Dating Brittle Deformation with Hematite (U-Th)/He Chronometry

Alexis Ault, University of Arizona
Peter Reiners, University of Arizona

Direct, robust timing constraints on fault slip are necessary to reconstruct structural-tectonic histories, understand mountain building and landscape evolution processes, and document ancient seismicity to assess modern seismic hazards, earthquake forecasting, and fault mechanics. Fault activity is commonly indirectly deduced by dating geologic units that predate or postdate faults or a range of possible fault activity ages is inferred from variations in bedrock low-temperature thermochronology dates and/or extrapolated cooling rates. This relies on the assumption that exhumational cooling is directly tied to fault motion at a regional scale, which is often not the case. Radioisotopic methods used to place direct temporal constraints on brittle fault activity include 40Ar/39Ar dating of neo-formed fault gouge clay and U-Th dating of syntectonic carbonate and opal.

Hematite is among the most common synkinematic minerals associated with fault surfaces and is amenable to (U-Th)/He dating, presenting a new method for dating brittle deformation. (U-Th)/He dates from hematite associated with faults record brittle faulting events by constraining the timing of either synkinematic hematite formation or the rapid cooling from fault surface frictional heating during faulting. In some cases, these dates may track regional cooling, yielding a new tool to quantify tectonic exhumation or erosion linked to broader fault zone evolution. We present hematite (U-Th)/He data from faults from several case studies including the footwall damage zone of the well-characterized Wasatch fault, UT and the Gower Peninsula, Wales to demonstrate the utility and complexities in applying this technique to constrain brittle deformation. For example, hematite (U-Th)/He dates from, mirrored, metallic, locally iridescent, hematite-coated small fault surfaces in the footwall damage zone of the southern Brigham City segment of the Wasatch fault zone are interpreted as direct constraints on fault slip. Pliocene hematite He dates overlap published and new apatite (U-Th)/He and apatite fission-track data from several locations in the host rock. Texturally identical hematite from iridescent regions separated by tens of cm on a single fault surface yield significantly different hematite (U-Th)/He dates, ruling out the interpretation of the dates as conventional thermochronologic ambient cooling ages. Fault surface iridescence is associated with the high temperature conversion hematite to Fe2+ (magnetite) from seismic slip and flash heating at geometric asperities. We suggest that these hematite (U-Th)/He dates reflect rapid cooling from this heating during localized seismic slip events, documenting evidence of ancient microseismicity in the exhuming footwall damage zone at depths of ~2-3 km.

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