Record of seismic and aseismic deformation in exhumed western North America fault damage zones: insights from hematite microtextures and fault rock thermochronometry
Alexis Ault, Utah State University
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Hematite fault mirrors and striated, metallic slip surfaces in exhumed fault zones record disparate thermal and mechanical processes. Nano to microscale textural and chemical tools can document evidence for dynamic weakening and re-strengthening of fault surface hematite during an earthquake or evidence of aseismic creep. Hematite is amenable to (U-Th)/He (He) thermochronometry and accurate interpretation of hematite He data requires hematite textural characterization, grain-size (closure temperature) distribution data, and constraints on ambient thermal conditions during and after hematite formation from apatite He thermochronometry. Here, we present examples of this workflow from hematite-coated fault surfaces in two seismogenic basement-hosted fault damage zone systems. Data from the Wasatch fault, UT, and Mecca Hills fault damage zones, CA reveal evidence for seismic and aseismic deformation processes, respectively.
A network of high gloss, light reflective hematite fault mirrors in the exhumed Wasatch fault zone preserve textural and thermochronometric evidence for elevated temperatures during fault slip. Polygonal hematite crystal morphology, coupled with hematite He data patterns from these surfaces, host rock apatite He data, and thermomechanical modeling are best explained by friction-generated heat (T ~≥1200 °C), hematite recrystallization, and attendant He loss (thermal resetting) at geometric asperities <4.5 Ma. In the Mecca Hills, adjacent to the San Andreas Fault, fault damage zones cut Plio-Pleistocene sediments and crystalline basement. In the Painted Canyon fault damage zone, hematite slip surfaces comprise thin, shingled, hematite plates in anastomosing patterns akin to "scaly fabrics." Reproducible mean hematite He dates are ~0.7-0.4 Ma and are younger than ~1.2 Ma apatite He dates from adjacent undeformed crystalline host rock. Hematite textures and thermochronometric data collectively document Late Pleistocene episodes of syn-kinematic hematite mineralization via cyclic crack-seal and creep processes.