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

In the ultramafic Erro-Tobbio unit (Voltri-Massif; Western Alps) a set of o verprinting structures in serpentinite mylonites is related to Alpine subdu ction to about 80 km depth and to subsequent exhumation. Antigorite mylonit es are cut by en-echelon olivine veins, which in turn are dissected by mult iple sets of shear bands containing olivine and titanian clinohumite. The t ransition from olivine-free to olivine-bearing structures indicates recryst allization during prograde metamorphism. All structures display the same to p-to-the-NW kinematics providing evidence for a continuous non-coaxial defo rmation. The serpentinite mylonites surround km-scale bodies of pre-Alpine peridotite which show only minor Alpine overprint. This indicates that duri ng subduction-related deformation, recrystallization and fluid flow were st rongly localized within serpentinite mylonites. Olivine-bearing, discontinuous shear planes with top-to-the-SE sense of mov ement crosscut the prograde structures. The inversion of shear sense sugges ts 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, t he shear sense inversion marks the change from burial to exhumation of the serpentinites. The low density of antigorite serpentinites (2.75 g/cm(3)) c auses strong buoyancy, thus providing a mechanism for the exhumation of dee ply subducted rocks. It is suggested that serpentinites may act as carriers for the uprise of eclogite bodies, which have higher densities than the pe ridotitic upper mantle. (C) 2000 Elsevier Science B.V. All rights reserved.