EVIDENCE FOR LATE-STAGE N-S SHORTENING DURING LARAMIDE DEFORMATION ALONG THE SOUTHEASTERN MARGIN OF THE WIND RIVER MOUNTAINS, WYOMING

Uplift of the basement-cored Wind River Mountains during the Laramide orogeny (ca. 75-45 Ma) produced a series of subsidiary NW-SE trending folds in Paleozoic and Mesozoic strata along the adjacent southwestern margin of the Wind River basin. These folds generally parallel the trend of the Wind River uplift and are consistent with the regional NE-SW shortening that produced the uplift. However, individual folds along the trend display a local left-stepping en echelon pattern that becomes more complex in the southeastern most folds of the series. The interchange between Derby Dome and Sheep Mountain anticline (SMA), the two southernmost folds in this sequence, contains a complex series of faults and folds that are consistent with a late stage of N-S Laramide shortening. Similar evidence for N-S shortening occurs along the southern termination of SMA, to the south of the Clear Creek fault (CCF), a major ENE trending reverse fault.

Faults associated with the folding of SMA, to the south of the interchange between SMA and Derby Dome, trend NW-SE. However, in the interchange area and the region of the northern closure of SMA, the NW-SE-trending Carr Reservoir fault and several smaller E-W trending faults cut across the axial trace of the SMA and clearly post date its formation. The Carr Reservoir fault, which is the most extensive fault across the northern closure of SMA, is a steep northeasterly dipping, NW-SE trending reverse fault that displays both hanging wall and foot wall folding and is inferred here to have formed during a transition from NE-SW shortening to more N-S shortening. The E-W trending faults include the south verging Red Bluff fault that repeats parts of the Triassic Chugwater formation across the northern extent of SMA. To the north of the Red Bluff fault, a complex set of N and S dipping E-W trending faults within the Muddy Sandstone produce a local pop-up structure and E-W trending hanging wall folds. The E-W trending Schoettlin Mountain fold, to the south of the CCF, indicates that N-S shortening is not constrained to the interchange zone, but occurs both in the N and S terminations of SMA.

We believe that these E-W trending structures are evidence for a change in Laramide shortening direction from NE-SW to N-S, similar to that describe by Bergh and Snoke (1992) in the Shirley Mountains ~ 165 km to the SE.

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