ASPERITY FLASH HEATING DURING LABORATORY EARTHQUAKES DETECTED WITH HEMATITE (U-TH)/HE THERMOCHRONOMETRY

Co-seismic heat generation exerts a fundamental role in some dynamic weakening mechanisms and thus fault strength evolution during the seismic cycle. Hematite-coated slip surfaces, including high-gloss, light-reflective fault “mirrors,” may record thermal and textural evidence of these processes. Hematite (U-Th)/He (He) thermochronometry is sensitive to transient high temperatures and can detect thermal signatures of seismic slip in the rock record. To investigate this hypothesis, we conducted frictional sliding experiments on specular hematite at seismic to sub-seismic slip velocities. Owing to the material’s fragile nature, we used a rotary shear geometry with an annular ring of SiC sliding against a specularite plate. Hematite material from each plate was characterized before and after sliding via textural and hematite He analyses to quantify He loss (T proxy) over variable experimental conditions. Experiments were run at 8.5 MPa normal stress, 0.01-340 mm/s sliding velocity, and 35-1500 mm slip distance. High slip velocity experiments yield compacted, ~5-30 μm-thick slip surfaces comprising hematite gouge of sub-angular particles ~50 nm-2 μm in diameter and a sharp boundary with undeformed specularite plates. These surfaces have localized, mm-diameter, mirrored zones of sintered nanoparticles, analogous to natural hematite and carbonate fault mirrors. Hematite He analyses of starting material (n=25) is compared with fault mirror (n=4) and gouge (n=6) run products from high slip-velocity experiments, showing up to 71 ± 1 % (1σ) and 18 ± 3 % He loss, respectively. Results indicate fault mirror zones experienced enhanced He loss from friction-generated heat. The spatial heterogeneity of and 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 confirms hematite He thermochronometry can detect asperity flash heating signatures on natural fault surfaces, depending on the degree of slip-related grain size (diffusion domain) reduction and the post-earthquake thermal history. Results provide new empirical evidence describing micro-asperity theory and highlight the role of co-seismic temperature rise in the development of fault mirrors.