PSEUDOTACHYLYTE IN THE SUBDUCTING SLAB MANTLE FROM ALPINE OPHIOLITES REVEAL FOSSIL INTERMEDIATE-DEPTH EARTHQUAKES LINKED TO DIFFERENTIAL STRESS RELEASE (Invited Presentation)

It is now ascertained that intermediate-depth subduction-zone earthquakes concentrate either inside subducting slabs, or in hydrated layers along the plate interface. The cause of this seismicity is uncertain, but is typically attributed to rock embrittlement associated with fluid pressurization, or to thermal runaway instabilities.

We document the exceptional preservation of glass in pseudotachylyte (product of frictional melting during seismic faulting) formed at 60-70 km subduction depths in oceanic gabbro-peridotite of the Western Alps, a fossil analogue to oceanic mantle undergoing present-day subduction. Up to 90 vol% of this gabbro-peridotite section is poorly hydrated to dry: it shows high-T mantle-to-oceanic foliations, but escaped crystal-plastic deformation (and largely metamorphism) during Alpine subduction. The remaining minor volumes of hydrated metaperidotite and metagabbro record static eclogitization.

In the dry gabbros, pseudotachylytes preserve glass hosting microlites of olivine, plagioclase, clinopyroxene and locally pyrope garnet. Raman analysis shows the glass is dry. Pseudotachylytes also cut the eclogitized metagabbro and metaperidotite: development of cataclastic clinopyroxene cemented by omphacite and overgrowth of damage microfaults by eclogitic garnet indicate that pseudotachylytes formed at P-T conditions of 550 °C, 2.1 GPa. Pseudotachylytes formation in near anhydrous lithosphere free of coeval ductile deformation, excludes an origin by dehydration embrittlement or thermal runaway. We explain seismicity by the release of differential stresses accumulated in strong, dry, metastable rocks. Survival of glass and absence of subduction-related ductile deformation show the key control of fluids on reaction kinetics and rheology. These rocks represent a proxy for the rheological behaviour of a subducting dry oceanic slab, which is rarely exposed in exhumed high-P terrains that mainly incorporate material from the fluid-rich subduction channel.