P-T-T ESTIMATION OF THE FOOTWALL AND HANGING WALL OF THE VINCENT FAULT, SAN GABRIEL MTNS, SOUTHERN CALIFORNIA

The Vincent Fault (VF), widely interpreted as the original subduction interface of the Laramide flat-slab subduction event, separates a Meso-Proterozoic gneiss complex and Mesozoic granitoid rocks in the hanging wall from Late Cretaceous Pelona Schist in the footwall. New pressure, temperature and chronological data from the Pelona Schist close to the Fault and hanging-wall mylonites imply later modification of the VF during exhumation.

The Pelona Schist and hanging-wall mylonites preserve greenschist facies metamorphism. The dominant deformation mechanism in the Pelona Schist is pressure solution, strongly modified by dislocation creep close to the VF; the mylonites deformed mainly by dislocation creep. Raman spectroscopy on carbonaceous material indicates peak metamorphic temperature in the Pelona Schist from the San Gabriel Mtns of 515.2 ± 22.6 °C. Phengite geobarometry based on the silicon content in white mica and TitaniQ geothermobarometry were applied to the Pelona Schist and the mylonites. The results show that the top of the Pelona Schist and lower section of the mylonite zone were equilibrated at ~4.7 kbar and 372 to 425 °C while the upper section of the mylonite zone were equilibrated at ~2.4 kbar and ~365 °C.

Zircon fission track ages of 15 samples from the Pelona Schist and the hanging wall of the VF in the East Fork of the San Gabriel River cluster around 46.7 ± 5.9 Ma, indicating that no brittle faults in the hanging wall contributed to the exhumation of the Pelona Schist. U-Pb dating of the detrital zircon in the Pelona Schist gives a minimum age of 71 Ma, close to the previously published age of 68 Ma in this area.

Combined with the published peak metamorphic pressure of 10 kbar and Ar-Ar muscovite age of 57.8 ± 0.1 Ma in the Pelona Schist, a P-T-t path of the Pelona Schist and the overlying mylonites can be established. The protolith of the Pelona Schist was subducted by 68 Ma, after which a peak P-T of 10 kbar and ~515 °C was achieved. Part of the Pelona Schist was underplated, the top of which was mylonitized together with the base of the hanging wall of the VF at a P-T condition of ~4.7 to 2.4 kbar and 425 to 365 °C. The mylonitization should have had started by 57.8 ± 0.1 Ma and ceased by 46.7 ± 5.9 Ma.

The dominant top-to-ESE shear sense preserved in the top of the Pelona Schist and the hanging-wall mylonites could be related to exhumation.