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°SW. The average width of the main slip surface is ~  5 mm. A dense network of thin (20–200 μm) injection and ultramylonite‐like veins decorates the fault walls. In the injection veins, Raman microspectrometry mapping reveals pockets of still preserved amorphous silicate, containing ≈ 1% of structurally bound H 2 O. In the ultramylonite‐like veins, electron backscattered diffraction mapping reveals that ultrafine (0.2–2 μm) olivine grains exhibit a strong fabric with (010) planes parallel to shearing, consistent with temperatures above 1250°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 40 km. 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 (≈ 1 Pa·s), 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 (>40 km) earthquake of 6 <  M w  < 7. Its focal mechanism is deduced from the crystal preferred orientation due to late coseismic creep in ultramylonite‐like veins and deciphered by electron backscattered diffraction.