DEFORMATION OF THE SUBDUCTING SLAB: INHERITANCE THROUGH RETROGRESSION IN THE MONVISO OPHIOLITE

The Monviso ophiolite complex is a well-preserved slice of exhumed oceanic lithosphere that reached eclogite facies during the Alpine subduction, providing a record of the metamorphic processes that occurred throughout its journey. This project focuses on detailed mapping of the ophiolite complex to reconstruct its progressive deformation, metamorphism and retrogression throughout subduction and exhumation, and its relationship to fluid flow as a crucial driver of these processes. We conducted meter-scale field mapping of an area dominated by Mg-Al metagabbros, constructing cross-sections through the lower crust of the downgoing plate. The metagabbro is bounded by two shear zones separating it from metabasites above, and serpentinite below (upper oceanic crust and slab mantle, respectively). The Mg-Al metagabbros contain megacrystic, static clinopyroxene inherited from their igneous protolith. Increasing metamorphism and deformation results in smaller clinopyroxene grain sizes with increased strain and the development of eclogite-facies mylonite. Repeating strain gradient patterns of unstrained to strained layers show throughout the cross-sections. Additionally, retrogressed sections are characterized by the alteration of clinopyroxene to amphibole. Our observations suggest that retrogression becomes increasingly prominent in the direction of both bounding shear zones, but with no clear relationship to internal strain gradients.This suggests deformation of the protolith began during prograde metamorphism and was only partially modified during retrogression. Veins are only observed in partially to fully retrogressed rock and at a high angle to the foliation, suggesting fluid flow occurred after initial deformation, and likely prior toretrogression. This mapping reveals the influence of inherited igneous texture and the interplay of metamorphism, fluid infiltration and deformation in controlling the rheology of the downgoing slab throughout its evolution.