Integrating hematite (U-Th)/He dating, microtextural analysis, and thermomechanical modeling to date seismic slip

Friday, 18 December 2015
Poster Hall (Moscone South)
Robert McDermott1, Alexis K Ault1, James P Evans1 and Peter W Reiners2, (1)Utah State University, Logan, UT, United States, (2)University of Arizona, Tucson, AZ, United States
Linking petrologic and geochronologic evidence for seismicity in the rock record is challenging, yet critical for understanding slip mechanics in natural faults, structural histories, and modern seismic hazards. We couple hematite (U-Th)/He (HeHe) dating with microtextural analysis and thermomechanical modeling to decipher this record from locally iridescent, hematite-coated fault surfaces in the seismogenic Wasatch fault zone (WFZ), Utah. Prior study of one fault surface linked textural evidence for elevated temperatures with a pattern of HeHe dates to hypothesize that this surface preserves evidence of multiple seismic slip events. New scanning electron microscopy (SEM) and HeHe data from a larger sample suite test this hypothesis. The SEM images reveal the presence of <500 nm polygonal hematite crystals at some iridescent regions, suggesting co- to post-seismic hematite annealing and recrystallization at temperatures >800 °C. Fault surface samples yield 3.8 ± 0.03 to 1.5 ± 0.1 Ma dates, with younger dates in iridescent regions. These results are younger than 88.5 ± 15.0 Ma and 10.8 ± 0.8 Ma dates from veins associated with initial hematite mineralization as well as new apatite (U-Th)/He dates of 4.0 ± 0.6 Ma-5.4±1.1 Ma that constrain the footwall thermal history. Reproducible but statistically different HeHe dates from samples on the same fault surface are consistent with prior observations. Collectively, these observations suggest that hematite He dates record rapid cooling from localized shear heating at asperities to temperatures hot enough to reset the hematite He system. Models incorporate rate-dependent friction and half-space cooling to constrain shear zone temperature evolution. Results reveal temperatures >800 °C are sufficient to reset hematite up to 200 µm from the fault surface and HeHe dates may represent patches of rate-strengthening friction during seismic slip. Ongoing work utilizes SEM to target aliquots with textural evidence for elevated temperatures and fast rupture and/or slip rates on the fault surface prior to He dating and refining models to incorporate such data.