Extractable organic material in fault zones as a tool to investigate frictional stress

The detection of a frictional heating signature is an attractive, but elusive, way to measure frictional stress on exhumed faults with no evidence of melting. We present a new paleothermometer for fault zones that utilizes the thermal maturity of extractable organic material to determine the maximum frictional heating experienced by the fault. Because there are no retrograde reactions in these organic systems, maximum heating is preserved. We investigate the Punchbowl Fault, an ancient strand of the San Andreas Fault in southern California. According to the methylphenanthrene index for thermal maturity, there is no differential heating between the fault and off-fault samples, indicating that the fault did not get hotter than the surrounding rock during slip. Additionally, the thermal maturity of the rocks falls within the range of heating expected from the bounds on burial depth and time, and adds some constraint to the burial depth range. Simple heating models indicate that the fault must have had either 1) an extremely low shear stress during slip, 2) an active slipping zone thicker than 1 mm, or 3) earthquakes smaller than ~ M6, to lack a differential heating signature after 44 km slip.