METEORIC INFILTRATION ALONG THRUST FAULTS WITHIN THE WYOMING SALIENT OF THE SEVIER FOLD-THRUST BELT

Fluids transported through fold-thrust belts (FTBs) include connate, metamorphic, and meteoric waters. In an effort to quantify the sources, pathways, and driving forces of these fluids, we studied veins and their surrounding host rock along a transect through the center of the Wyoming Salient of the Sevier FTB. We focus on veins within fault zones and limestone hydrostratigraphic units (J. Twin Creek, Tr. Thaynes, and M. Lodgepole Formations), identifying systematic suites of mesoscopic structures, including vein sets and multiple minor fault sets. These structures, and the fluids from which they formed were characterized (e.g. timing, temperature, salinity, isotopic composition) using structural, petrologic, fluid inclusion, and stable-isotope (C and O) geochemical data. Using this data, we were able to identify both which structures (regional folds and faults, as well as mesoscopic structure networks) are active pathways to fluid flow, and when they were active conduits.

These fluids and their resulting veins can broadly be characterized as endogenous or exogenous to the FTB fluids. Isotopic analysis indicate that endogenous fluids were predominantly connate waters, or waters that have been modified to closely resemble such by extensive exposure to host rocks. Exogenous fluids are dominated by meteoric fluids. However, these fluids are not equally distributed in time or space. The median δ¹⁸O within thrust sheets for the hosts rock is -7.6 (interquartile range = -6.0 to -9.5) and the median for the veins is -9.0 (IQR = -7.0 to 11.1). Within fault zones the median δ¹⁸O host rock value is -5.7 (IQR = -4.7 to -6.7) and median vein value is -10.6 (IQR = -6.5 to -13.5). These data, in conjunction with fluid inclusion temperature of homogenization values for thrust sheets, with a median of 124.1°C (IQR = 98.5 to 139.9), and thrust zones, with a median of 81.5°C (IQR = 59.6 to 101.5), indicate that endogenous fluids dominate within thrust sheets and that exogenous fluids of meteoric origin were primarily channelized along regional structures, namely thrust faults.

These data contradict existing numerical models, which predict wholesale infiltration of meteoric water across the entire FTB wedge and through thrust sheets. Therefore, new models of meteoric water infiltration to depth within a FTB should be developed.