Patrick Rolin

Palaeozoic evolution of the Plateau d’Aigurande (NW Massif Central, France)

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.

Progressive shortening axis rotation recorded by Variscan synkinematic granites : example of the South Armorican shear zone in the Vendée (France)

The Variscan continental collision is expressed by large shear zones in Western Europe. The synthesis of granite ages, related to different deformation fields in the Vendée area, suggests a geodynamic model for the tectonic evolution of this part of the Variscan belt between 370 Ma – 320 Ma. After the first step of the continental collision, leading to high temperature and anatexis at 375-360 Ma, the south-eastern part of the Armorican massif underwent large dextral shearing along N110-N125 trending shear zones, related to a bulk NNW-SSE shortening direction, up to early Visean time. Large-scale displacements progressively decreased at around 345-340Ma. During middle Visean time, the shortening axis direction rotated towards a NNE-SSW position implying changes in the regional deformation field. The occurrence of N70-N100 sinistral and N110-N130 dextral conjugate shear zones within leucogranites are related to that epoch. Finally, a new dextral shear zone system, trending N130-N150 along the Parthenay shear zone, occurs during late Visean time. This progressive middle Visean change of shortening direction probably corresponds to a major change in the Iberian plate motion and indentation during the Mississipian collision.