Deformation Mechanisms and Kinematics of a Crystalline-Cored Thrust Sheet: The Washakie Thrust System, Wyoming

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Geological Society of America Speical Papers



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The EA thrust system, northwestern Wyoming, consists of a southwest-directed, Precambrian-cored thrust sheet that formed above the northeast-dipping EA and North EA thrusts. The Precambrian-Cambrian contact is broadly folded, both at the leading edge of the Precambrian-cored portion of the system, and within the thrust sheet. The leading edge of the thrust sheet collapsed by slip along normal faults. Deformation of Precambrian rocks in the thrust sheet occurred by slip along northeast and steeply southwest–dipping faults. Deformation mechanisms in the faults include brittle fracture, cataclasis, and syntectonic alteration of feldspar to clays. Slip was concentrated in gouge zones and foliated cataclasites; relatively small amounts of slip occurred along narrow fracture surfaces. Faults in Precambrian rocks bound relatively intact, nondeformed blocks that range in size from 100 m to greater than 1 km. The combining of documented deformation mechanisms with kinematic restorations of cross sections shows that the broadly folded form resulted from slip along reverse faults within Precambrian rocks. The broad folds may mark the early stage of thrust-sheet formation, and subsequent slip on thrusts cut the limb of the Precambrian-cored fold to translate the thrust sheet southwestward over sedimentary rocks in the Wind River basin. Folded rocks may also have developed in the hanging wall of the EA thrusts, with simultaneous fold growth and thrust slip. Restorations also account for collapse of the thrust sheet and show that the primary reverse fault responsible for emplacement of the thrust sheet dipped 40° to 50° northeast, and the present change in dip of the fault is due mostly to hanging-wall collapse by slip along normal faults.

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