NEW SLIP-RATE ESTIMATES ON THE SOUTHERN CALICO FAULT WITHIN THE EASTERN CALIFORNIA SHEAR ZONE

The Eastern California shear zone accommodates ~25% of slip along the transform plate boundary, however the distribution of strain across its numerous faults is poorly constrained. Geodetically derived rates for the ECSZ range from 12-18 mm/yr, in some cases, more than doubling the summed geologic slip-rates for the region. This difference may be attributed to various factors such as the contribution of off-fault deformation or temporal variations in rate, however more fault studies are needed to address these uncertainties. Within the ECSZ, the Calico fault has the highest estimated geologic slip-rate. The several published rates vary from ~1.8 to 3.6 mm/yr which were all measured along a small 15 km stretch on the north-central portion of the fault (Oskin et al., 2007; Selander, 2015; Xie et al., 2018). The apparent disagreement between these rates and lack of spatial distribution highlights the uncertainty to which the behavior and seismic hazard of the Calico fault system is understood as well as the ECSZ itself. We provide new preliminary slip-rate estimates and geomorphic mapping of the southern Calico fault within the Marine Corps Air Ground Combat Center. We measure offsets ranging from ~5 to 65 m across six alluvial surfaces, mainly from displaced risers and channels cut into alluvial fans. The alluvial deposits range in age from 2 to 70 ka as measured from Optically Stimulated Luminescence (n=6) and ¹⁰Be surface exposure dating (n=11). Offset alluvial fans derived from catchments of deformed Quaternary sediments are Holocene in age while fans from bedrock catchments are Late Pleistocene reflecting the influence of provenance in alluvial fan production and longevity. The resulting six independent age and offset estimations provide a preliminary slip-rate range of 1-2 mm/yr. Our findings agree with the lower previously estimated rates and suggest that the Calico fault may maintain a relatively constant slip-rate that is spatially and temporally invariant. Furthermore, these findings maintain a lower geologically summed regional slip-rate for the ECSZ indicating that the difference from geodetically derived rates may be primarily due to the contribution of off-fault deformation, slip on secondary structures, or temporal differences of other faults.