Luminescence dating of alluvium deposits to investigate Holocene slip potential along the Mission Creek fault strand in Southern California, USA

Abstract The Mission Creek fault strand (MCF) of the San Andreas Fault is proposed to be the primary strand accommodating slip between the Pacific and North American plates in Southern California, but its Holocene activity northwest of Indio is debated. This study presents new depositional ages for alluvial deposits near the mouth of the Mission Creek drainage to investigate potential Holocene activity of the MCF. We estimate a mean depositional age of 0.7 ± 0.2 ka for the alluvium deposited in the drainage valley, contradicting previously inferred ages of >3–18 ka. The young ages for alluvium, often indistinguishable from the age of modern alluvium, suggest that grains are transported during high-energy flash flood–like events. A comparison of luminescence and cosmogenic 10 Be ages from alluvial surfaces adjacent to the Mission Creek suggests a possible reworking event at ∼30 ka. Young alluvium ages, together with evidence for frequent flash floods, suggest that this site is rapidly resurfacing and therefore unlikely to preserve surficial rupture signatures older than a few hundred years. Therefore, the lack of observable offsets in deposits overlying the MCF does not imply Holocene inactivity.