LATERAL VARIATION OF SYNOROGENIC FLUIDS WITHIN THE WYOMING SALIENT OF THE SEVIER FOLD-THRUST BELT

Fluid migrations that take place within orogenic belts affect both orogenic evolution and the rheological, mechanical, and geochemical properties of rocks. Within foreland fold-thrust belts, deformation varies laterally, with changes in structures as varied as mesoscopic veins to regional faults; variations that may be reflected in the pathways, sources, and driving forces of fluid systems. Two major fluid systems were studied along a central and northern transect through the Wyoming salient of the Sevier fold-thrust belt: fluid systems associated with (1) the regional fracture network found within thrust sheets, and (2) the fracture network associated with regional faults.

The dominant structures of the regional fracture network are cross-strike, strike-parallel, and bed-parallel veins. These structures were nearly identical in their distribution and orientation between the central and northern transect but were rotated ~30° counterclockwise along the northern transect during thrusting. Structural data indicates the fracture network developed within the toe of the fold-thrust belt and was sealed by calcite precipitation prior to thrusting. Fluid inclusion, stable isotope, and calculated fluid isotopic data from thrust-sheet veins also show nearly identical trends and overall ranges between the central and northern transects. Geochemical data indicate the fluid system acted primarily as a closed system, with a formational fluid source derived by compaction.

Regional-fault fluids systems stand in sharp contrast to thrust-sheet fluid systems. Fault-related fractures are highly connected and chaotic. Fluid inclusion and stable isotope data are variable both within faults and between the central and northern transects. Geochemical data indicates that faults fluid systems act primarily as open systems, with fluids being dominated by meteoric fluids with components of formational and metamorphic fluids driven by a hydraulic head. In addition to the varying fluid sources, fault fluids can (1) follow a relatively direct or tortured fluid pathway, (2) be relatively open or closed to outside fluids, and (3) pass through a wide variety of host rocks. This leads to multiple fault fluid types with variable sources, pathways, and interactions with the surrounding wall rock.