Detecting asperity flash heating on hematite faults from deformation experiments and (U-Th)/He thermochronometry

Alexis K. Ault, Gabriele Calzolari, Greg Hirth, & Robert McDermott

Published August 9, 2019, SCEC Contribution #9409, 2019 SCEC Annual Meeting Poster #157

Co-seismic temperature rise, which induces dynamic weakening, is challenging to directly document in natural and experimental fault rocks. Hematite (U-Th)/He (He) thermochronometry is sensitive to transient high temperature events. Thus, hematite-coated slip surfaces, including high-gloss, light-reflective fault mirrors (FMs), may record thermal and textural evidence of these processes. To test this hypothesis, we conducted frictional sliding experiments on specular hematite at seismic slip velocities. We used a rotary shear geometry with an annular ring of SiC sliding against a hematite slab at 8.5 MPa normal stress. Two types of experiments were conducted. To produce the maximum frictional heat, continuous slip experiments impose 40 cycles of slip with a maximum slip velocity (320 mm s−1), with no interruption between cycles, to a total displacement of 1500 mm. Interrupted slip experiments were conducted to the same total displacement, but comprise 40 cycles with the protocol: 2 mm-displacement at 0.01 mm s−1, followed by slip at ~320 mm s−1 for the remainder of the cycle and a two-minute rest between cycles. Hematite material was characterized before and after sliding via textural and hematite He analysis to quantify He loss (temperature proxy) over variable experimental conditions. Both types of experiments produce an ~5-30 μm-thick gouge layer with angular, ~50 nm-2 μm-diameter particles. These layers contain localized, mm-diameter, FM zones of sintered nanoparticles, analogous to natural hematite and carbonate FMs. Hematite He analyses of the starting material (n=22) are compared with FM (n=4) and gouge (n=6) run products and FM aliquots show ~71 % and ~35 % He loss for continuous slip and interrupted slip experiments, respectively. Gouge aliquots do not exhibit appreciable He loss. Results indicate FM zones experienced enhanced He loss from friction-generated heat. Spatial heterogeneity of observed He loss patterns from these zones are consistent with asperity flash heating. Observed He loss requires asperities >200-300 μm in diameter, producing flash temperatures >1000 ˚C for ~1 ms. This work provides new empirical evidence for the application of micro-asperity theory and highlights the role of co-seismic temperature rise in FM formation. We also confirm hematite He thermochronometry can detect asperity flash heating signatures on natural faults, providing new insights into interpretation of fault-specific non-monotonic thermal histories.

Key Words
hematite, thermochronometry, asperity flash heating, deformation experiments

Citation
Ault, A. K., Calzolari, G., Hirth, G., & McDermott, R. (2019, 08). Detecting asperity flash heating on hematite faults from deformation experiments and (U-Th)/He thermochronometry. Poster Presentation at 2019 SCEC Annual Meeting.

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