Stable isotope and modeling studies of fluid-rock interaction associated with the Snake Range and Mormon Peak detachment faults, Nevada

Abstract

Stable isotope and fluid-inclusion data were obtained from rocks from traverses within and above the Snake Range and Mormon Peak detachments in Nevada in order to evaluate fluid sources and the nature of fluid flow associated with detachment faults during faulting, and to determine whether the initial depth of the detachment fault influenced the nature of syntectonic fluid flow. Oxygen and hydrogen isotope data indicate the detachment faults were infiltrated by meteoric water over a range of structural levels; however, only the upper-plate rocks and brittlely deformed portions of the faults exhibit significant O-isotopic shift. Although all traverses included limestones, δ 18 O of detachment fault breccia, veins, and upper-plate rocks differed significantly depending on the specific limestone involved. In one traverse in the Snake Range, where thin-bedded limestone of the Cambrian Lincoln Peak Formation was sampled, the δ 18 O of detachment fault breccia, veins, and stylolitically deformed upper-plate limestone near the detachment fault is typically 15‰±2‰ standard mean ocean water (unexchanged limestone has δ 18 O of ≈20‰), and the matrix was in O-isotopic equilibrium with vein fluid. Elsewhere in the Snake Range, where the detachment fault lay in massive medium-grained Cambrian and Ordovician limestones, δ 18 O values of detachment fault breccia and veins was much lower, typically 2‰±3‰, whereas δ 18 O of limestone matrix was between 16‰ and 20‰, far out of O-isotopic equilibrium with vein fluid. The sampled portion of the Mormon Peak detachment lay in medium-bedded Cambrian and late Paleozoic limestone: early veins have high δ 18 O values of 23‰–28‰ (unexchanged limestone has δ 18 O of ≈28‰), whereas later veins and detachment fault breccia have δ 18 O between 6.6‰ and 17.9‰. Thus, in the Snake Range, fluids were either in isotopic equilibrium with wall rock throughout the sampled fault history, implying intergranular flow, or were far out of equilibrium with it, implying channeling via a fracture network. The fluids in the Mormon Peak detachment were initially in isotopic equilibrium with wall rock, becoming increasingly 18 O depleted and out of O-isotopic equilibrium with wall rock with time. The difference in isotopic exchange history in the detachment faults and related rocks is evidently not a function of initial structural depth, but of permeability and its distribution between matrix and fractures. Combined thermal, fluid flow, and oxygen isotope exchange modeling demonstrate that the observed isotopic composition of rocks in and associated with the detachment fault could have been produced either by influx of meteoric water from topographically high areas downdip of the sampled area during detachment faulting, or by convection in the upper plate induced by elevated geothermal gradient and deformation-enhanced permeability. Locally derived meteoric fluids also infiltrated the detachment fault system in places, but are not required in order to account for low δ 18 O detachment breccias and veins. The model computations indicate that cumulative syntectonic fluid flux along the detachment faults was between 1700 and 11 000 kg/cm 2 , depending on location. Time-averaged fault permeabilities are estimated by modeling to have been between 2 and 20 mD. Thus, the model results not only verify fluid source, but also provide insight into fluid flow mechanism and important hydrogeologic properties of the detachment faults during deformation.