Tectonics From Topography: Strong Correlation Between Mountain Front Steepness and Holocene Slip Rates Along the Wasatch Normal Fault, USA.

Abstract:

The footwalls of active normal faults are often decorated with facet slopes: semi-planar, steeply dipping slopes made of bedrock, or bedrock thinly mantled by regolith, that rise up out of the fault trace. Here we test two hypotheses to determine whether normal-fault facet surfaces record information about the local fault slip rate on a 10-100 ky timescale. 1) If rock-mass strength is spatially uniform, relative variations in slip rate along strike of a normal fault can be estimated directly from facet slope angle. 2) If erosion rate is independently known, estimates of absolute slip rates can be obtained. These hypotheses are based on a simple mathematical model of footwall development that predicts that facet slope angle is set by the ratio of bedrock erosion rate to fault slip rate. We tested these hypotheses by first compiling data from the Wasatch Fault Zone (Utah, USA) where the rates of fault slip are unusually well known as a result of decades of intensive paleoseismology studies, and where millennial-scale erosion rates along the range front have been measured using cosmogenic radionuclides. We then mapped spatial variations in facet morphology along the entire length of the range front using 1 m resolution topographic data. We find a strong correlation between along-strike measurements of facet angle and Holocene slip rate. The mean facet steepness of each fault segment varies systematically from > 35 degrees in the center of the fault array to < 20 degrees at the southern end. Assuming characteristic fault dips of 50-60 degrees and erosion rates of 0.1-0.2 mm/yr, our predictions of absolute 100 ka average slip rates are consistent with estimates previously made from offset geomorphic features. These results demonstrate the feasibility of facet-slope analysis as a low-cost paleoseismology tool that can extract information about the rates of slip on range-bounding normal faults, and hence seismic hazard, directly from topography.