Cryptic evolution of fault rocks along the shallow southern San Andreas fault system? Inherited material, shallow burial, and neoformed phases

Alexandra A. DiMonte, Lydia R. Bailey, Alexis K. Ault, Dennis L. Newell, Audrey M. Warren, Stephen E. Cox, Sidney R. Hemming, & Greg Hirth

Submitted August 30, 2026, SCEC Contribution #15355, 2026 SCEC Annual Meeting Poster #TBD

Shallow slip on the southern San Andreas fault (SSAF) system is accommodated in clay-rich gouge, but how clay was incorporated into different faults varies with lithology. It may be inherited from the material the fault cuts or neoformed. We evaluate these scenarios in the Mecca Hills where the SSAF cuts Pliocene to Holocene sedimentary rock and compare with the adjacent Painted Canyon and Hidden Springs faults that cut sedimentary and crystalline rocks, including at greater exhumed depths. We pair field observations with mineralogical, microstructural, geochemical, and geochronologic analyses to resolve the provenance, burial, evolution, and alteration of SSAF gouge.
Bulk X-ray diffraction (XRD) analyses indicate fault-related samples are more clay-rich than comparable off-fault sedimentary and crystalline samples, and XRD of clay-size fractions (<2 µm) indicates neoformed clay is low temperature, non-K-bearing smectite. Fault-related clay-size fractions have more negative δ2H values (-100 to -125‰ VSMOW) than off-fault samples. Forward modelling of hydrogen isotopic data, which reflect mixing of phases formed at various temperatures, suggest that low-temperature clays formed in meteoric waters.
K-Ar dates from clay-size fractions from the SSAF gouge and adjacent sedimentary rocks overlap and are too old (20-219 Ma) for in situ illite growth during strike-slip faulting. Instead, these dates reflect detrital sources: locally-derived micaceous Salton Trough sediment (20-41 Ma), or local sediment mixed with Colorado River detritus (71-219 Ma). Detrital apatite (U-Th)/He dates (1-94 Ma) are consistent with this provenance and suggest shallow exhumation (<1.5 km). In contrast, K-Ar dates from the Hidden Springs fault reflect illite formation during pre-San Andreas regional detachment-related exhumation (45-20 Ma), and Painted Canyon dates (6-10 Ma) record fault-related illite growth at depths >2.5 km.
Basement-hosted, more deeply exhumed faults generated gouge by comminution and alteration of their host rock. Colorado River sediment mixed with locally-derived sediment, likely from the Orocopia Mountains and emergent Mecca Hills, was deposited in the Salton Trough, incorporated into and transported along the SSAF system, and never buried >1.5 km. Shallow fault slip was pervasive within fine-grained, clay-rich sediment, and on-going interseismic fluid-fault rock alteration formed the low-temperature smectite that governs present-day mechanical behavior.

Key Words
San Andreas fault, geochemistry, thermochronometry, geology, fault rock, fault gouge, detrital sediment

Citation
DiMonte, A. A., Bailey, L. R., Ault, A. K., Newell, D. L., Warren, A. M., Cox, S. E., Hemming, S. R., & Hirth, G. (2026, 08). Cryptic evolution of fault rocks along the shallow southern San Andreas fault system? Inherited material, shallow burial, and neoformed phases. Poster Presentation at 2026 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology