Temperatures and fluids on faults based on carbonate clumped-isotope thermometry

We present results from a carbonate clumped-isotope thermometric study of 42 carbonate samples collected within ∼1 m or less of the Mormon Peak detachment, a large-slip Miocene normal fault in the Basin and Range province of southern Nevada. Samples include cataclastic rocks, narrow vein fillings and larger void-filling carbonates. Our results are consistent with earlier measurements of O and C isotopic ratios and fluid inclusion temperatures, and provide independent constraints on the isotopic composition and temperature of both syntectonic and post-tectonic pore waters. The results reveal a wide range of precipitation temperatures (24 to 137 °C) associated with deformation, and indicate that the pore waters were meteoric, with δ^18^O as low as −11.6 permil (VSMOW) and δ^13^C as low as −8.0 permil (VPDB). The results do not provide any direct evidence for high-temperature thermal decarbonation reactions (∼500 to 800 °C) that are widely expected to result from flash heating along upper crustal faults, although they do not rule them out so long as recarbonation occurs at very low temperature, or the products of these reactions are volumetrically minor. The results are difficult to reconcile with recent suggestions that the detachment is the base of one or more catastrophically emplaced, surficial landslides. In concert with other lines of evidence, the data are most simply interpreted as recording deformation and precipitation events through a long history of slip, accompanied by relatively deep (\>3 km) circulation of meteoric pore waters along the detachment plane.