Tectonic Significance of a Major Detachment Zone at the Crust-Mantle Boundary of an Ophiolite: Cima di Gratera, Corsica

The crust - mantle boundary of the oceanic lithosphere has been identified as a major petrological and geochemical discontinuity since 1972. This boundary also form a major mechanical and seismic discontinuity across which both seismic velocities and density sharply increase by more than 10%. In most ophiolites, the boundary occurs as a transition zone with a mixture of dunites, gabbros, pyroxenites, wehrlites, and chromitites up to 600 m thick. The rheological behavior of this major boundary during ophiolite nappe obduction or lithosphere subduction appears to be broadly homogeneous, i.e., it does not localize strain. Recent observations on the Eocene Cima di Gratera ophiolite of the Schistes Lustrés nappe in Corsica show that this boundary, previously known for hosting abundant pseudotachylyte veins, also hosts a ~20 mm-thick and continuous ultramylonite zone that extends at least tens of meter along the gabbro-peridotite contact. The timing of formation, shear strain and sense of shear of this ductile shear zone are being investigated to elucidate its significance. The rheological decoupling attested by this narrow shear zone may have taken place at different stages during the tectonic history of the ophiolite: i) early, during the subduction part of the Eocene subduction, in thrust kinematics; ii) late, during the exhumation of the subducted lithosphere. These new observations also raise the question of why some and not all subducted ophiolites present this type of decoupling and how much strain is actually accommodated along this zone (10's of meters of kilometers). To address these questions we present new structural and microstructural / scanning electron microscopy observations to constrain the kinematic of deformation and the deformation conditions. Our observations are compared with the few known examples of similar strain localization along the oceanic crust - mantle boundary: a) in the Fish massif of the Oman ophiolite, b) in the Addie-Webster ring of North Carolina.