LARAMIDE FOLD AND FAULT GEOMETRIES ON DERBY DOME AND ITS EN ECHELON CONNECTION TO DALLAS DOME, SOUTHERN WIND RIVER BASIN, WYOMING

Detailed geologic mapping and structural analysis of the doubly plunging Derby Dome anticline indicates that the en echelon disposition between Derby and Dallas Domes occurred early in their development, prior to the formation of back-limb thrusting that is continuous between the two domes.

The domes have been inferred to be a result of back-limb, out-of-the basin shortening associated with uplift of the Wind River Mountains above the Wind River thrust. The earliest stage of this shortening produced strongly asymmetric SW-verging folding of the domes with local reverse faulting along the tight synclinal trace of the fold. Continued shortening produced a back-limb thrust on the northeastern (basin-ward dipping) limb of both Derby and Dallas Domes. This thrust is continuous across and unaffected by left-lateral offset on the foot wall of the thrust that produced the en echelon disposition of Derby and Dallas Domes.

The sedimentary rocks on the footwall of the back-limb thrust, which are exposed along the en echelon intersection between Derby and Dallas Domes, undergo an abrupt change in orientation with significant stretching and tear faulting along a trend of 270°, approximately normal to the trend of the Wind River Mountains. The tear faulting locally offsets steeply dipping units of the Frontier, Mowry, Muddy Sandstone, Thermopolis and Cloverly Formations along the southwest corner of Dallas Dome with left-lateral north side up displacement. This tearing is then dispersed by stretching within incompetent units along its trend to the east. The stretching to accommodate displacement occurs primarily within relatively incompetent units of the Morrison/Cloverly and Thermopolis Formations, where unit thicknesses are reduced by as much as 50%. Competent units of the Muddy Sandstone and Morrison/Cloverly become locally dismembered and serve as prominent markers of fault dislocation.

The inferred deformation sequence implies that the en echelon offset between Dallas and Derby Domes occurred early in the deformation history of the domes. On-going studies suggest that a similar timing of en echelon displacement may also have occurred on the southern margin of Derby Dome along its interface with Sheep Mountain Anticline.