Deformation fabrics of olivine in Val Malenco peridotite found in Italy and implications for the seismic anisotropy in the upper mantle

An exhumed huge body of ultramafic rocks is preserved in Val Malenco, Italy. In this study, petrofabrics and deformation microstructures of olivine in the Val Malenco peridotite were examined, and new observations of lattice preferred orientation (LPO) of olivine such as type-B and type-E LPO in the peridotite are reported. Three types, depending on the grain size, of olivine fabrics were observed: type-A fabric exhibiting the (010)[100] slip system in the large relic grain area, and type-B and type-E fabrics exhibiting the (010)[001] and (001)[100] slip systems, respectively, in the widely distributed recrystallized-grain area. The slips occurred mainly on (010)[001], and the (001)[100] slip system has only rarely been reported to be the dominant slip system of olivine found in natural rocks. Further, seismic velocities and seismic anisotropies were calculated for the three types of fabrics found. The seismic anisotropies in the upper mantle can be attributed to the type-B, type-E, or type-A LPOs of olivine. Moreover, the anomaly in seismic low velocity in the mantle wedge can be attributed to the serpentinization as well as the mixed fabrics of the type-A, type-B, and type-E.