Late Quaternary Erosion Rates in the San Gorgonio Pass: Insights From Thermoluminescence Thermochronology

Ayush Joshi, Nathan D. Brown, Seulgi Moon, & Marina O. Argueta

Submitted September 7, 2025, SCEC Contribution #14852, 2025 SCEC Annual Meeting Poster #TBD

The San Gorgonio Pass (SGP), situated within the southern San Andreas Fault zone, is a tectonically complex region that may control earthquake rupture propagation from the Coachella Valley segment to the San Bernardino segment, making it crucial for local seismic hazard assessment. Uncertainty remains regarding slip partitioning across the fault strands within the SGP region during the Late Quaternary. Previous investigations have utilised low-temperature thermochronometers such as apatite U–Th/He (AHe) and catchment-averaged cosmogenic ¹⁰Be concentrations to infer fault activity. Both methods have limitations: AHe averages erosion over million-year timescales, masking short-term tectonic and climatic signals, whereas millennial-scale ¹⁰Be estimates can be biased by stochastic landsliding in steep terrain or by relict catchment topography. Here, we apply thermoluminescence (TL) thermochronology—an ultra-low-temperature thermochronometer—to derive erosion rates at individual bedrock outcrops. We resolve erosion rates at timescales of 10 to 100 ka, thereby bridging the temporal gap between cosmogenic 10Be (103 years) and AHe (106 years). We report erosion rates for outcrops on either side of the Mill Creek, Mission Creek, Galena Peak, and San Bernardino strands of the San Andreas Fault in the southern San Bernardino Mountains to test whether there is differential erosion across these fault strands due to tectonic uplift, or whether erosion rates relate to fault strand proximity or other topographic metrics. We plot these erosion rates against various topographic metrics including hillslope angle, local relief, elevation, curvature, distance to fault, and precipitation from the 30-year PRISM dataset to investigate potential controls. We find that erosion rates are positively correlated with hillslope angle and topographic relief. Erosion rates also increase with negative concavity (ex. valley bottoms). Samples exhibit higher erosion rates closer to the Galena Peak, western Mill Creek, and San Bernardino strands of the San Andreas Fault, consistent with either ongoing tectonic uplift or enhanced erosion due to inherited rock damage from past activity. Near the Mill Creek catchment headwall (the “Mill Creek Jumpoff”), we observe an upstream-migrating knickpoint, consistent with fluvial adjustment to base-level fall due to tectonic reorganisation. Our results demonstrate the utility of TL-derived erosion rates in tectonically complex settings.

Key Words
San Gorgonio Pass, Thermoluminescence Thermochronology

Citation
Joshi, A., Brown, N. D., Moon, S., & Argueta, M. O. (2025, 09). Late Quaternary Erosion Rates in the San Gorgonio Pass: Insights From Thermoluminescence Thermochronology. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology