SCEC Award Number 22097 View PDF
Proposal Category Collaborative Proposal (Data Gathering and Products)
Proposal Title Using luminescence dating to constrain the most recent slip along the Mission Creek strand of the San Andreas fault near Mission Creek
Investigator(s)
Name Organization
Nathan Brown University of Texas, Arlington Seulgi Moon University of California, Los Angeles
Other Participants Ayush Joshi (PhD student of Brown)
Marina Argueta (PhD student of Moon)
SCEC Priorities 1a, 5b, 5c SCEC Groups Geology, EFP, SAFS
Report Due Date 03/15/2023 Date Report Submitted 03/15/2023
Project Abstract
 We collected eight sediment samples and one bedrock outcrop sample from the
Mission Creek field site for luminescence dating analysis. Our goal is to constrain the timing of most recent slip at this site by dating alluvium associated with a deflected stream channel as well as sediment that appears to be unfaulted along the expected trace of the fault strand. The 50-m offset drainage channel hosts at least two alluvial units. The younger
alluvium is dated at 0.9±0.2 ka and occupies the channel upstream of, and within, the fault trace.
The older alluvium was sampled upstream of the fault trace and yields an age of 2.0±0.5 ka. We
could not identify datable sediments downstream of the fault trace, so we cannot conclude whether
the older alluvium predates offset. If it does, this would correspond to a mean slip rate of 25 mm/yr,
consistent with independent constraints from previous studies.
All dated alluvium adjacent to Mission Creek was younger than 1.2 ka, with the tributary alluvial
fan that covers the expected trace of the Mission Creek strand (Yule et al., 2021) giving an age of
0.7±0.1 ka. The modern sand bar and terrace deposits of the active channel are measured to have
similar apparent ages of 0.7±0.1 and 0.5±0.1 ka, respectively, indicating that the tributary alluvial
fan is indistinguishable from active. Therefore, the absence of apparent offset at this site may be
simply due to active sedimentation across the fault trace and not necessarily due to fault inactivity
during the Holocene.
Intellectual Merit This study provides eight new luminescence depositional ages and one bedrock low-temperature thermochronology age that inform most recent activity at the Mission Creek site of the Mission Creek strand of the SSAF. We demonstrate that all sampled sediment units adjacent to the modern Mission creek channel are younger than 2 ka. This finding negates previous assertions that unbroken alluvium at this site implies strand inactivity since 40 ka based on assumed, not measured, depositional ages. This result directly supports SCEC research priority 1a.
Broader Impacts This study constitutes one of three Ph.D. chapters for my graduate student at UTA, Ayush Joshi. He presented our findings at the SCEC Annual Meeting in 2022 as well as a luminescence dating conference for the Americas this past fall. Project funding enabled his first field visit to the Mission Creek site as well as to the conferences. And with my salary support from this project, I have trained him on luminescence dating techniques from sample preparation through data reduction and age analysis.
Exemplary Figure Figure 1: Luminescence depositional ages are shown for all analyzed samples. Panels (a)-(h) show
site photographs; (i) shows the inset map for the offset channel; (j) shows the regional overview map,
including a star indicating surface Fm1b of Owen et al. (2014); and (h) shows the samples taken
adjacent to Mission Creek. Sample UTA0088 is a perched alluvial package and sample UTA0090 is
a granitic gneiss sampled from a bedrock outcrop for luminescence thermochronology.
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