termination. AMS data (blue cones) are also shown (Sánchez-Moreno et al. 2013). For every sector, the cone axis represents the mean of the palaeomagnetic vector (or the Kmax axis in case of AMS tensors) and the semiapical angle the confidence (α 95 by Fisher, 1953). Several sites are averaged out in every sector using the VPD software. Stereoplots represent the palaeomagnetic means (VPD) before (in situ) and after restoration (bedding correction) to the palaeohorizontal. Mean bedding poles (S0) are also shown to characterize the fold axis (see also Fig. 11). The Luzientes thrust is also mapped as well as the hanging-wall and footwall cut-offs in the Yeste-Arrés Fm (red lines)

Pyrenees and in the Ebro Basin (Larrasoaña et al. 2004; Pueyo-Anchuela et al. 2010 and 2012; Pocoví et al. 2014). On the other hand, the San Marzal pericline also shows the westernmost evidence of thrusting in the External Sierras. The Yeste-Arrés sandstones on top of the Arguis marls, where the topographic depression of Luzientes is located, clearly display the

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hanging-wall and footwall cut-offs of this thrust (red lines in Fig. 13), affecting therefore to the entire Bartonian sequence (at least) and attesting for ~1 km of additional shortening in the Santo Domingo anticline. The Luzientes thrust, here defined, is the last (outcropping) imbricate of the External Sierras fold and thrust system and was never mapped before. Apparently, the thrust seems to trend pseudo-parallel to the main Pyrenean grain in this sector, although its original orientation (pre-conical folding) is difficult to assess because of the complex restoration needed in these geometries. Its relationship with the core of the anticline is obscured partially because of a set of vertical faults in the northern part of the pericline that preclude to stablish its continuation (or not) with the basal detachment level. In any case, the Luzientes thrust is clearly linked to the transference of deformation to the Campodarbe unit, where a complex intra-formational multilayer detachment system was defined and it is responsible for the Santo Domingo hinge collapses mapped by Oliva et al. (2012). Restoration of palaeomagnetic vectors (or any linear data) in this kind of geometries is not a simple task and must be carried out considering the kinematics of the fold. The San Marzal and Santo Domingo western termination inspired analogue simulations (Ramón et al. 2013) and numerical modelling (Ramón et al. 2012, 2016a, b) focused on the understanding of this complex geometry as well as on the use of palaeomagnetic vectors as an auxiliary tool during 3D restoration of non-cylindrical and non-coaxial structures. Stop 3: Southwestern Pyrenees at the Santo Domingo summit 42.442949 °N; −0.916468 °W Panoramic view of the Southwestern Pyrenees Regional Geology Looking to the north from the Santo Domingo summit (stop #3), the Jaca piggyback Basin and the Internal Sierras are noted. This basin forms a part of the South Pyrenean Zone and it is limited to the north by the Internal Sierras (although some remains of the Axial Zone can be observed in the background), and to the south by the External Sierras (Santo Domingo isoclinal anticline in this area). The sediments that crop out in the Jaca piggy-back Basin are the turbiditic deposits in the north (Hecho Group, Mutti 1984; Payros et al. 1999; Remacha and Fernández 2003), and the molasse sequence, of fluvial (mainly) and alluvial fan origin (Campodarbe and Bernués Fms; Puigdefàbregas 1975) (Fig. 14). The turbiditic deposits lie on the carbonatic marine platform rocks (Upper Cretaceous-Palaeocene). These Upper Cretaceous-Palaeocene rocks crop out in the Internal Sierras directly over the Palaeozoic sequences. They are ~1000 m thick and are incorporated into the Larra cover thrust system, which connects towards the north with the Lakora basement thrust (Teixell 1996). The Upper Cretaceous-Palaeocene cropping out in the External Sierras is 45 km along trend. To the south of this fold, there is the Ebro Basin. The contact between the two basins is considered to be at places the stratigraphic contact between the Campodarbe Fm (Jaca-Pamplona Basin) and the Uncastillo Fm (Ebro Basin) (Puigdefàbregas 1975, Arenas1993). There is a relay of folds and an increasing number of them towards the west of the molassic piggy-back basin, where the thickness of the Campodarbe Fm increases. This is consistent with the behaviour of multilayer folding, which depends, among other parameters, on the number and thickness of the layers and their mechanical strength (Ramberg 1963; Frehner et al. 2006; Oliva-Urcia et al. 2012a). A detailed description of the geological structure of the area can be found in OlivaUrcia et al. (2012a) (Figs. 3 and 4) and it is as follows: the Santo Domingo-Tafalla anticline is interpreted to link at depth with a north-verging blind thrust rooted in the Upper Triassic evaporites. The thrust cuts across the anticline hinge affecting from Triassic rocks to the lower part of Campodarbe Fm. The amplitude of the Santo Domingo-Tafalla anticline decreases towards W, in the same direction of the plunge of the fold axis (Oliva-Urcia et al. 2012a and references therein). At surface, it appears as a tight fold involving the Campodarbe Fm, with layer-parallel slip. As a result, hinge collapse occurs (saddle reef structures, Ramsay 1974). Hinge collapses are typical in multilayer systems when kink-type folds occur (Ramsay 1974). To the north, the Longás syncline and the Botaya thrust (anticline in hanging wall) show a southward vergence. The sedimentary wedge composed mainly by the Hecho Group and the Arguis-Pamplona Fm thickens from the Bailo syncline towards the north. In the northern limb of the Bailo syncline, marine platform Mesozoic-Palaeogene rocks crop out, defining an imbricate thrust system (Leire-Illón) rooted in the Upper Triassic detachment level. These thrust sheets have individual displacements up to ~

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2 km, and a general southward vergence, although bi-vergent anticlines can be found at surface, forming pop-up structures in the hanging walls of thrusts. The shortening calculated for the piggy-back basin is accounted by the Guarga thrust sheet (southernmost basement thrust in Fig. 3) and probably, some of it is related to the Gavarnie basement thrust sheet, since the Leyre-Illón system passes laterally to folds of Late Eocene-Early Oligocene activity (Teixell 1996). However, most of the post-Triassic cover thrust slip in the area can be accounted by the basement displacement represented in the cross section of Fig. 3. The displacement of the basement in this area, contrary to eastern or western areas, cannot be transferred towards the south, probably due to the lower (and therefore, non-effective) thickness of a Triassic detachment level (Keuper facies) or to the absence of evaporates in the Eocene sequences in the Ebro Basin. Then, the folds in the Campodarbe Fm were the pin line for the Mesozoic-Cenozoic cover, and consequently displacement was transferred to the north. This interpretation implies that the mechanical behaviour of the different lithologies strongly affects the structure of the sedimentary cover: structures in the cover relay along-strike, but shortening is balanced in a non-rotational area (Oliva-Urcia et al. 2012a). Kinematics of the vertical axis rotations in the External Sierras Differences in shortening along-strike within fold and thrust belts are the main driving cause to produce vertical axis rotations (VAR) (McCaig and McClelland 1992; Allerton 1998; Soto et al. 2006; Sussman et al. 2012). A truly 4D understanding of fold and thrust belts and orogenic systems, in a larger scale, must be necessarily based on the study of this elusive kinematic indicator (Mukherjee 2019). Rotational kinematics is usually tackled by characterizing the rotation magnitudes (VAR) by means of palaeomagnetic analysis. Palaeomagnetism is a proven reliable and accurate way to estimate VARs at different scales and, in particular, at the fold and thrust belt scale (Norris and Black 1961; Van der Voo and Channel 1980; Pueyo et al. 2016b; Oliva-Urcia and Pueyo 2019), with numerous data available nowadays in almost any orogenic region. Apart from the VAR characterization, the timing of the rotational movements and the estimation of the velocities of rotation (both, key kinematic variables) are very scarce (Speranza et al. 1999; Duermeijer et al. 2000; Mattei et al. 2004; see also Mukherjee and Khonsari 2018 and Mukherjee and Tayade 2019 for review on various rotation rates of crustal blocks). This is partially due to the difficulty in combining the occurrence of two factors; the existence of syn-rotational sediments and the availability of semi-continuous dating records (i.e. magnetostratigraphy). The Southwestern Pyrenees is a unique natural laboratory to tackle these strict requirements and where a complete rotational kinematic record was firstly published for an individual thrust system (the South Pyrenean sole thrust) at the Pico del Águila area in the Central External Sierras (Pueyo et al. 2002). Coming back to the panoramic view, one of the densest palaeomagnetic data sets in orogenic systems can be observed from the Santo Domingo summit (Fig. 15 and Table 1). All visible structural units have been sampled and studied for palaeomagnetic purposes in the frame of numerous academic (Bentham 1992; Hogan

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Fig. 15 Distribution of palaeomagnetic sites and magnetostratigraphic sections in the Western Pyrenees (red dots) (IGME-Geode map superposed to Google Earth). Palaeomagnetic rotations (mean values) in the southwestern Pyrenees (equal-area projection). Data obtained from the Pyrenean palaeomagnetic database, Pueyo et al. (2017b). Structural units with and without significant rotations are split

1993; Meigs 1995; Pueyo 2000; Larrasoaña 2000; Oliva-Urcia 2004; Fernández 2004; Mochales 2011; Pueyo-Anchuela 2012; Teletzke 2012; Rodríguez-Pintó 2013; Ramón 2013; Beamud 2013; Izquierdo-Llavall 2014; Pérez-Rivarés 2016; SilvaCasal 2017) and other research works: Ebro Basin (Larrasoaña et al. 2006; PérezRivarés et al. 2002, 2004, 2016), External Sierras and the Jaca Basin (Hogan and Burbank 1996; Pueyo et al. 1997; 2003a and b, 2020; Oms et al. 2003; Kodama et al. 2010; Pueyo-Anchuela et al. 2012; Oliva-Urcia et al. 2012a, 2016 and 2019; Rodríguez-Pintó et al. 2008, 2012; Silva-Casal et al. 2019; Anastasio et al. 2020), Internal Sierras (Oliva-Urcia and Pueyo 2007a and 2007b; Oliva-Urcia et al. 2008,

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Table 1 Mean palaeomagnetic vectors in different structural units of the Southwestern Pyrenees. Sites/sections: number of sites (means) and magnetostratigraphic sections considered. Polarity; magnetic polarity (N: normal, R: reverse) P/S; Primary/secondary (remagnetized). Mean vector data: Dec (declination) and Inc (inclination) together with Fisher’s (1953) statistical parameters (α 95 , k and R). VAR: Robust vertical axis rotation mean estimation. A more detailed grouping of data was performed for the External Sierras. VARs can be significantly larger in individual sites since means tend to smooth them

2009, 2012b and 2018; Izquierdo Llavall et al. 2015 and 2018), Northwestern Pyrenean zone (Oliva-Urcia et al. 2010; Menant et al. 2016; Aubourg et al. 2019; Izquierdo-Llavall et al. 2020), Pamplona Eocene Basin (Larrasoaña et al. 2003a, b, c and 2004), as well as the westernmost locations of the Aínsa Oblique Zone (Bentham and Burbank 1996; Mochales et al. 2010, 2012a and b, 2016; Muñoz et al. 2013; Rodríguez-Pintó et al. 2013a, b, 2016). All in all, more than 550 mean palaeomagnetic vectors (several thousand demagnetized rocks) are synthetized in Table 1 (Fig. 15 and Table 1). Unrotated domains, as expected from the non-rotational convergence of Iberia and Eurasia from Late Cretaceous to Miocene (Sibuet et al. 2004; Vissers and Meijer 2012), include all magnetostratigraphic studies from the Ebro Basin (see compilations by Pérez-Rivarés et al. 2018 and Garcés et al. 2020), the unrotated Pamplona Basin (Larrasoaña et al. 2003a) and the data located in the southern limb of the Santo Domingo anticline (in structural continuity with the Ebro foreland Basin; see overview by Pueyo et al. 2020). These data can be considered as the local palaeomagnetic reference (Fig. 15 and Table 1); a vector pointing almost straight northwards (N358E). Rotated domains comprise the data from the Jaca molasse and turbiditic Basin (about 10–15° CW rotation), the Internal Sierras and the External Sierras, except for the westernmost sector (≈20° CW) and the Aínsa Oblique Zone and the westernmost sector of the External Sierras displaying significant rotations (35° CW). Beyond the Pico del Aguila complete rotational kinematic control, additional synrotational records have been investigated during the last years to the east, in the Balzes

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(Rodríguez-Pintó et al. 2016) and the Boltaña anticlines (Mochales et al. 2012a) as well as in the Aínsa Basin (Muñoz et al. 2013). A scientific debate on the isochrony or diachrony (Pueyo et al. 1997) of the main rotational movement in relation to the shortening ages of the main South Pyrenean basement units is still open (see the recent overview by Oliva-Urcia and Pueyo 2019) and the end-members of this system will potentially shed light on the problem; the Mediano anticline (preliminary data published by Beamud et al. 2017) to the east and the Santo Domingo anticline to the west (here introduced). The panoramic view in stop #3 allows us to observe the Salinas magnetostratigraphic section (Hogan 1993) located in the northern limb of the Santo Domingo anticline (in the road to Sta. Bárbara pass). Furthermore, 25 previous palaeomagnetic data drilled in the Campodarbe Fm fall along the same section and 20 additional sites from the marine Bartonian-Priabonian rocks underneath (Pueyo 2000 PueyoAnchuela et al. 2012; Pueyo et al. 2020). This set of results (Fig. 16 and Table 2) allows us to infer the rotational kinematics of the Santo Domingo anticline, the westernmost structure of the External Sierras front (100 km away from Mediano, 75 km from Boltaña anticlines). After the stratigraphic correlation of the individual sites (Fig. 15), we have grouped them according to the magnetic chron age; C17&C16, C15&C13, C12, C11&C10. These four groups present similar time spans (1–1.3 Ma). On the other hand, the palaeomagnetic means denote an inclination shallowing (typical in continental fluvial rocks; Garcés et al. 1996) and moderate-good qualities. The rotation ages and magnitudes have been plotted (Fig. 17) as well as the data from other studied structures to the east (Mediano, Boltaña, Balzes and Pico del Águila anticlines together with the Guara thrust system). In the eastern structures, the rotation ages, or at least the main pulse, are almost isochronous and took place during the Bartonian-Priabonian period. However, the Santo Domingo anticline displays a significant younger age (Upper Rupelian times) and reinforces the diachronism of the rotational activity along-strike the Pyrenean basal thrust along the External Sierras front. Specifically, the end of the rotational movement laterally vanishes along-strike at a rate of ~9 km Ma−1 , considering the distance between the Balzes and Santo Domingo anticlines. With respect to the onset of rotation, similar magnitudes are expected if the lateral migration pattern of the deformation along the External Sierras front is taken into account as well (Millán et al. 2000). Further data in the Mediano anticline (in progress) may shed light on the other end-member of the system. Interestingly, the magnitudes of the rotation velocity are very similar among the better examples; up to 13° Ma−1 in the Balzes anticline (10° Ma−1 in average, Rodriguez-Pintó et al. 2016), between 2.3–10° Ma−1 in Boltaña (Mochales et al. 2012a), 2–7° Ma−1 ; in the Pico del Águila anticline (Pueyo et al. 2002 and RodríguezPintó et al. 2008) and between 2–12° in the Santo Domingo anticline with two distinct periods. This catalogue of complete folding and rotational kinematics will help understanding the 4D architecture of the External Sierras thrust system and, in general, the one of the Southwestern Pyrenees.

Fig. 16 Palaeomagnetic sites along the Sta. Bárbara road (geological map and orthophoto quad) and the stratigraphic correlation with the Salinas section (Hogan 1993)

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Table 2 Robust palaeomagnetic vectors in different magnetic chrons along the Salinas and Santa Bárbara Section. n/N: number of sites used/measured. Mean vector data: Dec (declination) and Inc (inclination) together with Fisher’s (1953) statistical parameters (α 95 , k and R). Time intervals (chron duration) also display the error N

N

Dec

Inc

a95 (°)

k

R

Age and error (Ma)

Chron

24

27

039

37

6.3

23.2

0.9586

36.9 ± (1.3)

C17 & C16

6

6

036

35

20.1

14.5

0.9310

34.4 ± (1)

C15 & C13

7

7

029

30

16.9

15.9

0.9373

31.9 ± (1.3)

C12

4

5

351

12

13.5

62.8

0.9841

29.6 ± (1)

C11 & C10

Fig. 17 Rotation magnitudes versus age in the Southern Pyrenees. Estimating of the rotation velocity and timing of some oblique structures; Light blue line) Mediano anticline (preliminary data by Muñoz et al. 2013 and Beamud et al. 2017). Red line) Pico del Águila (data from Pueyo et al. 2002 and Rodriguez et al. 2008). Blue line) Northern sector of the Guara thrust system (Pueyo 2000; Pueyo et al. 2003b). Purple line) Balzes anticline (Rodriguez-Pintó et al. 2016, 2020). Orange line) Boltaña anticline (Mochales et al. 2012a) and the Santo Domingo anticline (green line) published in this work. Folding kinematics is also shown for the later anticline

Stop 4: The Campo Fenero section 42.431119 °N; −0.898484 °W Stratigraphy and sedimentology of the middle Eocene Santo Domingo Mb South from the Santo Domingo summit, the Upper Lutetian-Lower Bartonian limestones constitute a continuous ridge along the southern flank of the Santo Domingo anticline. This ridge can be easily tracked on aerial photography. This is because both the underlying continental materials from the Tremp Fm and the overlying marine

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Fig. 18 Outcrop of the Santo Domingo Mb in the area of El Portillo (west of stop #3)

marls of the Arguis Fm are usually covered by vegetation, whereas the middle Eocene marine limestones stand out in the Santo Domingo Range due to the differential erosion (Fig. 18). This ridge is constituted by two litostratigraphic units, the Guara Fm and the Santo Domigo Mb, the latter included within the Arguis Fm. The Guara Fm pinches out towards the centre of the Santo Domingo sector and only the Santo Domingo Mb crops out westwards (on the mentioned Campo Fenero section and on San Marzal). The Santo Domingo Mb can be mapped both along the southern flank of the Santo Domingo anticline and in the San Felices and Punta Común nappes (outcropping in the southern flank of the Santo Domingo anticline). The most representative sections of this unit are La Osqueta (Figs. 19, 20) and Campo Fenero (Fig. 21). The unconformity between the Guara Fm and the Santo Domingo Mb can be observed in La Osqueta section (Fig. 20) as a complex surface representing the previous carbonate ramp system (Guara Fm) sunk due to the interaction of eustasy and flexural subsidence (Silva-Casal et al. 2019). Indeed, this tectonic subsidence created the necessary accommodation space for the development of a new carbonate ramp system on a former foreland area (i.e. the Santo Domingo Mb). The Campo Fenero section This section is easily accessible and contains impressive outcrops of the Santo Domingo Mb that essentially have been used as a reference for the lithostratigraphic

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Fig. 19 Lithostratigraphic units outcropping in La Osqueta section in the southern flank of the anticline. ( Modified from Silva-Casal 2017) viewed from the south

Fig. 20 Base of the glauconite layer at the lower part of Santo Domingo Mb and unconformity in La Osqueta section. Outcrop view (a and b) and polished hand sample of the unconformity c (Modified from Silva-Casal 2017)

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Fig. 21 Stratigraphic profile as in the Campo Fenero section ( Modified from Silva-Casal 2017)

unit (Silva-Casal et al. 2019). The absence of Guara Fm rocks in the area precludes observing the unconformity between both units in this section and here the Santo Domingo Mb lies on top of the red beds of the Tremp Fm. Instead, it is a characteristic glauconitic-rich interval marking the onset of the carbonate sedimentation renewal throughout the base of the Santo Domingo Mb. This basal glauconitic layer rests above the unconformity with Guara Fm in La Osqueta area (Fig. 20), as well as in many western sections, such as Campo Fenero and San Marzal ones. In the

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Campo Fenero section (Fig. 21), it is constituted by skeletal facies, mainly packstones, with centimetric components such as bryozoans, bivalves, equinoids and larger foraminifera (Discocyclina). A 5 m-thick interval of Discocyclina rudstone in a wackestone-mudstone matrix is observed after a covered area. The skeletal components and the overall muddy texture of this basal layer on Campo Fenero evidence oligophotic, middle ramp conditions for the deposit. At around one-fourth of the Sect. (25 m; Fig. 21), a sharp facies change separates the glauconitic level from the next interval, constituted by packtones with a large amount of acervulinid foraminera and echinoid plate fragments. This level ranges from massive facies at the base to cross-bedded towards the top and represents shallow subtidal environments. The large concentration of acervulinid foraminifera suggests vegetated environments, and the cross-bedding to the top points to the influence of the wave agitation. Above, the carbonatic sedimentation is interrupted by a siliciclastic interval (38–53 m in Fig. 21), with few bioclasts and more concentration of organic matter. This part seems to correspond with lagoon-related environments, and constitutes the top of an overall regressive sequence. The following succession (53–95 m) with grainstone–packstone texture (Fig. 22) is evenly stratified and intercalated with cross-bedded and bioturbated intervals. Acervulinid foraminifera, echinoid plate fragments, bryozoans and occasionally larger foraminiera (e.g. Operculina) can be observed here. To the top, the most common components are articulated coralline algae, which constitute the uppermost

Fig. 22 Cross-bedding in shallow marine facies, upper part of Campo Fenero section

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grainstone almost entirely. The facies described in this interval were deposited in an inner ramp environment with swell influence and covered at some degree by submarine vegetation, as the abundance of acervulinid foraminifera and articulated coralline algae suggests (Silva-Casal 2017). Despite the relatively homogeneous upper part of the Campo Fenero section, the presence of tractive structures towards the top suggests an upwards trend to shallower conditions. On the very top, the boundary between the limestones of Santo Domingo Mb and the blue marls of the Arguis Fm (typical crumbly aspect in outcrop) is not observed in this section, although it is conformable in the San Marzal section, and interpreted as a progressive deepening of the carbonate ramp in the area of Santo Domingo. Larger foraminifera within the Santo Domingo Mb are scarce and the only biostratigraphic data in the unit came from this specific section (a single sample containing N. biarritzensis and N. beaumonti) allowing to interpret a Bartonian age (SBZ 17 in Serra-Kiel et al. 1998). Photodependent larger foraminifera (i.e. Alveolina, Orbitolites) are characteristically scarce along the Santo Domingo Mb facies, particularly in the shallower domains. This change in carbonate generation is probably related to a nutrient-rich environment associated to dominantly deltaic deposits in the South Pyrenean Basin during the Bartonian (Silva-Casal 2017). The Santo Domingo Mb represents the last stage in the development of the peripheral shallow marine carbonate environments from the Jaca-Pamplona foreland Basin, coeval with the beginning of the foreland basin overfill (Silva-Casal 2017).

6 Conclusions In this book chapter, we focus on the western edge of the External Sierras (Southern Pyrenees) as an outstanding example of complete fold kinematics in a complex fold and thrust system termination, proposing a field trip with 4 stops. The Santo Domingo anticline evolution is fully recorded by syntectonic sedimentation (both limb-tilting and rotational) allowing for a fully reconstruction of its deformational history. Folding kinematics is witnessed by two different unconformities: (1) the San Felices complex angular unconformity (passing gradually to the west to a progressive one; La Peña flexure) attests for the onset of folding of the Santo Domingo anticline in Priabonian times in association to the late emergence of the Gavarnie thrust in this sector of the foreland. (2) Just atop, the Uncastillo progressive unconformity allows the dating of the main folding event that took place during Miocene in relation to the emplacement of the Guarga thrust sheet in the cover units. On the other hand, the conical geometry characterized by an abrupt fold termination of the anticline to the west (San Marzal pericline) was induced by a significant clockwise rotational movement of the basal thrust sheet (determined by a vast palaeomagnetic dataset in the region). (3) The Santa Bárbara section (in the northern limb of the anticline) has recorded the rotational kinematics of this movement during Rupelian times, younger than other oblique structures (Boltaña, Balzes, Pico del Aguila) in the Eastern External Sierras (Bartonian-Priabonian).

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The three syntectonic records are fully dated by long magnetostratigraphic sections (Luesia, Fuencalderas, Salinas and Sos profiles) that allow for an accurate dating of the deformation to be achieved. The field trip, centred in Luesia village (southern limb of the anticline) proposes three stops with panoramic views of these unique records. Besides, a fourth stop is devoted to the sedimentology of the recently defined Santo Domingo Mb of the Arguis Fm. Acknowledgements This research was supported by the projects CGL2006-02289, CGL200914214 (Pmag3Drest) CGL2014-54118-C2-2-R (DR3AM), CGL2017-90632-REDT (MABIGER Network), PRX17/00462 and PID2019-104693GB-I00/CTA (UKRIA4D) by the Spanish Ministry of Science and Universities and the support given by the Applied Geology and Geotransfer Research Groups (GeoAP-E0117R) by the Aragón Government. This work also focuses on the southern sector of the target area of the GeoERA project 3DGeoEU (ERANET Cofund action 731166 [H2020], Project code GeoE.171.005) and it is also acknowledged. We are very grateful for the thorough corrections and improvements of Gonzalo Pardo as well as those by Soumyajit Mukherjee who reviewed and edited this article. Dutta and Mukherjee (2021) encapsulate this work.

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B. Henry, M. Jackson, T. Werner & F. Lagroix (Ed.), ASM in deformed rocks a tribute to Graham J. Borradaile. Tectonophysics, 629, 303–318. Pueyo Anchuela, Ó. (2012) Estudio de fábricas magnéticas y su relación con la deformación en el sector centro-occidental del Pirineo central (Aragón y Navarra). Serie Tesis de la Universidad de Zaragoza (Vol. 68, pp. 391). https://zaguan.unizar.es/record/9908?ln=es Pueyo Anchuela, Ó., Pocoví Juan, A., & Gil Imaz, A. (2010) Tectonic imprint in magnetic fabrics in Foreland Basin settings. Study in the Southern Pyrenees Foreland Basin, Ebro Basin, Spain) Tectonophysics, 492(1–4), 150–163. Pueyo-Anchuela, O., Pueyo, E. L., Pocoví, A., & Gil-Imaz, A. (2012). Vertical axis rotations in fold and thrust belts: Comparison of AMS and paleomagnetic data in the western external sierras (Southern Pyrenees). Tectonophysics, 532–535, 119–133. Pueyo, E. L. (2000). Rotaciones paleomagnéticas en sistemas de pliegues y cabalgamientos. Tipos, causas, significado y aplicaciones (ejemplos del Pirineo Aragonés). Unpublished PhD thesis. Universidad de Zaragoza (pp. 296). Pueyo, E. L., Millán, H., & Pocoví, A. (1997). Rotational kinematics of the southpyrenean basal thrust at the Sierras Exteriores Aragonesas: Magnetotectonic data. Acta Geológica Hispánica, 32(3–4), 119–138. Pueyo, E. L., Millán, H., & Pocovı, A. (2002). Rotation velocity of a thrust: A paleomagnetic study in the external sierras (Southern Pyrenees). Sedimentary Geology, 146(1–2), 191–208. Pueyo, E. L., Parés, J. M., Millán, H., & Pocoví, A. (2003a). Conical folds and apparent rotations in paleomagnetism (A case studied in the Southern Pyrenees). In C. Mac Niocaill, T. H. Torsvik & B. A. van der Pluijm (Ed.), Paleomagnetism applied to tectonics; a tribute to Rob Van der Voo. Tectonophysics 362(1–4), 345–366. Pueyo, E. L., Pocoví, A., Parés, J. M., Millán, H., & Larrasoaña, J. C. (2003b). Thrust ramp geometry and spurious rotations of paleomagnetic vectors. Studia Geophysica et Geodaetica, 47(2), 331–357. Pueyo, E. L., Pocoví, A., Millán, H., & Sussman, A. (2004). Map-view models for correcting and calculating shortening estimates in rotated thrust fronts using paleomagnetic data. In A. Weil & A. Sussman (Ed.), Special publication on orogenic curvature: Integrating paleomagnetic and structural analyses (Vol. 383, pp. 57–71). Geological Society of America Special Publication Pueyo, E. L., Sánchez, E., Canudo, J. I., Pereda-Suberbiola, X., Puértolas-Pascual, E., Parrilla-Bel, J., et al. (2016a). Magnetoestratigrafía del Cretácico Superior del sector Occidental de las Sierras Exteriores (Pirineo Occidental), implicaciones bioestratigráficas. Geotemas, 16(1), 909–912. Pueyo, E. L., Oliva-Urcia, B., Sussman, A. J., & Cifelli, F. (2016b). Palaeomagnetism in fold and thrust belts: Use with caution. In E. L. Pueyo, F. Cifelli, A. Sussman, & B. Oliva-Urcia (Ed.), Geological society of London special publication on palaeomagnetism in fold and thrust belts: New perspectives (Vol. 425(1), pp. 259–276). Pueyo, E. L., Compaired, F., & Sánchez, E. (2017a). A large-scale conical fold termination in the southwestern Pyrenees. Journal of Structural Geology, 98. (Picture of the month) Pueyo, E. L., García-Lasanta, C., López, M. A., Oliván, C., San Miguel, G., Gil-Garbi, H et al. the GeoKin3DPyr working group (2017b). Metodología para el desarrollo de la BBDD paleomagnética de Iberia (EPOS-DDSS Iberian Paleomagnetism). In MAGIBER X. (pp. 94–99). Instituto de Investigación en Ciencias Ambientales (IUCA). Universidad de Zaragoza. ISBN: 978-84-16723-40-9 Pueyo, E. L., Santolaria, P., Calvín, P., Casas, A. M., Sánchez-Moreno, E. M., Oliva-Urcia, B., et al. (2020). Local and apparent vertical-axis rotations, detachment folding and diapirism in the southwestern Pyrenean termination. Tectonics. Puigdefábregas, C. (1975). La sedimentación molásica en la cuenca de Jaca. Pirineos, 104, 1–188. Puigdefàbregas, C., & Soler, M. (1973). Estructura de las Sierras Exteriores pirenaicas en el corte del río Gállego. (prov. de Huesca). Pirineos, 109, 5–15. Puigdefabregas, C., & Souquet, P. (1986). Tecto-sedimentary cycles and depositional sequences of the mesozoic and tertiary from the pyrenees. In E. Banda, & S. M. Wickham (Ed.), The geological evolution of the Pyrenees. Tectonophysics (Vol. 129(1–4) pp. 173–203).

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Miocene-Quaternary Strain Partitioning and Relief Segmentation Along the Arcuate Betic Fold-and-Thrust Belt: A Field Trip Along the Western Gibraltar Arc Northern Branch (Southern Spain) Alejandro Jiménez-Bonilla, Manuel Díaz-Azpiroz, Inmaculada Expósito, and Juan Carlos Balanyá Abstract The Western Gibraltar Arc (WGA), located at the western end of the Mediterranean Alpine orogenic belt, was built up by the westward emplacement of the Alboran hinterland domain upon the South Iberian and Maghrebian paleomargin domains. This guide is focused on the strain partitioning modes developed in the fold-and-thrust belt (FTB) of the northern branch of the WGA. The proposed itineraries aim to show the distinctive strain partitioning modes of this arcuate FTB from the hinge (itinerary 1) to the lateral (itinerary 2) zones of the WGA. We focus our observations on the structural evolution from the late Miocene onwards. This short-term (a few My) temporal frame favours that the structures accommodating strain partitioning may greatly control a number of first-order topographic traits, for instance, relief segmentation. Itinerary1, located in the northern WGA hinge zone, allows one to visit a representative area of the inner FTB characterized by a set of parallel mountain ranges enclosing the Ronda intermontane basin. In this area, the topography is conditioned by late Miocene to Pliocene km-scale, upright or doubly vergent, NE-SW trending folds and associated reverse faults. These structures, parallel to the orogenic grain, gave rise to a conformable relief. Additionally, first-order along-strike relief segmentation controlled by normal faults has accommodated arc-parallel extension. The itinerary includes stops in the two main normal fault zones located at the margins of the Ronda basin, the most important intermontane basin of the northern WGA. Itinerary2 runs along the lateral, eastern end of the WGA, marked by a km-scale, E-W oriented brittle-ductile shear zone, the Torcal Shear Zone (TSZ), which is formed by a set of en-echelon morphostructural highs that define a 70 km long topographic alignment. The itinerary starts at the TSZ western end and continues eastwards to the main structural domains of its central part: the Valle de Abdalajís Massif and the Torcal de Antequera Massif. This permits us to visit the main structures that characterize this highly partitioned shear zone: NE–SW folds and reverse faults, NW–SE normal faults, and E–W dextral strikeslip faults, especially localized along the northern and southern walls of the TSZ. A. Jiménez-Bonilla · M. Díaz-Azpiroz (B) · I. Expósito · J. C. Balanyá Department of Physical, Chemical and Natural Systems, Universidad Pablo de Olavide. Crtra, Utrera km 1, 41013 Sevilla, Spain e-mail: [email protected] © Springer Nature Switzerland AG 2021 S. Mukherjee (ed.), Structural Geology and Tectonics Field Guidebook—Volume 1, Springer Geology, https://doi.org/10.1007/978-3-030-60143-0_4

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The overall kinematics corresponds to dextral transpression with a nearly vertical significant extrusion and associated surface uplift.

1 Introduction Fold-and-thrust belts (FTBs) associated with arcuate orogens (e.g., Western Alps, Carpathians, Calabrian and Gibraltar arcs) acquire a wide variety of map-scale geometries that seem to be controlled by several factors (e.g., indenter shape, predeformational thickness of the sedimentary sequence and detachment strength). These orogens show convergence trajectories or radial tectonic transport directions along with the arcuate structural trend (Macedo and Marshak 1999; Marshak 2004). The degree of divergence of tectonic transport directions, which occurs by eventual vertical axis rotation of independent blocks, is closely related to the arc formation mechanism, defining contrasting patterns when primary FTBs are compared to secondary (oroclinal) or progressive arcuate FTBs (e.g., Hindle and Burkhard 1999; Weil and Sussman 2004). Due to the combination of the above-mentioned factors, arcuate FTBs display map-view shapes with a number of linked second-order salients and recesses, commonly connected by arc lateral transition zones. As a result, the angle of plate convergence changes with respect to the considered FTB segment, thereby leading to variations of the strain partitioning mode along the orogenic grain. At arc hinges, the strain is mainly partitioned into orthogonal shortening, accommodated by arc-parallel folds and thrusts, and arc-parallel stretching, commonly accommodated by arc-perpendicular normal faults and/or arc-oblique strike-slip faults (e.g., Marshak 1988; McCaffrey 1991; Balanyá et al. 2007). By contrast, arc lateral segments usually localize strike-slip dominated tectonics, often developing transpressive zones, oblique to the overall arcuate orogenic trend. In active or recent arcuate FTBs, the structures accommodating strain partitioning in upper crustal levels may exert significant control on landscape and topography, usually involving relief segmentation. In this respect, arc-orthogonal shortening gives way to rugged across-strike topography, whereas arc-parallel stretching is associated with significant along-strike relief drops. In this guide, we visit the northern branch of the Western Gibraltar Arc (Fig. 1). Our itinerary runs along the FTB orogenic grain and seeks to illustrate the contrasting strain partitioning modes exhibited by arcuate FTBs from hinge to lateral zones. The young age of the structures accommodating deformation and the occurrence of fault zones traverse to the orogenic grain lead to a significant relief segmentation that makes this trip a good opportunity to use landscape as a tectonic tool.

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Fig. 1 Geological map locating the two itineraries included in this field guide. Modified from Jiménez-Bonilla (2017)

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2 Tectonic Setting The Mediterranean-type arcs (e.g., Gibraltar, Calabria, Eastern Carpathians) are characterized by high curvatures, relatively low topography and strongly attenuated upper plate thickness due to coeval extension and collisional tectonics (Royden and Burchfiel 1989). The Gibraltar Arc formed during the last 25 My due to the collision of the Alboran Domain against the South Iberian and Maghrebian paleomargins, building up the Betic and Rif chains, respectively (Vera et al. 2004). Between the Alboran Domain (internal zone) and both paleomargins, Flysch Trough units were deposited and later sandwiched. Both paleomargins and Flysch units formed the external zones and were later deformed into the Gibraltar Arc fold-and-thrust belt (e.g. Expósito et al. 2012). The radius of curvature in the hinge zone of the arc, measured at the internal-external zones boundary, is ~120 km, and its concave side (hangingwall of the suture) contains a large extensional basin (the Alboran basin), where the continental crust was thinned down to 14 km (Comas et al. 1999; Torné et al. 2000). The Western Gibraltar Arc (WGA; Balanyá et al. 2007, 2012) has been formally defined as the portion of the Gibraltar Arc west of the 4° 30 meridian. Its structural grain draws a second-order salient that ends at two transpressional zones (Fig. 1): the Torcal Shear Zone (TSZ) to the north (Betics) and the Jebba Fault Zone to the south (Rif). Deformation within the FTB around the WGA occurred since the early Miocene to the Holocene. During this period, overall kinematics has been governed by the ca. N–S Europe-Africa convergence (Mazzoli and Helman 1994) and the westward migration of the arc/back-arc system probably related to slab retreat (Royden 1993; Spakman and Wortel 2004) and/or asymmetric delamination (García-Dueñas et al. 1992; Duggen et al. 2004). In the northern branch of the WGA, the FTB is made up of two types of detached covers containing Mesozoic to Paleogene marine sedimentary sequences (Fig. 1): (I) the Subbetic units, which correspond to the South Iberian palaeomargin, mainly deposited onto a continental platform; and, thrust to the NW upon these, the Flysch Trough units, mainly composed of deep-water marine sequences that were deposited partially onto an oceanic lithosphere or an attenuated continental lithosphere (Durand-Delga et al. 2000). Most of the observations proposed in this guide are located within the inner Subbetic units, namely the Penibetic, which, from bottom to top include (MartínAlgarra 1987): (1) Middle Triassic limestones and dolostones, followed by Upper Triassic, evaporite-rich, marls with dolostones and limestones at the top; (2) Lower and Middle Jurassic dolostones and oolitic limestones; (3) Upper Jurassic nodular and oolitic limestones; (4) Lower Cretaceous whitish marly limestones (“White Beds”); and (5) Upper Cretaceous to Paleogene redish marly limestones (“Red Beds”). The northern FTB mountain front borders the Guadalquivir foreland basin. Marine deposits filled this basin during the middle and late Miocene, whereas a progressive westward emersion took place from the Pliocene onwards (González-Delgado et al. 2004). Several intermontane basins developed within the FTB, some of them

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connected with the Guadalquivir basin. Their inception took place during the late Miocene when the current general topographic traits of the Betic Chain started to emerge (Braga et al. 2003; Sanz de Galdeano and Alfaro 2004). Examples of these are the Ronda and the El Chorro intermontane basins, both of them included in this field trip. Located in the concave side of the Gibraltar Arc, the Internal Zones (Alboran Domain, Fig. 1) of the Betic-Rif orogen are made up, from structural bottom to top, of the Frontal Units, the Alpujarride and the Malaguide complexes. The Frontal Units consist of strongly deformed, Triassic to Palaeogene rocks, which crop out in the periphery of the Alboran Domain, having been thrust upon the Flysch Trough units. The Alpujarride and the Malaguide complexes, tectonically placed onto the Frontal Units, contain Palaeozoic and Mesozoic sequences. The Alpujarride complex comprise a very thick metamorphic (mostly metapelitic) sequence that contains eclogites and carpholite-bearing HP-LT assemblages (Tubía and Gil Ibarguchi 1991) and peridotites (Sánchez-Gómez et al. 2002). Different metamorphic stages took place in the Alpujarride complex from the Permian to the early Miocene (Platt and Whitehouse 1999; Sosson et al. 1998; Acosta-Vigil et al. 2014). The Malaguide complex, on top of the Alpujarride complex, is mainly composed of a slightly deformed and metamorphosed Ordovician-Permian sequence. La Joya Complex is a turbiditic unit that frequently overlies the Alboran Domain (Fig. 2). The deformation within the Betic FTB shows a typical thin-skinned style and its evolution can be divided into two major tectonic steps. (1) A main accretionary event occurred at early to middle Miocene time, coevally with NW-verging folds in the Subbetic units (Expósito et al. 2012) and a thrust system affecting the Flyschs Trough units (Luján et al. 2006). (2) From late Miocene onwards, the WGA forms as a second-order arc of the Gibraltar Arc, and two domains with distinctive strain partitioning modes are developed within the FTB in this sector of the Betics: the WGA hinge zone and the Torcal Shear Zone (TSZ). The WGA hinge zone is characterized by arc-orthogonal shortening structures (fold, thrusts and reverse faults) coeval with arc-parallel stretching, mainly accommodated by arc-perpendicular normal faults and conjugate strike-slip faults (Balanyá et al. 2007, 2012; Jiménez-Bonilla et al. 2015, 2017). Thrusting transport sense describes an outward divergent pattern. Conversely, deformation in the TSZ was highly partitioned into E–W major dextral strike-slip dominated faults and transpressive zones, NE–SW oriented shortening structures and NW–SE strikes normal faults that accommodated subordinate extension parallel to fold axes (Díaz-Azpiroz et al. 2014; Barcos et al. 2015). In order to illustrate the contrasting strain partitioning modes described above, we propose to split the field trip into two itineraries (Fig. 1): Itinerary1- devoted to the WGA hinge zone, and Itinerary2- focused on the TSZ.

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Fig. 2 Geological map of Itinerary 1 with the location of its stops. Modified from Jiménez-Bonilla et al. (2015). The red line shows the location of cross-Sect. 7b

3 Useful Information to Plan Your Trip In addition to its geological heritage, the area described in this field guide offers a broad range of natural and cultural values. The route goes across several protected natural sites, such as Parque Nacional de Sierra de las Nieves, Parque Natural de Grazalema, Paraje Natural del Torcal de Antequera, or Paraje Natural del Desfiladero de Los Gaitanes (Caminito del Rey). Regarding its cultural interest, it is well worth visiting, among others, the monumental towns of Antequera and Ronda,

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as well as the Antequera Dolmens Archaeological Site, included in the UNESCO World Heritage Sites list. There is a wide offer for accommodation in the area of the field trip. The touristic towns of Ronda and Antequera are on our route, and you can find plenty of hotels and restaurants. Málaga, the main city of the Costa del Sol, is located 50 and 100 km away from Antequera and Ronda, respectively. Málaga also provides the closest international airport, though you can also fly to Sevilla. Note that the summer is quite warm in southern Spain and the temperature can easily exceed 35-40°. Furthermore, the whole region is a common touristic hotspot, being particularly busy in summer when some places proposed as stops in the guide can be crowded with visitors. In this respect, if you intend to visit the “Caminito del Rey” (see stop 2.3), you must take into account that it is a protected area with restricted access, which needs booking in advance (tickets may be sold out months before your planned date). You can purchase tickets through the official webpage (http://www.caminitodelrey.info/en/) or via tourist agencies or local hotels.

4 Itineraries Description 4.1 Itinerary 1. The Northern Branch of the Western Gibraltar Arc This route runs along the FTB of the northern WGA hinge zone, which is an area characterized by a set of parallel mountain ranges (up to 1918 m high) that encloses the Ronda intermontane basin (Fig. 2). The relief of this area was built up after the Tortonian. Its cross-strike topography is conditioned by late Miocene to Pliocene km-scale folds displaying a conformable relief. Therefore, mountain ranges made up of Penibetic units coincide with largescale NE–SW oriented late antiformal cores, whereas floor valleys coincide with synformal cores (Jiménez-Bonilla et al. 2015, 2017). Along-strike relief segmentation also occurs, greatly controlled by arc-parallel extension (Jiménez-Bonilla et al. 2015). One of the main relief discontinuities along the orogenic grain is the Ronda basin, the most significant intermontane basin of the Western Betics, whose marine sedimentary infill ranges from the late Tortonian to the Messinian. The basin itself is characterized by a wavy relief with altitudes between 450 and 850 m, being limited to the NE and SW by an abruptly rising relief, with altitudes up to 1550 m. These sharp boundaries are controlled by post-Serravalian, NW–SE striking normal faults, sub-orthogonal to the post-Serravallian large-scale fold axes (Jiménez-Bonilla et al. 2015). Fault surfaces dip basinward and hanging walls throw down the Penibetic and Flysch Trough units that constitute the Ronda basin basement. Conversely, the Ronda basin NW boundary is a dextral transpressive band. Geomorphic indices, useful to test the recent tectonic activity in areas with low to medium deformation rates, point

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to a current tectonic activity of some of the structures affecting the Ronda basin (Jiménez-Bonilla et al. 2015, 2017). When kinematic patterns of this area are integrated within the entire WGA (including the Western Betics and Northern Rif), the Ronda basin tectonic evolution appears clearly controlled by the characteristic strain partitioning patterns linked to the evolution of a progressive arc with outward divergent thrusting (Balanyá et al. 2007; Jiménez-Bonilla et al. 2015; Jiménez-Bonilla 2017). Accordingly, the aim of this itinerary is to identify the dominant strain partitioning modes within this FTB kinematic domain and their relationships with the development of the Ronda basin. Stop 1.1A is devoted to introducing the tectonic organization of this FTB segment. Stops 1.1B and 1.4 show good examples of characteristic arc-perpendicular shortening structures within the Subbetic units and the Ronda basin infill, respectively. Stops 1.2, 1.3 and 1.5 present normal faults that accommodate arc-parallel stretching at the Ronda basin margins. The relationships between the Ronda basin infill and its basement are illustrated in stop 1.6. Finally, stop 1.7 focuses on the development of transpressional-related structural highs in the northern boundary of the Ronda basin.

4.1.1

Stop 1.1. the Alboran Domain Mountain Front and the Innermost Fold-and-Thrust Belt

Guarda Forestal viewpoint (36° 46 54.36 N 4° 59 14.19 W). Ronda-El Burgo road (A-366). Stop 1.1A. Looking to the south from the viewpoint, there is a panoramic view over the Alboran Domain mountain front at the northern side of Sierra de las Nieves (up to 1918 m). From bottom to top in the tectonic pile, we observe (Fig. 3): (a) the Penibetic units, mostly represented by the Red Beds formation (Upper Cretaceous to Paleogene); (b) the Flysch Trough units (Jurassic to Paleogene), here severely thinned by low angle normal faults that produce the tectonic inversion of the Alboran Domain basal thrust; and (c) the Frontal Units, mainly composed of Triassic and Jurassic

Fig. 3 General view of the Alboran Domain mountain front (left), and the Penibetic ranges (right). Both pictures were taken nearby Stop 1.1 (see location in Fig. 2)

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Fig. 4 a The geologist Manuel Díaz-Azpiroz at an example of NW-verging fold affecting the Red Beds formation, b associated stylolitic axial plane cleavage and c calcite veins perpendicular to the fold axes. Pictures are taken nearby stop 1.1, 600 m towards Ronda along the road A-366 (see location in Fig. 2)

carbonatic rocks. Bedding, which is especially apparent in the Penibetic, dips to the SE all along the cross-section. Looking to the west, we observe the Penibetic reliefs of Sierra Blanquilla (up to 1430 m), in which differences between pale grey competent rocks (Jurassic limestones) and soft rocks (Cretaceous marls) are easily distinguished. Very good exposures of the Jurassic sequence can be found along the road A-366 between this stop and Puerto del Viento. Stop 1.1B. Observations within the Penibetic can be completed walking 600 m towards Ronda along the road A-366. Starting from the viewpoint, we observe first the White Beds formation (white cherty marly limestones) and later the Red Beds formation (red marly limestones). Both formations are affected by NW-verging folds, early to middle Miocene in age (Expósito et al. 2012). Metric to decametric chevron folds develop a pressure-solution, axial plane cleavage, which is often stylolitic (Fig. 4). Flexural-slip-folding is evidenced by typical chevron-type morphologies and the occurrence of calcite fibers on flexural-slip planes (bedding). Pervasive calcite veins, mostly orthogonal to the hinge lines, accommodate fold-axis parallel extension (Fig. 4; Expósito et al. 2012).

4.1.2

Stop 1.2. The Grazalema Fault

This stop is located on the road A-372, close to Grazalema (Fig. 5a). There is a parking place nearby (36° 45 17.74 N 5° 21 31.10 W). At this stop, we can observe the Grazalema fault scarp, which is marked by a 300 m topographic drop of the NE block (Fig. 5b). The SW block of the Grazalema

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Fig. 5 a Location of the observations and main structures of the Grazalema fault, b main scarp and c equal area, lower hemisphere plot of fault planes and related slickenlines

fault, topographically uplifted, is composed of Jurassic Penibetic limestones and dolostones, whilst the NE block is made up of Cretaceous to Palaeogene Penibetic marly limestones and Miocene sandstones of the Flychs Trough units. Most fault planes dip steeply towards the NE. In the Cretaceous marly limestones, some S–C structures and slickenlines on C planes show dominant normal fault slip (Fig. 5c). The Grazalema fault cuts across the Endrinal range, which corresponds with a late Miocene to Pliocene, upright antiform (Fig. 5a, b). The vertical slip of the Grazalema fault is higher than 1.5 km (Jiménez-Bonilla et al. 2015). Apart from the Grazalema fault, we can also observe some features related to the early to middle Miocene deformation event: (1) Close to the village of Grazalema, along road CA-5311, there is a fold train composed of metric to decametric folds, verging to the NW and affecting an outcrop of Cretaceous marly limestones. See 1A in Fig. 5a for location (JiménezBonilla et al. 2015). (2) Continuing to road A-372, there is another fold train made up of NW-verging, decametric folds, but deforming in this case the Flysch Trough units. See 1B in Fig. 5a for location (Jiménez-Bonilla et al. 2015). 4.1.3

Stop 1.3. The El Republicano Fault

Taking the road A-374 towards Villaluenga and before arriving in this town, there is a dirty road to the left where parking is possible (36° 41 25.81 N 5° 22 19.86 W). From this parking, we have a nice view of the El Republicano fault trace (Fig. 6). This fault also downthrows the NE block and cuts across another NE-SW late Miocene to Pliocene antiform. All fault planes dip steeply to the NE, where younger

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Fig. 6 a Picture and equal area, lower hemisphere plot (dots are slickenlines on fault planes) of the El Republicano normal fault. b S–C structures in the normal fault zone

rocks crop out (Miocene Flyschs Trough units, see Jiménez-Bonilla et al. 2015). Both slickenlines and kinematic criteria show that this is a dip-slip dominated, normal fault (Fig. 6). The vertical slip of this fault surpasses 1 km. An early to middle Miocene event deformed the Flyschs Trough units that crop out in the fault hanging wall, giving way to a west-verging imbricate stack (JiménezBonilla et al. 2015). This stack is evidenced by the alternation of clays (without vegetation) and sandstones (with vegetation cover). The Flysch Trough units are currently hosted in a synform in-between the antiforms of stops 2 and 3 (Fig. 2). This imbricate stack is cut by both the El Republicano and Grazalema normal faults, which are the most important normal faults in the SW boundary of the Ronda basin (Figs. 2 and 5, see also Jiménez-Bonilla et al. 2015).

4.1.4

Stop 1.4. The Salinas Fold

This stop is located at road A-374 close to the town of Ronda, in the diversion to a petrol station (36° 45 33.20 N 5° 09 43.36 W). Looking to the north, we observe a range within the upper Miocene infill of the Ronda basin (Fig. 7). This NE–SW range coincides with an antiform whose fold limbs dip 50°, as observed in resistant calcarenites eroded by deep gullies. Around this stop, rocks of the Ronda basin infill crop out: calcarenites, conglomerates and silts. From this stop, we also have a wonderful panoramic view of the Ronda basin boundaries: the SW boundary (stops 2 and 3) and the NE boundary, which will be

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Fig. 7 a Picture of the Salinas range and b cross-section including both boundaries of the Ronda basin (Jiménez-Bonilla 2017; See location in Fig. 2)

visited in stops 5 and 6, looking to the west and east, respectively. Both the SW and NE boundaries downthrow the Ronda basin and have contributed to accommodate the WGA-parallel stretching since the late Miocene (Fig. 7). Mountain ranges at both basin boundaries are mostly NE–SW oriented and coincide with upright late Miocene to Pliocene antiforms cored in Jurassic limestones. The NW–SE shortening, together with NE–SW stretching accommodated by the normal faults enlighten the post-late Miocene strain partitioning mode in this FTB segment of the WGA (Balanyá et al. 2007; Jiménez-Bonilla et al. 2015).

4.1.5

Stop 1.5. Normal Fault Zone at the NE Boundary of the Ronda Basin

This stop is located along road CA-414, close to the Cuevas del Becerro village (36° 51 49.11 N 5° 03 18.63 ). The NE boundary of the Ronda basin is partially faulted. This stop coincides with one of the segments affected by a NW–SE normal fault zone, which have produced SW-facing topographic scarps in the Penibetic limestones (Fig. 8). Faults have listric geometry with decametric vertical displacements. The cumulative vertical displacement is around 250 m. S–C type structures and slicken linespitches indicate a dominant dip-slip, normal component of the fault (Jiménez-Bonilla et al. 2015).

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Fig. 8 Normal faults at the NE boundary of the Ronda basin (stop 1.5, see location in Fig. 2). The discontinuous blue line shows the contact between Lower and Middle Jurassic limestones

4.1.6

Stop 1.6. Ronda Basin Basal Unconformity

This stop is located along the road CA-414, between the villages of Cuevas del Becerro and Setenil. There is a dirty road to the right (36° 51 49.97 N 5° 06 0.12 W). The best place to park is at the entrance to this dirty road. This stop shows a cross-section of a segment of the NE boundary of the Ronda basin, which corresponds here with an unconformity. Walking 250 m to the east from the parking place, we observe the Ronda basin basement composed of Jurassic Penibetic limestones with gentle dips to the west. Walking westwards along the road, we can see these limestones, but with intense karstification. Some of these karstified channels are filled with conglomerates with rounded limestone clasts, which constitute the basal deposits of the basin. Finally, well-organized beds of similar conglomerates are present further west (Fig. 9, see also Jiménez-Bonilla et al. 2015).

Fig. 9 Unconformity between the Penibetic limestones and the conglomerates of the Ronda basin infill nearby Stop 1.6 (see location in Fig. 2)

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Stop 1.7. NW Boundary of the Ronda Basin

This stop is located at the Olvera castle, where we have a panoramic view (36° 56 06.56 5° 16 04.80 ). This stop is located at the NW boundary of the Ronda basin, which corresponds to a dextral transpressive fault zone partitioned into strike-slip faults and folds. Looking from SW to SE many WSW–ENE-oriented rock alignments, limited by faults, can be noticed. They define a 20 km long dextral strike-slip fault zone (Fig. 10). This stop is located in one of these rock alignments where some dextral strike-slip fault planes can be observed. Looking to the west, we observe a NE–SW mountain range that coincides with a late Miocene to Pliocene antiform (Fig. 10). To the south, the Ronda basin infill overlies a basement composed of Triassic Penibetic rocks. The basin infill dips to the south. Hence, a structural high bounds the basin to the north, and separates the Ronda intermontane basin from the Guadalquivir foreland basin (Fig. 2; located 30 km to the NW; Jiménez-Bonilla et al. 2015).

4.2 Itinerary 2.—The Torcal Shear Zone The Torcal Shear Zone (TSZ) is identified by several disconnected, en-echelon morpho-structural highs that define an E–W, 70 km long and 5–8 km-wide alignment (Fig. 11). To the south, the TSZ limits with the Alboran Domain, which in this area is partially covered by lower-middle Miocene sediments, including in its upper part the

Fig. 10 Geological map of the NW boundary of the Ronda basin (see location in Fig. 2) and equal area, lower hemisphere plot of faults bounding rock alignments

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Fig. 11 Geological map of the TSZ with its four principal sectors Díaz-Azpiroz et al. (2014), from west to east: Teba-Peñarrubia ridges (TPS), Valle de Abdalajís Massif (VAM), Torcal de Antequera Massif (TAM) and Cabras-Camorolos ridges (CCS). The inset shows the location of the TSZ within the Betic-Rif orogen. Rectangles mark the location of Figs. 14, 16 and 19, where stops of itinerary 2 are shown

so-called La Joya olistostromic complex (Fig. 2, Suades and Crespo-Blanc 2013). To the north, the TSZ is in contact with Triassic marls and evaporites, as well as upper Miocene calcarenites. The structural pattern of the TSZ includes a great variety of brittle-ductile structures that are interpreted, as a whole, as resulting from a late Miocene (or younger) dextral transpression (Díaz-Azpiroz et al. 2014; Barcos et al. 2015). This age is based on different criteria: (a) the interference patterns of the TSZ structures with earlymiddle Miocene folds (Expósito et al. 2012, see stop 1.1B); (b) the evidence of transpressive structures affecting upper Miocene calcarenites; and (c) the calculation of focal mechanisms from crustal earthquakes and geomorphic indices, both indicating recent strike-slip tectonics within the TSZ (Balanyá et al. 2012; Barcos et al. 2012a; Díaz-Azpiroz et al. 2014). However, upper Miocene sediments of the El Chorro basin locally cover some of these structures in the VAM, likely accounting for some deformation diachrony. Transpression was strongly partitioned at several scales. At the km-scale, several sectors with contrasting structural patterns and kinematics appear along-strike (Fig. 11). The westernmost sector (Teba-Peñarrubia) represents the transition between the WGA (itinerary 1) and the TSZ (Jiménez-Bonilla et al. 2013). Two massifs at the central sector (Valle de Abdalajís and Torcal de Antequera massifs, VAM and TAM, respectively) accommodate the main, dextral transpressional strain, and are the principal focus of itinerary 2. The Cabras-Camorolos sector acted as a termination zone that links the TSZ with the central Betics (Barcos et al. 2012b). At lower scales, late Miocene dextral transpression was partitioned into kinematically homogeneous domains that accommodated different components of the bulk strain (Fig. 12). The TAM presents two main structural domains: an internal domain (TAM-ID) occupying most of the massif and two narrow outer domains (TAM-OD)

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Fig. 12 Structural features of the TSZ central sector (Díaz-Azpiroz et al. 2014; Barcos et al. 2015). a Geological and structural map of the TAM and VAM. Within the TAM, the inner domain (ID), and the northern (NOD) and southern (SOD) outer domains are shown. The location of cross-sections of the VAM (I-I’, Fig. 18b) and TAM (II-II’, Fig. 22b) is included. b and c Structural data from the TAM b and VAM c, shown as a lower hemisphere, equal area plots of fault planes with related slickenlines and slickenfibers (black circles), fold facing directions (asterisks) and finite strain axes (white triangles in TAM, black squares in VAM)

that define the northern and southern limits. The TAM inner domain presents mainly shortening structures (folds and reverse faults) oblique (NE-SW striking) to the main TAM limits (E-W oriented). The TAM outer domains are uplifted with respect to the neighbouring units, and dextral strike-slip dominated fault zones separate them. The general structure of the VAM is markedly different. Its southern limit is a discrete dextral fault zone. In turn, its inner part presents a general imbricate structure defined by dextral-reverse fault zones, developed occasionally at antiform short-limbs. Both structures in the inner part are subparallel to the VAM boundaries (ENE-WSW) and homogeneously distributed within this massif. In both the TAM and the VAM, NWSE striking normal faults produce subordinate extension orthogonal to the principal, shortening structures. We analyzed this transpression in three stages: 1) description of the finite strain geometry (orientation and, when possible, magnitude of the three main finite strain axes) in each domain; (2) comparison of finite strain geometry with results from the model of triclinic transpression with oblique extrusion (Fernández and DíazAzpiroz 2009) to obtain the main kinematic parameters and boundary conditions (Díaz-Azpiroz et al. 2014; Barcos et al. 2015); and (3) analogue modelling based on the kinematic results to reproduce the main structures observed in the TAM (Barcos et al. 2016). This analysis suggests that the different structural patterns

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observed, resulting from multi-scale strain partitioning, match with a single far-field vector oriented N099°–109°E (according to the current position of the TSZ; Fig. 13). Bulk kinematic differences between the TAM and VAM (mainly in convergence obliquity) would likely arise from local variations in the main boundaries of each sector (N086°E/73 N at the TAM and N064°E/82S at the VAM). In turn, distinct strain partitioning styles could be related to the specific kinematics of each sector and/or to lateral variations of the underlying plastic layer (Triassic evaporite-rich materials) thickness (Barcos et al. 2015). We integrate the TSZ kinematics in the general Betic-Rif evolution. After the main, early-middle Miocene convergent tectonic event that structured the Betic foldand-thrust belt (see itinerary 1), oblique convergence with respect to the Alboran Domain continued active from the late Miocene onwards (Balanyá et al. 2012; Barcos et al. 2012a). The contact with the Alboran Domain represented a major rheological boundary that localized deformation along with a narrow band that would have resulted in the TSZ. Transpression at this shear zone uplifted the Penibetic formations respecting the Alboran Domain to the south and the Miocene deposits to the north. Furthermore, the dextral lateral displacement at the TSZ accommodated the progressive westward migration of the WGA with respect to the rest of the Betic-Rif chain, thus increasing the protrusion degree of this arc. Analogue models of progressive arcs reproduce a similar kinematic evolution (Jiménez-Bonilla et al. 2020). Itinerary 2 (Figs. 14, 16, 19) is easily accessible from either Olvera, the ending point of itinerary 1, using road A-384 to Campillos, or directly from Ronda via road

Fig. 13 Conceptual model for the late Miocene transpressional event and the multi-scale strain partitioning in the TSZ central sector, with the main kinematic and far-field parameters, in block diagram a and equal area, lower hemisphere projection. b Scales in a are approximate and vertical scale within the TSZ is double than out of this shear zone (Barcos et al. 2015)

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Fig. 14 Google Earth image showing the Teba-Peñarrubia ridges and the location of stops 2.1 and 2.2 (see location in Fig. 11)

A-367. Alternatively, it is possible to start this itinerary at Antequera. We present here an overall view of the TSZ and the main structures that accommodate dextral triclinic transpression at its central sector, in the VAM and TAM. The visitor will see how along-strike partitioning of transpression produced different structural styles in both massifs. The route would ideally be split into two days. The first day (stops 2.1–2.4) would be dedicated to a general view of the TSZ, the termination zone that links the TSZ with the WGA, visited in itinerary 1, and the structures of the VAM. The second day (stops 2.5–2.11) is focused on the TAM. A good option would be to stay at Antequera. All field observations are based on previous works by the authors (Díaz-Azpiroz et al. 2014, 2019; Barcos et al. 2015, 2016; Ramírez Prior et al. 2018). The visitor is referred to these papers for further information.

4.2.1

Stop 2.1. The Teba-Peñarrubia Sector

This stop is located at the Teba castle (36°58 46.78 N 4°55 06.70 W). Jurassic limestones crop out at the hill where the castle is located. This hill is the western termination zone of the TSZ, which links with the WGA through the Almargen ridge. Looking west and south, two NE–SW mountain ranges with lengths exceeding 10 km, and corresponding to the WGA fold-and-thrust belt, are cut at their NE end by a WNW–ESE dextral strike-slip fault. This strike-slip fault also marks the western end of the Teba ridge. Looking east in the foreground, a change in the structural trend from WSW–ENE in the Teba ridge to WNW–ESE in the Peñarrubia ridge is noted. Both ridges correspond to hm-scale open antiforms. The Peñarrubia antiform (with NNE and SSW dipping limbs) is cut at its NE limb by a dextral

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Fig. 15 The majestic Tajo del Molino gorge (looking southwards) crosscutting the Peñarrubia ridge. Note the south-dipping bedding on the right cliff

Fig. 16 Google Earth image showing the VAM and the location (stop 2.3) of the starting point of the ca. 7 km Caminito del Rey trail (see location in Fig. 11)

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Fig. 17 a Panoramic view from the main entrance to the Caminito del Rey and the Gaitanejos gorge entry. From this point northwards, we can observe the uppermost part of the pre-orogenic sequence: Middle-Upper Jurassic oolitic limestones, Lower Cretaceous White Beds and Upper CretaceousPaleogene Red Beds. The upper Miocene calcarenites unconformably lie over the pre-orogenic sequence and show spectacular taffoni (upper left). b Giants’ kettles hanged several meters above the current riverbed. c Early-middle Miocene, north-verging chevron folds deforming the Red Beds. d Panoramic view from the western margin of the El Hoyo valley with the interpretation of the main structures that define the two southernmost tectonic imbrications of the VAM (Díaz-Azpiroz et al. 2019). See the text for further details

strike-slip fault, whilst the Teba antiform (with NW and SE dipping limbs) is cut by a reverse fault at its SE limb. These faults can be appreciated by the associated scarps. The Teba antiform and reverse fault are interpreted as a compressive bridge limited by two WNW–ESE strike-slip faults (Jiménez-Bonilla et al. 2013). Looking ESE, in the background, two separate topographic highs correspond to the VAM (right, stops 2.3 and 2.4) and TAM (left, stops 2.5–2.11).

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Fig. 18 a Panoramic view of the north-verging, imbricate structure of the VAM north of Sierra del Valle de Abdalajís. b Interpretative cross-section of the whole massif with the location of Fig. 18a. (Díaz-Azpiroz et al. 2019)

Fig. 19 Google Earth image showing the TAM and the location of stops 2.4–2.11 (See location in Fig. 11)

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Stop 2.2. The Tajo Del Molino Gorge

Take road MA-465 from Teba and then C-341 to Campillos. After ~1.5 km, park to the right (36° 58 52.4 N 4° 52 56.7 W). Walk eastwards along the road 120 m to a bridge over Tajo del Molino. This deep gorge (Fig. 15) is a product of an antecedent stream prior to the uplift of the Peñarrubia ridge (Jiménez-Bonilla et al. 2013). This gorge permits us to see the smooth SSW dip of the Jurassic limestones. Continuing ~500 m to the north along the gorge, bedding dip progressively decreases defining the Peñarrubia antiform, which is cut by a WNW-ESE dextral strike-slip fault. At the northern end of the gorge, an outcrop of upper Miocene conglomerates can be observed. These sediments are deformed by a WNW-ENE synform with beds dipping 30° to the NNE.

4.2.3

Stop 2.3. Transect Across the Valle de Abdalajís Massif

This stop consists of a ca. 7 km walk along the so-called Caminito del Rey pathway (Fig. 16), which transects across the VAM. The trail goes through two spectacular gorges, named Gaitanejos and Los Gaitanes. You may park at 36° 55 44.8 N 4° 48 07.6 W and walk to the entrance (36° 55 56.1 N 4° 47 21.9 W). From this point, walking is only permitted in one direction, towards the south. After exiting the trail, you may walk to the El Chorro railway station (36°54 24.7 N 4°45 33.2 W), where a shuttle will take you back to the parking area. The whole route will take ~4 h. The best roads to reach the parking lot from Teba (stop 2.2) are C-341, then A-357 to Campillos, A-7286 to Embalses, and Parque Guadalteba, the road to Embalses del Guadalhorce and the road to Presa Conde de Guadalhorce. This way is ~15 km in total. Along the Caminito del Rey it is possible to have almost continuous geological observations of the complete pre-orogenic sedimentary sequence, from the Upper Triassic to the Paleogene, and of the general imbricate structure of the VAM (see stop 2.4). However, such structure is somewhat different in this sector (compare to observations at stops 2.4 and 2.7C), which shows fewer imbrications, occasionally affected by ENE–WSW striking normal faults. It is also possible to appreciate upper Miocene sediments of the El Chorro basin unconformably lying over the pre-orogenic sequence. (A) From the parking lot to the main entrance, there is a ca. 1.4 km walk where upper Miocene calcirudites and calcarenites extensively crop out. Calcarenites present striking cross-bedding and spectacular taffoni (Fig. 17a). (B) At the beginning of the trail (36°55 57.58 N 4°47 17.63 W), from the main entrance to the entry to the first gorge (Gaitanejos), it is possible to observe, from south to north (Fig. 17a), the uppermost part of the pre-orogenic sequence: Upper Jurassic oolitic limestones, Lower Cretaceous whitish marly limestones (White Beds) and Upper Cretaceous-Paleogene reddish marly limestones (Red

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Beds). Bedding dips moderately to the north, defining the northern limb of a hmscale, ENE-WSW trending antiform. Upper Miocene calcareous conglomerates and sandstones lay unconformably over the Cretaceous-Paleogene rocks. The rest of the Jurassic sequence (oolitic and nodular limestones) crops out along the Gaitanejos gorge. Also at this gorge (36° 55 52.44 N 4° 47 11.96 W), giants kettles appear hanging above the Guadalhorce current riverbed (Fig. 17b), which suggests rapid incision. In the El Hoyo valley, which lies between the two main gorges, mainly White and Red Beds crop out. These rocks are affected by early-middle Miocene, north-verging chevron folds (Fig. 17c, looking WNW from 36° 55 28.64 N 4° 46 58.41 W), which are identified along the entire WGA and observed in stop 1.1B. Looking east from 36°55 16.59 N4°46 48.15 W, it is possible to appreciate part of the characteristic imbricate structure of the VAM (Fig. 17d). The Sierra the Huma is in the background of the view. An open synform with Jurassic limestones at the nucleus defines the characteristic flat summit of this range. Triassic evaporite-rich marls and dolostones crop out at the northern limb, and overthrust Cretaceous-Paleogene marly limestones. The southern limb dips gently to the north and rapidly recovers to define an antiform (out of the image) with a subvertical southern limb, which appears in the foreground, to the right, at the other side of the river. The slope shows, to the south, dolostone beds that overthrust the Cretaceous-Paleogene marly limestones. Further to the south, it is possible to observe the conformable contact between the Triassic formations and the Jurassic limestones, which are incised by the Los Gaitanes gorge. In the central part of the view, the Cretaceous-Paleogene marly limestones lie conformably over the Jurassic limestones, which show north-verging shortening structures: an antiform and a reverse fault. This structure constitutes the base of the next imbrication to the north after the Sierra de Huma. Just before entering the Los Gaitanes gorge (36°55 2.75 N4°46 27.45 W), it can be seen, at the subvertical southern limb of the Sierra de Huma antiform (see above), the stratigraphic contact between the Triassic and Lower-Middle Jurassic formations. The path along this gorge crosscuts the entire Jurassic sequence (oolitic—nodular—oolitic imestones). The opening of the gorge to the El Chorro reservoir (36° 54 56.85 N4° 46 18.66 W) coincides with the Jurassic top layers in contact with the CretaceousPaleogene formations. Exiting the trail, the White Beds (with chert nodules, 36° 54 57.06 N4° 46 10.75 W) underlie the Red Beds (36° 54 55.74 N4° 46 10.63 W). In the village of El Chorro (36° 54 30.64 N4° 45 36.41 W), where the route ends, it is possible to observe phyllites belonging to the Malaguide complex (Alboran Domain). The contact with the Penibetic formations does not crop out along the path, but it is the dextral transpressive fault zone that defines the southern limit of the TSZ in the VAM.

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Stop 2.4. Panoramic View of the Northern VAM Imbricate Structure

From the parking lot of the Caminito del Rey main entrance, make a left turn and follow this road eastwards. After ca. 1 km, take right following signs to Antequera and Campillos. From this point onwards, follow signs to Antequera and V. Abdalajis. In the first part of the way (3.5 km, up to 36° 56 47.1 N 4° 47 31.5 W), the road crosscuts upper Miocene calcareous conglomerates and sandstones with striking cross-bedding. Between 3.5 and 9.5 km, the road goes along the northern edge of the VAM, with either Jurassic limestones or Cretaceous-Paleogene marly limestones to your right. At 36° 58 17.3 N 4° 44 27.8 W, looking south, there is a panoramic view of the northern edge of the VAM. A Jurassic limestone cliff corresponds to the hanging wall of a WSW–ESE striking, southward dipping reverse fault that puts this formation over the Cretaceous-Paleogene marly limestones (vegetated gentle slope below the cliff). This is the northernmost imbrication of this massif. In the background, limestone crests mark other imbrications. Looking SSE, the Sierra de Huma (see stop 2.3E) summit is apparent. After 16 km from the starting point, at the crossroad (36° 57 26.1 N 4° 41 26.7 W), take road A-343 to the left. Right before km 19 (36° 58 45.2 N 4° 39 00.4 W), there is a sign to the left to Las Lagunillas. You may stop here or at the farmer entrance to your right. Looking westwards, there is a panoramic view of the northernmost imbricate structure of the VAM (see also stop 2.7C). From south to north, three of these imbrications appear. They are all limited to the north by south-dipping reverse faults that put Jurassic limestones over Cretaceous-Paleogene marly limestones (Fig. 18).

4.2.5

Stop 2.5. Northern Boundary of the Torcal de Antequera Massif

Drive southwards from Antequera along road A-343. After km 9 benchmark, at the crossroad, continue ahead along road A-7075 towards Torcal de Antequera and Villanueva de la Concepción. Short after the camping, at around km 47.5 benchmark there are two dirt tracks, one at each side of the road. Stop on the left one (36° 59 06.7 N 4° 31 28.6 W). Looking westwards (Fig. 20), the northern TAM outer domain can be observed. In an east-facing outcrop, two reddish limestone layers can serve as markers, dipping to the north (to the right of the view). A subvertical fault descends these layers with respect to the rest of the TAM outer domain. Upwards, the fault zone describes a splay structure with curved, south-dipping surfaces. Further to the north, at the lower part of the outcrop, the reddish layers seem to define a very open, incomplete synform. However, a thorough look shows this is an apparent effect; the structure results actually from the combined displacements of several small-scale, south-dipping faults, which resemble that of the main fault surface. Overall, these faults define what we could describe as a positive flower-like structure (we only see its northern half), although it is not a typical one, as we will see in stop 2.8.

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Fig. 20 Northern limit of the TAM with the interpretation of the asymmetric flower-like structure (Díaz-Azpiroz et al. 2014). Note that the structure has an important right-lateral component

4.2.6

Stop 2.6. The Boca Del Asno antiform

Continue along road A-7075 heading to the Torcal de Antequera. After km 45 benchmark, there are three consecutive closed curves in a place known as Boca del Asno. Between the first two curves (36° 58 24.2 N 4° 29 42.2 W) there is an excellent panoramic view of the eastern edge of the TAM (Fig. 21). In the central part of the slope, grey limestone beds dip gently to the north, and define (down left) an antiform hinge (Boca del Asno antiform, Fig. 21), which is kinematically equivalent to the El Camorro antiform (stop 2.7B), but located further to the SE, in an en-échelon array (Fig. 12). Short after km 42 benchmark, at the crossroad (36° 57 51.3 N 4° 30 39.5 W), make a right turn. This road finishes at the Torcal de Antequera Interpretation Center. In the first kilometer, to the right, grey carbonatic rocks with very thick bedding (occasionally massive) crop out. The lower part corresponds to uppermost Triassic dolostones, which are followed by Lower-Middle Jurassic oolitic limestones. Leave the car at the parking lot at the end of the road. Before going to stop 2.7, it is possible to have a panoramic view to the south of the TAM from the Mirador de las Ventanillas, 50 m away from the Interpretation Center.

4.2.7

Stop 2.7. Structural Pattern of the TAM Inner Domain

This stop consists of a 1.5 km walk from the Interpretation Center. Follow the road from the parking lot to the NE. After ca. 300 m (36° 57 20.3 N 4° 32 29.7 W), abandon the road and take a narrow track to the left. Walk towards the NW along the foot of the Camorro de las Siete Mesas (an NW-SE elongated topographic high to

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Fig. 21 SE limit of the TAM inner domain viewed from the east and structural interpretation (DíazAzpiroz et al. 2014). Rocks cropping out at the slope correspond to the lowermost layers of the Penibetic sequence: Upper Triassic dolostones and Lower Jurassic limestones. Gently north-dipping limestone layers define to the south an open SE-verging antiform (Boca del Asno antiform). The overturned limb is cut by a reverse fault

your right). After ca. 1.3 km (36° 57 37.7 N 4° 32 52.5 W), there is an excellent panoramic view to the west. Three different observations can be made at this stop. (A) This is a good place to observe the Upper Jurassic formations of the Penibetic stratigraphic sequence (already presented in stop 2.3F–G). Looking eastwards, the Camorro de las Siete Mesas presents nodular limestones at its base and oolitic limestones at its top, both Upper Jurassic. Nodular limestones are reddish colored and rich in ammonite fossils (it is possible to see some in this outcrop). Weathering has created a conspicuous and contrasted layered structure that represents the most typical morphological feature of the Torcal de Antequera (locally known as bollos). Looking to the WSW, the greycolored Upper Jurassic oolitic limestones produce a different weathering morphology with dolines defining an intricate pattern locally known as callejones. The contact between these two formations is an NW-SE striking fault with the uplifted wall (nodular limestones) to the NE. This is one of the NW-SE striking normal faults that produce extension parallel to the main shortening structures of the TAM inner domain (see stop 2.9). The fault plane is not apparent here. (B) Looking to the WNW, in the middle part, there is an excellent panoramic view of the main shortening structures of the TAM inner domain (Fig. 22). To the right, the higher hill with limestones is the Camorro Alto, corresponding to an hm-scale, NE–SW striking, SE-verging antiform similar to that observed at Boca del Asno (stop 2.6A). Limestone beds mark the

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Fig. 22 a Shortening structures of the TAM inner domain. The hill coincides with the SE-verging Camorro Alto antiform, whose short limb defines the SE slope. Related to this limb, the reverse El Navazo fault puts Jurassic limestones (grey rocks) over Cretaceous-Paleogene Red Beds (cultivated fields), which in turn define the El Navazo synform hinge zone. b Schematic cross-section showing the main structural pattern of the TAM (see the location in Fig. 12) with the location of the panoramic view in Fig. 22a (Díaz-Azpiroz et al. 2014)

subvertical SE limb. A NE–SW striking, SE-verging reverse fault (El Navazo fault) is developed at this limb, and puts Jurassic limestones over Cretaceous-Paleogene marly limestones, recognizable by the gentle relief (the El Navazo valley). These same rocks appear at the hinge of the next synform to the SE (El Navazo synform). (C) Looking to the west, in the background, there is a traverse panoramic view of the neighbouring VAM imbricate structure (see Fig. 18). From south to north, one can observe the Sierra de Huma synform (see stop 2.3E) with its characteristic flat summit, the Sierra del Valle de Abdalajís antiform, and two smaller tectonic imbrications limited to the north by south-dipping reverse faults (see stop 2.4).

4.2.8

Stop 2.8. Panoramic View to the North of the TAM

From the previous stop walk to the NE leaving the Camorro de las Siete Mesas to your right. After ca. 400 m, there is an excellent view to the NNE (36° 57 55.9 N 4° 33 00.9 W).

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Fig. 23 NE looking panoramic view from the Camorro de las Siete Mesas. It is possible to observe the Triassic rocks to the north of the TAM and the southern view of the flower-like structure that defines the northern TAM outer domain, whose northern view was observed at stop 2.5

From this point, we it is possible to observe the transpression-related uplift of the TSZ with respect to the northern formations, upper Miocene calcarenites in the foreground to the NNW, and Triassic evaporite-rich marls to the NNE (Fig. 23). The relative uplift of the northern TAM outer domain (see stop 2.5) respecting the rocks located immediately to the south is also evident (Fig. 23).

4.2.9

Stop 2.9. Normal Faults of the TAM Inner Domain

Go back to the road and turn left to the east. Walk ~200 m to 36° 57 23.7 N 4° 32 19.9 W. You can also go back to the parking lot, take the car and drive to 36° 57 27.3 N 4° 32 14.5 W, where parking is also possible. Looking to the SE, there is an almost orthogonal view of two of the NW-SE striking normal faults of the TAM inner domain, clearly marked by the stratigraphic top of the nodular limestones (Fig. 24). These faults dip to the NE (left), present a somewhat listric geometry that tilted the hanging walls and contribute to extend the TAM inner domain along a NE–SW direction. The subtle difference between the tectonic slip and the topographic drop suggests a recent age for these structures.

4.2.10

Stop 2.10. Southern Boundary of the Torcal de Antequera Massif

Go back to the road and then drive to road A-7075. Turn right to Villanueva de la Concepción, cross the village along Blas Infante av., and at the crossroad, continue ahead following signs to Almogía. Drive less than 2 km along road MA-3403 and take a right to La Joya. After 5 km along this road, where a Point of Geological Interest (Punto de interés geologico) is marked (36° 56 01.6 N 4° 35 08.3 W), make a right turn. Continue 2 km more to Cortijo del Robledillo (36° 56 49.6 N 4° 35 02.1 W). Leave the car, abandon the dirt track, and walk eastwards around 700 m leaving the limestone cliff to your left (36° 56 49.3 N 4° 34 38.1 W).

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Fig. 24 NW-SE striking, NE-dipping normal fault. Its slip is estimated from the stratigraphic top of the reddish nodular limestones. Note the small difference between the fault throw and the resulting topographic drop (Díaz-Azpiroz et al. 2014)

This is the southern limit of the southern TAM outer domain, which is uplifted respecting the Alboran Domain units, including the middle Miocene La Joya olistostromic complex, to the south. The limit is defined by a 20 m-thick shear zone affecting the contact between the Jurassic limestones to the north and CretaceousPaleogene Red Beds to the south. There are remarkable differences in the structural style depending on the rock type: discrete, meter- to decimeter-spaced fault planes in limestones, whereas marly limestones show centimeter- to decimeter-spaced SC like structures. This rheological partitioning is similar for all strike-slip, reverse (see stop 2.11) and oblique faults of the TSZ, which suggests that regardless of the kinematics, deformation occurred under similar conditions and, thus, coevally. In all cases, slickenlines and slickenfibers are clearly seen on discrete fault and C planes. In this outcrop, such lines present a fairly constant attitude, with low to moderate west plunges (Fig. 25a). This, in combination with the asymmetry of S–C structures on subhorizontal observation surfaces (Fig. 25b), points to a dominant dextral displacement with a subordinate reverse slip component.

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Fig. 25 Outcrop-scale structures at the southern TAM outer domain (Díaz-Azpiroz et al. 2014). a Calcite slickenfibers on a limestone fault plane. b Asymmetric, dextral S–C like structures in marly limestones (red beds)

4.2.11

Stop 2.11. NE–SW Reverse Fault Within the TAM Inner Domain

Go back to the vehicle and continue along the dirt track for around 3 km, until close to Cortijo de la Fuenfría (36° 57 37.3 N 4° 35 55.9 W). In this stop, the El Navazo reverse fault (Fig. 26), which we observed in a panoramic view from the Camorro de las Siete Mesas (stop 2.7B), can be examined in detail. The Jurassic limestones dip to the south, defining the SE limb of the El Camorro antiform, and overthrust the Cretaceous-Paleogene Red Beds. Some meters to the SE, the bedding of these same marly limestones is subvertical or even dipping

Fig. 26 a The reverse El Navazo fault (see stop 2.7B) puts Jurassic limestones (grey rocks in the background) over Cretaceous-Paleogene Red Beds (in the foreground), which show S–C like structures. b Detailed view (location marked by red square) of S-C like structures, whose asymmetry suggests apparent top-to-the-SE displacement (Díaz-Azpiroz et al. 2019)

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steeply to the north, which is coherent with their position in the short, eventually overturned, limb. In the fault zone, S–C like structures obliterated the bedding of the Red Beds. The high plunge of slickenfibers on C planes and the asymmetry of structures suggest main reverse kinematics for this fault (Fig. 26b). Acknowledgements Financial support from Project PGC2018-100914-B-100 of the Spanish Ministry of Science, Innovation, and Universities, and Project UPO-1259543 of the Andalusian Office of Economy and Knowledge. Edition guidance by S. Mukherjee and thorough review by A. Azor are gratefully acknowledged. Thanks to Marion Schneider, Annett Buettener, Boopalan Renu, Alexis Vizcaino, Doer the Mennecke-Buehler, and the proofreading team (Springer). Dutta and Mukherjee (2021) summarize this article.

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Durand-Delga, M., Rossi, P., Olivier, P., & Puglisi, D. (2000). Situation structurale et nature ophiolitique des roches basiques jurassiques associées aux flyschs maghrébin du Rif (Maroc) et de Sicile (Italie). Comptes Rendues de la Academie des Sciences, 331, 29–38. Dutta, D., & Mukherjee, S. (2021). Introduction to Structural Geology and Tectonics Field Guidebook—Volume 1. In S. Mukherjee (Ed.), Structural Geology and Tectonics Field Guidebook— Volume 1. Springer Nature Switzerland AG. Cham. pp. xi-xvi. ISBN: 978-3-030-60142-3. Expósito, I., Balanyá, J. C., Crespo-Blanc, A., Díaz-Azpiroz, M., & Luján, M. (2012). Overthrust shear folding and contrasting deformation styles in a multiple decollement setting, Gibraltar Arc external wedge. Tectonophysics, 576–577, 86–98. Fernández, C., & Díaz-Azpiroz, M. (2009). Triclinic transpression zones with inclined extrusion. Journal of Structural Geology, 31(10), 1255–1269. García-Dueñas, V., Balanyá, J. C., & Martínez-Martínez, J. M. (1992). Miocene extensional detachments in the outcropping basement of the Northern Alborán basin and their tectonic implications. Geo-Marine Letters, 12, 88–95. González-Delgado, J. A. (coord.), Civis, J., Dabrio, C. J., Goy, J. L., Ledesma, S., País, J., et al. (2004). Cuenca del Guadalquivir. In J. A. Vera (Ed.), Geología de España (pp. 543–550). Madrid: SGE-IGME. Hindle, D., & Burkhard, M. (1999). Strain, displacement and rotation associated with the formation of curvature in fold belts; the example of the Jura Arc. Journal of Structural Geology, 21, 1089– 1101. Jiménez-Bonilla, A. (2017). Along-strike segmentation and basin evolution in curved fold-andthrust belts: The study case of the northern Gibraltar Arc. (Ph.D. Thesis), Pablo de Olavide University, 276pp. Jiménez-Bonilla, A., Barcos, L., Expósito, I., Balanyá, J. C., & Díaz-Azpiroz, M. (2013). La Zona Transversal de Peñarrubia-Almargen (Béticas): Tectónica transpresiva tardía y segmentación del relieve. Geogaceta, 55, 7–10. Jiménez-Bonilla, A., Expósito, I., Balanyá, J. C., Díaz-Azpiroz, M., & Barcos, L. (2015). The role of strain partitioning on intermontane basin inception and isolation, External Western Gibraltar. Journal of Geodynamics, 92, 1–17. https://doi.org/10.1016/j.jog.2015.09.001. Jiménez-Bonilla, A., Expósito, I., Balanyá, J. C., & Díaz-Azpiroz, M. (2017). Strain partitioning and relief segmentation in arcuate fold-and-thrust belts: A case study from the Western Betics. Journal of Iberian Geology, 43, 497–518. Jiménez-Bonilla, A., Crespo-Blanc, A., Balanyá, J. C., Expósito, I., & Díaz-Azpiroz, M. (2020). Analog models of fold-and-thrust wedges in progressive arcs: A comparison with the Gibraltar Arc external wedge. Frontiers in Earth Science, 8. https://doi.org/10.3389/feart.2020.00072. Luján, M., Crespo-Blanc, A., & Balanyá, J. C. (2006). The Flysch Trough thrust imbricate (Betic Cordillera): A key element of the Gibraltar Arc orogenic wedge. Tectonics, 25, 1–17. Macedo, J., & Marshak, S. (1999). Controls on the geometry of fold-thrust belt salient. Geological Society of America Bulletin, 111, 1808–1822. Marshak, S. (1988). Kinematics of orocline and arc formation in thin-skinned orogens, Tectonics, 7, 73–86. Marshak, S. (2004). Salients, recesses, arcs, oroclines, and syntaxes—A review of ideas concerning the formation of map-view curves in fold-thrust belts. In K.R. McClay (Ed.), Thrust tectonics and hydrocarbon systems (Vol. 82, pp. 131–156). AAPG Memoir. Martín-Algarra, A. (1987). Evolución geológica alpina del contacto entre las zonas internas y externas de la Cordillera Bética. (Tesis Doctoral), Universidad de Granada, 1171pp. Mazzoli, S., & Helman, M. (1994). Neogene patterns of relative plate motion for Africa-Europe: Some implications for recent central Mediterranean tectonics. Geologische Rundschau, 83, 464– 68. McCaffrey, R. (1991). Slip vectors and stretching of the Sumatra fore arc. Geology, 19, 881–884. Platt, J. P., & Whitehouse, M. J. (1999). Early Miocene high-temperature metamorphism and rapid exhumation in the Betic Cordillera (Spain): Evidence from U-Pb zircon ages. Earth and Planetary Science Letters, 171, 591–605.

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Ramírez Prior, A., Díaz-Azpiroz, M., & Balanyá, J. C. (2018). Caracterización geológica del cinturón de pliegues y cabalgamientos bético en el transecto del “Caminito del Rey” (Málaga). Bases para la interpretación de su patrimonio geológico. Geogaceta, 63, 75–78. Royden, L. H. (1993). Evolution of retreating subduction boundaries formed during continental collision. Tectonics, 12, 629–638. Royden, L. H., & Burchfiel, B. C. (1989). Are systematic variations in thrust belt styles related to plate boundary processes? (The western Alps versus the Carpatians). Tectonics, 8, 51–61. Sánchez-Gómez, M., Balanyá, J. C., García-Dueñas, V., & Azañon, J. M. (2002). Intracrustal tectonic evolution of large lithosphere mantle slabs in the western end of the Mediterranean orogen (Gibraltar Arc). In G. Rosenbaum, G. S. Lister (Eds.), Reconstruction of the Alpine-Himalayan Orogen. Journal of Virtual Explorer, 8, 23–34. Sanz de Galdeano, C., & Alfaro, P. (2004). Tectonic significance of the present relief of the BeticCordillera. Geomorphology, 63, 175–190. Sosson, M., Morillon, A. C., Bourgois, J., Feraud, G., Poupeau, G., & Saint-Marc, P. (1998). Late exhumation stages of the Alpujarride Complex (western Betic Cordillera, Spain): New thermochronological and structural data on Los Reales and Ojen nappes. Tectonophysics, 285, 253–273. Spakman, W., & Wortel, M. J. R. (2004). A tomographic view on western Mediterranean geodynamics. In W. Cavazza, F. Roure, W. Spakman, G.M. Stampfli, & P. Ziegler (Eds.), The TRANSMED Atlas-The Mediterranean region from crust to mantle (pp. 31–52). Berlin, Heidelberg: Springer. Suades, E., & Crespo-Blanc, A. (2013). Gravitational dismantling of the Miocene mountain front of the Gibraltar Arc system deduced from the analysis of an olistostromic complex (western Betics). Geologica Acta, 11, 215–229. Torné, M., Fernández, M., Comas, M. C., & Soto, J. I. (2000). Lithospheric structure beneath Alboran Basin: Results from 3D gravity modelling and tectonic relevance. Journal of Geophysical Research, 105, 3209–3228. Tubía, J. M., & Gil Ibarguchi, J. I. (1991). Eclogites of the Ojén nappe: A record of subduction in the Alpujarride Complex (Betic Cordilleras, southern Spain). Journal of the Geological Society, London, 148, 801–804. Weil, A. B., & Sussman, A. J. (2004). Classifying curved orogens based on timing relationships between structural development and vertical-axis rotations. Geological Society of America Special Paper, 383, 1–15. Vera, J. A., Arias, C., García-Hernández, M., López-Garrido, A. C., Martín-Algarra, A., MartínChivelet, J., et al. (2004). Las zonas externas Béticas y el paleomargen Sudibérico. In J. A. Vera (Ed.), Geología de España (pp. 354–361). Madrid: Sociedad Geológica de España-Instituto Geológico y Minero de España.

The Southern Iberian Shear Zone (SW Spain): Inclined Transpression Related to Variscan Oblique Convergence in a HT/LP Metamorphic Belt Manuel Díaz-Azpiroz and Carlos Fernández

Abstract In this field guide, we present the boundary between the Ossa-Morena and South Portuguese Zones, two of the main domains of the Iberian Massif, in the northern central Huelva province (SW Spain). This boundary forms part of the Rheic Ocean suture, one of the major sutures of the Variscan belt in Europe, and was formed by the Paleozoic sinistral oblique convergence and collision between Avalonia and Armorica in SW Iberia. Strain was partitioned between mainly shortening structures and left-lateral, eventually transpressional, shear zones. The former suture portrayed in this guide presents an oceanic domain with metasediments and MORB-derived metabasites, and a former continental margin with HT/LP metamorphic rocks. The oceanic-derived rocks are partially deformed by the Southern Iberian Shear Zone, an excellent example of ductile triclinic transpression. The field guide is organized in four transects across the Ossa-Morena–South Portuguese boundary, particularly focused on the oceanic metabasic unit and the Southern Iberian Shear Zone, to show: (1) the old, deep structure of this orogenic suture; (2) ductile mesostructures related to complex 3D transpression; and (3) interesting HT/LP rocks. Transects are achievable on foot and are close to each other, thus the whole trip could be completed in 2–3 days. The field area is located in the Sierra de Aracena and Picos de Aroche Natural Park. It is easy to reach and has varied accommodation facilities and additional geological, natural, cultural and gastronomic attractions. This field trip could be combined with that to the Western Gibraltar Arc northern branch, just 220 km away, and presented also in this issue.

M. Díaz-Azpiroz (B) Department of Physical, Chemical and Natural Systems, Universidad Pablo de Olavide, Crtra. Utrera Km 1, 41013 Seville, Spain e-mail: [email protected] C. Fernández Department of Earth Sciences, Universidad de Huelva, Campus de El Carmen, 21071 Huelva, Spain e-mail: [email protected] © Springer Nature Switzerland AG 2021 S. Mukherjee (ed.), Structural Geology and Tectonics Field Guidebook—Volume 1, Springer Geology, https://doi.org/10.1007/978-3-030-60143-0_5

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1 Introduction The displacement of tectonic plates on the Earth’s spherical surface is defined by small circles around specific rotation poles, thus it is normally oblique to tectonic boundaries (e.g., Philippon and Corti 2016). Equivalent oblique situations are produced extensively at smaller-scale structures, such as major orogenic boundaries and shear zones. In oblique convergent cases, deformation can be partitioned between mainly shortening and strike-slip zones or accommodated at transpressional zones where shortening normal to the zone is combined with zone-parallel simple shearing. At the usual scale of these structures, we can assume that the displacement vector between deforming blocks related by the transpressional zone (i.e., far-field vector, Jones et al. 2004) is presumably subhorizontal (subparallel to the overall Earth surface at a specific place). Consequently, the orientation of the simple shearing direction on the shear zone plane is directly related to the inclination of that shear zone. As such, simple shear is horizontal in vertical transpressional zones (Sanderson and Marchini 1984; Fossen and Tikoff 1993) and oblique when these are inclined (e.g., Lin et al. 1998). In the latter case, the resulting deformation is typically triclinic, that is, none of the three finite deformation axes coincides with the external reference frame defined by the deformed zone and the vorticity vector. This situation can be observed in several natural ductile shear zones where foliation attitude remains roughly constant whereas the orientation of tectonic lineations is strongly variable. After the definition and modeling of such structures, the reported number of natural cases has greatly increased in the last years (Díaz Azpiroz et al. 2019). However, well-studied examples are still needed. The Paleozoic convergence and collision between Avalonia and Armorica at SW Iberia were mostly oblique and produced strain partitioning between mainly shortening structures and left-lateral shear zones, and also to more complex transpressional deformation zones (e.g., Crespo-Blanc and Orozco 1988; Díaz-Azpiroz and Fernández 2005; Díaz-Azpiroz et al. 2006; Simancas et al. 2003, 2005), thus representing an excellent place to analyze triclinic transpression. Moreover, lithologically, uncommon HT/LP rocks characterize this former suture (Bard 1969; Crespo-Blanc and Orozco 1991; Díaz-Azpiroz et al. 2006) and make it even more interesting. Accordingly, in this field guide, we present a view of this old, eroded boundary (loosely identified as the Rheic Ocean suture, although its exact former location is still controversial, see for example, Nance et al. 2010; Kroner and Romer 2013; Pérez-Cáceres et al. 2015), with particular attention to the Southern Iberian Shear Zone (Crespo-Blanc and Orozco 1988).

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2 Geological Setting The Variscan belt in Western Europe (Fig. 1) resulted from the Paleozoic convergence and subsequent collision between Gondwana, along with some related microcontinents (e.g., Armorica), and Avalonia (e.g., Matte 2001; Simancas et al. 2005; Nance et al. 2010; Kroner and Romer 2013). The Iberian Massif represents the largest and best exposed portion of the European Variscan orogen. Based on the seminal works of Lotze (1945) and Julivert et al. (1972), the Iberian Massif was divided into several domains with distinctive stratigraphic, paleontological, petrological, and structural features. The two major domains appearing at the SW-most sector of the Iberian Massif are the Ossa-Morena Zone (OMZ) and the South Portuguese Zone (SPZ), which represent two former continental blocks (likely Armorica and Avalonia, respectively) once separated by the Rheic Ocean, that collided during the Variscan orogenic cycle in Devono–Carboniferous times. Therefore, the OMZ, the SPZ, and their boundary constitute an excellent example of an old orogenic belt with an oceanic suture, now profoundly eroded. The IBERSEIS seismic profile (Simancas et al. 2003) shows a clear image of the deep crust of these two zones and their contact. Three main units appear at the major tectonic boundary between the OMZ and the SPZ (Fig. 2): (1) the southernmost edge of the OMZ, whose metasedimentary sequence would represent the former continental margin of Armorica; (2) the BejaAcebuches metabasites (BAM), with oceanic petrological and chemical affinities; and (3) the Pulo do Lobo unit (Bard 1969; Ribeiro and Silva 1983), a cryptic unit interpreted either as a former accretionary prism (e.g., Eden 1991; Silva et al. 1990; Onézime et al. 2003) or as the northernmost edge of the SPZ (Oliveira 1990; PérezCáceres et al. 2015). The first two units constitute a high temperature/low pressure (HT/LP) belt defined by Bard (1969) as the Aracena metamorphic belt (AMB); they were defined, respectively, as the continental and oceanic domains of this belt (Castro et al. 1996a, b). Further to the north, the Cubito-Moura unit, with high-pressure Fig. 1 Possible plate configuration of the Variscan belt at late Carboniferous times (simplified from Matte 2001; Simancas et al. 2005 and Pérez-Cáceres et al. 2015). The orange star marks the approximate location of the field area

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Fig. 2 Geological map of the boundary between Ossa-Morena and South Portuguese Zones at SW Iberian Peninsula (from Castro et al. 1996a), with the main tectonic domains and shear zones (SISZ: Southern Iberian Shear Zone; CSZ: Calabazares Shear Zone; CASZ: Cortegana-Aguafría Shear Zone; BVFZ: Beja-Valdelarco fault zone). The four proposed transects (T1 to T4) and the approximate location of Fig. 3 (dark blue square) are also shown

metamorphism and intercalations of MORB-derived metabasites, is interpreted as an allochtonous domain that could be derived from the emplacement of the Rheic Ocean (Pérez-Cáceres et al. 2015 and references therein). This field guide does not include visits to this latter unit. The HT/LP metamorphic rocks of the southernmost edge of the OMZ are interpreted as the high-grade equivalents of the Precambrian continental shelf broken by a bimodal volcano-sedimentary sequence produced during the lower–middle Cambrian rifting stage (Expósito et al. 2003; Chichorro et al. 2008). The main lithostratigraphy is defined from correlations with similar rock units described in the central, low-grade OMZ. Accordingly, the visited sector includes from bottom to top: mainly pelitic gneisses, migmatites, nebulites, and charnockites with black quartzites and minor kinzigites (corresponding to the Ediacaran Serie Negra, Bard 1969; Crespo-Blanc and Orozco 1991; Giese et al. 1994; Chichorro et al. 2008); lower Cambrian marbles and an overlying complex unit comprising feldspathic and throndhjemitic leucogneisses, calc-silicate rocks, amphibolitic migmatites, amphibolites, and marbles, dated between 526 and 505 Ma (Chichorro et al. 2008). Synto late-metamorphic noritic bodies (Castro et al. 1996b) intrude these metasedimentary rocks. Geothermobarometric estimations have yielded maximum temperatures of 850–1000 °C at pressures of 4–6 kbar (Díaz-Azpiroz et al. 2006 and references therein). According to structure superposition criteria (Díaz-Azpiroz et al. 2006), the HT/LP metamorphism, dated as Mississippian (351–323 Ma, Castro et al. 1999; 350–335 Ma, Pereira et al. 2009), was coeval with an extensional deformation stage (OMZ-D2 ), described elsewhere in the OMZ and interpreted as an intraorogenic mainly transtensional phase in SW Iberia (e.g., Pereira et al. 2009; Pérez-Cáceres et al. 2015). Folds at microlithons account for a contractional deformation (OMZ-D1 )

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Fig. 3 Block diagram: Main structural features of the different domains at the OMZ-SPZ boundary (from Díaz-Azpiroz et al. 2006). CD-D3 and CD-D4 correspond to the composite final deformation stage affecting the HT/LP metamorphic rocks of the southernmost edge of the OMZ

previous to this extension (Díaz-Azpiroz et al. 2006). OMZ-D1 has been related to the upper Devonian subduction/closure of the Rheic Ocean at SW Iberia (cf. DíazAzpiroz et al. 2006; Pérez-Cáceres et al. 2015). The main HT/LP foliation (OMZ-S2 ) was affected by a composite left-lateral transpressional deformation (OMZ-D3−4 ) that produced complex fold interference patterns and ductile shear zones (Fig. 3). This stage likely resulted from the final, Upper Carboniferous, oblique continental collision. The BAM is a narrow (up to 2 km wide) band that extends discontinuously from Beja (Portugal) to Almadén de la Plata (Spain) along ca. 150 km. It strikes WNW-ESE (N100-110°E in average) and dips moderately (average dip angle of 60–70°) to the NE (Fig. 4a). There are two sectors, at Beja (Portugal) and the northern Huelva province (Spain), where the BAM is better represented. However, specific features described in the former, such as serpentinized mantle rocks, older N-directed thrusting structures and a large-scale fold affecting the unit (Fonseca and Ribeiro 1993; Quesada et al. 1994; Pérez-Cáceres et al. 2015), are not observed in the Spanish sector. The BAM are mainly amphibolites, with mafic schists near the southern contact with the Pulo do Lobo unit, and were formed from the metamorphism of basic magmatic rocks likely formed at a mid-ocean ridge, as suggested by geochemical studies (Bard and Moine 1979; Dupuy et al. 1979; Quesada et al. 1994; Castro et al. 1996a; Azor et al. 2009). Zircon ages, interpreted as those of the magmatic event (Azor et al. 2008), and Ar/Ar ages supposedly dating the metamorphism (Castro et al. 1999) yielded similar results (ca. 340 Ma), which has led to some controversy on their tectonic interpretation (Azor et al. 2008, 2009; Pin and Rodríguez-Aller 2009). The northern half of the BAM unit is represented by medium to coarse-grained banded Hb-Pl amphibolites that include also diopside (Di) in its uppermost levels (upper amphibolites–granulites facies transition), near the contact with the OMZ. Pressures are estimated to be around 4–5 kbar (Castro et al. 1996a) and temperatures range from near 800 °C at the uppermost levels, consisting of hornblende–plagioclase– diopside (Hb–Pl–Di) amphibolites, to ca. 700 °C at the Hb–Pl amphibolites (Castro

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Fig. 4 a Schematic map of the Beja-Acebuches metabasites and the four proposed transects. Equal area, lower-hemisphere projections of the mylonitic foliation (BAM-S2 ) and tectonic lineation (BAM-L2 ) from three along-strike domains (see Fernández et al. 2013 for plotting details). Note that BAM-L2 plunges to the E-SE at western and eastern domains and to the NW at the central domain. b Detailed map of the Beja-Acebuches metabasites and the Southern Iberian Shear Zone (location in Fig. 4a), corresponding to transects 1–3 (modified from Fernández et al. 2013). c Inverted field metamorphic gradient of the BAM as illustrated from Pl-Hb temperatures (Díaz-Azpiroz et al. 2006). Different strain parameters indicate fabric strength decrease away from the SISZ base. These are: Concentration parameter of the distribution of long axes of amphibole crystals (k Amp), shear folds interlimb angle, axial ratio of amphibole crystals (R Amp), crystal size of plagioclase crystals ( Pl), and slope angle of CSD diagrams of plagioclase crystals (β CSD Pl). See Fernández et al. (2013) for details

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et al. 1996a, Díaz-Azpiroz et al. 2006). Therefore, these rocks present a field inverted metamorphic gradient (Figs. 4b, c and 5), rendering tectonic interpretation problematic (cf., Castro et al. 1996a; Díaz-Azpiroz and Fernández 2005; Pérez-Cáceres et al. 2015). A syn-metamorphic deformation (BAM-D1 ) produced the main mesostructure of these amphibolites, a grain-size layering, often remarked by the preferred orientation of amphibole blasts (BAM-S1 ), subparallel to the main boundaries of the unit, WNW-ESE striking with moderate dips to the N. Occasionally, in the uppermost levels, a strongly NE-plunging, faint mineral lineation (BAM-L1 ) is apparent. Shear-sense criteria suggest SW-verging thrusting for BAM-D1 (Díaz-Azpiroz and Fernández 2005). The uppermost BAM contact is affected by a meter-thick late thrust, viz., the Calabazares Shear Zone (CSZ), which trends ca. WNW-ESE, dips to the north and extend discontinuously along at least 10 km (Díaz-Azpiroz and Fernández 2005). The southernmost half of the BAM unit is affected by the Southern Iberian Shear Zone (SISZ, Crespo-Blanc and Orozco 1988) and by a related retrograde metamorphism that reached the lower amphibolite–greenschist facies transition at the lowermost levels (mafic schists with Pl–Hb–Ac–Ep–Chl), thus enhancing the inverted field metamorphic gradient. The deformation associated with the SISZ (BAM-D2 ) is described extensively by Díaz-Azpiroz and Fernández (2005) and is summarized here. BAM-D2 produced shear folding of the BAM-S1 observable at the upper structural levels of the SISZ. Toward the structural base, intense stretching at fold limbs and strong grain-size reduction produced by BAM-D2 resulted in a new, penetrative mylonitic foliation (BAM-S2 ), remarked in the mafic schists by EpChl rich layers, which obliterated BAM-S1 . Progressive strain also increases fabric strength (Díaz-Azpiroz and Fernández 2003; Díaz-Azpiroz et al. 2007). Therefore, both the deformation and the retrometamorphism produced at the SISZ were more intense at the structural base of the shear zone and progressively decreased upward (Figs. 4 and 5). However, meter-scale heterogeneous distribution of this activity produced lozenges of higher grade, less deformed rocks surrounded by lower grade, strongly foliated bands. A conspicuous lineation (BAM-L2 ) defined by plagioclase ribbons and the preferred orientation of amphibole blasts is often observed. BAM-S2 presents a rather constant attitude (WNW–ESE striking and dipping moderately to the NE), whereas BAM-L2 shows gentle to moderate plunges either to the E–SE or to the NW, depending on the analyzed sector, covering a wide range >90° (Fig. 4). These structural features fit well with a triclinic, simple shear-dominated transpression with along-strike variations in the extrusion direction of the coaxial component (Fernández et al. 2013). Numerous shear-sense criteria observed on gently S-dipping surfaces (i.e., close to the vorticity normal section, VNS) indicate a main left-lateral slip for the simple shear component of transpression. Both the SISZ and the CSZ are kinematically compatible with the late, left-lateral collisional stage registered in the OMZ (OMZ-D3−4 ). The SISZ marks the contact between the BAM and the Pulo do Lobo unit, affecting also a decameter-thick band of the latter. Here the Pulo do Lobo unit is composed of phyllites to fine-grained quartz-potassium feldspar-muscovite (Qtz-Kfs-Ms) schists, with intercalated, cm- to m-thick quartzite layers. A meter-thick band of the Pulo do Lobo uppermost levels occasionally presents also garnet (Grt), andalusite (And) and,

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Fig. 5 Schematic log of the Beja-Acebuches metabasites, including microstructural features, constructed with combined information from transects 1–3 (Fernández et al. 2012)

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more sporadically, cordierite (Crd). This assemblage suggests the BAM may have induced some sort of contact metamorphism to the underlying Pulo do Lobo unit (Crespo-Blanc and Orozco 1991; Díaz-Azpiroz et al. 2006). The phyllites-quartzites of the Pulo do Lobo unit affected by the SISZ show a penetrative S-C structure, with strongly N-dipping S-planes and gently N-dipping C-planes, compatible with SW-verging thrusting. This field guide presents four different transects across the OMZ-SPZ boundary, particularly focused on the oceanic metabasitic unit (BAM), including the Southern Iberian Shear Zone, which also affects the oceanic metasediments of the Pulo do Lobo unit and with some outcrops of the HT/LP metamorphic rocks from the former Armorica continental margin (OMZ). The main goals are to present: (1) the old, deep structure of a Paleozoic orogenic suture; (2) mesostructures related to complex 3D deformation produced at a transpressional, mainly ductile shear zone; and (3) a suite of rocks that underwent a somewhat particular HT/LP metamorphic event

3 Location, Accessibility, and Useful Information This field guide is organized in four transects (Figs. 2, 4 and 6) located at the northern Huelva province, in the Sierra de Aracena and Picos de Aroche Natural Park (SW Spain). The closest international airport is Sevilla (100 km), with flights to several destinations in Europe and domestic connections with Madrid, Barcelona, and Palma. Sevilla has also high-velocity railway connections with Madrid, Barcelona, and Málaga, with airports including transoceanic flights. The closest towns to the proposed routes are Almonaster la Real (transects 1 and 2), Cortegana (transect 3) and Aracena (transect 4). They all have facilities for long stays, such as hotels and touristic apartments. Localities are communicated by local roads, with a maximum

Fig. 6 Satellite image of the field area showing the four proposed transects (T1 to T4)

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distance (between Stops 3.5 and 4.4) of 35 km and 50 min. Each transect is achievable on foot, although in transects 3 and 4, a car may be preferred. Transects 2 to 4 are mainly located along roads, where wearing reflective safety vests is compulsory. The perfect season to accomplish this field trip is March–May or September–November, although some rain could be expected in both periods. Southern Spain is particularly hot from late May–early June to August–early September (maximum temperature >35 °C at this time of year), so doing this field trip is not recommended in this season (however, you may check weather conditions in Aracena, the main town in the area, at the Spanish meteorological agency Web page, http://www.aemet.es/es/ eltiempo/prediccion/municipios/aracena-id21007). Winter is just cool, so it is also an alternative moment of the year for this fieldtrip. There are no water springs along the proposed routes, so carrying your own water is recommended. This field guide is focused on few particular geological issues, but the area presents some other interesting geological localities, such as the Cumbres Mayores, lower Cambrian pillow-lavas, the Peña de Arias Montano spring and travertine formation, the Aroche wollastonite skarns and stripped marbles or the many mines of the Iberian Pyrite belt. The visitor is referred to Olías Álvarez et al. (2008) and Sáez et al. (2012) for detailed information about these and other geological sites. Almadén de la Plata, 60 km eastward of Aracena, is the easternmost outcrop of the OMZ-SPZ boundary portrayed in this field guide and one of the entry points to the Sierra Norte de Sevilla UNESCO Geopark, which offers a wide variety of interesting geological sites. Besides its great geology, the Sierra de Aracena y Picos de Aroche Natural Park possesses also relevant natural and cultural offers. The Gruta de las Maravillas (Grotto of the Wonders), which is included here as part of transect 4 (see Stop 4.1), is a world famous cave on Cambrian marbles (http://www.aracena.es/es/municipio/ gruta/). Furthermore, several small, charming villages, with white houses and stoned streets, are worth a visit. Our favorites are Aracena, Almonaster La Real, Linares de la Sierra, Alájar, Los Marines, and Valdelarco, but all of them have nice things to offer. This region is one of the most important in Spain for the raising of Iberian pork and includes the village of Jabugo, the world’s capital of the celebrated Iberian ham (Jamón Ibérico). It is also a well-known region among mushrooms collectors, and it is possible to find several succulent species from fall to spring.

4 Routes Description 4.1 Transect 1: Almonaster La Real Along this route (Fig. 7), it is possible to observe several examples of the main lithologies from the HT/LP metamorphic belt located at the southernmost edge of the OMZ; a transect to the Beja-Acebuches metabasites, from the HT, Di-bearing amphibolites to the mafic schists; and also rocks of the Pulo do Lobo unit affected by the SISZ.

The Southern Iberian Shear Zone (SW Spain): Inclined …

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Fig. 7 Transect 1 with proposed stops on satellite image (a) and geological map (b). c Longitudinal profile of this route

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This route mostly goes on dirt tracks and trekking trails. However, the last part from Stops 1.11–1.15 goes mostly along roads, where wearing visible vest is mandatory. Three stops (1.3, 1.8, and 1.10) are located in private land; the owners normally accept crossing these fences, but they must be kept closed at all times. Practical information Route type: circular Length: 4400 m (with additional 800 m to return back to the starting point) Total upward walk: 1750 m Average upward slope: 12% Maximum upward slope: 30% Difficulty: medium (some walks on trekking trails, steep slopes) Estimated time: 5.5 h (including final return from Stop 1.15 to the starting point) Start walking NNE from the village of Almonaster La Real, near the restaurant “Las Palmeras” (37° 52 23.93 N 6° 47 2.08 W), where a signpost indicates a marked track. Stop 1.1: After a short walk of ca. 200 m, go up a zigzag path from 37° 52 26.3 N 6° 46 55.8 W. On the way up, observe the medium to coarse-grained banded amphibolites of the BAM. Looking carefully with lenses, green diopside crystals can be detected. Stop 1.2: At the top of the zigzag path, at the junction, take left and walk around 50 m to 37° 52 29.7 N 6° 46 57.5 W. There is a small outcrop of amphibolites with diopside from the uppermost levels of the BAM affected by the Calabazares Shear Zone (Fig. 8a). This is a meter-thick ductile shear zone, with reverse kinematics, which could be related to the latest collisional stage between the SPZ and the OMZ. Heterogeneous deformation at this shear zone produced lozenges of folded amphibolites surrounded by mylonitic bands where intense dynamic recrystallization (Díaz-Azpiroz and Fernández 2005) produced strong grain-size reduction (compare with Stop 1.1). Stop 1.3: Go back and walk 250 m to the NE. At 37° 52 35.22 N 6° 46 50.27 W, there is an outcrop on the left side, across a fence. Cambrian marbles show a HT foliation (OMZ-S2 ) defined by the preferred orientation of amphiboles (black) and pyroxenes (green) and remarked by differential solution produced by weathering. This foliation is affected by SW-verging recumbent folds of the OMZ-D3-4 , which is related to the final continental collision stage (Fig. 8b). Stop 1.4: Go back to the south around 50 m and reach the junction with a small track to the left at 37° 52 34.1 N 6° 46 50.9 W. About 20 m to the south of the track junction, at 37° 52 33.3 N 6° 46 51.5 W, it is possible to observe calc-silicates (with Pl–Hb–Di), with cm-thick intercalations of marbles. Continue 20 m to the north and take the small track to the right. At 37° 52 34.3 N 6° 46 50.3 W,