TEMPERATURES AND FLUIDS ON FAULTS BASED ON CARBONATE CLUMPED–ISOTOPE THERMOMETRY

We present results from a carbonate clumped-isotope thermometric study of 41 carbonate samples collected within ~1 m or less of the Mormon Peak detachment, a large-slip Miocene normal fault in the Basin and Range of southern Nevada. We focused on the detachment because it has a long history of investigation, and is among the most extensively exposed low-angle normal faults in the world that is developed almost entirely within carbonate rock. Samples include cataclastic rocks (breccia, fault plane gouge and injected gouge), 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 syntectonic pore waters. These materials yield carbonate precipitation temperatures ranging from 10 °C to 139 °C, δ¹³C (VPDB) ranging from –8‰ to +2‰, and calculated pore fluid δ¹⁸O (VSMOW) ranging from –11‰ to +9‰. The variations are coherent with sample location and fabric, and so the method appears to be a viable new tool for constraining the marked variations in both temperature and fluid sources inherent to this system. The initial results indicate that meteoric waters were present in the system across nearly the entire observed temperature spectrum, as observed in the temperature range of 10 °C to 115 °C for void-filling carbonate samples, an observation that is not apparent on the basis of the conventional stable isotopic data alone for these rocks. Based on these data, the fault system was characterized by relatively free circulation of meteoric waters along the fault to depths of at least 3-4 km. This in turn implies that, at meter-scale near the fault, long-term pore pressure was at most hydrostatic, although transient overpressure cannot be ruled out. The recorded temperatures do not provide any direct evidence for high-temperature thermal decomposition reactions (ca. 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 relatively low temperature, or the products of these reactions are volumetrically minor.