Crustal structure of the Bighorn Mountains region: Precambrian influence on Laramide shortening and uplift in north‐central Wyoming

Abstract

The crustal structure of north-central Wyoming records a history of complex lithospheric evolution from Precambrian accretion to Cretaceous-Paleogene Laramide shortening. We present two active source P wave velocity model profiles collected as part of the Bighorn Arch Seismic Experiment in 2010. Analyses of these velocity models and single-fold reflection data, together with potential field modeling of regional gravity and magnetic signals, constrain crustal structure and thickness of the Bighorn region. We image a west dipping reflection boundary and model a sharp magnetic contact east of the Bighorn Arch that together may delineate a previously undetected Precambrian suture zone. Localized patches of a high-velocity, high-density lower crustal layer (the "7.× layer") occur across the study area but are largely absent beneath the Bighorn Arch culmination. Moho topography is relatively smooth with no large-scale offsets, with depths ranging from~50 to 37 km, and is largely decoupled from Laramide basement topography. These observations suggest that (1) the edge of the Archean Wyoming craton lies just east of the Bighorn Mountains, approximately 300 km west of previous interpretations, and (2) Laramide deformation localized in an area with thin or absent 7.× layer, due to its relatively weak lower crust, leading to detachment faulting. Our findings show that Precambrian tectonics in northern Wyoming may be more complicated than previously determined and subsequent Laramide deformation may have been critically dependent on laterally heterogeneous crustal structure that can be linked to Precambrian origins.The potential role of Archean (>2.5 Ga) and Proterozoic (0.541-2.5 Ga) structures in the development of the Laramide orogeny (~75 Ma to~40 Ma) has been suggested by numerous studies that note contrasting structural patterns in the southern and northern Rocky Mountain provinces [Brown, 1993;Erslev and Koenig, 2009;Stone, 1986]. Deformation, probably driven by flat-slab subduction [e.g., Bird, 1988;Coney and Reynolds, 1977;Dickinson and Snyder, 1978], led to basement-involved thick-skinned shortening during the Laramide orogeny (~75 Ma to~40 Ma) [Dickinson et al., 1988], forming basement-cored foreland arches. In the Proterozoic basement of Colorado, Laramide arches typically strike north-northwest, are separated by narrow basins, and form a well-defined, en echelon deformation front on their eastern margin. In the large Archean basement of Wyoming, Laramide structures exhibit a larger variety of strikes and are separated by wide basins, with no well-defined deformation front. These contrasting geometries of thick-skinned deformation have been attributed to the greater diversity of structural fabrics in the Archean Wyoming basement [Brown, 1993;Erslev and Koenig, 2009] and different crustal architectures. Most notably, the high-velocity, high-density lower crustal WORTHINGTON ET AL.CRUSTAL STRUCTURE OF THE BIGHORN ARCH 208

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

“…The nature of these faults at greater depths is even more uncertain. Previous studies in the region have identified vertical seismic velocity gradient increases at ~25 km depth, thus defining a midcrustal transition zone (Gorman et al, 2002;Worthington, et al, 2016). This midcrustal transition has been identified at ~6.5 km/s and has been interpreted as either a crustal detachment surface (Erslev, 1993) or, where velocities exceed 7 km/s, a mafic underplate (Snelson et al, 1998;Gorman et al, 2002; see also Fig.…”

“…The major thrusts are the thick-skinned, basement-involved, Laramide structures, with dips of 30°-45° at the surface (Smithson et al, 1979;Stone, 2003;Weil et al, 2016), and movement on these faults formed the prototypical Laramide arches of Wyoming. Subsurface studies have suggested that these structures may sole out into a midcrustal detachment zone at depths of ~25 km (Blackstone, 1990a;Erslev, 1993;Yeck et al, 2014;Worthington et al, 2016). However, the Consortium for Continental Reflection Profiling (COCORP) line across the southern Wind River thrust shows a relatively consistent apparent fault dip (38°) to ~25 km, below which the reflector disappears (Smithson et al, 1979).…”

“…Thus, there is no direct evidence of a listric geometry with the fault soling into a crustal detachment. In addition, recent seismic studies have not imaged a planar detachment surface at midcrustal depths for the Bighorn arch or Piney Creek thrust (Worthington et al, 2016).…”

The distribution and composition of high‐velocity lower crust across the continental U.S.: Comparison of seismic and xenolith data and implications for lithospheric dynamics and history

“…Doublets are predominant in this area in Figure b, and SW Vs models show some of the highest velocities and thickest high‐velocity crust here. However, a recent refraction experiment in northern Wyoming (profile 44 in Figure a) suggests that high‐velocity lower crust is locally discontinuous in the region and apparently absent directly beneath the Bighorn Mountains [ Worthington et al ., ; Yeck et al , ].…”

U–Pb baddeleyite crystallization age for a Corson diabase intrusion: possible Midcontinent Rift magmatism in eastern South Dakota

“…Studies of the Precambrian development in this region are hindered by the lack of basement outcrops (with the exception of the Black Hills), lack of high resolution aeromagnetic and other geophysical data (for the Dakotas, in particular), and a lack of drill holes to basement across much of the area. Yet, underlying this region are significant basement terranes and tectonic boundaries, including the suture between the Wyoming and Superior cratons beneath the Dakotas (Dahl et al 2005;Worthington et al 2015;Kilian et al 2016), the southern terminus of the Trans-Hudson orogen, and the southwestern margin of the Superior craton (Figures 1, 2). The nature of these boundaries is of fundamental importance to unraveling the Precambrian evolution of the North American craton.…”