RHEOLOGICAL EVOLUTION OF THE LARAMIDE SUBDUCTION CHANNEL IN SOUTHERN CALIFORNIA

The ocean-affiliated Pelona schist of Late Cretaceous age in southern California was formed during the Laramide flat-slab subduction event. It preserves deformation structures formed during subduction and two stages of exhumation: 1) return flow in the subduction channel, and 2) late-stage extension caused by the Vincent fault.

Schistosity defined by trails of mineral inclusions in albite porphyroblasts may have been formed during subduction, associated with high-P low-T metamorphism. Mica, chlorite and other sheet silicate minerals wrapped around albite and epidote porphyroblasts are well aligned and were likely formed during return flow within the subduction channel, accompanied by decompression and greenschist facies metamorphism with a peak condition of 10.5 ± 1 kbar and 515 to 550 °C. Strong differentiation of sheet silicates into P-domains, and equant quartz grains in the pressure shadows of albite and epidote porphyroblasts, indicate that pressure solution was the dominant deformation mechanism. Pelona schist right underneath the Vincent fault has a mylonitic microstructure formed at 4.7 ± 0.6 kbar and 372 to 430 °C, with crystal plastic deformation of quartz, which overprinted the pre-existing structures. This is related to normal-sense shear along the Vincent fault at relatively shallow depths. Quartz in the mylonite is recrystallized by subgrain rotation, suggesting that climb-assisted dislocation creep was the dominant deformation mechanism.

Published geochronological data yield that subduction started after 68 Ma and the first stage exhumation of the Pelona schist occurred between 60.3 Ma and 57.8 ± 0.1 Ma. With an assumed width of 10 km and a dip of 30° of the subduction zone, the maximum first-stage exhumation rate is 17.5 mm/yr, corresponding to a strain rate of 1.25 × 10^-13 s^-1. The estimated differential stress in the upper part of the subduction channel based on quartz grain size is 10 to 15 MPa, which is consistent with the maximum differential stress in a buoyancy-driven channel flow model.