The importance of serpentinite mylonites for subduction and exhumation of oceanic crust.

Abstract

In the ultramatic Erro-Tobbio unit (Voltri-Massif: Western Alps) a set of overprinting structures in serpentinite mylonites is related to Alpine subduction to about 80 km depth and to subsequent exhumation. Antigorite mylonites are cut by en-echelon olivine veins, which in turn are dissected by multiple sets of shear bands containing olivine and titanian clinohumite. The transition from olivine-free to olivine-bearing structures indicates recrystallization during prograde metamorphism. All structures display the same top-to-the-NW kinematics providing evidence for a continuous non-coaxial deformation. The serpentinite mylonites surround km-scale bodies of pre-Alpine peridotite which show only minor Alpine overprint. This indicates that during subduction-related deformation, recrystallization and fluid flow were strongly localized within serpentinite mylonites. Olivine-bearing, discontinuous shear planes with top-to-the-SE sense of movement crosscut the prograde structures. The inversion of shear sense suggests a change in position of the serpentinites relative to the downgoing slab, i.e. from the subducted slab to the upper plate during accretion. Thus, the shear sense inversion marks the change from burial to exhumation of the serpentinites. The low density of antigorite serpentinites (2.75 g/cm3) causes strong bouyancy, thus providing a mechanism for the exhumation of deeply subducted rocks. It is suggested that serpentinites may act as carriers for the uprise of eclogite bodies, which have higher densities than the peridotitic upper mantle.