Shallow, transient slow slip in the southern San Andreas fault system: insights from natural and experimental hematite and hematite-clay slip surfaces

Alexis K. Ault, Alexandra A. DiMonte, Jordan L. Jensen, Greg Hirth, Cameron D. Meyers, & Kelly K. Bradbury

Submitted September 11, 2022, SCEC Contribution #12289, 2022 SCEC Annual Meeting Poster #146

Documenting modes of past deformation in exhumed fault rocks and comparison of natural and experimental faults are required to constrain the rheology and distribution of slip that occurs in the shallow crust during the modern earthquake cycle. Our recent, SCEC-supported work has focused on networks of small, basement-hosted hematite slip surfaces in exhumed strike-slip faults that connect to the southern San Andreas fault (SSAF) in a positive flower structure in Mecca Hills, CA. Scanning electron microscopy shows these mm-thick surfaces have basal hematite injection veins and layered veinlets, with nanoscale, high-aspect ratio plates that mirror the morphology of phyllosilicates. New field observations reveal the spatial association of hematite and clay at all scales along the SSAF system in Mecca Hills. Basement damage includes hematite-clay gouge and discrete slip surfaces. Fe-oxide-rich clay gouge in sedimentary rock defines the mapped trace of the Coachella segment of the main SSAF.

Single-velocity and velocity-step experiments on coarse-grained specular hematite use an Instron rotary-shear apparatus with rates of 0.85 m/s-320 mm/s for 1-45 cm displacement at 5 and 8.5 MPa normal stress, with diabase, granite, and SiC upper annuli. The coefficient of friction is ~0.25-0.45, and hematite exhibits primarily velocity-strengthening and velocity-neutral behavior at rates <1 mm/s, like some phyllosilicates. Experiments produce gouge with a >7200% increase in surface area. Comparison of hematite (U-Th)/He dates from undeformed specularite and gouge shows >10% He loss during comminution without bulk fault surface temperature rise. However, natural Mecca Hills hematite surfaces did not experience post-formation comminution and instead likely deformed by interplate sliding. Hematite He dates (n=87 new, 88 previously reported individual analyses) from 33 natural hematite surfaces record discrete mineralization events between ~2.2-0.4 Ma at <1.5 km depth. Reproducible intrasample hematite He dates require reactivation of these surfaces occurred at slip rates slow enough to not cause thermally-activated He loss. We suggest hematite faults initially form by fluid overpressure and fluid-rock interaction during transient slow slip. Once formed, anisotropic hematite continues to deform by transient slow slip. Ongoing work investigates the textures and frictional behavior of hematite-clay faults and if and how hematite localizes deformation in these fault rocks.

Key Words
hematite, clay, slow slip, southern San Andreas fault, Mecca Hills, deformation experiments

Citation
Ault, A. K., DiMonte, A. A., Jensen, J. L., Hirth, G., Meyers, C. D., & Bradbury, K. K. (2022, 09). Shallow, transient slow slip in the southern San Andreas fault system: insights from natural and experimental hematite and hematite-clay slip surfaces. Poster Presentation at 2022 SCEC Annual Meeting.

Related Projects & Working Groups
Fault and Rupture Mechanics (FARM)