Group , Poster #156, Fault and Rupture Mechanics (FARM)

Characterizing shallow slow slip with natural and experimental hematite fault surfaces

Alexandra A. DiMonte, Alexis K. Ault, Greg Hirth, & Cameron D. Meyers
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Poster Presentation

2021 SCEC Annual Meeting, Poster #156, SCEC Contribution #11520 VIEW PDF
Exhumed hematite fault surfaces record microstructural and chemical signatures, including (U-Th)/He dates and He loss patterns, associated with mineralization and prior slip at depth. Comparisons with experimental slip surfaces refine slip rates and deformation mechanisms associated with these signatures and inform ongoing deformation at depth. Hematite-coated slip surfaces occur within the shallowly exhumed Painted Canyon fault (PCF) basement damage zone, Mecca Hills, CA. Scanning electron microscopy displays anastomosing, platy, nm-scale hematite morphologies that resemble clay minerals. Hematite slip surface microstructures, including injection veins, S-C fabric, layered precipitates, and... reworked clasts of hematite, indicate failure via fluid overpressure and subsequent slip at subseismic rates. Hematite (U-Th)/He dates record two periods of mineralization and slip at depths of <1.5 km during exhumation prior to ~0.8 Ma and from ~0.8-0.4 Ma. Together, fault microstructures and thermochronometry document periods of shallow slow slip events in the past ~1 Ma within the PCF damage zone.

Deformation experiments inform He loss and microstructures associated with controlled, observable slip rates. Prior high-velocity (320 mm/s) rotary shear experiments (Calzolari et al., 2020) were conducted with specular hematite comprising μm-thick plates and a SiC upper annulus at 8.5 MPa load for 1.5 m displacement. Resulting slip surfaces have fault mirrors and gouge, and (U-Th)/He dates from mirrored material indicate >70% He loss associated with friction-generated temperature rise. New low-velocity and low-displacement experiments utilize the same apparatus, load, and starting material, but with a diabase upper annulus and 0.85 um/s to 1 cm/s slip rate for 1 cm to 1.5 m displacement. These experiments yield a ~0.3-0.4 coefficient of friction and lower volume of gouge compared to high-velocity, high-displacement experiments. Ongoing work will identify gouge morphology and He loss associated with these slow slip experiments for comparison with results from high-velocity and natural PCF surfaces. We will also explore He loss in fine-grained gouge prepared from specularite boulders, as well as microstructures and frictional properties, including the velocity-dependence of friction, in gouge prepared from coarser-grained specularite and natural Mecca Hills hematite slip surfaces to assess the impact of hematite morphology and interstitial phases on slip accommodation.

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