Structural evolution along the Susa Shear Zone: the role of a first-order shear zone in the exhumation of meta-ophiolite units (Western Alps)

In the Western Alps, different shear zones acting at different depths have been investigated for explaining multistage exhumation of (U)HP units, and several exhumation models have been proposed for explaining present-day stacking of different tectonometamorphic units. This study aims to reconstruct the tectonic evolution of the Susa Shear Zone (SSZ), a polyphasic first-order shear zone, outcropping in the Susa Valley. The SSZ consists of a thick mylonitic zone, along which units characterized by different Alpine metamorphic P–T peaks are coupled. In the study area, the footwall of the SSZ mostly consists of oceanic units (i.e., Internal Piedmont Zone), which record eclogitic conditions, whereas the hanging wall consists of oceanic units (i.e., External Piedmont Zone), which record blueschist-facies conditions. These tectonic units were deformed during subduction- and exhumation-related Alpine history, throughout four main regional deformation phases (from D1 to D4), and were coupled along the SSZ, wherein two shearing events have been distinguished (T1 and T2). T1 occurred during early exhumation and was characterized by “apparent reverse” Top-to-E kinematics, whereas T2 occurred during late exhumation and was characterized by Top-to-W kinematics. Detailed fieldwork and structural analysis allowed us to describe the main features of the different deformation stages and define the deformation relative timing. As final result, we propose a four-step geodynamic model, focused on the different stages developed along the SSZ, from pre-T1 to syn-T2, showing the geometrical relationships between the tectonic units involved in the exhumation. The model aims at explaining the role of the SSZ in the axial sector of the Western Alps.