Shear zone structures and microstructures in mantle peridotites from the Voltri massif, Ligurian Alps, N.W. Italy

Abstract

In the Erro-Tobbio peridotite three generations of shear zone structures are developed in granular peridotite: peridotite tectonites, peridotite mylonites and serpentinite mylonites. Peridotite tectonites occur in a kilometre-scale shear zone developed under estimated conditions of 1100 to 1220°C and 16 ± 6 kbar, and flow stresses between 4 and 11 MPa. The deformation in this shear zone involved dislocation creep and concurrent recrystallization of olivine by a combination of subgrain rotation and extensive grain boundary migration. With decreasing pressure and temperature and increasing flow stress, spinel peridotite mylonites (T= 800-925°C, P = 6-8 kbar, σ= 93-153 MPa) and chlorite peridotite mylonites (T = 550-800°C, P = 4-6 kbar, σ = 331-786 MPa) developed in up to hundred metre scale shear zones transecting the peridotite tectonites. The mylonitic microstructures suggest that dynamic recrystallization of olivine occurred by a mechanism involving grain boundary bulging to nucleate new strain free grains. In addition, 'fluidal' microstructures dominated by pyroxene clasts with tails of fine-grained amphiboles suggest that superplastic mechanisms may have been operative in these mylonites. The sequence of tectonite shear zones followed by peridotite mylonites is inferred to result from the progressive ascent of asthenospheric peridotites during opening of the Mesozoic Piemonte-Ligurian ocean. Serpentinite mylonites, developed at temperatures of 300-550°C and pressures of about 4kbar, are observed in shear zones mostly associated with ductile overthrusting and imbrication. These structures are clearly related to nappe emplacement during Alpine collision.