--> Paleostress Analysis of the Black Hills Uplift — Powder River Basin Margin of the Newcastle Area, Weston County, Wyoming

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Paleostress Analysis of the Black Hills Uplift — Powder River Basin Margin of the Newcastle Area, Weston County, Wyoming

Abstract

The Black Hills uplift and Powder River Basin are separated by the Black Hills (BHM) and Fanny Peak (FPM) monoclines that intersect about seven miles east of Newcastle, Wyoming. The northwest and north-south trends of these structures, combined with an assumed regional Laramide horizontal stress field oriented approximately N70°E, suggests possible high-angle, oblique movement for basement faults that form the cores of these structures. A previous study focused on the geometry of structures within the monoclines and related them to features formed in a strike-slip environment, e.g. restraining bends, in-line anticlines, oblique faults and lenses such as the Newcastle terrace. To test the idea of a strike slip component, this study utilizes the kinematics of mesoscopic faults and joint data collected along the monoclines and manipulated in fault kinematic software (FaultKin 6). Preliminary results show a minor clockwise, eastward rotation of maximum compressive stress from N25°E to N45°E along the Black Hills monocline from west of the Newcastle terrace to the intersection of the BHM and FPM. Conjugate fault orientations suggest pre- to syn-tectonic formation of two thrust fault systems with dips which were subsequently rotated with bedding during rotation of the sedimentary layers. The intersection of monoclines of this magnitude is rare in the Rocky Mountains allowing for this research to add to the current knowledge of Laramide tectonics and to help further reconstruct the tectonic evolution of the Black Hills uplift.