We document new geological indicators of ancient seismicity in the form of highly reflective, 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 m2 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 (Fe3+ to Fe2+) and associated conversion of hematite to magnetite. We propose 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 surfaces represent coseismic or aftershock deformation down to magnitude –3 in the exhumed footwall damage zone of the Wasatch fault.
- Wasatch fault,
- damage zone,
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