Hematite nano- to micro-textures and (U-Th)/He thermochronometry inform seismic and aseismic fault damage zone processes

Alexis K. Ault, Robert G. McDermott, Amy C. Moser, & James P. Evans

Published August 14, 2017, SCEC Contribution #7576, 2017 SCEC Annual Meeting Poster #111

Hematite fault mirrors and striated, metallic slip surfaces in exhumed fault zones record disparate thermal and mechanical processes. Hematite is amenable to (U-Th)/He (He) dating and accurate interpretation of hematite He data requires hematite textural characterization, grain-size (closure T) 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 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 and host rock apatite He data, are best explained by friction-generated heat, hematite recrystallization, and attendant He loss (thermal resetting) at geometric asperities. Models of asperity flash heating and coupled fractional hematite He loss yield temperatures of ~≥1200 °C and 85-100% He loss at fault surfaces. Results imply asperity flash heating and dynamic weakening promoted seismicity on these surfaces <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.

Hematite textures from exhumed fault rocks provide a foundation for comparison with future SCEC-supported hematite deformation experiments to quantify the temperature and frictional properties of hematite-coated fault surfaces. Fault rock thermochronometry and textural analysis of experimental products will (1) link experimental and natural hematite fault observations and (2) identify potential deformation mechanisms in natural settings.

Key Words
hematite, exhumed faults, (U-Th)/He dating, textures, Wasatch Fault, San Andreas Fault, Mecca Hills

Ault, A. K., McDermott, R. G., Moser, A. C., & Evans, J. P. (2017, 08). Hematite nano- to micro-textures and (U-Th)/He thermochronometry inform seismic and aseismic fault damage zone processes. Poster Presentation at 2017 SCEC Annual Meeting.

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