Abstract

To better understand the relationship between geomorphology and fault slip, we investigated the origins of topographic depressions previously interpreted as beheaded channels representing small offsets at Van Matre Ranch (VMR) along the San Andreas fault, Carrizo Plain, California. We excavated four fault‐parallel trenches (T1–T4) across depressions and sampled for single‐grain postinfrared infrared‐stimulated luminescence (p‐IR IRSL) age estimates of channel fill. Only T2 sediments are young enough (0.38±0.06  ka) to be associated with a nearby drainage (sourced 12  m southeast [SE]), providing a short‐term slip rate of 31.6+9/6.6  mm/yr. The age of the T2 channel fill falls within the uncertainty ranges of the penultimate through fourth event back as dated at Bidart Fan 12  km northwest (NW). Hand‐excavated exposures at nearby T1 indicate that the T2 channel sediments have experienced at least two earthquake events and that the T1 beheaded gully is a fosse between two small offset alluvial fans (10  m radius). Reconstructing the alluvial fan apex shows that offset at this location in the 1857 Mw 7.8 Fort Tejon earthquake was 4  m. Therefore, offset in the penultimate earthquake is <8  m at the VMR site because we cannot discount that T2 channel sediments experienced four earthquakes. Interestingly, buried channel ages are older at other trenches (4.26–8.12 ka), indicating distant, larger drainage basin sources SE of the study area. Our results indicate that for the Carrizo Plain, (a) there may be appreciable high‐frequency variation in paleoearthquake offset along strike and in successive earthquakes at a point; (b) beheaded topographic depressions on the downstream side of the fault have the potential to, but do not necessarily, capture drainage basins on the upstream side of the fault with continued slip; and (c) small catchments may not produce channel landforms or deposits as frequently as has been suggested.

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