OXYGEN ISOTOPE CONSTRAINTS ON THE EARLY SLIP HISTORY OF THE MOHAVE WASH FAULT, CHEMEHUEVI MOUNTAINS, SE CA

Fluids are likely significant during the life-cycle of low-angle normal faults (LANFs), but the role of those fluids and their source at fault initiation are unclear. The Mohave Wash Fault (MWF), a LANF situated within the Chemehuevi Mountains core complex (SE CA), offers a key site to evaluate this question. Sampling was focused along 15 km down dip along the MWF, which slipped 1-2 km during the Miocene before being denuded passively to the surface by extension preferentially localized on the higher-level Chemehuevi Detachment Fault. To evaluate fluid-rock interactions during initiation of this fault system, δ¹⁸O values of quartz and epidote were measured by CO₂-laser fluorination.

The MWF cuts Cretaceous granitic rocks and Precambrian gneiss. The damage zone is variable in thickness and characterized by cracked granitic rocks hosting mineralized fractures, cohesive cataclasites, thin foliated shear zones, and rare pseudotachylite. δ¹⁸O of quartz hosted by undeformed granite ranges from 9.0-10.3‰, defining predeformation values. Foliated shear zones and quartz veins extend to lower δ¹⁸O_Qtz from 10.1-6.1‰, while cataclasites record the lowest δ¹⁸O_Qtz values down to 1.1‰. The δ¹⁸O values of epidote (from all types) ranges from 5.3‰ to -0.4‰; the lowest values are generally observed in cataclasites. The shifts to lower δ¹⁸O (mineral) are explained by interaction with heated, low δ¹⁸O fluids (evolved meteoric fluids or basin brines). Apparent temperatures from stable isotope thermometry on coexisting quartz and epidote (from 0.5 cc of rock) from the footwall are replicated by samples collected near one another, and are typically 50-150˚C higher than ambient footwall temperatures at 23 Ma (fault initiation) determined using ⁴⁰Ar/³⁹Ar closure temperatures (John and Foster, 1993). Temperatures defined by both methods generally increase in the paleodip direction. The temperature difference across the footwall estimated from Δ¹⁸O_(Qtz-Ep) versus Ar/Ar closure temperatures either indicate the mineralization occurred prior to Ar/Ar closure or reflect localized upwelling of hot, deep-seated fluids during slip along the MWF. Calculated δ¹⁸O_H2O in equilibrium with mineral pairs decreases with lower temperature, consistent with influx of progressively lower δ¹⁸O fluids with decreasing temperature and depth.