TRANSIENT LANDSCAPES IN STRIKE-SLIP SETTINGS

In this study, we use a landscape evolution model to investigate the longer-term, catchment-wide landscape response to horizontal fault motion. Our results show that strike-slip faulting induces a persistent state of landscape disequilibrium in the modeled landscapes brought about by river lengthening along the fault alternating with abrupt shortening due to stream capture. Trunk channels that drain across the fault record this cycle of transience in the form of knickpoints and convexities along the channel profile. Although all of the trunk channels modeled show some evidence of horizontal fault motion, the magnitude and character of perturbations to channel form appear to be slip-rate dependent. The models also predict that, in some cases, ridges oriented perpendicular to the fault migrate laterally in conjunction with fault motion. We find that ridge migration happens when slip rate is slow enough and/or soil creep and river incision are efficient enough that the landscape can respond to the disequilibrium brought about by strike-slip motion. Regional rock uplift relative to baselevel also plays a role, as the generation of topographic relief is required for ridge migration. In models with faster horizontal slip rates, stronger rocks or less efficient hillslope transport, ridge mobility is limited or arrested despite the continuance of river lengthening and capture. In these cases, prominent steep, fault-facing facets form along well developed fault valleys. Comparison of landscapes adjacent to fast-slipping (>30 mm/yr) and slower-slipping (≤ 1 mm/yr or less) strike-slip faults in California, USA, reveals features that are consistent with model predictions. Our results highlight a potential suite of recognizable geomorphic signatures that can be used as indicators of horizontal crustal motion and geomorphic processes in strike-slip settings even after cycles of river capture have diminished or erased apparent offset along the fault.