Fossil earthquakes recorded by pseudotachylytes in mantle peridotite from the Alpine subduction complex of Corsica

Paleo-earthquakes recorded by pseudotachylytes have recently been discovered in the blueschist facies subduction complex of Alpine Corsica. Pseudotachylytes occur in ophiolite gabbro and mantle peridotite belonging to the Schistes Lustrés of Cape Corse. Ultramafic pseudotachylyte fault- and injection veins are found within well-preserved peridotite lenses and are progressively hydrated together with the host rock along the margins of the lenses. Numerous pseudotachylytes ranging in thickness from less than 1 to 380 mm have been identified. Veins thicker than 3 mm may show flow banded chilled glassy margins and cores with dendritic to spherulitic quench textures. The newly formed minerals are zoned olivine (Fo93–89), clino- and ortho-pyroxene with compositions indicative of high crystallization temperatures (1300–1400 °C), zoned Cr-spinel, and a glassy to micro-vesicular hydrous matrix showing that frictional melts contained up to 4% water. Frictional heating on co-seismic faults raised the temperature from ambient blueschist facies conditions (450 °C and 1–1.5 GPa) to more than 1700 °C, which is required for ∼75% disequilibrium melting of spinel peridotite at 1.5 GPa. The characteristic fault-vein thicknesses observed are 1 to 3 cm, but several fault-veins are thicker than 10 cm. Co-seismic displacement of 1 m, a stress of 300 MPa, and seismic efficiency of 5%, may melt ca 60 kg peridotite pr. m2 fault surface corresponding to 20 mm thick layer of ultramafic pseudotachylyte. The ultramafic pseudotachylytes described here formed by disequilibrium melting of peridotite in the upper part of the Alpine subduction zone. If the interpretations of typical displacements of approximately 1 m are correct, the most common pseudotachylyte fault-veins are related to magnitude ca. 7 or larger subduction earthquakes.