TEMPORAL VARIATIONS IN FAULT SLIP REVEALED BY STREAM PROFILE ANALYSIS: AN EXAMPLE FROM SALINE VALLEY, CALIFORNIA

Deciphering the temporal evolution of slip on fault systems requires reconstruction of fault displacement using markers of varying age. Preservation of such markers over the lifespan of individual faults is rare, however, and is typically limited to relatively young geomorphic features. An alternative approach to reconstructing fault slip utilizes the erosional response of channels in the footwall blocks of such fault systems. Here, we explore the conditions under which footwall topography and channel profiles may preserve a record of variations in fault slip through time. The approach is motivated and illustrated using an example from active oblique-slip faults in eastern California.

Normal-oblique slip on the range-bounding faults along the western margin of Saline Valley is linked to strike-slip displacement on the Hunter Mountain fault (average slip rate of 2 – 2.5 mm/yr since ~ 4 Ma). Channel profiles of bedrock stream crossing the normal fault, however, exhibit pronounced knickpoints in the middle of the profiles that are systematically distributed in elevation along the range and do not correspond to lithologic variations. Knickpoints define a transition between moderate-gradient channels upstream mantled with alluvium and high-gradient bedrock gorges downstream. Normalized steepness indices of upstream reaches are consistent across the range, and imply a quasi-equilibrium landscape that was perturbed by an increase in relative base-level fall. Reconstruction of former profiles implies that ~1500 m of differential rock uplift is recorded by this transient adjustment. Preliminary estimates of the response time of detachment-limited incision models suggest that transient profiles reflect an increase in fault slip at ca. 0.7 – 1.4 Ma. We are currently testing this hypothesis via erosion rates inferred from cosmogenic isotope inventories in modern sediment from above and below knickpoints, but our preliminary results appear provide a means to reconcile geodetic velocities and geologic slip rates across this fault system