FLUID FLOW SYSTEMS ASSOCIATED WITH THE MEADE THRUST, WYOMING SALIENT, SEVIER FOLD-THRUST BELT

Although overall fault zone architecture has been shown to affect fluid transport, questions remain on spatio-temporal evolution of fluid pathways within fold - thrust belts. By looking at multiple exposures of the Meade thrust fault in the Wyoming salient of the Sevier fold - thrust belt, focusing on mineralized structures and their host rock, we address three basic questions about fluid systems along regional thrust faults. First, at what point in the development of a fold thrust - belt do faults become active conduits, particularly in relation to other structures, and how long are these pathways open. Second, what structures (e.g. faults, fractures, or breccia) within a fault zone are conduits to fluid flow, and how does the fault as a whole affect fluid transport within its vicinity. Third, what are the sources of fluids found along the fault zone (e.g. meteoric, connate, and metamorphic). We integrate structural, petrologic, SEM/EDS, fluid inclusion, and stable-isotope (C and O) geochemical data to provide a more complete picture than is possible with any one method. Focusing on limestones in the footwall and hanging wall of the Meade thrust (Jurassic Twin Creek and Mississippian Lodgepole Formations), we identified systematic suites of mesoscopic structures, including veins and multiple minor fault sets. Even at relatively small distances from the fault (100s of m), veins are generally well grouped into discernable sets that closely mirror those seen within the body of the sheet; closer to the fault core, these well behaved sets are still present, but are supplemented by chaotic veining and mineralized minor faults, finally giving way to calcite cemented breccias. C-O isotopic analysis of material within the fault zone reveals lower values of δ¹⁸O within veins relative to host rock (-12.5‰ vs. -5.8‰), a distinct fluid signature that implies channelized flow and possible meteoric influence. This signature is reinforced by fluid inclusion data that show lower mean homogenization temperatures (79°C vs. 159°C), and also more variable temperatures than are seen within the thrust sheet. Together these data indicate an evolving fluid system along a major thrust fault zone during regional fluid migration, with pathways being highly concentrated within the fault zone, reactivated many times, and at elevated fluid pressures.