Thrust Geometry of Northern Absaroka Sheet, Idaho and Wyoming: ABSTRACT

Abstract

Seven major thrusts comprise the Absaroka-St. John thrust system in the Snake River Range southwest of Jackson Hole. The Absaroka thrust forms the base of this system in which earlier, higher thrusts are folded by later, lower thrusts. Shortening within the system is 17 km, excluding the basal detachment. Recent mapping requires a revised nomenclature for the thrusts to recognize that thrust sheets substitute for one another along strike. There are three different thrust geometries in the area: (1) upward opening wedge-shaped imbricates; (2) horses; (3) complex systems of horses and imbricates. The St. John, Elk, Ferry Peak, and Baldy thrust sheets are wedge-shaped, thickening from about 700 m in the west to 800+ m in the east. These sheets all carry Cambrian limestones and their fault surfaces merge with the basal Absaroka detachment to the west. The Absaroka thrust sheet is isolated as a large horse between the Absaroka and St. John thrusts, which merge at depth to the west and on the surface to the east. The Thompson thrust lies at the base of a complex fault-zone of horses and imbricates resulting from the partitioning of fault slip throughout a well-developed karst and solution breccia zone in the upper Mission Ca yon formation. The Absaroka-St. John thrust system was uplifted and partly eroded as a result of later deformation associated End_Page 805------------------------------ with motion on the Darby and Prospect-Jackson thrust, and because of the uplift of the Tetons. The style of thrusting in this system then should be typical of the regional Absaroka thrust sheet. End_of_Article - Last_Page 806------------