In the ultramafic 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/cm(3)) causes strong buoyancy, 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.
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