Chemical changes and fluid-rock interaction in faults of crystalline thrust sheets, northwestern Wyoming, U.S.A.

Abstract

We investigate the degree of fluid-rock interaction in two thrust faults which formed at 4–7 km and 10–12 km depth in crystalline thrust sheets in northwestern Wyoming using whole-rock geochemistry. The fault zones consist of undeformed and unaltered protolith which bounds damaged zones with increased fracture, fault and vein density and chemical alteration, and a fault core comprised of zones of gouge, cataclasite and ultracataclasite. Whole-rock geochemical analyses of major, minor and trace elements in 26 samples document the fluid-rock interactions in the fault zones. Fault-related rocks from the shallow East Fork fault exhibit 10–40% depletion of Si, Al, K, Na and Ca as measured against immobile Ti as reference in the damaged zone, 40–60% depletion of Ca and Na, and 0–20% depletion of Si, Al and K in the fault core. Rocks from the deeper level White Rock thrust exhibit 10–30% depletion of these elements in protocataclasites at the edge of the fault core and up to 65% depletion in the cataclasites and ultracataclasites of the fault core. We interpret these results to show that soluble elements were removed from the fault zones by syntectonic fluid flow. Estimated volume losses in the shallow fault range from 0 to 22%, whereas the deeper level fault experienced 50–65% volume loss in the fault core. Silica loss is used to estimate fluid-rock ratios in the fault zones by assuming a range of temperatures and silica solubilities in the faults during deformation. Volumetric F/R ratios range from 10 2-10 4 for both faults, which yield fluid fluxes in the fault zones of 10 −6-10 −9 m s −1 using geologic constraints on the life and geometry of the fault. Depletion and volume loss at deep levels was greater due to hotter fluids and finer grain-size generated by mixed brittle-plastic deformation, whereas at shallow levels, cooler temperatures and purely brittle deformation resulted in vein formation and less chemical fluid-rock interaction.