EVIDENCE FOR ANCIENT SEISMICITY ALONG EXHUMED FAULTS: IRIDESCENT-METALLIC SLIP SURFACES

Most of the energy produced by seismic slip is consumed by geologic processes such as the generation of heat. We examine iridescent highly polished slip surfaces that may record seismic slip in the rock record.

Hematite-coated fault surfaces with metallic to highly polished luster and gloss are exposed over an ~ 4 km long, and 300 m wide footwall damage zone of the Brigham City segment of the Wasatch fault, in the Paleoproterozoic Farmington Canyon Complex. Fault surfaces range from shiny and highly reflecting to dull purple surfaces. Fault surfaces range from irregular rough surfaces with steps, well-developed striae, and small centimeter scale mullions, to remarkably smooth, highly-polished slip surfaces (HPSS) that form large dipping planar ledges up to over 10’s cm² to 5-10 m². Slip surfaces are composed of hematite and minor amounts of quartz and feldspar. Spots and wavy zones of Iridescence are common on rough and highly polished fault surfaces. Euhedral hematite laths in the faults are 2-10 μm-long and 0.25-2 μm-thick and fill < 1 mm-thick fractures and faults in cross-sectional SEM images. The combination of hematite fracture-fill and activation during slip along the Wasatch fault generated patchy iridescence on metallic surfaces. The micro-scale Fe₂O₃ crystals that comprise the iridescent surfaces are preferentially aligned parallel to the HPSS and are 2-3 crystals thick. SEM images reveal overlapping layers of hematite with different slip vectors embedded in each layer, with up to seven striae orientations present on some surfaces, and others with perpendicular striae. Engineering experiments and data from ceramics studies indicate that iridescent hematite forms at temperatures > 400 ^oC in reducing conditions. The roughness of HPSS was measured from the nm to cm scales using three surface metrology techniques. The roughness is uniform in the slip-parallel and perpendicular directions for HPSS. This lack of roughness anisotropy may be the result of multiple slip events in orthogonal or near-orthogonal directions. We propose that the iridescent-metallic slip surfaces in the Wasatch fault zone result from frictional heat at seismic slip rates and such iridescence is therefore a high-quality indicator of ancient seismicity.