Seismically-induced serpentine dehydration as a possible mechanism of water release in subduction zones. Insights from the Alpine Corsica pseudotachylyte-bearing Monte Maggiore ophiolitic unit

Abstract The Monte Maggiore ophiolitic unit of Alpine Corsica consists of an intact to variably hydrated spinel lherzolite intruded by Jurassic gabbro dykes. A widespread serpentinization, marked by low-pressure serpentine polymorphs, results from sea water interaction with mantle rocks during Jurassic sea-floor spreading of the Piemonte-Liguria ocean. A second serpentinization event, marked by the presence of antigorite, is related to the Cretaceous to Paleogene subduction of the Piemonte-Liguria oceanic lithosphere. Tectonic pseudotachylyte veins locally, crosscutting the unit, display several original characteristics. (1) They are hosted by serpentinite, showing that seismic ruptures can propagate through hydrated mantle rocks. (2) The host serpentinite in contact with pseudotachylyte veins shows a thin (500 μm or less) rim of secondary olivine newly crystallized at the expense of serpentine. The heat necessary for serpentine dehydration is likely provided by the frictional melt. Antigorite-bearing clasts (themselves partly dehydrated along their boundaries) reworked in pseudotachylyte veins on one hand and antigorite veins crossing pseudotachylyte veins on the other hand show that frictional melting took place at pressure and temperature conditions compatible with antigorite stability. This indicates that frictional meting occurred at pressures between 0.7 and 0.85 GPa, that is, at ~20 to ~30 km depths in the subducting Piemonte-Liguria oceanic slab. Since the dehydration of serpentine into olivine is only observed at the host-rock selvages of pseudotachylyte veins, it cannot be related to the crossing, by the subducting slab, of the regional 610 ± 100 °C isotherm of the subduction zone, temperature at which antigorite starts to dehydrate into olivine. An estimate of the water released by the dehydration reaction suggests that for a magnitude 6 earthquake, the average amount is about 1.36 L/m2 or 1.36 × 105 m3 for the entire rupture surface (assumed to be 100 km2). If the released water is not incorporated in the nearby frictional melt, it can flow away from the slip zone and contribute to the various fluid-rock interactions active in subduction zones. Alternatively, if the water cannot escape from the slip zone and if the pore pressure is locally high enough, it could trigger aftershocks following the initial seismic rupture.