New multi-approach constrains for the poliphase evolution of eclogite-blueschist boundary between ophiolitic units in the Susa Valley (Western Alps)

New structural and petrological data are collected in the Mid-Susa Valley, along the boundary between eclogite and blueschist ophiolitic units. This boundary is marked by a huge (300-600 m thick) shear zone, named Susa Shear Zone (SSZ, Gasco et al., 2011), that allow eclogitic units (Zermatt-Saas-like Units and Dora Maira Unit) to be exhumed under blueschist units (Combin-like Units). The SSZ is regionally located in the western limb of the dome structure involving Dora Maira Unit and the overlying Zermatt-Saas Units. The shear zone shows a block-in-matrix structure and well recalls a tectonic mélange (sensu Festa et al., 2010). The blocks consist of different litotypes (mafic and ultramafic rocks, marble, paragneiss) embedded in a less competent matrix made up of mylonitic calcschist. Two mylonitic event sare observed both on the field and in oriented thin sections. The first event occurred during early exhumation stage and is marked by remnants of a tectonic contact with Top-to-E kinematism. The latter are particularly well-defined by S-C structures, and porphyroblasts rotated within a mylonitic foliation. The second event is related to late exhumation and is marked by a shear zones with Top-to-W kinematism. The two mylonitic events occur under different metamorphic conditions. The analyses of the Si content in the white mica, shows a gap of almost 10-13 kbar between the two mylonytic events, with P of almost 15-18 kbar for the first event(Top-to-E), and 5 kbar for the second event(Top-to-W). Different textural features in deformation, kinematism and P-T conditions allow to infer the geodynamic evolution as a two main stages: (i) a pre-doming extensional Top-to-E shear zone, corresponding to the early stage exhumation of the eclogitic units along the subduction channel, and (ii) a Top-to-W extensional shear zone, due to the growth of the regional dome structure, corresponding to the late stage of exhumation. The original geometry of the structures referred to first mylonitic event are now significantly modified due to doming of eclogitic units. In fact kinematic indicators show apparently reverse sense of shear. In conclusion, it is to be emphasized the extensional Top-to-E kinematism of early movement along the SSZ. This early shear zone was subsequently overturned by a deformation phase, testified by large-scale folds giving the dome structure to the axial belt. Finally, a further extensional phase developed by extensional movement Top-to-W and related to the onset of the tectonic collapse of the Alpine orogen.