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

Summary: Fossil earthquakes recorded by pseudotachylytes in mantle peridotite
from the Alpine subduction complex of Corsica
Torgeir B. Andersen *, Ha°kon Austrheim
Physics of Geological Processes and Department of Geosciences, University of Oslo, P.O.Box 1047, Blindern, 0316 Oslo, Norway
Received 17 March 2005; received in revised form 28 October 2005; accepted 28 November 2005
Available online 10 January 2006
Editor: E. Boyle
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 Lustre´s 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 (Fo93­89), clino- and ortho-pyroxene with
compositions indicative of high crystallization temperatures (1300­1400 8C), 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 8C and 1­1.5 GPa) to more than 1700 8C, 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


Source: Andersen, Torgeir Bjørge - Centre for Physics of Geological Processes & Department of Geosciences, Universitetet i Oslo


Collections: Geosciences