No abstract

The Variscan continental collision has led to the development of large strike-slip shear zones in western Europe. Our study focuses on the regional deformation and shear zone patterns in the Massif Armoricain and the French Massif Central. The synthesis of granite emplacement ages associated to granite deformation fields, allow us to propose a geodynamic model for the tectonic evolution of this part of the Variscan belt between 370 Ma -320 Ma (Late Devonian -Namurian).After the first steps of the continental subduction-collision, leading to high temperature and anatexis associated with N-S shortening at 380-370 Ma (Frasnian to Famennian), the southern part of the Massif Armoricain and western part of French Massif Central underwent large dextral shearing along N100-N130 trending shear zones up to early Visean time. These large-scale displacements progressively decreased at around 350-340 Ma, during the first emplacements of biotite bearing granites (Moulins-les Aubiers-Gourgé massif and Guéret massif intrusions).During middle Visean times, the shortening axis direction rotated towards a NNE-SSW direction implying changes in the regional deformation field. The occurrence of N070-N100 sinistral and N110-N130 dextral conjugate shear zones within leucogranites are related to that time. Finally, new N150-N160 dextral shear zones appeared in middle to late Visean times: as for examples, the Parthenay and the Pradines shear zones in the SE Massif Armoricain and the Millevaches massif, respectively. These shear zones were conjugated to the sinistral N020 Sillon Houiller in the French Massif Central. They reflect large scale brittle continental indentation in the French Variscan belt during the middle to late Visean. Tectoniques transcurrentes famenno-namuriennes (370-320 Ma) enregistrées par les plutons syn-cinématiques dans la chaîne varisque française (Massifs armoricain et central) Mots-clés. -Varisque, Massif armoricain, Massif central, Zone de cisaillement, Granites, Datations.Résumé. -La collision continentale varisque évolue en Europe occidentale par le développement de zones de cisaillement d'échelle crustale jalonnées d'intrusions granitiques syn-et post-cinématiques. Cette étude se focalise sur les zones de cisaillement qui disloquent cet orogène varisque du Massif armoricain au Massif central, et sur la déformation des granites qui les jalonnent. La synthèse de nombreux âges récents obtenus sur ces granites syn-et post-cinématiques, qui ont enregistrés les différents champs de déformation, permet de proposer un modèle géodynamique pour cette partie de l'orogène entre 370-320 Ma, soit sur la période Dévonien supérieur -Carbonifère inférieur. Au Dévonien moyen-supérieur, la fin de la collision continentale varisque est marquée dans la partie méridionale du Massif armoricain (Vendée) et la partie occidentale du Massif central par une évolution métamorphique de haute température accompagnée d'une anatexie vers 380-370 Ma (Frasnien-Famennnien). Cette évolution est relayée par le développement d'une tectonique trans...

Summary The Plateau d’Aigurande represents the north-westernmost part of the French Massif Central. It is overlain by the sediments of the Paris Basin to the north and bounded by the La Marche shear zone to the south. Detailed mapping, mainly from the Creuse Valley region, indicates thrust and nappe tectonics. Beginning at the base of the sequence one can recognize: (1) the Fougères unit (schists of low to medium grade) intruded by syntectonic leucogranites; (2) the Eguzon unit (medium- to high-grade schists which are partly diaphtoretic in the lower part of the unit); (3) the Gargilesse unit (high-grade schists); and (4) the Migmatitic unit. The metasediments of the Fougères, Eguzon and Gargilesse units were originally coarse grained clastics. They are interbanded with magmatic units (orthogneisses and amphibolites). The migmatites are derived from greywacke-type rocks intruded by magmatic rocks (orthogneisses and amphibolites). Following a high-pressure (Silurian?) tectonometamorphic event, the piling up of tectonostratigraphic units occurred during two periods of deformation. The main one, probably of late Caledonian (Acadian?) age, was synchronous with or slightly after the climax of metamorphism. The second, of Westphalian age, was accompanied by the emplacement of leucogranitic magma and by retrograde metamorphism. The shear-sense appears to have been from SW to NE during the Acadian phase and from NW to SE during the Westphalian. Structural and lithological studies suggest that the three lowermost units may have been derived from the same palaeogeographic domain while the migmatites have a distinctly different history.

No abstract