Large earthquakes generate frictional heat, and the magnitude of heating is related to the slip magnitude, the applied effective normal stress, and the frictional strength of the fault. We looked for evidence of this heating in apatite fission-track age and track-length distributions of samples from adjacent to and within the San Gabriel fault zone in southern California. The fault is thought to be an abandoned major trace of the San Andreas fault system active from 13 to 4 Ma and has since been exhumed from depths of 2–5 km. At our sample locality, as much as 40 km of total slip is thought to have accumulated along a localized ultracataclasite layer just 1–8 cm thick. We see no evidence of a localized thermal anomaly in either fission-track ages or track lengths—even in samples within just 2 cm of the ultracataclasite. Because of the absence of any measurable impact on fission tracks, we have been able to use forward modeling of heat generation, heat transport, and fission-track annealing to constrain the frictional properties of the fault. We find that either there has never been an earthquake with >4 m of slip at this locality, or the average apparent coefficient of friction must have been <0.4.

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