Crustal-scale strain partitioning: footwall deformation below the Alpine Oligo-Miocene detachment of Corsica

The Alpine metamorphic units of Corsica were exhumed in the footwall of an Oligo-Miocene detachment. New structural data are used to describe the ductile deformation within the footwall and its evolution from greenschist facies metamorphic conditions to the surface. Regional variations in the type of kinematic indicators show a partitioning between vertical coaxial shortening and localized non-coaxial flow on extensional ductile shear zones. Coaxial flow results in folding with axial-plane crenulation cleavage that is horizontal or gently W dipping. Slightly curved fold axes and a general asymmetry indicate a minor component of eastward shear. Non-coaxial flow results in the formation of a mylonitic foliation dipping to the E associated with top-to-the-east sense of shear. This distribution is seen at all scales from the thin section to that of the crust. At crustal-scale, vertical coaxial shortening occurs within large boudins bounded by plurikilometric extensional shear zones. As exhumation proceeded, the footwall entered the brittle field. Late deformation is then characterized by subvertical shear and large scale folding, interpreted in terms of isostatic re-equilibration during denudation. Tectonic activity ends with normal faulting and deposition of marine sediments in asymmetric grabens. This description illustrates the behaviour of the ductile crust beneath upper crustal tilted blocks.