Abstract

The Gros Ventre Range is a northwest-trending, basement-cored, Laramide uplift in western Wyoming. The range is bound on the southwest by the northwest-striking Cache Creek thrust fault, which dips northeast beneath the range. The ancestral (Laramide) Gros Ventre Range extended much further to the northwest than the present Gros Ventre Range. as evidenced by the length of the Cache Creek thrust.

The present Gros Ventre Range is subdivided into two blocks of uplifted basement, the (Shoal Creek) eastern block and the (Skyline Trail) western block. The eastern block is bound by the Shoal Creek fault, interpreted to be a downward-flattening reverse or thrust fault. Fault zone slickenside lineations show that the Shoal Creek fault moved to the west, oblique to the Cache Creek thrust and the trend of the range. The western block is bound by the Cache Creek thrust, which also shows oblique-slip to the west. Therefore, the crest of the eastern Gros Ventre Range was uplifted through west-directed shortening, not by southwest-directed dip-slip on the Cache Creek thrust. The notion that foreland thrust faults experienced purely dip-slip displacement normal to strike is incorrect for this uplift, and may be incorrect for other foreland uplifts.

The Neogene Teton Range and normal fault are superimposed at a high-angle across the northwest end of the ancestral, Laramide Gros Ventre Range. The Teton normal fault does not appear to reactivate or exploit pre-existing surface structures associated with the Gros Ventre Range. However, two models are presented that may explain the orientation of the Teton normal fault. The first model (Sales, 1983) proposes that the Cache Creek thrust ramped up-section to the west at the west end of the ancestral Gros Ventre Range, as modeled at a small-scale by the Shoal Creek fault. This ramp was then reactivated in Neogene time by normal, dip-slip displacement on the Teton normal fault. The second model proposes that the Teton normal fault reactivated north-trending basement shear zones, similar to those at the northwest end of the Wind River Range. Both models could explain the trend and attitude of the Teton normal fault.