FLUID SOURCES, PATHWAYS, AND DRIVING FORCES WITHIN FOLD-THRUST BELTS: THE CENTRAL WYOMING SALIENT OF THE SEVIER OROGEN

Fluid transport through fold-thrust belts (FTBs) is well established, but questions remain about the sources, pathways, and driving forces of these fluids. By studying over 200 field locations along a transect through the center of the Wyoming Salient of the Sevier FTB, focusing on mineralized structures and their host rock, we address three questions. First, what are the properties of the fluids (e.g. temperature, salinity, isotopic composition), and how do they vary throughout the FTB? Second, what are the sources of these fluids (connate, metamorphic, or meteoric)? Third, which structures (regional folds and faults, as well as mesoscopic structure networks) are active pathways to fluid flow, and when are they active conduits? To answer these questions, we integrate structural, petrologic, fluid inclusion, and stable-isotope (C and O) geochemical data.

To characterize fluid pathways within individual thrust sheets, we focus on limestone hydrostratigraphic units (J. Twin Creek, Tr. Thaynes, and M. Lodgepole Formations), identifying systematic suites of mesoscopic structures, including veins sets (bed-parallel, cleavage-parallel, and dominant cross-strike sets) and multiple minor fault sets. Isotopic analysis indicates that the fluids that precipitated these veins (δ¹⁸O -5.1 to -9.5‰) were predominantly connate waters, or waters that have been modified to closely resemble such by extensive exposure to host rocks. Fluid inclusions within veins reveal a decrease in T_h from more interior to exterior thrust sheets (162 to 124°C), suggesting a combination of migrational cooling, shallower structural depths, and meteoric fluid influence.

Along major thrust faults and folds, regional vein sets are supplemented by chaotic veining and calcite-cemented breccias. Analysis of vein material reveals highly variable fluid inclusion temperatures (46.5 to 209.4°C) and δ¹⁸O values (-2.8 to -17.5‰), as well as a difference between vein and host rock values (avg -12.4 vs. -4.5‰), implying channelized flow and exogenous fluid sources.

Together, the data indicate an evolving fluid system. Fluid migrates ahead of the FTB wedge, weakening the rock and leading to LPS, with deformation and fluid flow eventually concentrating along major faults and folds.