MICROSTRUCTURAL ANALYSIS OF DEFORMATION AND METAMORPHISM ALONG THE VINCENT THRUST IN THE EASTERN SAN GABRIEL MOUNTAINS, CALIFORNIA

The Vincent thrust in the San Gabriel Mountains juxtaposes continental arc rocks and Proterozoic gneisses, now mylonitized, above the Pelona schist, thought to be a subducted Late Cretaceous-Early Tertiary oceanic sedimentary and volcanic complex. There has been debate on whether subduction was E- or W-directed, and whether the thrust itself is the original subduction interface. In this study, we examined microstructures from the upper and lower plates of the Vincent thrust to establish the relative timing and kinematics of deformation and metamorphism.

We distinguish three phases of deformation and two phases of metamorphism within the Pelona schist. An early foliation (S1) defined by elongate minerals and graphite inclusions is preserved only in plagioclase porphyroblasts that are rotated with respect to the external fabric (S2). Garnet is also present within the plagioclase porphyroblasts and contains pressure shadows parallel to S1, indicating that D1 formed during peak upper greenschist facies metamorphism (M1). D2 is defined by isoclinal folds with a strong planar foliation and is associated with retrogressive lower greenschist-facies mineral assemblages in both the Pelona schist and upper plate mylonites (M2). D2 shear-sense indicators, including shape- and crystallographic-preferred orientations in quartz, asymmetric porphyroclasts/blasts, extensional crenulation cleavages, and brittle fractures, all suggest top-SE sense of shear. D3 is localized beneath the Vincent thrust, crenulates D2, and is defined by passively concentrated platy minerals and weak shape-preferred orientation of quartz.

D1 may represent subduction and underplating to considerable depths to form S1. The low-T deformation in the mylonites and the retrogression in the schists during D2 suggest that D2 involved exhumation and cooling; the top-SE shear direction, antithetic to the probable subduction direction, suggests that D2 was extensional. This implies that the Vincent ‘thrust' was either cut or reactivated by an extensional fault, and should perhaps be called the Vincent fault, rather than the Vincent thrust. D3 represents minor later reactivation of the fault.