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