Graduation Semester and Year
Summer 2026
Language
English
Document Type
Dissertation
Degree Name
Doctor of Philosophy in Earth and Environmental Science
Department
Earth and Environmental Sciences
First Advisor
Nathan D. Brown
Second Advisor
Seul Gi Moon
Third Advisor
Majie Fan
Fourth Advisor
Ricardo Sanchez Murillo
Abstract
In southern California, near the Salton Trough, the San Andreas fault (SAF) bifurcates into the Banning strand to the south and the Mission Creek strand to the north. As these strands extend northwest, they are further subdivided into multiple subparallel fault strands, including the Mill Creek and Galena Peak strands to the north and the Garnet Hill strand and San Gorgonio Pass fault zone to the south, giving rise to a multistranded southern San Andreas fault system. The southern SAF continues to accommodate 20–30 mm/yr of Holocene slip; however, given its multistranded structure, researchers have long debated how slip is partitioned across this complex tectonic architecture, which strands accommodate the most slip, and which might pose the greatest seismic hazard. In this study, we constrain the late Pleistocene–Holocene activity along the Mission Creek strand near the southeastern San Bernardino Mountains, where Mission Creek exits its bedrock drainage, and along the Mill Creek strand near the southern San Bernardino Mountains in the San Gorgonio Pass region. We used single-grain K-feldspar luminescence dating to constrain the depositional age of alluvial deposits overlying the Mission Creek fault trace at this site, to test whether the fault ruptured these deposits during the Holocene. Our results showed that these deposits are very young, with a mean depositional age of 0.7 ± 0.2 ka; accounting for the residual age constrained from bleaching experiments reduces this further, to approximately 0.4 ka, much younger than the previously presumed ages of >3–18 ka at this site. Additionally, we observe that this region is geomorphically very active, with evidence of reworking across multiple abandoned alluvial fan surfaces. This means that the site is undergoing rapid resurfacing and is unlikely to preserve the surficial rupture signatures of a Holocene earthquake. In the bedrock-dominated southern San Bernardino Mountains, we used thermoluminescence (TL) thermochronology to constrain the late Pleistocene–Holocene activity along the Mill Creek and associated fault strands in the San Gorgonio Pass region. The 10–100 ka temporal resolution of TL, coupled with its shallow depth scale, allows us to probe in-situ positional variability in erosion rates that is not biased by surface processes operating at much larger spatial scales. Our results are consistent with previous findings of topographic disequilibrium, in which erosion is concentrated in the valleys while high-elevation reaches of the landscape remain disconnected from the ongoing erosion. Elevated erosion rates are observed in the 10–20 ka period near the western Mill Creek strand, whereas the eastern Mill Creek strand exhibits a marked reduction in erosion within the past 100 ka. Six TL samples collected near the catchment headwall showed constant erosion rates despite increasing relief and were better understood in the framework of a dynamic erosional response of the landscape to a drop in base level. The cooling histories of these samples record the passage of an upstream-migrating knickpoint: a distinctive pattern of high erosion rates followed by a marked drop in erosion, which correlates with downstream distance from the Mill Creek Jumpoff, the assumed present-day position of the knickpoint. We estimate a lateral migration velocity of 0.07 ± 0.02 m/yr for this knickpoint, which places the base-level drop at the Mill Creek–Galena Peak strand junction at 100–189 ka. We speculate that the base level at this junction could have dropped through either differential uplift or drainage reorganisation driven by slip along the Mill Creek–Galena Peak strand configuration within the 100–189 ka period. In the absence of direct geologic slip constraints along the Mill Creek and Mission Creek strands, we cannot definitively conclude that these strands were active during the late Quaternary. However, when read alongside existing geologic and geophysical constraints, our new ages and erosion rate estimates show that absence of direct constraints does not rule out activity either. Our study allows for a more informed interpretation of recent activity along these fault strands and identifies clear targets for future work.
Keywords
San Andreas Fault, Seismic hazard, Luminescence dating, Single grain dating, Sediment reworking, San Gorgonio Pass, San Bernardino Mountains, Thermoluminescence Thermochronology, Knickpoint migration, Tectonic reorganisation
Disciplines
Geology | Geomorphology | Tectonics and Structure
License
This work is licensed under a Creative Commons Attribution 4.0 International License.
Recommended Citation
Joshi, Ayush, "LATE PLEISTOCENE – HOLOCENE FAULT ACTIVITY AND LANDSCAPE EVOLUTION IN SOUTHERN CALIFORNIA: INSIGHTS FROM LUMINESCENCE BURIAL DATING AND THERMOCHRONOLOGY" (2026). Earth & Environmental Sciences Dissertations. 2.
https://mavmatrix.uta.edu/ees_dissertations2/2