Stress and temperature variations in the hangingwall of the Laramide subduction megathrust, San Gabriel Mtns, California

The Vincent Thrust in southern California places a Meso-Proterozoic gneiss complex and Mesozoic granitoid intrusive rocks above the Late Cretaceous Pelona Schist, and is widely interpreted as the subduction zone megathrust during the Laramide flat-slab subduction event. Preliminary data from the 500 - 1000m thick mylonite zone in the hangingwall of the thrust suggests that it shows a complex spatial and temporal variation in temperature and deviatoric stress. The typical mineral assemblage of the mylonite is quartz + feldspar + hornblende + chlorite + epidote + mica + calcite; but in the upper part of the mylonite zone there are layers several tens of meters thick of very massive chlorite-rich ultramylonite. From the base to the top of the mylonite zone, the microstructures change dramatically. Close to the Vincent Thrust quartz has been recrystallized dominantly by subgrain rotation, and has an average dynamically recrystallized grain-size close to 50 μm, while at the top of the zone quartz exhibits characteristics of bulging recrystallization, and the grainsize drops to ~ 10 μm. The quartz c-axis pole figures change from X-maxima close to the Vincent Thrust to crossed girdles, and to peripheral Z-maxima in the upper part of the mylonite zone. Differential stress was calculated using the currently accepted quartz recrystallized grainsize piezometer. Recrystallized quartz grains were distinguished from relic grains by microstructural criteria including the absence of subgrain boundaries within them. The grainsize of the mylonite in the upper plate of the Vincent Thrust generally decreases with increasing distance to the fault. Consequently, the calculated differential stress increases from 22 MPa close to the thrust to ~ 200 MPa near the top of the mylonite zone. The microstructure in the Pelona Schist at the top of the lower plate is composite, with a relict grainsize similar to the recrystallized grainsize in the lower mylonites, but with a younger, higher stress, dynamically recrystallized grainsize of about 15 μm. The quartz recrystallization mechanisms, c-axis fabrics, and grainsize evolution from the base to the top of mylonite zone indicate significant changes in temperature, differential stress, and strain rate across the relative narrow width of the mylonite zone. This may indicate temporal changes in deformation conditions: further work will aim to determine the significance of these changes, and to the resolve spatial variations at any one time.