THE KINEMATICS OF FRICTIONAL MELTS IN A SUBDUCTION ZONE:PILOT STUDY ON CORSICAN PSEUDOTACHYLYTES

Pseudotachylytes from the Schistes Lustrés in Corsica may preserve the record of large earthquakes in subduction zones. These rocks, formed along the oceanic crust – mantle boundary, offer opportunities to further our knowledge of subduction seismic processes. Our study on ultramafic pseudotachylytes uses the mini-AMS method and shows a consistent flow plane, flow direction and sense of shear. These fabrics result from coseismic viscous shear in frictional melts. The sense of shear recorded along slip planes is crucial to assess whether slip occurred during a reverse or normal kinematics. The kinematic analysis of the seismic rupture represented by these generation veins is problematic due to the paucity of external offset markers and the lack of internal shear sense criteria in these extremely fine grained materials. The absence of garnet or its relicts in the peridotites of this locality suggests a maximum pressure of equilibration for the host-rock of 1.2 GPa (≈40 km). Four representative oriented samples of these rocks display characteristic microstructures of frictional melts, including skeletal grains, microlites, vesicles, and corroded clast. The material consists of a crypto-crystalline matrix of olivine and pyroxenes, and a small percentage of clasts. The samples of this study are free of serpentinization. The magnetic properties of the pseudotachylytes indicate the presence of pseudo-single domain to single domain stoichiometric magnetite grains, along with a minor contribution of various paramagnetic silicates. The magnetic fabric of these rocks is very consistent within specimens (3.5 mm cubes) from the same vein. The magnetic assemblage results from breakdown of olivine, pyroxenes and chrome spinel, followed by rapid nucleation of magnetite. The magnetite grains form during coseismic viscous shear in a manner similar with those formed in frictional laboratory experiments on the same material. The AMS of these rocks arises from the SPO of elongated magnetite grains. Finally, the AMS fabric obliquity with respect to the seismic slip plane is used as a shear sense indicator. Preliminary results show the dominance of normal senses of slip, hence support that seismic slip took place during the late exhumation of the peridotitic units.