Seismogenic shear zones in the lithospheric mantle: Ultramaphic Pseudotachylytes in the Lanzo Peridotite ( Western Alps, NW Italy)

At Mt. Moncuni (Lanzo Massif,Western Alps) plagioclase peridotites and early mid-ocean ridge basalt (MORB) gabbroic dykes are deformed by shear zones containing cataclastic bands and both fault-vein and injection-vein pseudotachylytes, which are crosscut by late MORB porphyritic dykes. Fault-vein pseudotachylytes have thicknesses of the order of 1mm; injection-vein pseudotachylytes have a typical thickness of 1-10 cm and contain spinifex textures. Structural, petrological and geochemical data show that the pseudotachylytes formed by near-complete melting of the host peridotite, at ambient temperature-pressure conditions (T=600 ± 100 °C, P< 0.5 GPa) close to the brittle-ductile transition of ultramafic rocks, during exhumation of the lithospheric mantle in the early stages of formation of the LigurianTethys oceanic basin. Estimates of the average volume fraction of unmelted clasts and of the ambient and liquidus temperature, together with thermophysical parameters, allow the determination of the melting energy per unit volume. Coseismic displacement is not observable at Mt. Moncuni, and consequently the dynamic shear resistance cannot be inferred.We show that commonly proposed relations between fault-vein thickness and displacement are of limited value, given the difficulty in identifying ‘single-event’ pseudotachylytes and the mobility of the melt. However, we also show that dynamic shear resistance can be predicted to decrease sharply if the melt coats the whole fault plane, partly as a consequence of the nonlinear viscosity of silicate melts at high strain rates. The Mt. Moncuni pseudotachylytes are the result of upper mantle seismicity at shallow depth (z<20 km) over a time period of at most 5 Myr. Estimation of the total seismic energy release and moment (caused by an unspecified number of small to moderate earthquakes) requires an assessment of the total pseudotachylyte volume. This is highly uncertain, with a probable qualitative error margin of ± 1 order of magnitude.The inferred values of cumulative seismic energy release and moment are of the order of 10apex15±1 J and 10apex19±1 N m, respectively, resulting in a seismic energy release rate of approximately 10apex8±1 J/a. This value is compatible with present-day seismic rates at extensional plate margins.