“MIRRORED” HEMATITE AND SILICA FAULT SURFACES REVEAL TEXTURAL EVIDENCE OF COSEISMIC(?) ELEVATED TEMPERATURES IN FAULT DAMAGE ZONES

Mirrored fault surfaces are thin, high gloss, light reflective, and commonly composed of layers of nano-sized grains. Silica, hematite, and carbonate high gloss slip surfaces are common in damage zone minor fault networks. Minerals on and below these surfaces may preserve textures produced by friction generated elevated temperatures during fault slip. Detailed micro- and nano-scale characterization of crystal morphology and textures are critical for interpreting fault surface thermochronometry data and potential coseismic slip processes. Petrographic and scanning electron microscopy (SEM) analysis of hematite- and silica-coated, mirrored fault surfaces from different fault zones, including the Wasatch fault zone of north-central Utah, reveals equant, hexagonal Fe-oxide and quartz nanocrystals (commonly sub-micron in diameter) adjacent to the slip surfaces. Polygonal crystals within 5-100 μm of slip surfaces exhibit triple-junction forming grain boundaries and lack shape-preferred orientation. Other surfaces preserve lobate grain boundary textures. A gradation from polygonal to lobate grain boundaries and/or a decrease in polygonal grain size with increasing distance characterize some fault surfaces. In the Wasatch example, polygonal and lobate textures are commonly isolated in ~20 μm zones adjacent to principal slip surfaces. Polygonal hematite texture is analogous to experimentally deformed hematite in dry heating or torsion experiments at ~1000 °C. Lobate boundaries appear similar to grain morphologies observed in experiments conducted at lower temperatures. Quartz morphology is analogous to that observed on other silica-rich fault surfaces, where gel lubrication is suggested as a potential dynamic weakening mechanism. In our mirrored fault examples, comparable hematite and silica crystal morphologies and textures imply similar high temperature modes of formation, such as thermal annealing, sintering, and/or dynamic recrystallization, possibly a result of flash heating and associated reduction in fault strength. Multi-scale correlated microscopy via FIB-SEM and TEM analysis will help discriminate between these formation mechanisms with implications for damage zone coseismic processes.