Tectonic evolution of a Laramide transverse structural zone: Sweetwater Arch, south central Wyoming

Abstract

Structural, anisotropy of magnetic susceptibility (AMS), and paleomagnetic data record patterns of layer‐parallel shortening (LPS), vertical‐axis rotation, and regional fault‐fold evolution across the Sweetwater Arch, a major west to WNW trending, basement‐cored Laramide uplift in Wyoming. The southern arch flank is bounded by a WNW striking reverse fault zone that imbricated basement and cover rocks, the northern flank is bounded by a west striking fault zone with a component of strike‐slip and NW trending en echelon folds, and the eastern plunge transitions into an area of multiple‐trending faults and folds. Synorogenic strata record major arch uplift from Maastrichtian to Early Eocene time, followed by arch collapse. LPS, with development of systematic minor fault sets and AMS lineations, preceded large‐scale folding. LPS directions, estimated from both minor fault and AMS data, were oriented WSW along the northern flank, subparallel to Laramide regional shortening, but were refracted to the SSW along the southern flank, and to the west along the eastern arch plunge. Additional minor faults developed along steep fold limbs during continued shortening, with directions remaining SSW along the southern flank but becoming more variable along the eastern plunge where an increasingly heterogeneous stress field developed as additional faults were activated along basement heterogeneities. Vertical‐axis rotation was limited along the arch flanks, whereas the eastern plunge underwent counterclockwise rotation. Deflections in shortening directions were partly related to basement heterogeneities, including weak supracrustal belts on the arch flanks, a strong granitic core, and local reactivation of Precambrian shear zones.

Structural inheritance and the role of basement anisotropies in the Laramide structural and tectonic evolution of the North American Cordilleran foreland, Wyoming

“…Studies of structural and anisotropy of magnetic susceptibility data for the southern Bighorn arch and the Sweetwater uplift area (Weil et al, 2014(Weil et al, , 2016 provided definitive evidence supporting model 3. Weil et al (2016) suggested that reactivation of preexisting (Precambrian) basement weakness and heterogeneities, and flat-slab subduction during regional WSW-ENE shortening were the major influences on regional Laramide stress/strain, along with lesser amounts of transpression and creation of localized wrench-related structures in sedimentary cover across Wyoming.…”

“…This interconnected pattern of deformation is present across Wyoming ( Fig. 2; Erslev, 1993), and it suggests that the Laramide-style arches of Wyoming may be the product of a fundamental, multicomponent structural/tectonic system that I postulate had beginnings with initial formation during the Archean and culminated in the Laramide orogeny, as suggested by Weil et al (2014Weil et al ( , 2016. The late Archean (2.9-2.5 Ga) convergent history and continental growth of the Wyoming Province are discussed with specific emphasis on the relation of pre-Laramide cratonic deformation to Laramide orogenesis.…”

“…Three general kinematic models have been proposed to explain the structural style and various orientations of Laramide arches and uplifts across Wyoming (Weil et al, 2016).…”

“…Folding of sedimentary cover ranges in trend from N-S to E-W, exhibiting varying degrees of vergence, with both right-and left-stepping en échelon zones formed locally (Fig. 3;Weil et al, 2016). Reference to "Laramide" in this paper refers to this structural style only.…”

“…Some major Laramide faults, such as the Piney Creek and Wind River thrusts, have dips of 30°-45° (i.e., moderate-angle reverse faults; Smithson et al, 1979;Stone, 2003), and movement on these faults has created kilometers of structural relief relative to surrounding basins (Snoke, 1993). Based on the observed configuration (attitude and slip) of major Laramide faults on the surface and in basement rocks of Wyoming (Paylor and Yin, 1993;Ver Ploeg and Greer, 1997;Molzer and Erslev, 1995;Stone, 2002 and Weil et al, 2014Weil et al, , 2016, numerous oblique-slip faults (uplifts striking WNW and NE) appear to be present and are interconnected with large thrust and/or moderate-angle reverse faults ("prototypical" arches striking NNW). This interconnected pattern of deformation is present across Wyoming ( Fig.…”