Energy Balance From a Mantle Pseudotachylyte, Balmuccia, Italy
Résumé
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 1Pa
| Origine | Fichiers éditeurs autorisés sur une archive ouverte |
|---|---|
| Licence |