Abstract

Quantifying off-fault deformation (OFD) rates on geomorphic time scales (102–105 yr) along strike-slip faults is critical for resolving discrepancies between geologic and geodetic slip-rate estimates, improving knowledge of seismic hazard, and understanding the influence of tectonic motion on landscapes. Quantifying OFD over these time scales is challenging without displacement markers such as offset terraces or geologic contacts. We present a landscape evolution model coupled with distributed lateral tectonic shear to show how drainage basins sheared by lateral tectonic motion can reveal OFD rates. The model shows that OFD rate can control the orientation of drainage basin topography: the faster the OFD rate, the greater the deflection of drainage basins toward a fault-parallel orientation. We apply the model to the southern San Andreas fault near the Mecca Hills (southern California, USA), where drainage basins change in orientation with proximity to the fault. Comparison of observed and modeled topography suggests that the OFD rate in the Mecca Hills follows an exponential-like spatial pattern with a maximum rate nearest the fault of 3.5 ± 1.5 mm/yr, which decays to ∼0 at ∼600 m distance from the fault. This rate is applicable since the initiation of differential rock uplift in the Mecca Hills ca. 760 ka. Our results suggest that OFD in this 800 m study area may be as high as 10% of total plate motion. This example demonstrates that curved drainage basins may be used to estimate OFD rates along strike-slip faults.

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