The Variscan Belt of Western Europe, Volume 2 E-Book
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- Herausgeber: John Wiley & Sons
- Kategorie: Wissenschaft und neue Technologien
- Sprache: Englisch
This book deals with the geological record and the evolution of ideas concerning the Variscan orogenic belt in France and neighboring regions. Volume 1 is based on a general introduction concerning the imprint of the Variscan period on the geology of France, as well as on the particularities of the study of this ancient orogen. A history of the concepts applied to the Variscan belt is proposed in order to consider this orogen in the history of Earth Sciences. A paleogeodynamic analysis of the Variscan cycle sets the general framework for the evolution of the orogen, which is then tackled through the prism of the magmatic, metamorphic and tectonic record of the early phases (from Cambrian to Lower Carboniferous). Volume 2 proposes an analysis of the late evolution of the Variscan orogenic belt, reflecting its dismantling in a high-temperature context during the Upper Carboniferous and Permian. The sedimentary archives are described, as well as the questions raised by the specificities of this ancient orogen.
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Table of Contents
Cover
Table of Contents
Title Page
Copyright Page
Preface
Introduction
List of Figures
1 High-Temperature Metamorphism and Syn- to Late-Orogenic Magmatism
1.1 Introduction
1.2 High-temperature/low- to medium-pressure metamorphism and partial melting of the Variscan crust
1.3 Late-orogenic magmatism
1.4 Causes and geodynamic implications of crustal melting
1.5 References
2 Late-Orogenic Structural Reorganization of the Variscan Belt during the Upper Carboniferous
2.1 Introduction
2.2 Major structures of the Variscan belt of Western Europe
2.3 Ibero-Armorican Arc
2.4 Late orogenic deformation in the internal domain
2.5 Late-orogenic deformation in the southern foreland domain
2.6 Summary and discussion on the restructuring of the Variscan belt in the Carboniferous
2.7 Conclusion
2.8 References
3 Sedimentary Basins and Evolution of Reliefs Associated with the Variscan Cycle in France and Adjacent Countries
3.1 Introduction
3.2 Foreland basins along the Southern Laurussian Margin
3.3 Basins in the Inner Variscan zone
3.4 Regions of the Gondwanan Margin and Foreland Basin
3.5 Post-orogenic stage: Stephanian–Permian basins
3.6 Conclusion
3.7 References
Conclusion
C.1. An orogen related to subduction processes involving specific margins
C.2. A segmented, non-cylindrical mountain belt controlled by oblique convergence
C.3. A mountain belt characterized by a dismantling stage involving a hot and weak lithosphere
C.4. The relief of the Variscan belt in question
C.5. References
List of Authors
Index
Summary of Volume 1
End User License Agreement
List of Illustrations
Chapter 1
Figure 1.1. Distribution of terrains that have undergone high-temperature meta...
Figure 1.2. Details of the geological map of France to the 1/1,000,000th repre...
Figure 1.3. Details of the geological map of France to the 1/1,000,000th repre...
Figure 1.4. Summary of melting conditions and syn- to late-orogenic pressure– ...
Figure 1.5. Synthesis of ages and conditions of melting episodes in French Var...
Figure 1.6. Phase diagram of continental crust melting reactions. Ky: kyanite;...
Figure 1.7. Composition of granitoids and vaugnerites in the Eastern French Ma...
Figure 1.8. Synthesis of the ages obtained on Variscan granitoids according to...
Figure 1.9. Schematic representation of high-temperature syn- to post-collisio...
Chapter 2
Figure 2.1. Geological map of the Variscan belt of Western Europe showing stru...
Figure 2.2. Photograph of a shear band affecting paragneisses in an extensiona...
Figure 2.3. Synthetic geological maps illustrating the evolution of deformatio...
Figure 2.4. Structural map of the Variscan belt of Western Europe representing...
Figure 2.5. Structural map of the Cantabrian Zone highlighting the curved thru...
Figure 2.6. Structural maps (a, b, c) and synthetic table (d) representing the...
Figure 2.7. Geological map of the Armorican Massif with two crustal-scale cros...
Figure 2.8. Structural map of the Armorican domain showing the relationships b...
Figure 2.9. Photograph of a thin section of the Angers quartz mylonite (thin s...
Figure 2.10. Left: structural map of the Armorican Massif with a representatio...
Figure 2.11. Model of the evolution of the Armorican Massif during the Carboni...
Figure 2.12. Structural map of the South Armorican domain and structural cross...
Figure 2.13. Structural map of the Gulf of Morbihan. GM: Gulf of Morbihan; Gu:...
Figure 2.14. Schematic view of a gneiss dome controlled by gravity instability...
Figure 2.15. Schematic representation of a metamorphic core complex (modified ...
Figure 2.16. Structural map of the French Massif Central representing the stre...
Figure 2.17. Structural scheme showing geometric relationships between ductile...
Figure 2.18. Geological map of the northwestern part of the French Massif Cent...
Figure 2.19. EW interpretative geological cross-section at the latitude of St-...
Figure 2.20. 3D model of the Mont Lozère pluton on which the magnetic lineatio...
Figure 2.21. Geological map of the Velay dome. Red foliation trajectories repr...
Figure 2.22. Structural map and geological cross section of the Pilat detachme...
Figure 2.23. Geological map of the Montagne Noire Massif (modified from Trap e...
Figure 2.24. Synthetic structural map of the dome of Montagne Noire. The carto...
Figure 2.25. Petrostructural map of the Variscan Massifs of the Pyrenees. The ...
Figure 2.26. Synthetic structural map showing the strain field in the Variscan...
Figure 2.27. Illustration of the mode of deformation in transpression between ...
Figure 2.28. Structural model in two steps of the Variscan segment of the Pyre...
Figure 2.29. Synthesis of chronological data on various deformed domains of th...
Figure 2.30. The Variscan belt during the closure of the Ibero-Armorican Arc i...
Chapter 3
Figure 3.1. Distribution of major sedimentary basins in the Variscan belt duri...
Figure 3.2. Simplified geological map of southern Belgium illustrating the dis...
Figure 3.3. Lower Devonian lithostratigraphic succession in the Namur–Dinant B...
Figure 3.4. Lithostratigraphic succession of the Middle and Upper Devonian in ...
Figure 3.5. a. View towards the west of the Carrière du Nord at Frasnes exposi...
Figure 3.6. Schematic map of sedimentation areas and lithostratigraphic succes...
Figure 3.7. Lithostratigraphic succession of the lower Carboniferous in the Na...
Figure 3.8. a. East–west transect through the Franco-Belgian coal basins (...
Figure 3.9. Rhenish Massif and the eastern end of the Ardenne. The Rhine forms...
Figure 3.10. Map showing the distribution of the Famennian to Visean strata in...
Figure 3.11. a. Bromberg Formation (upper Visean) in its stratotype, the Bromb...
Figure 3.12. Conceptual diagram of the migration of depocenters in the Rhenish...
Figure 3.13. Schematic reconstruction of deposit environments and processes th...
Figure 3.14. Map of Devonian and Carboniferous sedimentary outcrops within the...
Figure 3.15. Lithostratigraphic units of selected regions in the Armorican Mas...
Figure 3.16. Examples of Devonian facies of the Crozon peninsula. a., b. Lande...
Figure 3.17. Simplified geological map of the southern slope of the Montagne N...
Figure 3.18. a. Caunes marble: red micritic limestones with complex cavities f...
Figure 3.19. a. Upper oolithic unit in the La Serre outcrop (near Cabrières): ...
Figure 3.20. Theoretical reconstruction of the mixed siliciclastic-carbonate r...
Figure 3.21. Simplified geological map of the Pyrenees locating Devonian and C...
Figure 3.22. Composite lithostratigraphy of representative regions of the Fren...
Figure 3.23. Examples of Pyrenean Devonian facies. a. Siltstone and limestone ...
Figure 3.24. Synthetic drawing of the Lower Devonian to Carboniferous successi...
Figure 3.25. a. Stratigraphic representation of the Pyrenees from the upper Vi...
Figure 3.26. Distribution of Stephanian–Permian basins in France. Massifs: A: ...
Figure 3.27. Geological cross-sections of the Carmaux (according to Delsahut 1...
Figure 3.28. Working face of the Pradènes open pit mine (Graissac basin). Ruff...
Conclusion
Figure C.1. Upper Carboniferous paleogeographic map of the Variscan belt. This...
Guide
Cover Page
Table of Contents
Title Page
Copyright Page
Preface
Introduction
List of Figures
Begin Reading
Conclusion
Index
Summary of Volume 1
WILEY END USER LICENSE AGREEMENT
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SCIENCES
Geoscience, Field Director – Yves Lagabrielle
Dynamics of the Continental Lithosphere, Subject Head – Sylvie Leroy
The Variscan Belt of Western Europe 2
Late Magmatic, Metamorphic and Tectonic Events and the Sedimentary Record
Coordinated by
Yoann Denèle
Julien Berger
First published 2023 in Great Britain and the United States by ISTE Ltd and John Wiley & Sons, Inc.
Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act 1988, this publication may only be reproduced, stored or transmitted, in any form or by any means, with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms and licenses issued by the CLA. Enquiries concerning reproduction outside these terms should be sent to the publishers at the undermentioned address:
ISTE Ltd27-37 St George’s RoadLondon SW19 4EUUK
www.iste.co.uk
John Wiley & Sons, Inc.111 River StreetHoboken, NJ 07030USA
www.wiley.com
© ISTE Ltd 2023 The rights of Yoann Denèle and Julien Berger to be identified as the authors of this work have been asserted by them in accordance with the Copyright, Designs and Patents Act 1988.
Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s), contributor(s) or editor(s) and do not necessarily reflect the views of ISTE Group.
Library of Congress Control Number: 2022947648
British Library Cataloguing-in-Publication DataA CIP record for this book is available from the British LibraryISBN 978-1-78945-100-9
ERC code:
PE10 Earth System Science PE10_5 Geology, tectonics, volcanology PE10_7 Physics of earth’s interior, seismology, volcanology PE10_13 Physical geography
Preface
Yoann DENÈLE and Julien BERGER
Géosciences Environnement Toulouse (GET), Paul Sabatier University, Toulouse, France
The idea behind this collective work stems from the request sent to us in September 2018 from Sylvie Leroy, subject head of “Dynamics of the Continendtal Lithosphere”, part of the SCIENCES collection produced by ISTE Group, to coordinate a book on the Variscan belt. It seemed appropriate to propose a book on this mountain range by focusing on the French crystalline massifs, while avoiding imposing strict geographical limits in order to devise the paleogeographical and geodynamic framework. As this book aims to reach a wide audience of teachers and students, we have chosen to provide chapters that are based on recent advances in research, with a certain degree of popularization, including detailed descriptions of key zones of the Variscan massifs in France.
The chapters of this book have been written by a number of individuals; it would therefore not have seen the light of day without these various authors who were enthusiastically and passionately involved in this collective work. The author or authors take responsibility for their respective chapter, which primarily sets out their personal vision, while taking into account other opinions. The relative diversity of points of view reflects ongoing debates and shows that the study of the Variscan belt is not complete, but an active field of Earth Sciences. Science is progressing very quickly and as the finalization of this book took longer than expected, with significant differences concerning the writing periods of certain chapters, it seems important to us to specify here, the dates of receipt of the first versions of the various chapters so that they may be inscribed in their historical framework, in particular in relation to recent or current publications, the results of which the authors of the chapters were unable to integrate. The yellow jersey of this book thus goes to Chapter 4 of Volume 1, the first version of which was proposed in August 2019. Chapter 2 of Volume 1 and Chapter 1 of Volume 2 were sent in February 2020, and finally the period running through the end of 2020 and the beginning of 2021 was the most prolific with the successive reception of Chapters 1 and 3 of Volume 1 and Chapter 3 of Volume 2, and then Chapter 2 of Volume 2. The overall rereading and finalization of the Introduction and Conclusion, which are challenging steps, led to the submission of this project at the end of 2021.
March 2023
Introduction
Yoann DENÈLE and Julien BERGER
Géosciences Environnement Toulouse (GET), Paul Sabatier University, Toulouse, France
The rocks forming the crystalline basement of France, Western Europe, parts of America (the Appalachians) and North Africa (the Meseta) are essentially Ediacaran to Carboniferous. They were formed or transformed during a major orogenic period extending from the Devonian to the Carboniferous (see Chapter 1), termed Variscan, but Hercynian may also be used for historical reasons. At the end of the 19th century, the term Variscan, with reference to the Latin name Curia Variscorum of the city of Hof in Bavaria, was suggested by the Austrian geologist Eduard Suess (1831–1914) to describe an ante-Permian mountain range of Central Europe with distinct structural directions from other Paleozoic belts observed in Armorica and Iberia. During the same period, Marcel Bertrand (1847–1907) proposed, with reference to the ancient forest of Central Europe Hercynia silva, the term Hercynian, to characterize all of these domains whose formation seemed synchronous. However, in 1924, Hans Stille (1876–1966) used the term Variscan to describe the same system. Thus, the terms Variscan and Hercynian were subsequently used by geologists whether they were of Anglo-Saxon or Latin influence, respectively, before the geological sciences were progressively dominated by the use of Anglo-Saxon terms, and thus Variscan became informally the scientific term of reference.
The topography of French regions is marked by high mountain ranges with sharp peaks (Alps, Pyrenees), large areas of medium mountain ranges (French Massif Central, Vosges) or low hills with rounded peaks (Armorican Massif, Ardennes), and vast plains (Paris and Aquitaine basins). The duality of relief in mountainous areas is sometimes associated with the age of the orogenic phenomena that has affected these massifs (Variscan vs. Alpine). This simplification does not reflect the richness of France’s geological history and does not explain certain morphological features. For example, there is a dichotomy between the slightly accentuated topography of hills and valleys of the Armorican Massif, which rises to 384 m in Tuchen Gador, and the steep reliefs of the Cévennes, which reach an altitude of almost 1,700 m in Mont Lozère. These two regions with contrasting topography are, however, apparently preserved from alpine orogenic phenomena, and show the outcrop of crystalline rocks deformed and metamorphosed during the Variscan orogeny, and could thus be considered to belong to the “old massifs of France”. In fact, the Alpine cycle led to the development of high reliefs in hinterland orogenic domains (Alps, Pyrenees) and large flexural foreland basins (Aquitaine Basin, Southeast France Basin), as well as of a discontinuous thinned crustal domain around the Alpine arc. The topography of French regions is also controlled by the development, from the Upper Eocene to the Lower Miocene, of this rift system to which belong various basins of the West European rift and the Gulf of Lion margin. The rift basins (e.g. Rhine and Limagne grabens in the West European rift, Alès graben on the Gulf of Lion margin) are surrounded by summits of medium altitudes (Vosges, eastern and southern parts of the Massif Central), which belong lato sensu to the shoulders of the rift basins, on which the Variscan crystalline rocks were exposed. Exhumation and incision of these domains also seem to be controlled by a deep dynamic topography (mantle upwellings) as attested by the Mio-Pliocene (Cantal, Aubrac) or Quaternary (Chaîne des Puys) volcanism in the French Massif Central, as well as evidence of significant uplifts over the same period. The formation of the Cévennes mountains is thus linked to a slow but significant Cenozoic exhumation on the border of rift systems, which, compared to the essentially epeirogenic processes undergone by the Armorican Massif since the Permian, explains the difference in topography between these two areas. Thus, unlike the Pyrenees and the Alps, which form high mountain ranges from a structural and morphological point of view, it is not appropriate to consider in physical geography of France a Variscan mountain range, but only Variscan crystalline massifs (Armorican Massif, French Massif Central, Vosges) surrounded by basin domains (Paris and Aquitaine Basins), sometimes forming discontinuous nuclei in the hinterland of mountain ranges (Alps, Pyrenees). The present-day topography of these massifs is not a direct consequence of the initial Carboniferous orogenic period. Nevertheless, although the formation of the Variscan belt around the 300 Ma period has little or no influence on the relief of France, the exposed crystalline rocks testify to the building up of a major mountain range at the end of the Carboniferous, which extends in Europe to more than 3,000 km long and 700 km wide, and whose relief, although subject to debate, was undoubtedly significant, and had a remarkable influence on the climate of the Earth in the Carboniferous and Permian.
This book describes the geological evolution of this paleo-mountain belt through six chapters (Chapters 2–4 of Volume 1 and Chapters 1–3 of Volume 2) mostly focused on French Variscan massifs with a seventh chapter (Chapter 1 of Volume 1) reviewing the evolution of ideas since the 18th century.
The study of ancient mountain belts must take into account several specificities. First, the geologist must restore a system that has undergone significant transformation during subsequent geodynamic events. In the case of the Variscan belt, and in particular for its European branch, it is necessary to restore the Atlantic Ocean system, and more particularly the Bay of Biscay, as well as the Alpine orogenic belts. In this book, this problem is not dealt with head-on in a dedicated chapter. We should remember that although the restoration of the opening of the Bay of Biscay is relatively simple, this is not necessarily the case for the Pyrenean–Alpine belts and the Mediterranean marginal basins, especially when it comes to determining the precise position of isolated crystalline massifs, such as the Maures and the crystalline massifs of the Alps, Corsica and the Pyrenees at the Carboniferous–Permian boundary. Thus, correlations between these massifs and the different units of the main branch of the Variscan belt in France (French Massif Central and Armorican Massif) are not yet clearly established. This explains why these isolated crystalline massifs are discussed separately in the various chapters of this book. Another particularity of the study of ancient orogens is to consider a system for which the geodynamic framework is difficult to establish, because the lithospheric plates involved and their kinematics cannot be as clearly defined as in Cenozoic orogenic systems. The geophysical record is more limited, and the absence of preserved oceanic sea floor prevents the determination of high-resolution kinematics of plates over long periods of time. As illustrated in Chapter 2 of Volume 1, which summarizes the paleo-geodynamic context of the Variscan chain, geodynamic constraints are essentially the result of confrontation between paleomagnetic data, which make it possible to determine paleo-latitudes, and “geological” data, in particular geochronological, paleontological and geochemical. Despite all of this information, the proposed geodynamic evolution during the ancient Variscan orogeny remains incomplete, and comparison with the more recent systems is a potential source for scientific progress. Furthermore, the reconstitution of an ancient orogen requires the consideration of data derived mainly from the middle and lower crustal structural levels. Indeed, in the more recent chains, such as the peri-Tethysian systems (e.g. Alps, Himalaya), the upper structural level, marked by an association of fold and thrust belts barely affected by metamorphism, is most often exposed. This is not the case for the core of the ancient belts for which the lower structural level, marked by rocks with penetrative ductile deformation and high metamorphic grades, is most often observed. This pattern is, in part, related to the late orogenic events that have induced an exhumation of these structural levels upon the return to equilibrium of the previously thickened and gravitationally unstable orogenic crust. This pattern is accentuated by the Meso-Cenozoic history, which may have resulted in significant local exhumation and erosion of Variscan crystalline rocks in rift shoulder and hinterland domains of the Pyrenean–Alpine belts. Thus, the study of crystalline rocks that have recorded the construction of the Variscan belt (and ancient orogens in general) necessarily calls for geologists with expertise on this lower structural level. This is particularly the case for magmatic and metamorphic petrology, ductile tectonics and high-temperature geochronology. This fact explains why four chapters of this collective work are devoted to summarizing this type of study, whether concerning pre-collisional magmatism (Chapter 3 of Volume 1), metamorphism and early deformations (Chapter 4 of Volume 1), or late-orogenic magmatism (Chapter 1 of Volume 2) and deformation (Chapter 2 of Volume 2). In the uppermost structural level, sedimentary archives are partial and generally observed in spatially restricted external domains (Ardennes, Pyrenees, Cantabrian). In Chapter 3 of Volume 2, a group of authors propose an exhaustive summary of sedimentary record associated with the Variscan cycle and show the power of this archive as a vector of our understanding of the morphological evolution of the belt. Nevertheless, the scarcity of sedimentary archives preserved in internal domains, and in particular records associated with the early phases of orogenic construction, implies that the analysis of these domains is largely dependent on the evolution of geochronological methods, as well as on our ability to interpret the results they provide. This book does not contain a chapter dedicated to the geochronology of the Variscan belt, but what appears to be a gap is also an advantage, as all chapters describe the geochronological data and discuss them in the light of knowledge from the field, which are indispensable constraints for interpreting “absolute ages”. Of course, the various experts who wrote the chapters of this book do not necessarily have the same way of interpreting these ages, leading to controversies that are thus illustrated.
All of the chapters of this book highlight the contribution of analytical data to constrain the history of the Variscan belt. Nevertheless, the study of mountain ranges still depends on high-quality field observations, which should provide the general framework for analytical studies, whether paleomagnetic, geochronological, geochemical or geophysical. Field analyses are dependent on the quality of outcrops, which are not homogeneous in the French crystalline massifs. Without going into the details related to microclimates, the crystalline massifs, which have recently been exhumed in the core of mountain ranges (Axial Zone of the Pyrenees and internal/external crystalline massifs in the Alps), benefit from excellent qualities of outcrop, which make it possible to study the structures of the rocks with a good continuity. This is also the case in the eastern and southern parts of the French Massif Central affected by slow but significant exhumation, particularly in the Cenozoic. In the Armorican Massif, coastlines present excellent conditions of outcrops but give way to bocages in the hinterland, in which it is rare to observe the bedrock. The pattern is quite similar to that of the Western French Central Massif, where outcrops on the altered crystalline plateaus are rare. In these areas, the accumulation of observations on temporary outcrops (quarries, major road or rail infrastructure) has fortunately made it possible to produce remarkable mapping work, without which any synthesis on the Variscan belt in France would seem incongruous.
In Chapter 1 of Volume 1, Olivier Vanderhaeghe, Professor at Université Paul Sabatier, Toulouse, presents the history of the evolution of methods and concepts applied to the Variscan belt, and thus of our vision of this paleomountain belt. This history also highlights the influence of the study of this ancient orogen as a source of inspiration in the history of Earth Sciences. The first methodological developments concern cartography and stratigraphy, and the appearance of the first synthetic geological maps at the beginning of the 19th century (United States of America, Great Britain and then France) is thus followed by the development of more detailed maps, at the regional scale, at the end of the 19th and beginning of the 20th century. The study of rocks by optical microscopy was gradually developed during the second half of the 19th century and made it possible to develop the analysis of minerals and structures on a microscopic scale, which are essential methods for studying crystalline rocks. These methodological developments made it possible to develop a number of concepts. In ancient concepts, if there is one that emerged in the Variscan belt before the advent of plate tectonics, it is indeed the theory of geosynclines that was introduced from the study of the Appalachians in the early 19th century. Geosynclines are defined as submerged domains, accumulating large quantities of sediments and localized deformation and the effects of metamorphism, thus constituting a favorable place for the transformation of rocks. This concept explains the association of three rock types in mountain ranges: primitive rocks, transition rocks and sedimentary rocks. In the geosynclinal framework, sometimes in contradiction with this paradigm, the concepts of zoneography of metamorphism, horizontal displacements of tectonic units (nappes) or partial melting (migmatites) also find their source in the Variscan belt in the middle of the 20th century. Subsequently, geology underwent the plate tectonic revolution in the late 1960s and early 1970s. Although this theory was initially developed remotely from the Variscan crystalline massifs, since it was essentially the result of geophysical observations of the globe, it was soon adapted to the study of this belt from the mid-1970s. During this period, various proposals emerged concerning the identification of suture zones, and thus of the continental and oceanic domains involved in the Variscan orogeny, and therefore regarding its paleogeographical context.
In Chapter 2 of Volume 1, Jean-Marc Lardeaux and Karel Schulmann, Professors at the Universities of Nice and Strasbourg, respectively, offer a summary of the paleogeographical and paleo-geodynamic context of the Variscan belt. In this chapter, which is essential for understanding the evolution of the belt, these two authors, based on a summary of litho-tectonic data as well as of paleontological, paleoclimatic, and mineralogical constraints and modern paleomagnetic models, highlight essential points of reference and open questions concerning Variscan geodynamics. Thus, it is shown that the Variscan belt results from the Devonian–Carboniferous convergence of the two mega-plates, Laurussia and Gondwana, whose collision led to the formation of the upper Carboniferous Pangea. Several micro-continents and oceanic domains, whose size and geometry vary in different reconstructions, are highlighted between Laurussia and Gondwana. The “waltz” of these micro-continents (Avalonia, Armorica), which belong to what some authors call a “Variscan continental ribbon”, located on the northern periphery of Gondwana in the Cadomian, before being transversally displaced towards Laurussia during a major rifting episode in the Cambro-Ordovician, is thus highlighted. The involvement of these domains in the construction of the belt led to the formation of four litho-tectonic zones (the northern foreland, Rheno-Hercynian, Saxothuringian and Moldanubian), which reflect as many paleogeographic units whose accretion, during the continental collision following the closure of at least two oceanic domains in two subduction zones with opposite dip, gradually built up the Variscan belt. High-resolution data over the period 360–280 Ma suggest large variations in the kinematics of mega-plates, including a major kinematic reorganization at the beginning of the Permian, and thus emphasize that the gravitational collapse of the belt is not the exclusive cause of post-orogenic extension. Finally, paleogeographical studies show the existence of an oceanic domain, the Paleo-Tethys, between Gondwana and the North Gondwanan micro-continents, which is supposed to be in subduction from the Visean but whose impact on the evolution of the Variscan belt is still ignored.
In Chapter 3 of Volume 1, Julien Berger, Associate Professor at Université Paul Sabatier in Toulouse, offers a summary of the characteristics of pre-collisional magmatism, and illustrates that the lithosphere of the units involved in the construction of the Variscan belt was deeply affected by pre-orogenic geodynamic events during the Cadomian cycle, as well as during a major Cambro-Ordovician episode, which is still largely misunderstood. Markers of this Cambro-Ordovician magmatism correspond to ophiolitic associations and also to massifs of basic rocks, mafic–felsic associations (the famous leptyno-amphibolitic complex) and especially felsic rocks forming massifs of orthogneiss, whose ubiquitous presence is a characteristic of the Variscan belt. In this chapter, various recognized ophiolitic massifs are described and analyzed, in light of recent scientific advances from the ocean floor (mid-oceanic ridges, rifted continental margins). The author shows that the combination of basic and ultrabasic rocks did not necessarily reflect the presence of an ancient mid-oceanic ridge. The presence of oceanic sutures in the Variscan massifs of France is thus checked. This synthesis of cartographic, geochronological and geochemical data on Cambro-Ordovician magmatism in France reveals an extremely heterogeneous magmatism, which reflects either an active subduction from the Cadomian, involving back-arc domains, or, what seems the most probable hypothesis, a rifting phase affecting a fertile mantle, inherited from Cadomian subduction. The impact of this continental margin magmatism, remarkable by the abundance of felsic rocks, implies that the Variscan belt can no longer be considered simply as the result of the convergence of continental margin domains with Cadomian basement. Compilation of U–Pb ages on zircons reflects the formation of a transitional (i.e. partly juvenile) crust in the Cambro-Ordovician continental margins. This chapter also provides a summary of Devonian magmatism, which reflects the formation of continental arcs as well as local back-arc domains during a period of active subduction.
In Chapter 4 of Volume 1, Michel Faure, Professor at the University of Orléans, describes exhaustively the evidence of metamorphism and early deformation in the French Variscan belt, and thus proposes a summary of the characteristics of the phases of subduction and collision, which took place in the Devonian and Carboniferous. The evidence of these early phases is mainly observed in the Moldanubian domain of the belt, and in particular in the French Massif Central and the South Armorican domain for the main branch in France. In the French Massif Central, the Moldanubian domain is marked by a stack of tectonic nappes, with an upper allochthonous unit (Upper Gneiss Unit) forming tectonic klippes and a lower allochthonous unit (Lower Gneiss Unit) and a para-autochthonous unit forming tectonic windows in the Limousin and exposed in the southern front of the belt. Four early tectono-metamorphic events are recognized in the main branch: D0, D1, D2 and D3. Phases D0 and D1, highlighted in the upper allochthonous unit, correspond, respectively, to an episode of burial under HP conditions and to an exhumation stage. Geothermal gradients of the burial episode, between 7 and 15°C/km, are typical of subduction zones. A metamorphic peak pressure of up to 3 GPa represents a burial of units at a depth of more than 90 km. The retromorphosis associated with episode D1 is associated with the construction of an early southwesterly vergent crustal prism during a period of continental subduction of Gondwanan margins involving strictly continental units (high-pressure granulites) or ocean–continent transition units (eclogites of the leptyno-amphibolitic complex). D2 and D3 events are associated with continental collision and are expressed preferentially in the Gondwana margin, illustrating a strength contrast with the Armorica lithosphere, more marked by Cadomian orogeny. Phase D2 is associated with a Barrovian metamorphism, with an inversion of the metamorphic isograds in the Lower Gneiss Unit, which emphasizes the ongoing thrusting of the Upper Gneiss Unit. The stretching associated with this event has a longitudinal direction with respect to the suture zones, suggesting a strong oblique component of the collisional deformation, which is still poorly understood. Tectono-metamorphic events associated with phase D3 are recorded in the southern part of the main branch, illustrating the propagation of collisional deformation towards the southern foreland. While the absolute chronology of D1, D2 and D3 events is relatively well constrained between 380 and 340 Ma, the age of the HP event (D0) is discussed in light of the most recent geochronological datations that appear to converge towards an estimate of this event at the Late Devonian (380–360 Ma). This chapter also proposes a summary of the characteristics of the isolated French crystalline Variscan massifs such as the Maures, Corsican and Alpine crystalline massifs, which record early episodes involving subduction and collision phenomena quite comparable to the main branch.
In Chapter 1 of Volume 2, Arnaud Villaros, Oscar Laurent, Simon Couzinié and Jean-François Moyen, PhD in Earth Sciences, Research Fellow at the Géosciences Environnement Toulouse laboratory, Assistant Professor at the University of Lorraine, and Professor at the University of Lyon, respectively, describe the evidence and implications of high-temperature metamorphism and syn- to late orogenic magmatism record in the Variscan belt in France. In this chapter, the preponderance of areas that have been affected by high-temperature Carboniferous metamorphism and intruded by large granitic massifs is highlighted from synthetic maps. The synthesis of petrological and geochronological data from these domains shows that the first episode of high-temperature metamorphism (denoted M2) affected deep rocks of the internal zones around 360–340 Ma, that is, during their exhumation associated with the formation of a crustal orogenic prism. This metamorphic episode, synchronous of the main compressive phase (D2), is described as syn-orogenic. Subsequently, between 330 and 290 Ma, Variscan units underwent a new high-temperature metamorphic stage that led to the formation of a large quantity of migmatites initially with stable biotite (this stage denoted M3), then marked by a total destabilization of the biotites and spatially associated with LP granulites (denoted M4). These latter metamorphic events being synchronous with extensional events affecting the internal zone of the chain are referred to as late orogenic events. High-temperature metamorphic rocks developed during stages M3 and M4 are spatially and temporally associated with numerous granitoid massifs. Granites of crustal origin are predominant; they include peraluminous two-mica leucogranites, and peraluminous granites with biotite and cordierite whose source magmas were formed at higher temperature. Plutons of high-K calc-alkaline mafic to intermediate rocks locally called vaugnerites or durbachites, associated with metaluminous granitoids, were also observed. The signature of these plutons implies a mantle source previously enriched by crustal materials. These associations show that the large volume of migmatites and plutons of crustal origin in the Variscan crystalline massifs of France reflects thermal maturation of an overthickened continental crust. However, this process is not exclusive and it is now clearly established that it is assisted by partial melting affecting the mantle. In the eastern Massif Central, migration of plutons from north to south between 335 and 300 Ma suggests a process of asymmetric delamination of the lithospheric mantle during late Variscan phases. In this context, the sequence of emplacement of magmas, at a given latitude, reflects the migration of the fusion front from the base to the top of the orogenic crust.
In Chapter 2 of Volume 2, Yoann Denèle and Bryan Cochelin, Assistant Professor at Université Paul Sabatier in Toulouse and Postdoctoral Fellow at the State Key Laboratory in Nanjing, respectively, describe the evidence of late orogenic deformation that led to a structural reorganization of the Variscan chain in the upper Carboniferous. This chapter is a synthesis of the structural data observed in the units that were formed during the upper Carboniferous (plutons, migmatites M3 and M4) in the main Variscan branch in France, as well as the Pyrenees and the Cantabrian. These data document the formation of late crustal-scale or even lithospheric-scale structures that developed between 320 and 295 Ma, that is, 40 Ma after the beginning of the continental collision. They are mainly localized south of the Nort-sur-Erdre fault, which is considered a major Devonian suture zone (Armorica– Gondwana). They have affected a thermally mature orogenic crust as evidenced by high to ultra-high temperature metamorphism, crustal melting and the emplacement of large volumes of magmas over the same period. The diversity of late-orogenic structures (shear zones, mid-lower crustal flow zones, detachments, gneissic domes, oroclines), associated with this thermal recording illustrates large-scale strain partitioning in a significant part of the “Variscan continental ribbon” whose lithosphere, framed by the mega-continents Laurussia and Gondwana which impose an oblique convergence movement, is weakened by delamination phenomena. The deformation pointed out by these structures is described in this chapter as an episode of restructuring because it has a first-order impact on the structure of the Variscan belt as we know it today. This deformation is in particular associated with considerable relative displacements (several tens to several hundreds of kilometers) between the litho-tectonic units or inside these units. These displacements are accommodated by the formation of major strike– slip shear zone as well as by plate-scale rotations around the vertical axis (i.e. oroclinal bending). This Carboniferous tectonic event also leads to a reorganization of various structural levels of the orogenic crust, marked by the rapid exhumation of migmatites in gneiss domes, which can be observed both in the internal zone of the belt and in what constituted the southern foreland during the early stages.
In Chapter 3 of Volume 2, Markus Aretz, Élise Nardin, Frédéric Chistophoul and Julien Denayer, Associate Professor at Université Paul Sabatier in Toulouse, Research Fellow at the Géosciences Environnement Toulouse laboratory, Associate Professor at Université Paul Sabatier in Toulouse, and Associate Professor at the University of Liège, respectively, describe sedimentary basins and the evolution of the topography associated with the Variscan cycle in France and neighboring countries. This chapter is an unprecedented synthesis of sedimentary data on the Variscan cycle. It includes a description of the the pre-orogenic (Silurian), syn-orogenic (Devonian and lower Carboniferous), late orogenic (upper Carboniferous) and post-orogenic (Lower Permian) sedimentary record in all the Variscan massifs of France, as well as in regions bordering northern France. On the basis of synthetic lithostratigraphic logs, illustrations of key outcrops and various geological maps, these authors describe the sedimentary successions and their implications in terms of evolution of the Variscan topography. Sedimentary archives are well preserved in the large peripheral basins, whether to the north (Ardennes, Rhenish Massif) or to the south of the mountain range (Montagne Noire, Pyrenees) and show, despite their clearly distinct paleogeographical positioning, a surprisingly comparable sedimentary evolution. Indeed, whether on the Laurussia margin in the north or on the Gondwana margin in the south, the history of the Variscan cycle is marked by a major Silurian–lower Devonian transgression, then by the gradual installation of vast carbonate-dominated platforms, which will undergo a major episode of reorganization in the Upper Devonian, possibly linked to the formation of reliefs in hinterland of the belt. In both basins, the transition from passive margin systems to flexural foreland basins is clearly established from the Visean. During this period the rapid progression of flysch facies, which gradually invaded and filled the basins, clearly shows the presence of reliefs linked to the formation of a mountain range. The continuation of this evolution led to the formation of large paralic basins in the Namurian and Westphalian, particularly in the north of the chain, resulting in the formation of numerous layers of coal. In the more internal parts of the belt, the sedimentary record in the Devonian and Carboniferous is more disparate and often incomplete, reflecting the influence of early but discontinuous reliefs since a large part of the sedimentary successions have marine affinities. Finally, the formation of intra-mountain basins in the internal zone of the chain, in a transtensional tectonic context in the Gzhelian, testifies to the late reorganization of the belt. The floristic assembly of these basins questions the amplitude of the relief in the core of the belt, before the formation of the vast Permian plains, which marked the end of the evolution of the Variscan mountain range.
List of Figures
Chapter 1
Figure 1.1
–
Distribution of terrains that have undergone high-temperature metamorphism and associated magmatic rocks. They correspond to the majority of Variscan outcrops
Figure 1.2
–
Details of the geological map of France to the 1/1,000,000
th
representing the distribution of high-temperature metamorphism and Carboniferous magmatic rocks in the French Massif Central
Figure 1.3
–
Details of the geological map of France to the 1/1,000,000th representing the distribution of HT–LP metamorphic rocks and the associated magmatism in other French Variscan Massifs: Armorican Massif, Morvan, Vosges, Corsica, Pyrenees, Montagne Noire, Maures Tannerons and External Crystalline Massifs
Figure 1.4
–
Summary of melting conditions and syn- to late-orogenic pressure–temperature changes affecting the Upper Gneiss Unit (UGU) and the Lower Gneiss Unit (LGU). The dotted path in the evolution of the LGU corresponds to the path followed by the Pyrenean granulite massifs
Figure 1.5
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Synthesis of ages and conditions of melting episodes in French Variscan terrains
Figure 1.6
–
Phase diagram of continental crust melting reactions
Figure 1.7
–
Composition of granitoids and vaugnerites in the Eastern French Massif Central (from Moyen
et al
. 2016). In this projection by Villaseca
et al
. (1998), the data is considered to be representative of other plutons of the same type
Figure 1.8
–
Synthesis of the ages obtained on Variscan granitoids according to their geochemical characteristics
Figure 1.9
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Schematic representation of high-temperature syn- to post-collision episodes that affected Variscan terrains in France
Chapter 2
Figure 2.1
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Geological map of the Variscan belt of Western Europe showing structural units (allochthonous and para-autochthonous) and major structures such as fault zone, shear zone, foliation trajectories and magnetic lineaments under the Meso-Cenozoic basins
Figure 2.2
–
Photograph of a shear band affecting paragneisses in an extensional tectonic regime
Figure 2.3
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Synthetic geological maps illustrating the evolution of deformation in the Variscan belt of Western Europe during the upper Carboniferous and Lower Permian, and emphasizing domains subjected to partial melting and crustal flow
Figure 2.4
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Structural map of the Variscan belt of Western Europe representing the vergence of major thrusts, the thrust trajectories and shear zone
Figure 2.5
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Structural map of the Cantabrian Zone highlighting the curved thrust faults and axial traces of early folds, as well as the presence of late EW folds
Figure 2.6
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Structural maps (a, b, c) and synthetic table (d) representing the joint sets that formed before (a), during (b), and after the formation of the Cantabrian orocline (d), and the directions of magnetization recorded over these three periods
Figure 2.7
–
Geological map of the Armorican Massif with two crustal-scale cross-sections
Figure 2.8
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Structural map of the Armorican domain showing the relationships between the SASZ and the syn-kinematic plutons
Figure 2.9
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Photograph of a thin section of the Angers quartz mylonite
Figure 2.10
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Structural map of the Armorican Massif with a representation of schistosity trajectories in the Central Armorican domain and restoration model
Figure 2.11
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Model of the evolution of the Armorican Massif during the Carboniferous period
Figure 2.12
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Structural map of the South Armorican domain and structural cross-section in the Gulf of Morbihan
Figure 2.13
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Structural map of the Gulf of Morbihan
Figure 2.14
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Schematic view of a gneiss dome controlled by gravity instability
Figure 2.15
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Schematic representation of a metamorphic core complex
Figure 2.16
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Structural map of the French Massif Central representing stretching direction during late-orogenic events
Figure 2.17
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Structural scheme showing geometric relationships between ductile shear zones, syn-tectonic plutons and the extensional Guéret allochthon in Northern Limousin
Figure 2.18
–
Geological map of the northwestern part of the French Massif Central
Figure 2.19
–
EW interpretative geological cross-section at the latitude of St-Germain-les-Belles (French department 87) integrating field, seismic, AMS and gravimetric data
Figure 2.20
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3D model of the Mont Lozère pluton on which the magnetic lineations and foliations are represented
Figure 2.21
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Geological map of the Velay dome
Figure 2.22
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Structural map and geological cross section of the Pilat detachment
Figure 2.23
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Geological map of the Montagne Noire Massif
Figure 2.24
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Synthetic structural map of the dome of Montagne Noire
Figure 2.25
–
Petrostructural map of the Variscan Massifs of the Pyrenees
Figure 2.26
–
Synthetic structural map showing the strain field in the Variscan domains of the Pyrenees
Figure 2.27
–
Illustration of the mode of deformation in transpression between two rigid plates
Figure 2.28
–
Structural model in two steps of the Variscan segment of the Pyrenees
Figure 2.29
–
Synthesis of chronological data on various deformed domains of the Variscan belt in France
Figure 2.30
–
The Variscan belt during the closure of the Ibero-Armorican Arc in the Cantabrian Zone (ca. 295 Ma)
Chapter 3
Figure 3.1
–
Distribution of major sedimentary basins in the Variscan belt during Carboniferous times
Figure 3.2
–
