--> The Missing Link: The Role of Basement Anisotropies in the Laramide Structural and Tectonic Evolution of the Wyoming Craton

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The Missing Link: The Role of Basement Anisotropies in the Laramide Structural and Tectonic Evolution of the Wyoming Craton

Abstract

Seismic studies (COCORP, BASE, EarthScope) over the past three decades have provided a better understanding of Laramide tectonism, especially at deeper crustal levels. However, deformational mechanisms in the upper crust related to Laramide contraction, mid-crustal detachment, and subsequent thick-skinned thrusting remain unclear. Internal controls on Laramide tectonism in the upper crust have been proposed to be related to basement anisotropies, which may provide the link between evolution of foreland arches at deeper crustal levels, and structures observed at the surface. This study examines Precambrian anisotropies of the Wyoming craton and provides a hypothesis on their potential role in Laramide orogenisis. Anisotropies are generally oriented in two directions; W-NW and N-NW. They have a complex/long history of deformation since the Precambrian, most recently, during the Laramide when NE-SW-contraction dominated the area. In Wyoming, it is proposed that W-NW structures were displaced as reverse, left-lateral oblique slip faults and, where connected, acted as lateral ramps facilitating major uplifts along the N-NW-trending structures. In Montana, where only W-NW basement structures are present, sinistral slip occurred along high-angle basement-seated faults without the associated vertical slip seen in Wyoming. This is likely due to the absence of N-NW-trending “connections” in basement rocks of Montana and thus explains the lack of Laramide arches in the state. Basement-seated faults are expressed at the surface as oblique, left-slip reverse faults/sinistral strike-slip faults (W-NW deformational zones in Wyoming/Montana) and medium-angle reverse faults/thrust faults (N-NW arches only in Wyoming) that are interconnected as part of a proposed cratonic deformational system. This system includes the Black Hills, which are bound on the east by the southern extension of the west-dipping Cedar Creek fault, likely representing the suture between Archean crust of the Wyoming craton and Proterozoic terranes of the Trans-Hudson Orogen. East-directed thrusting on the Cedar Creek fault during the Laramide likely facilitated uplift of the Black Hills with the Nye-Bowler fault acting as the sinistral lateral ramp. At depth, major thrust/reverse faults likely sole-out into a low-angle decollement in the middle crust. This cratonic deformational system is postulated to be the fundamental tectonic feature controlling formation of Laramide arches at upper crustal levels.