255 Broad Applications of Thermochronology to Understanding Geologic Rates and Processes Through the Sedimentary Record

This session showcases the many ways mid-to-low temperature thermochronometers can be used in isolation or with additional chronometers to understand geologic processes preserved in the sedimentary record such as sediment provenance, source exhumation, burial and/or erosion history.

Wednesday, 22 October 2025
8:00 AM - 12:00 PM
HBGCC, 304C

Provenance and deformation of southern San Andreas fault gouge: insights from illite K-Ar thermochronometry and stable isotope (δ2H) geochemistry

Paper number: 255-10
Program: Technical Sessions
Time: 10:45 AM – 11:00 AM

Authors

Abstract

The southern San Andreas fault (SSAF) system exposed in Mecca Hills, CA, is a natural laboratory to investigate the development of shallow on- and off-fault deformation at depth in space and time. Rocks actively deforming in the fault in the past are exposed at the surface today. We characterize these processes using field observations paired with detailed microstructural, mineralogical, H stable isotope, and geochronologic analyses. The main SSAF is exposed as a voluminous red fault gouge, and a series of subsidiary faults in a flower structure with the SSAF exhumed and deformed Plio-Pleistocene sediments, underlying crystalline basement, and Orocopia Schist. We targeted the main SSAF gouge and adjacent oblique thrust faults cutting Pleistocene sediments, Hidden Springs fault gouges dissecting sediments or Orocopia Schist, crystalline basement and sediment-hosted gouges of the Painted Canyon fault, and adjacent sedimentary and crystalline rock for comparison.

X-ray diffraction (XRD) analyses indicate all fault gouge samples contain significant (~20-60%) fine-grained detrital (2M1 illite, chlorite) and neoformed clays (1Md illite, kaolinite, smectite, palygorskite), as well as comminuted phases (silicates, carbonates) and evaporites; the specific mineralogy and textures vary with the host rock lithology. Preliminary illite K-Ar dates inform (1) the provenance of sediment and shallow sediment-hosted faults (~300-20 Ma source terrane exhumation); (2) detachment-related exhumation of crystalline basement and Orocopia Schist (~30-20 Ma), building on prior observations from (U-Th)/He thermochronometry from the Mecca Hills and 40Ar/39Ar analyses from the Orocopia Mountains; and (3) authigenic clay formation related to SSAF processes (<10 Ma). δ2H values (VSMOW) of detrital biotite (-100 to -90 ‰) from fault rocks and sediments overlap values of biotite (-110 to -100 ‰) and muscovite (-80 to -60 ‰) from crystalline rocks, suggesting a primary origin. Clay size fractions from fault rocks and sediments have δ2H values of -120 to -100 ‰, which reflect mixing of detrital (~5-30%) and exhumed (~15-65%) or surficial (~15-60%) authigenic clays. These observations suggest detrital micas did not experience significant hydrothermal alteration and recrystallization during burial, exhumation, or shallow fault-related processes. Authigenic clays in fault rocks formed and deformed during SSAF slip in the presence of meteoric water similar to modern Salton Trough groundwater and precipitation.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-414207

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