Preliminary data on detecting asperity flash heating on hematite faults with laboratory experiments and hematite (U-Th)/He thermochronometry

Gabriele V. Calzolari, Alexis K. Ault, & Greg Hirth

Submitted August 14, 2018, SCEC Contribution #8533, 2018 SCEC Annual Meeting Poster #176

Friction-generated heat is a primary by-product of seismic slip on faults and can activate various mechanisms that lead to low coseismic strength. Hematite mineralization is ubiquitous in fault zones as striated or “mirrored” (high gloss, light reflective) hematite-coated slip surfaces. These faults archive important components of the thermal, physical, and chemical conditions of faulting. Hematite is amenable to (U-Th)/He analysis and has a closure temperature range of ~50-250 °C, which is a function of the hematite crystal size. Thermally-activated volume diffusion of He in hematite, responds to short duration, high temperature thermal pulses. To evaluate the role of heat in development of thin hematite slip surfaces, we integrate hematite rotary-shear experiments and (U-Th)/He thermochronometry to quantify temperature, friction, microstructure, and He loss evolution over variable slip displacements and rates. Our workflow requires textural and (U-Th)/He characterization of hematite starting material, deformation experiments, and comparative microscopy and (U-Th)/He dating of experimental products. Scanning electron microscopy (SEM) shows undeformed hematite comprises specularite plates (~2-80 µm-thick). Hematite (U-Th)/He dates from individual crystals (41-77 µm-thick) are ~207-284 Ma, positively correlate with plate thickness, and have a sufficient He budget to document He loss during slip. Deformation experiments were conducted using an Instron apparatus at 5 MPa normal stress, 0.01-340 mm/s sliding velocity, and 35-2000 mm slip distance. Experiments reveal hematite is weak, with a low coefficient of friction from ~0.4-0.2 over a range of slip velocities, and exhibits dynamic weakening behavior. Experimentally-produced textures include a 20-100 µm-thick band of micron-scale, angular clasts and subrounded 0.5-0.2 µm particles. Low- and high-slip velocity runs yield mm-wide, high-gloss, light-reflective patches analogous to natural hematite fault mirrors. Preliminary SEM imaging of high speed runs shows some of these zones comprise sintered nanoparticles with polygonal grain boundaries, previously interpreted to represent recrystallization from transient, high temperatures. On-going nano-characterization and (U-Th)/He dating of experimental products will allow us to quantify friction-generated heat at subseismic to seismic slip rates, identify operative dynamic weakening mechanisms, and link experimental and natural hematite slip surface observables.

Citation
Calzolari, G. V., Ault, A. K., & Hirth, G. (2018, 08). Preliminary data on detecting asperity flash heating on hematite faults with laboratory experiments and hematite (U-Th)/He thermochronometry. Poster Presentation at 2018 SCEC Annual Meeting.


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