Learning from complexity: Paired U-series and (U-Th)/He analyses of hematite fault damage from the southern San Andreas fault

Jordan L. Jensen, Noah M. McLean, & Alexis K. Ault

Submitted September 7, 2025, SCEC Contribution #14572, 2025 SCEC Annual Meeting Poster #TBD

Hematite-coated slip surfaces in basement-hosted off-fault damage zones in Mecca Hills, CA, formed and deformed by slow slip in the southern San Andreas fault system (SSAF). Prior hematite (U-Th)/He geochronology revealed these slip episodes occurred between 800-400 ka. However, hematite that formed <1 Ma may be impacted by U-series disequilibrium, which violates the assumption of secular equilibrium made by the standard (U-Th)/He equation. Here, we advance prior work and account for variable U-series disequilibrium in young hematite by integrating U-series and (U-Th)/He geochronologic methods. We test this approach by targeting hematite slip surfaces from the same outcrops as DiMonte et al. (2022) and Moser et al. (2017) in Little Painted Canyon. New analyses (n=28) universally display [234U/238U] and [230Th/238U] activity ratios that are not consistent with secular equilibrium, indicating formation from fluids with disequilibrium amounts of long-lived U-series nuclides 234U and 230Th, as well as post-formation open-system behavior. Modern [234U/238U] activity ratios below unity imply fractional loss of 234U over its parent 238U in our polycrystalline aliquots that comprise hematite crystals with dimensions comparable to that of the α-recoil distance of 234U. Our numerical models, which take measured U-series and (U-Th)/He data as inputs, suggest hematite precipitated during slip events from groundwaters with U chemistry consistent with modern groundwaters in this region. We find that disequilibrium-corrected (U-Th)/He dates do not differ substantially from conventional (U-Th)/He dates for most samples because the effects of initial disequilibrium are mostly offset by progressive, α-recoil-driven loss of 234U. We infer hematite mineralization occurred largely between ~700-400 ka at depths shallower than ~1.5 km, consistent with prior studies. Some samples suggest earlier episodes of mineralization older than 700 ka, although these are associated with higher uncertainties. Evidence of open-system behavior in our samples has important implications for hematite (U-Th)/He dating across all ages, particularly for ultra-fine-grained varieties that are common in fault zones and most susceptible to α-recoil loss of intermediate daughter nuclides. Collectively, our results capture the geochemical evolution and associated exchange of radionuclides between pore fluids and near-surface wall rocks during the development of off-fault damage.

Key Words
paleoseismology, U-series dating, off-fault damage

Citation
Jensen, J. L., McLean, N. M., & Ault, A. K. (2025, 09). Learning from complexity: Paired U-series and (U-Th)/He analyses of hematite fault damage from the southern San Andreas fault. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology