Les cisaillements hercyniens de la Vendée au Limousin

Abstract

Located on either side of the Mesozoic sedimentary “Seuil du Poitou”, the study area extends from Vendée (southeast of the Armorican Massif) to the western part of the Massif Central. Most of the faults that crop out in Vendée and the Massif Central, but which are overlain by Mesozoic cover in the Seuil du Poitou area, belong to the South Armorican Shear Zone. Recent geologic-al surveys have enabled us to 1) propose a new structural map of the Variscan basement on either side of the “Seuil du Poitou”, and 2) correlate the south Armorican faults to the north with those in the western part of the Massif Central to the south. The Vendée - Massif Central basement lies within a large intracontinental horsetail shear zone open to the east and located along the prolongation of the South Armorican Shear Zone. The Parthenay Fault, one of the major faults of this system, separates an eastern domain with shallow-dipping structures from a western domain characterised by vertical structures. The Middle Variscan collision (400–370 Ma): the development of the internal Variscan zone is the result of polyphase tectonism recorded by the overthusting of high-grade metamorphic nappes during the Devonian. The structures in the Massif Central are the result of two successive phases of tangential deformation. The first, of Early Devonian age and having a south to southwest vergence, corresponds to the formation of the main nappes under HT-MP metamorphic conditions; high-temperature decompression of these nappes during their exhumation induced local anatexis (» 380-370 Ma). In Vendée, the early overthrusting was responsible for stacking of the Essarts eclogitic unit onto the Mervent unit; during the Middle Devonian, the northern Chantonnay units and the southern part of the Haut-Bocage unit were overthrust onto the Essarts–Mervent units. During the second deformation phase, of Late Devonian age, the nappe system evolved towards oblique collision with respect to the trend of the Variscan belt. In the Limousin area, the nappes reacted by northwestward shearing under intermediate pressure metamorphism that was either syn- or post-anatexis. Many K/Ar ages and P-T paths testify to the cooling (between 500 and 300 °C) and exhumation of this area around 360-350 Ma. Post collisional evolution, Late Devonian - Early Visean (» 370-345 Ma): oblique collision at the end of the Devonian (NW-SE shortening), marked by resumed northwestward thrusting, was followed by left-lateral wrenching at the Devonian-Tournaisian boundary and then by right-lateral wrenching during the Tournaisian and Visean. At this time, the Armorican faults dislocated the Variscan internal zone. This tectonism was coeval with a late melting event that generated calc-alkaline and peraluminous magma: a “tonalitic” magma, probably related to the melting of a mantle wedge inserted in the nappe stack, was emplaced at first, followed by granite magma (Guéret type) derived from lower crustal melting. The Late Tournaisian regional event generated N120° -140° -trending folds that deformed the nappes and certain intrusions. This folding, related to N-S to NNE-SSW shortening, may be linked to the development of dextral strike-slip faults responsible for local fold axis reorientation. During the Early Visean, the deformation affected the Bas-Bocage, suggesting a late involvement in the collision: the Essarts-Mervent tectonic units were then thrust onto the Bas-Bocage unit. Middle and Late Visean extensional tectonism (» 345-325 Ma): from the Middle Visean to Early Namurian(?) the orogenic belt underwent gravitational collapse causing local crustal thinning. Many listric normal faults induced gravity sliding with a vertical displacement of several kilometres, particularly along the granite margins. In Vendée, the Porphyroid nappe slid westward, contemporaneous with uplift of the central part of the parautochthon intensely intruded by perluminous granites. Namuro-Westphalian strike-slip tectonism (» 325-300 Ma): strike-slip tectonism became dominant during the Namurian and Westphalian. Initially dextral in the Namurian, this tectonism was characterised by reactivation of faults in the “Haut-Bocage” unit in Vendée and of the “La Marche” shear system along the edge of the internal zone. Leucogranites were emplaced along this large-scale structure (Mortagne, Parthenay and La Marche belt). Rotation of the shortening direction, which changed from NNW-SSE (dextral shearing) to NE-SW during the Westphalian, produced sinistral transcurrent shearing along the northern edge of the internal zone. During the Stephanian, this sinistral shearing was followed by dextral shearing under brittle conditions, as recorded throughout the Armorican fault system. Shearing locally induced the opening of narrow sedimentary basins. The succession of different shear directions (from dextral, to sinistral and back to dextral) along the major Ligerian–Moldanubian fault system leads us to consider several changes in the orientation of the finite strain axes (X, Y, Z) at this major plate boundary during the Carboniferous.

Key words: Strike-slip fault, Normal fault, Shear zone, Hercynian orogeny, Armorican massif, Vendée, French Massif Central, Limousin.