Fingerprinting and relocating tectonic slices along the plate interface: Evidence from the Lago Superiore unit at Monviso (Western Alps)

The Lago Superiore Unit (LSU, Monviso Massif, Italian Western Alps) is a section of fossil oceanic lithosphere equilibrated to eclogite facies conditions (550 °C - 2.8 GPa) during Alpine subduction (45-40 Ma). It is cut by two major shear zones, namely the Intermediate (ISZ) and Lower Shear Zone (LSZ), mostly consisting of serpentinite. The lowermost, serpentine-rich, section of the Lago Superiore Unit, the Basal Serpentinite, separates the HP ophiolite domain from the underlying continental Dora-Maira Unit. Here we show that the LSZ and the Basal Serpentinite were active at different stages of the subduction and exhumation history of the complex. Most of retrograde deformation and mineral re-equilibration were localized in the LSZ. Channelized fluids percolating during this phase chemically homogenized the LSZ serpentinites, that preserved their HP mineralogy only locally; the best-preserved relicts of the eclogite-facies high pressure stage within the LSZ serpentinite are nodules of magnesite (representing former veins) and eclogite blocks. Differently, the underlying Basal Serpentinite largely escaped the exhumation-related processes and still records the prograde chemical and petrological history of the LSU serpentinite, from ocean-floor hydration to HP metamorphic conditions. The Lago Superiore Unit thus represents a snapshot of major Alpine metamorphic and shearing events, from prograde subduction to exhumation. Its km-scale thickness, and the oriented antigorite fabric in the Lower Shear Zone and Basal Serpentinite makes it a good seismic reflector. This HP ophiolite complex can thus be used as proxy of a deep (70-80 km) Alpine-type subduction zone, and to better constrain and interpret seismic images of present-day convergent margins.