STRUCTURAL ANALYSIS OF THE TENSLEEP FAULT, BIGHORN BASIN, WYOMING (PALEOSTRESS, FORELAND, JOINTS)
The Tensleep Fault is an east-west trending Precambrian fault reactivated during the Laramide orogeny. This study of the fault west of the base of the Bighorn Mountains involves three levels of analysis: structural geometry, determination of motion, and interpretation of paleostress fields. Evidence of compression is pervasive along the fault, with the north side up except at David Dome where synclinal crowding has thrust beds on the downthrown (south) side northward over the Tensleep Fault. Motion on the Tensleep Fault is dominantly reverse with a possible left-slip component of less than 250m. A right stepover of the fault in the subsurface created enhanced fracture permeability in a compressive wedge on the updip edge of an Goose Egg Formation stratigraphic pinchout producing the oil reservoir in the Cottonwood Creek oilfield.
Paleostress directions are inferred from orientations of more than 12,000 mesoscopic structural features. Two distinct sets are evident in stereonets of joints, faults, slickensides, folds, stylolites, plumose structure and veins. The larger data set, inferring a N42-57(DEGREES)E stress direction, corresponds to the main Laramide compression direction. Application of this stress to the Tensleep Fault would be expected to produce thrusting and left-slip.
This work argues strongly for the horizontal compression origin of the Wyoming foreland and the nonpassive response of sedimentary rocks above the basement.
A second paleostress direction, N70(DEGREES)W directed, is younger than the northeast-directed event because it contains undeformed gypsum fibers in veins. This stress direction also parallels a prominent one in the northern Bighorn Basin and Beartooth Range that postdates the main Laramide event yet preceeds the Heart Mountain Thrust of 48-52 mya.
A causal relationship is implied between Farallon-Kula-North America plate convergences and the creation of mesoscopic structures along the Tensleep Fault more than 1,000 km from the plate margins. If the second stress event is also regional, it may correspond to an undetermined plate rotation, or the docking with North America of a microplate related to the Yellowstone Hotspot. A third paleostress field seen only in Tertiary rocks corresponds to the present day mid-continent stress field which is pervasive in the North American craton.