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Tectonics and Geodynamics of the Variscan Cycle in the Pyrenees

Bryan COCHELIN1, Charles GUMIAUX2,3, Benjamin LE BAYON3, Yoann DENÈLE4 and Thierry BAUDIN3

1 State Key Laboratory for Mineral Deposits Research, University of Nankin, China

2 ISTO, CNRS, Université d'Orléans, France

3 BRGM, Orléans, France

4 GET, CNRS, Université Toulouse-III-Paul-Sabatier, IRD, CNES, OMP, Toulouse, France

1.1. Introduction

The Pyrenees have long been overshadowed by the enthusiasm of research on the Alps; nevertheless, they are a first-class natural laboratory that can help us to better understand the Earth system. Due to the great diversity and geological richness that the Pyrenean chain provides, as well as the long and complex tectonic history of which it is the result, the study of the Pyrenean chain allows us to address many fundamental scientific questions that once animated, and still animate, the scientific community. Triggered by the Alpine tectonic uplift, the erosion of the Mesozoic to Cenozoic cover during the Tertiary period denudated a large portion of the Paleozoic basement of the Pyrenean chain. At first sight, the tectonic structuring of this basement is largely inherited from an earlier orogenic cycle: the Variscan orogeny.

Although the Pyrenean basement has been studied extensively since the 1970s, during the 2000s, the geological scientific community has focused on the study of other orogenic segments of the range, somewhat neglecting the progress of our knowledge and understanding of the structure of the Pyrenean basement. It is within this scientific framework that the Bureau de recherches géologiques et minières (BRGM) decided to initiate a major research program: the Geological Reference System of France (commonly referred to as RGF). This new program succeeded the 1:50,000 geological map program, while incorporating the advanced scientific methods and techniques of the 21st century, with the aim of restoring a continuous and coherent layer of geological knowledge throughout the country. The first of these RGF projects (2014-2019) focused on the Pyrenees, with the objective, in partnership with French research institutes, to propose a harmonized and customizable digital geological map, capable of integrating all new geological data acquisitions (cartographic, analytical, etc.). This chapter therefore aims to highlight the latest scientific advances concerning the Variscan cycle and its tectonic consequences in the Pyrenees, with regard to the recent work carried out ahead of and during the RGF Pyrenees project.

1.2. State of the knowledge at the start of the RGF Pyrenees project

1.2.1. General

In this section, the main characteristics of the Pyrenean basement inherited from the Variscan orogeny are summarized. For further information on works carried out before this date, an exhaustive synthesis is presented in Barnolas and Chiron (1996). The numerous cartographic surveys carried out over the course of the last century have found that the Paleozoic basement outcrops all along the Pyrenean range in the form of crustal blocks of variable size. The main block is located in the core of the Pyrenees and forms the Axial Zone (Figure 1.1(a)). The Axial Zone, which is about 330 km long and 60 km wide, is oriented parallel to the axis of the range and is bordered by the North Pyrenean Fault along its northern boundary (Figure 1.1(a)). To the north of this major fault, other much smaller basement blocks, called the North Pyrenean massifs, are found cartographically surrounded by the Mesozoic sedimentary series. The Variscan basement outcrops in the Basque massifs can be found in the western part of the Pyrenees (Figure 1.1(a)). All these massifs are constituted by metasedimentary rocks from the Ediacaran to Carboniferous age. The associated lithologies are essentially: (1) alternating sandstone and limestone, with rare conglomeratic levels, in the Cambro-Ordovician, (2) shales and black pelites associated with the Silurian, (3) alternating thick limestone series and sandstone and limestone levels in the Devonian and (4) silty and sandstone-conglomeratic series (turbidites) dating from the Carboniferous. The Lower Paleozoic metasedimentary series are sometimes intruded by Ordovician granitoids. Gneissic and/or migmatitic massifs from the Variscan orogeny are also observed locally. These gneisses, associated with migmatites, are evidence of Variscan high temperature metamorphism that affected the deepest rocks within the crust. These rocks, corresponding to the lower to middle crust, appear locally in the form of metamorphic domes, structurally below the weakly metamorphic sedimentary rocks from the upper Paleozoic (see section 1.2.2). The latter thus constituted the upper crust at the beginning of the Variscan orogeny. Numerous Variscan plutonic bodies of varying sizes and natures were emplaced within the formations described above (Figure 1.1(a)).

Figure 1.1. (a) Geological map of the Pyrenees showing the outcroping zones of the basement. The main granitic plutons and metamorphic domes of the Axial Zone, the North Pyrenean massifs and the Basque massifs are highlighted (modified from Cochelin et al. (2017)); (b) compilation of available dating for magmatism and late Variscan metamorphism along the Pyrenees

LEGEND FOR FIGURE 1.1. - Dates from 1: Denèle et al. 2012, 2: Vacherat et al. 2017, 3: Ternet et al. 2003, 4: Guerrot 1998, 5: Denèle et al. 2014, 6: Kilzi et al. 2016, 7: Lemirre et al. 2019, 8: Gleizes et al. 2006, 9: Esteban et al. 2015, 10: Lopez-Sanchez et al. 2018, 11: Mezger and Gerdes 2016, 12: Evans 1993, 13: Pereira et al. 2014, 14: Kilzi 2014, 15: Poitrenaud et al. 2020, 16: Paquette et al. 1997, 17: Schnapperelle et al. 2020, 18: Maurel et al. 2004, 19: Roberts et al. 2000, 20: Olivier et al. 2004, 21: Olivier et al. 2008, 22: Aguilar et al. 2014, 23: Druguet et al. 2014, 24: Van Lichtervelde et al. 2014, A: Vacherat et al. 2017, B: Hart et al. 2016, C: Lemirre et al. 2019, D: Esteban et al. 2015, E: Kilzi 2014, F: Delaperriere et al. 1994, G: de Hoÿm de Marien et al. 2019, H: Tournaire Guille et al. 2019, I: Siron et al. 2020, J: Aguilar et al. 2014, K: Druguet et al. 2014.

1.2.2. Magmatism and metamorphism

The most common and voluminous plutonic bodies were essentially emplaced in the upper crust. They often have a characteristic "mushroom-like" shape (Pouget et al. 1989). With a calc-alkaline geochemical signature, these plutons are most often composite in nature. Indeed, they are composed of both basic facies on their periphery (diorite, gabbro) and acid facies in their core (granite, rarely leucogranite). Similar plutons, albeit smaller in size (kilometers), are also frequently observed in the upper crust. When these calc-alkaline plutons are emplaced at the top of gneissic domes, they present a laccolitic form, as illustrated by the Mont-Louis pluton (Denèle et al. 2014) or the Lys-Caillaouas pluton (Pouget et al. 1989). The source of the magmas at the origin of these plutons has long remained uncertain, oscillating between purely crustal contribution and variable mantle involvement due to geochemical signatures that can be considered hybrid (Debon et al. 1996). The anatexis granites (leucogranites) of the Pyrenees are more modest in size, but widespread. They are essentially observable within metamorphic domes. They form decametric to plurikilometric lenticular bodies or appear as dykes. In migmatitic domains, they can be associated with small basic facies, as observed in the Gavarnie cirque or in the Aston massif (Pouget et al. 1989). The source of these basic rocks is described as mantelic, although the mechanism behind the partial melting of the mantle has long remained enigmatic and is still discussed to this day.

Until the democratization of U-Pb dating, established in the 1990s to 2000s, the emplacement of most plutonic massifs was interpreted as having taken place at the paroxysm of the Variscan orogeny (i.e. between 340 and 330 Ma), as described elsewhere in the internal zones of the chain (Massif Central, Bohemian Massif, etc.). However, new U-Pb dates have since revealed much younger emplacement ages than expected, between 310 and 290 Ma (Figure 1.1(b)), that is, at the transition between the Carboniferous and Permian. Nevertheless, some authors (Mezger 2010; Mezger and Gerdes 2016) maintain an older age, around 330-340 Ma, to some leucogranites. Still, the non-reproducibility of these results in more recent studies of the same magma bodies (Denèle et al. 2014; Lopez-Sanchez et al. 2018) (Figure 1.1(b)) leaves some doubt as to the significance of these older ages.

The partial melting of the lower Pyrenean crust is the result of a regional high temperature-low pressure (HT/LP) metamorphism, called "M2". In the Axial Zone, it is characterized in the middle and upper crust structural levels by, respectively, amphibolite and greenschist facies parageneses (Figures 1.2(a) and (b)). In the North Pyrenean massifs, the intensity of metamorphism seems more important than...