Constraints on Frictional Heating of Faults From Fission Track Thermochronology

Frictional heat generated during fault slip can be a useful way to derive information about the mechanics of fault rupture. We present a methodology for constraining frictional heat produced by large faults in nature that uses a combination of fission track thermochronology and simple numerical models. Our modeling indicates that temperature spikes from individual earthquakes will last only a few hours and affect an area localized to within a few centimeters of the slip surface. Previous efforts using thermochronology or surface heat flow to investigate frictional heating were not sensitive to these scales, but fission track annealing can record such events. We sample along an exhumed portion of the San Gabriel fault, which is believed to be an abandoned trace of the San Andreas fault system. At our sample locality, about 40 km of total slip accumulated along a localized ultracataclasite layer just 1 - 8 cm thick. We analyze samples from a transect across the fault with dimensions as narrow as 3 cm in the direction perpendicular to the fault. Along our transect, we observe no evidence for a localized reduction in fission track ages or track lengths even in samples as close as 3 cm to the fault surface. We use simple numerical models of heat flow and fission track annealing to calculate the maximum amount of heat generation that is consistent with these observations. Based on reasonable values of the paleogeothermal gradient and the timing of fault slip, we find that either: 1) the fault never experienced even a single earthquake with more than 4 m of slip at the sample locality, or 2) the fault is weaker than predicted by Byerlee's Law with hydrostatic pore pressure. >http://seismo.berkeley.edu/~burgmann/RESEARCH/HEAT</a>