STABLE-ISOTOPE AND MODELING STUDIES OF FLUID-ROCK INTERACTION ASSOCIATED WITH THE SNAKE-RANGE AND MORMON-PEAK DETACHMENT FAULTS, NEVADA

Stable isotope and fluid-inclusion data vc ere obtained from rocks fro m traverses within and above the Snake Range and Mor mon Peak detachme nts in Nevada in order to evaluate fluid sources and the nature of flu id flow associated with detachment faults during faulting, and to dete rmine whether the initial depth of the detachment fault influenced the nature of syntectonic fluid non. Oxygen and hydrogen isotope data ind icate the detachment faults mere infiltrated by meteoric water over a range of structural levels, but only the upper-plate rocks and brittle ly deformed portions of the faults exhibit significant O-isotopic shif t. Although all traverses included limestones. delta(18)O of detachmen t fault breccia, veins, and upper-plate rocks differed significantly d epending on the specific limestone involved. In one traverse in the Sn ake Range, where thin bedded limestone of the Cambrian Lincoln Peak Fo rmation was sampled, the delta(18)O of detachment fault breccia, veins , and stylolitically deformed upper-plate limestone near the detachmen t fault is typically 15 parts per thousand +/- 2 parts per thousand st andard mean ocean water (unexchanged limestone has delta(18)O of appro ximate to 20 parts per thousand), and the matrix Ras in O-isotopic equ ilibrium with vein fluid. Elsewhere in the Snake Range, where the deta chment fault lay in massive medium-grained Cambrian and Ordovician lim estones, delta(18)O values of detachment fault breccia and veins were much loner, typically 2 parts per thousand +/- 3 parts per thousand, w hereas delta(18)O of lime-stone matrix was between 16 parts per thousa nd and 20 parts per thousand, far out of O-isotopic equilibrium with r ein fluid. The sampled portion of the Mormon Peak detachment lag in me dium-bedded Cambrian and late Paleozoic limestone: early veins have hi gh delta(18)O values of 23 parts per thousand-28 parts per thousand (u nexchanged limestone has delta(18)O of approximate to 28 parts per tho usand), whereas later veins and detachment fault breccia have delta(18 )O between 6.6 parts per thousand and 17.9 parts per thousand. Thus, i n the Snake Range, fluids were either in isotopic equilibrium with wal l rock throughout the sampled fault history, implying intergranular fl ow, or were far out of equilibrium with it, implying channeling via a fracture network. The fluids in the Mormon Peak detachment were initia lly in isotopic equilibrium with wall rock, becoming increasingly O-18 depleted and out of O-isotopic equilibrium with wall rock with time. The difference in isotopic exchange history in the detachment faults a nd related rocks is evidently not a function of initial structural dep th, but of permeability and its distribution between matrix and fractu res. Combined thermal, fluid flow, and oxygen isotope exchange modelin g demonstrates that the observed isotopic composition of rocks in and associated with the detachment fault could have been produced either b y influx of meteoric water from topographically high areas downdip of the sampled area during detachment faulting, or by convection in the u pper plate induced by elevated geothermal gradient and deformation-enh anced permeability. Locally derived meteoric fluids also infiltrated t he detachment fault system in places, but are not required in order to account for lon delta(18)O detachment breccias and veins. The model c omputations indicate that cumulative syntectonic fluid flux along the detachment faults was between 1700 and 11 000 kg/cm(2), depending on l ocation. Time-averaged fault permeabilities are estimated by modeling between 2 and 20 mD. Thus, the model results not only verify fluid sou rce, but also properties insight into fluid flow mechanism and importa nt hydrogeologic properties of the detachment faults during deformatio n.