FLUID MIGRATION THROUGH THE WYOMING SALIENT OF THE SEVIER FOLD-THRUST BELT

Fluids migrations out of fold thrust belts and into foreland basins are well documented through hydrocarbon and ore deposits; however, questions remain regarding fluid systems within the fold thrust belts themselves. By inspecting over 100 exposures and the resulting data and samples along a central transect through 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 across the fold thrust belt. First, where do migrating fluids originate, and what are their sources (e.g. meteoric, connate, and metamorphic)? Second, does either primary or secondary permeability (e.g. pore space or structures such as fractures and faults), act as a conduit for fluid flow, and when are these pathways active and dominant during the progressive growth of the fold thrust belt? Third, how are these fluids modified as they migrate through the fold thrust belt, mixing with other fluids and interacting with the host rock.

We integrated structural, petrologic, SEM/EDS, fluid inclusion, and stable-isotope (C and O) geochemical data to provide a more complete picture than is possible using any one method. Focusing on limestone units across the fold thrust belt (Jurassic Twin Creek, Triassic Thaynes, and Mississippian Lodgepole Formations), we identified systematic suites of mesoscopic structures, including veins (a cross strike set, a strike parallel set, and a bed parallel set) and multiple minor fault sets. C-O isotopic analysis of vein material and host rock from within thrust sheets reveals paired values that are closely matched (avg. separation of 1.2‰); however, material within fault zones and prominent folds reveals lower δ¹⁸O values within veins relative to host rock (avg. separation of 6.7‰), 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 and more variable temperatures within fault zones than are seen within thrust sheets. Regionally, C-O isotopic values for both veins and host rock show a trend towards more negative values towards the foreland, indicating both regional fractionation and local equilibration, implying long distance and large magnitude migration of fluids.