Extensional tectonics in the Hercynian Armorican belt (France). An overview

A synthesis of existing geological, structural and geophysical data shows that the south Armorican Hercynian belt was marked by syn-convergence crustal thinning and dextral wrenching that were in part coeval in late Carboniferous times. Our kinematic model is further supported by new structural data and 40Ar/39Ar ages on synkinematic leucogranites. Extension and strike-slip followed earlier crustal thickening and exhumation of high-pressure metamorphic units in late Devonian-early Carboniferous times. Crustal extension led to the development of core complexes cored by migmatites and crust-derived granite laccoliths. At this time, the South Armorican shear zone acted as a transfer zone separating the extending domain of South Brittany from the non-extending domain of Central Brittany submitted to dextral wrenching. The overall structural pattern and attached kinematics are compared with recent numerical models and illustrated by a 3D interpretative model that integrates geological and deep seismic reflection data (ARMOR 2 profile).