CHARACTERIZING THE REGIONAL FLUID-FLOW SYSTEM OF THE WYOMING SALIENT, SEVIER FOLD-THRUST BELT

During mountain-belt formation, fluid migration plays an integral role in heat transport, mass transport, hydrocarbon accumulation, and rheology. To better understand the spatial and temporal patterns of regional fluid-flow systems, we integrate structural, petrologic, SEM/EDS, fluid inclusion, and stable-isotope (C and O) geochemical data for the well characterized Wyoming salient of the Sevier fold-thrust belt. We seek to characterize fluid sources (e.g. meteoric waters, connate waters, and metamorphic fluids), driving forces (e.g. topographic relief between the high hinterland and lower foreland, thermally and/or chemically generated gradients, and tectonic loading by thrust sheets and sediment burial), and fluid pathways (e.g. primary and secondary porosity, both within hydrostratigraphic units and along faults).

Focusing on limestone units in the Jurassic Twin Creek Formation, Triassic Thaynes Formation, and Missisippian Lodgepole Formation and correlatives, we identified systematic suites of mesoscopic structures, including veins and multiple minor fault sets. Surveys of cross-cutting relationships establish relative timing of these structures. SEM backscatter and X-ray analysis reveal minor variation in vein geochemistry and reactivation of previous structures. Image analysis of sample-area scans shows that cross-strike veins are most prevalent and have the largest apertures. Analysis of two-phase, aqueous fluid inclusions within veins reveals a decrease in T_h from more interior to exterior thrust sheets, suggesting a combination of migrational cooling, shallower structural depths, and meteoric fluid influence. C-O isotopes analysis of paired vein (cross-strike set) and host rock carbonate samples reveal equal δ¹³C values for vein-host rock pairs; δ¹⁸O values decrease for both veins and host rock from west to east, showing that host composition altered during regional flow. Along regional faults, analysis reveals low levels of δ¹⁸O within veins relative to host rock, a distinct fluid signature that implies channelized flow and meteoric influence. This data supports the hypothesis that fluid migration ahead of the wedge weakens rock, which undergoes distributed layer parallel shortening, and this deformation and fluid flow are then concentrated along major faults.