Microtextures of ultramafic pseudotachylyte fault veins from Corsica : an SEM-EBSD analysis

Present-day subduction zones exhibit intense seismic activity in the descending oceanic lithosphere. The mechanism behind shallow earthquakes is generally well-understood and related to frictional stick-slip on fault planes. Earthquakes originating deeper, at intermediate (50-300 km) or at even greater depths (up to ~700 km) can generally only be studied by the seismic energies released from earthquakes in subduction zones. The mechanism(s) facilitating seismic failure at such depths are not fully understood. Direct investigation of the deformation products formed by seismic faulting at depth is generally restricted by a lack of exposed examples. Paleoseismicity recorded by pseudotachylytes in the high-pressure and low temperature blueschist- to eclogite facies subduction complex of Alpine Corsica provide insights into earthquakes formed under conditions of approximately 1.5-1.8GPa at ~450°C. Pseudotachylytes occur in both ophiolite gabbro and mantle peridotite, along and in vicinity of the fossil Liguro-Piemontese MOHO. Microstructural investigation using the SEM and electron backscatter diffraction (EBSD) technique has been applied in this pilot EBSD study of selected samples from Alpine Ligurian peridotite host rocks, fault rocks and ultramafic pseudotachylyte. The abundance of ultramafic pseudotachylyte on small faults suggests that peridotite retains its strength to great depths. The EBSD work shows that the peridotites record highly inhomogeneous crystal-plasticity. Intracrystalline deformation features in orthopyroxene and clinopyroxene with curved exsollution lamellae, mechanical twins and kink bands coexist with common undulous olivine, which also coexist with high temperature slip systems in olivine. The co-seismic deformation occurred during presence of free water, and pseudotachylyte generation surfaces are associated with pre-existing or possibly syndeformational heterogeneities in the peridotite. After seismic stress-drop, the peridotite largely returned to ambient conditions. Pseudotachylyte in the studied localities preserves delicate quench-texture, including spherulites, dendrites and chilled margins. A more comprehensive EBSD study involving both the wall-rock peridotite, damage zones adjacent to pseudotachylyte fault-veins and the pseudotachylytes themselves are necessary to provide a detailed understanding of the microstructures related to the subduction faulting