Evidence for slow slip in Mecca Hills, CA, from microstructural and (U-Th)/He analysis of heterogeneous hematite coatings on shallow fault surfaces

Alexandra A. DiMonte, Alexis K. Ault, Kelly K. Bradbury, & Greg Hirth

Published August 14, 2020, SCEC Contribution #10564, 2020 SCEC Annual Meeting Poster #137 (PDF)

Geodetic and seismologic studies reveal slow slip events in the shallow portions of faults, but the influence of rheology on these events is poorly constrained. Outcrop to nanoscale structures and mineral textures observed in exhumed faults record signatures of varying slip rates and can be used to examine rheological properties. Hematite, a common secondary mineral in shallow faults, exhibits textures potentially diagnostic of slip rate and is amenable to (U-Th)/He thermochronometry. To evaluate the role of hematite in promoting shallow slow slip, we report initial field and scanning electron microscopy (SEM) observations of microstructures and textures from a network of minor fault surfaces in the exhumed Painted Canyon fault (PCF) in Mecca Hills, CA. Ten samples collected from five locations in the PCF have ubiquitous hematite slip surfaces with a range of orientations, textures, and crystal habits. SEM reveals multiple hematite morphologies, such as previously recognized high-aspect ratio plates, as well as serrated, stubby, cuspate or petal-like, and euhedral hexagonal plates. Hematite occurs (1) interlayered with calcite and phyllosilicates, (2) in microscale injection veins, (3) in layers of hematite-cemented breccia with a decreasing clast abundance and size toward the slip surface, (4) as tails on clasts, and (5) in reworked hematite clasts. Injection veins imply precipitation by pore-fluid overpressure. Hematite displays S-C fabrics, intra- and inter-plate folds, and shape preferred orientation that suggest ongoing deformation. Initial hematite (U-Th)/He dates document the timing of Pleistocene mineralization (Moser et al., 2017; EPSL). Reproducible mean dates for individual fault surfaces, different dates on separate slip surfaces, and the absence of recrystallization textures indicative of coseismic frictional heat support post-mineralization slow slip. Collectively these observations indicate multiple episodes of hematite precipitation, fault reactivation, and slow slip. On-going integrated hematite (U-Th)/He thermochronometry and textural analysis will evaluate this hypothesis with an expanded dataset from a cross-fault outside of the main PCF zone and high-density slip surfaces from individual outcrops. Future field work will document spatial relationships of hematite slip surfaces across specific basement lithologies and clay fault gouge, and the associated extent of alteration and fault reactivation.

Key Words
hematite, (U-Th)/He thermochronology, Mecca Hills, slow slip

Citation
DiMonte, A. A., Ault, A. K., Bradbury, K. K., & Hirth, G. (2020, 08). Evidence for slow slip in Mecca Hills, CA, from microstructural and (U-Th)/He analysis of heterogeneous hematite coatings on shallow fault surfaces. Poster Presentation at 2020 SCEC Annual Meeting.

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