THERMAL HISTORY OF THE SAFOD PILOT AND MAIN HOLES AT PARKFIELD, CA, CONSTRAINED WITH FISSION TRACK AND (U-TH)/HE THERMOCHRONOMETRY

The San Andreas Fault Observatory at Depth (SAFOD), which is located 1.8 km west of the San Andreas Fault near Parkfield, California, presents a unique opportunity to evaluate the thermal history of the active plate boundary in central California. The main hole, which is currently being drilled, traverses the upper ~3 km of the crust as it approaches the San Andreas Fault, first vertically, then at a shallow angle. We previously used fifteen apatite fission-track (FT) and 5 (U-Th)/He analyses from the SAFOD pilot hole samples to document the thermal history of the site (Blythe et al., 2003). Pilot hole FT and (U-Th)/He sample ages decrease with depth: FT ages range from ~60 Ma at the surface to ~ 3 Ma at the base of the hole (at 2.2 km depth and temperature of 90°C). (U-Th)/He ages at the base of the hole were ~1 Ma. These data were used to evaluate the burial and exhumation history of the site. Thermal modeling of the distribution of measured fission track lengths indicated (1) ~60°C of cooling occurred between 80 and 30 Ma, (2) reheating of ~50°C from ~30 to 8-4 Ma, probably as the result of basin subsidence and burial by ~2 km of sediments, and (3) finally, cooling of ~30°C has occurred as the result of ongoing Coast Range exhumation of ~1 km. Here we present new apatite and zircon FT and He analyses from samples collected at greater depths from the main hole, drilled adjacent to the pilot hole, but angling toward the San Andreas Fault at ~1.8 km depth. These new data allow us to better refine our thermal models and to better evaluate the evolution of the geothermal gradient over the last 80 Ma.