LUMINESCENCE AGES OF OFFSET AND UNFAULTED SEDIMENTS ALONG THE MISSION CREEK STRAND OF THE SOUTHERN SAN ANDREAS FAULT NEAR THE STONE HOUSE

In the region of the central Transverse Ranges, the San Andreas fault system is distributed into multiple fault strands. Geochronology datasets constraining slip histories along the individual fault strands can help to gauge the seismic hazard posed by these strands. The Mission Creek strand of the San Andreas fault has previously been thought to be inactive during the Holocene and Late Pleistocene. However, recent studies (Fosdick and Blisniuk, 2018; Blisniuk et al., 2021) argue that the Mission Creek strand could serve as the primary plate boundary fault at this latitude, capable of producing large earthquakes in the future. In this study, we examine whether there is evidence for Holocene or Late Pleistocene offset along the Mission Creek strand near the Mission Creek stone house site.

We use single-grain post-IR IRSL dating of K-feldspars to estimate the depositional ages of alluvial terraces that have been previously identified as unfaulted (Yule et al., 2021) as well as several nearby alluvium units suspected to be Quaternary in age (Waco, 2021) north of the Mission Creek stone house. We also date alluvium associated with a tectonically offset channel (Blisniuk et al., 2021). All of these sites fall within a ~1-km length of the Mission Creek strand of the San Andreas fault. The apparently unfaulted alluvial deposits overlie the fault zone, so their depositional ages should provide a minimum age for fault inactivity of the Mission Creek strand at these locations. Based on the regional sediment accumulation rates, soil development, and ¹⁰Be surface exposure ages of similar surfaces, Yule et al. (2021) have argued that these deposits constrain the most recent event to be older than >3 - 18 ka. Additionally, we sampled deposits associated with two fluvial channels offset by ~ 50 metres (Fosdick and Blisniuk, 2018) and ~ 60 metres respectively along the Mission Creek strand. These deposits should therefore pre-date the observed offset (Fosdick and Blisniuk, 2018). We report these depositional ages with implications for seismic hazard in the region.