Unravelling the development of regional-scale shear zones by a multidisciplinary approach: The case study of the Ferriere-Mollières Shear Zone (Argentera Massif, Western Alps)

Shear zone behavior is mainly controlled by deformation regime (brittle versus ductile), deformation temperature, strain rate and magnitude, and rheology of the deformed rocks. If a gradient of strain is established across a shear zone, softening phenomena can produce progressive localization of deformation in its core, resulting in the shear zone maintaining a constant thickness with increasing strain. In contrast, strain hardening processes may induce migration of deformation into the wall-rocks, causing an increase in shear zone thickness. In the Western Alps we have studied a NW-SE striking steeply dipping km-scale shear zone, the Ferriere-Mollières Shear Zone (FMSZ), that cross-cuts Variscan migmatites in the Argentera External Crystalline Massif. The shear zone is characterized by a deformation gradient, with strain increasing toward the center of the shear zone which we interpret to be associated with strain softening during Variscan retrograde metamorphism.