Subduction and Extrusion of an Ultra-High Pressure Oceanic Plate Interface (Lower Shear Zone, Monviso Massif, Western Alps)

The Lower Shear Zone (LSZ) of the Monviso Massif (Western Alps) constitutes a block-in-matrix remnant of the subduction plate interface, now tectonically emplaced into the Western Alpine orogenic belt. Here, we combine observations from cartographic to microscopic scales, focusing on garnet-bearing lithologies in both the mafic blocks and the serpentinite-metasedimentary matrix of the LSZ. Our study combines meso-to micro-scale study of different metamorphic foliations, identification of coesite in eclogitic garnets, geothermometry (Zr-in-rutile) and elastic geobarometry to constrain the LSZ activity during different stages of subduction and exhumation in the Alpine cycle. Garnet cores preserve a relict foliation related to the early stage of subduction along the plate interface, and it is restricted to the weaker metasedimentary layers. Both blocks and matrix within the LSZ record ultrahigh-pressure eclogite-facies conditions within the coesite stability field. This peak stage is also accompanied by the development of a high-pressure main foliation. During the early tectonic exhumation, favored by the return flow in the subduction channel, a fully transpositive, composite eclogite-facies foliation developed along the interface. Increasing viscosity contrast within the multi-layered plate -interface shear zone promoted progressive strain partitioning, producing a subduction-m & eacute;lange fabric. In the late stage of exhumation, the LSZ acts as the extraction-roof fault of an extruding wedge, and return-flow displacement is accommodated by progressive self-localizing tectonic disruption.