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KlappentextThis 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.

Yoann Denèle is a structural geologist, specializing in the tectonic evolution of the ductile domains of orogens.

Julien Berger is a petrologist, specializing in the magmatic and metamorphic evolution of ancient orogenic belts and magmatic arcs.

Details

Weitere ISBN/GTIN9781394264988

ProduktartE-Book

EinbandartE-Book

FormatEPUB

Format Hinweis2 - DRM Adobe / EPUB

FormatFormat mit automatischem Seitenumbruch (reflowable)

Verlag

John Wiley & Sons

Erscheinungsjahr2023

Erscheinungsdatum22.12.2023

Auflage1. Auflage

Seiten256 Seiten

SpracheEnglisch

Dateigrösse15340 Kbytes

Artikel-Nr.13388142

Rubriken

Genre9201

Inhalt/Kritik

Leseprobe

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...
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