Energy Balance From a Mantle Pseudotachylyte, Balmuccia, Italy

In the Balmuccia massif (NW Italy), a pseudotachylyte vein network (N068 trending) in a spinel lherzolite is interpreted as the product of frictional melting during a single M-w>6 earthquake. The subvertical fault underwent a metric dextral coseismic displacement, raking 60 degrees SW. The average width of the main slip surface is similar to 5mm. A dense network of thin (20-200m) injection and ultramylonite-like veins decorates the fault walls. In the injection veins, Raman microspectrometry mapping reveals pockets of still preserved amorphous silicate, containing approximate to 1% of structurally bound H2O. In the ultramylonite-like veins, electron backscattered diffraction mapping reveals that ultrafine (0.2-2m) olivine grains exhibit a strong fabric with (010) planes parallel to shearing, consistent with temperatures above 1250 degrees C during deformation and suggesting fast recrystallization from the frictional melt. The veins also exhibit pyroxene and recrystallized spinel, which proves that the earthquake occurred at a minimum depth of 40km. The energy balance demonstrates that complete fault lubrication must have occurred during coseismic sliding (i.e., dynamic friction coefficient <<0.1). Because of the low viscosity of slightly hydrated ultramafic liquids (approximate to 1Pas), we argue that lubrication was only transient, as the melt could rapidly flow into tensile fractures, which led to rapid cooling and promoted strength recovery and sliding arrest. Combined together, our observations suggest that this pseudotachylyte is the frozen record of a deep (>40km) earthquake of 6