Oblique fabrics in porphyroclastic Alpine-type peridotites: a shear-sense indicator for upper mantle flow

Abstract

A detailed structural study in Alpine-type peridotites of the ultramafic Voltri Massif in NW Italy reveals the existence of oblique fabrics in porphyroclastic tectonites. The dominant foliation of these tectonites is defined by stretched pyroxenes and aligned grains of spinel, which formed as a result of high-temperature flow in a km-scale shear zone bounded by a wall-rock of granular, virtually undeformed spinel-lherzolites. The oblique fabrics in these tectonites are characterized by elongate olivine grain shapes and a marked grain boundary alignment oriented at an angle of up to 40° to the tectonite foliation. In addition, kink-type subgrain boundaries are frequently parallel to (100), and show a preferred orientation at large angles to the tectonite foliation. Olivine lattice preferred orientation patterns in tectonites with this oblique grain shape fabric show bimodal distributions of [100] and [001]. The oblique fabrics and bimodal orientation distributions are inferred to result from the formation of high-angle kink-like grain boundaries followed by deformation-induced grain boundary migration at the expense of unfavourably oriented grains. The sense of shear derived from the oblique fabrics is consistent with the sense of shear derived from the lattice preferred orientation patterns, asymmetric pyroxene porphyroclast systems and the map-scale geometry of the shear zone in which these microstructures occur. It is therefore suggested that these oblique grain shape fabrics represent a reliable kinematic indicator for high-temperature flow in upper mantle shear zones.