The Mojave Section of the San Andreas Fault (California): 1. Shaping the Terrace Stratigraphy of Little Rock Creek Through the Competition Between Rapid Strike-Slip Faulting and Lateral Stream Erosion Over the Last 40 k.y.

To determine the post-40 ka slip-rate along the Mojave section of the San Andreas Fault (MSAF) we re-analyze the sedimentary record preserved where Little Rock (LR) Creek flows across the fault. At this location, interaction between the northeast-flowing stream and right-lateral fault has resulted in the abandonment and preservation of 11 strath terraces and one paleo-floodplain in the downstream trailing corner of the river, two of which are also preserved upstream to provide cross-fault matches. A new model of fault-induced river deflection, together with standard terrace riser restoration, yields strike-slip displacements of 1,140 +/- 160 m for the older terrace and 360 +/- 70 m for the younger one. When combined with new Be-10 dating and reinterpretation of prior measurements the displaced terraces yield right-lateral slip-rates of 27.7(+6.9/-3.5) and 26.8(+3.4/-3.0) mm/yr over the last 23 k.y. and last 40 k.y., where uncertainties are at 95% credible intervals. These new rate determinations are consistent with independent late Holocene estimates, indicating that the long-term rate of strain accumulation along the MSAF is relatively fast and does not vary significantly when averaged over timescales of 15-20 k.y. Using our new model of stream deflection, we find that the fluvial sequence was emplaced in two distinct periods, each characterized by a temporally stable but markedly different deflected river geometry. Each period coincides with a distinct stage of erosive power along LR Creek determined from independent paleoclimate proxies. Importantly, application of the new river-deflection model allows strike-slip displacements to be determined in the absence of upstream piercing points.