LATE-STAGE LARAMIDE REACTIVATION OF PRECAMBRIAN STRUCTURES: EVIDENCE FOR ~ N-S LARAMIDE SHORTENING ALONG THE SOUTHEASTERN MARGIN OF THE WIND RIVER MOUNTAINS

Along the SE margin of the Wind River Mountains, the trend of the Wind River thrust (WRT) changes from its typical NW-SE trend to a more E-W trend. To the north of this E-W trending segment, in the southernmost Wind River basin, two basement-involved fault zones show evidence of late-stage, ~ N-S Laramide shortening and reactivation of Precambrian brittle structures.

The Spring Creek fault (SCF) is a Precambrian ENE-trending feature that cuts across the southern margin of Sheep Mountain anticline and dips moderately to steeply to the south. It extends from the Archean core of the range, where it has inferred south-side-down normal displacement, eastward to the Beaver Rim where it is covered by undeformed Tertiary deposits along the southern margin of the Wind River basin. Laramide reactivation of the SCF southwest of the Beaver Rim produced reverse, south-side-up offset and steep to overturned sections of the Tensleep Ss and Madison Ls on the hanging wall of the fault. Adjacent hanging wall folding in the Gallatin Ls produced upright folds with ENE-striking axial planes, consistent with a compressional azimuth of ~ 160º.

The Beaver Creek thrust (BCT), just south of the SCF, is a largely E-W trending, northerly dipping Laramide fault that terminates at the SCF. It is the controlling fault along the southern margin of the Sweetwater Crossing Anticline (SCA), which folds the Paleozoic section from the Flathead Ss through the Tensleep Ss and is cored by Archean granitic gneiss. A massive Precambrian granite intruding the gneiss in the core of the SCA contains fracture orientations that are a subset of those in the adjacent gneiss. Laramide reactivation of a fracture set in the granite (average orientation N.48ºE. 81º SE) produced numerous en echelon tension fractures. Analysis of stress orientations required to produce the en echelon fractures indicate NNW-SSE compression during reactivation with an average orientations of σ₁ plunging 26 º to 153º, consistent with the compression azimuth of ~ 160º estimated from folds in the hanging wall of the SCF.

The ~E-W orientations of the BCT and the SCA, parallel to the ~E-W trace of the WRT to the south, are interpreted as late-stage hanging wall deformation above the Wind River thrust that is younger than the basin margin folding to the north that terminates with Sheep Mountain anticline.