Metamorphic and structural constrains for the exhumation of (U)HP – metaophiolites from the Susa Valley, Western Alps

High and ultra-high pressure metamorphic rocks have a crucial role in the construction of geodynamic models concerned with convergent plate margins. They offer the possibility of constructing P-T-t trajectories, and thus constrain the evolution of orogenic belts. In this work, new structural and metamorphic data were collected in the Susa Valley, wherein Zermatt-Saas-like meta-ophiolite units (i.e., the Internal Piedmont Zone, IPZ) are tectonically overlain by Combin-like meta-ophiolite units (i.e., the External Piedmont Zone, EPZ). The IPZ and EPZ record different metamorphic P-T peaks and are separated by a hundreds of meters thick shear zone (Susa Shear Zone, SSZ). The SSZ drove exhumation of the IPZ until coupling with the EPZ, and is characterized by two main mylonitic events. The younger one (ME2) develops a pervasive mylonitic foliation, with top to W extensional sense of shear, which wraps various-scale lithons wherein an older mylonitic foliation (ME1) developed under HP conditions (about 17 kbar) and characterized by top to E sense of shear, is preserved. In the IPZ four tectono-metamorphic events developed under variables metamorphic conditions. The first event (M1) shows UHP conditions, defined by the occurrence of microdiamonds (detected by micro-Raman spettroscopy analyses) included in Grt, Omp and Rt. The paragenesis is completed by Zo+Pg pseudomorphoses after Lws. For this relict metamorphic event P of about 33 kbar (microdiamonds occurrence) and T of 650 °C (Grt-Cpx geothermometers, and Zr contents in the Rt) were inferred. After the P peak, the IPZ was exhumed along the subduction channel following a near isosthermal decompression path, up to the development of the second metamorphic event, which still occurred under eclogitic conditions (14-16 kbar and 580-600 °C). This event is correlated with firstly exhumation along the SSZ (ME1). Following a retrograde and decompressional trajectory the IPZ was then re-equilibrated under greenschist facies conditions (M3, 5 kbar and 450-500 °C), that is related to the D2 main regional deformation phase. The last metamorphic event (M4) is defined by a rise of T up to about 550 °C, with formation of new stable paragenesis (Bt, Ep,Olig, Hbl-Prg, Ms) that partially replaced the previous. This late heating was registered also by SSZ mylonites, with formation of stable minerals in these metamorphic conditions developed along the shear planes (ME2). The inferred P-T path suggests that during the exhumation of the IPZ different processes occurred. A first fast exhumation stage mainly driven by buoyancy forces in a compressional regime from the base of the orogenic wedge towards shallow crustal level can be related to major cooling and was mainly controlled by E-verging compressional stage along the SSZ. Finally, a rapid unroofing of the hanginwall by W-directed extensional movement along the SSZ was responsible for the later heating.