Hot faults: Iridescent slip surfaces with metallic luster document high-temperature ancient seismicity in the Wasatch fault zone, Utah, USA

Abstract

and localized iridescence in the footwall dam- age zone of the Wasatch fault (Utah, USA) and document elevated temperatures on these faults. We propose that the formation of these iridescent slip surfaces requires frictional heat generated at geometric asperities at seismic slip rates. WASATCH FAULT ZONE ABSTRACT We document new geological indicators of ancient seismicity in the form of highly reflec- tive, iridescent, hematite-coated fault surfaces. Small faults that cut the Paleoproterozoic Farmington Canyon Complex in the footwall damage zone of the Brigham City segment of the Wasatch fault (Utah, USA) are smooth to striated surfaces, tens of square centimeters to 30 m 2 in area. The dull-rusty to high-metallic luster and moderate- to high-gloss surfaces exhibit multicolored elliptical iridescent patches ~0.5-3 cm across. Preexisting hematite crystals were deformed during slip on 1-200-mm-thick slip surfaces. Textural observations, X-ray diffraction, X-ray photoelectron spectroscopy, electron backscattered diffraction analysis, surface metrology, and similarity to experimentally formed iridescent spots in rocks and metals indicate that iridescence is associated with high-temperature (>300 °C) reduction of iron (Fe 3+ to Fe 2+ ) and associated conversion of hematite to magnetite. We pro- pose that the iridescent slip surfaces in the Wasatch fault damage zone are the result of seismic slip and flash heating at asperities along the small faults. The thousands of these sur - faces represent coseismic or aftershock deformation down to magnitude -3 in the exhumed footwall damage zone of the Wasatch fault.