Holocene geologic slip rate for the Mission Creek strand of the southern San Andreas fault, Indio Hills
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Slip on the southern San Andreas fault in the northwestern Coachella Valley in Southern California is partitioned among three fault strands: the Mission Creek, Garnet Hill, and Banning strands. The NW-striking Mission Creek strand extends from the Indio Hills into the San Bernardino Mountains, whereas the Banning and Garnet Hill strands strike W-NW and transfer slip into the San Gorgonio Pass region. Together, these three faults accommodate ~20 mm/yr of right-lateral motion based on geodetic slip rate estimates. Determining which strand accommodates the majority of fault slip and how slip rates on these strands have varied over Quaternary timescales is critical to seismic hazard assessment for this region. Here I present a new Holocene geologic slip rate estimate from an offset alluvial fan complex along the Mission Creek fault at the Three Palms site in the Indio Hills. The correlation of offset fans precisely located from a satellite image, confirmed by field mapping and B4 LiDAR indicates that the Three Palms fan complex is offset 50.0 ± 5.0 meters. U-series dating on pedogenic carbonate rinds collected at 25-100 cm depth within the deposit constrain the minimum depositional age to 3.49 ± 0.92 ka (95% CI), yielding a maximum slip rate of 19.2 ± 2.0 mm/yr. Cosmogenic ¹⁰Be surface exposure dating on boulders yields a scattered dataset with high apparent inheritance and a median age of 13.6 +6.9/-6.7 ka (1σ error). However, the minimum boulder age of 5.4 ± 1.0 ka (2σ error) was interpreted as the maximum depositional age for the fan. This age, along with the preferred offset of 50.0 meters, yields a minimum slip rate of 7.8 ± 0.8 mm/yr. We therefore estimate a Holocene geologic slip rate of 7-21 mm/yr, with a preferred rate of 9-14 mm/yr. This rate overlaps within error with a previously published late Pleistocene slip rate on the Mission Creek strand of 12-22 mm/yr calculated at Biskra Palms Oasis a few kilometers to the southeast.