Thermochronometric and textural evidence for seismicity via asperity flash heating on exhumed hematite fault mirrors, Wasatch fault zone, UT, USA

Title of Publication: 
Thermochronometric and textural evidence for seismicity via asperity flash heating on exhumed hematite fault mirrors, Wasatch fault zone, UT, USA
Author: 
McDermott, Robert G., Ault, Alexis K., Evans, James P., and Reiners, Peter W.
Publication Info: 
Earth and Planetary Science Letters. Volume 471, 1 August 2017, Pages 85–93
Abstract: 

Exhumed faults record the temperatures produced by earthquakes. We show that transient elevated fault surface temperatures preserved in the rock record are quantifiable through microtextural analysis, fault-rock thermochronometry, and thermomechanical modeling. We apply this approach to a network of mirrored, minor, hematite-coated fault surfaces in the exhumed, seismogenic Wasatch fault zone, UT, USA. Polygonal and lobate hematite crystal morphologies, coupled with hematite (U–Th)/He data patterns from these surfaces and host rock apatite (U–Th)/He data, are best explained by friction-generated heat at slip interface geometric asperities. These observations inform thermomechanical simulations of flash heating at frictional contacts and resulting fractional He loss over generated fault surface time–temperature histories. Temperatures of >∼700–1200 °C, depending on asperity size, are sufficient to induce 85–100% He loss from hematite within 200 μm of the fault surface. Spatially-isolated, high-temperature microtextures imply spatially-variable heat generation and decay. Our results reveal that flash heating of asperities and associated frictional weakening likely promote small earthquakes (Mw≈−3 to 3) on Wasatch hematite fault mirrors. We suggest that similar thermal processes and resultant dynamic weakening may facilitate larger earthquakes.

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Fig. 4. (A) Schematic of hematite fault mirror showing asperities of different diameters that produce spatially and thermally variable heat pulses and textures. Note different vertical and horizontal scales. (B) Thermomechanical simulation results showing time–temperature paths for hematite at fault surface (z = 0 mm) for flash heating at a 20 μm diameter asperity. Dashed portion of x-axis is schematic pre-slip time interval. Dashed and solid yellow bars beneath x-axis indicate pre- and post-slip periods, respectively. Red portion denotes slip event and associated temperature rise. (C) He loss with depth for diffusion domains of different radius r for 20 μm asperity flash heating scenario. Red shaded region indicates bulk He loss for dominant 0.1–0.5 μm grain radii in most fault surface aliquots. Vertical dashed line denotes typical ∼200 μm aliquot thickness. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)