Examining earthquake processes with microtextural analysis and (U-Th)/He thermochronometry: a case study from hematite fault mirrors in the Wasatch fault zone

Robert McDermott, Alexis K. Ault, & James P. Evans

Published August 8, 2017, SCEC Contribution #7438, 2017 SCEC Annual Meeting Poster #109

Advances in physically based models of earthquake rupture place an increasing importance on characterization of the physical processes that control dynamic friction in natural faults. We apply coupled textural analysis and hematite (U-Th)/He (He) thermochronometry, apatite He thermochronometry, and thermomechanical modeling to investigate the rock record of thermal processes operative on high-gloss (mirrored) hematite-coated fault surfaces in the damage zone of the Wasatch fault (WF). Fault mirrors exhibit ≤1-≥20 μm-diameter isolated patches of crystals with polygonal, triple junction-forming and/or lobate morphologies cross-cutting cataclasite at the fault surface. We interpret these textures as indicating localized temperature rise to >1000 °C and recrystallization. Hematite He dates are 5.0 ± 1.1 Ma to 2.4 ± 0.1 Ma and show variable intrasample scatter. In aliquots with polygonal/lobate grains, 72% yield hematite He dates younger than a host rock apatite He date of 4.5 ± 0.6 Ma. This He date relationship and calculated closure temperatures of 90-115 °C and 55 °C for the hematite and apatite He systems, respectively, suggest reheating of fault mirrors at depths shallower than the apatite He closure isotherm (≤2 km). Textural evidence of hot temperatures at the fault surface and correlation between these microtextures and young hematite He dates suggests He loss by frictional heating. Thermomechanical models show that observed microtextures and hematite He dates are best explained by flash heating of ~5-20 µm asperities to temperatures of ~600- ≥1200 °C, inducing 15-100% He loss from hematite. Model results and scattered hematite He dates suggest variable He loss on small spatial scales, highlighting spatial and temporal heterogeneity in flash temperatures and possibly dynamic strength during slip. We interpret these patterns as the thermal imprint of post-4.5 Ma paleoearthquakes, caused by asperity flash heating-assisted dynamic weakening. Estimates of displacement and rupture surface area yield paleoearthquake moment magnitudes of ~ -3 to 3. Self-similar seismicity recorded in other modern settings suggest processes recorded by these fault mirrors provide insight into genesis of larger seismic events along the WF and other major faults. Our study demonstrates the potential of microtextural analysis and fault rock thermochronometry to decipher the thermal and physical processes associated with seismic slip in natural settings.

Key Words
Hematite (U-Th)/He thermochronometry, hematite microtextures, fault mirror, asperity flash heating, Wasatch fault

Citation
McDermott, R., Ault, A. K., & Evans, J. P. (2017, 08). Examining earthquake processes with microtextural analysis and (U-Th)/He thermochronometry: a case study from hematite fault mirrors in the Wasatch fault zone. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology