Integrating structural, paleomagnetic, and thermo/geochronologic studies to understand evolution of the Sevier and Laramide belts, northern Utah to Wyoming

Abstract

Quantifying the kinematic evolution of an orogenic system, recorded in its 3-D displacement field, across a range of spatial and temporal scales is challenging, yet crucial for quantifying crustal shortening budgets, determining origins of mountain belt curvature, restoring paleostress-strain trajectories, and understanding relations of orogenesis to plate dynamics. In this review paper, we integrate structural, paleomagnetic, and thermo/geochronologic data sets to explore evolution of the Sevier fold-thrust and Laramide foreland belts within the North America Cordillera. These belts record an entire orogenic cycle, from onset of shortening to extensional collapse, and formed within different crustal rock packages and during changing subduction dynamics. Like other Cordilleran-style systems, these belts show progressive deformation stages that include a pre-orogenic stage in the distal foreland, an early layer-parallel-shortening (LPS) stage in the proximal foreland, and a main stage of large-scale faulting-folding. These belts also display differences in structural styles, timing, shortening directions, and rotation patterns. The Sevier belt has a thin-skin style with a western thrust system that developed in thick passive margin strata and an eastern thrust system that developed in transitional strata. The Laramide belt has a thick-skin style with reverse faults that continue to mid-crustal depths and developed in cratonic basement overlain by a thin sedimentary cover. Thrusts in the Sevier belt were emplaced progressively forelandward (W to E) spanning 125–50 Ma and formed a tapered wedge, whereas Laramide arches were uplifted from 70 to 50 Ma and separated by broad basins. Large-scale folds and thrusts of the Wyoming salient of the Sevier belt display curvature about a regional N–S trend and accommodated ∼170 km (∼50%) overall W-E shortening. The western thrust system experienced early layer-parallel shortening (LPS) at upper levels with a switch to shear and vertical flattening by viscous flow at deeper levels (T > 300 °C). The eastern system experienced early LPS (5–20%) with development of cleavage and minor faults. Paleomagnetic analysis shows systematic vertical-axis rotations that increased curvature and led to a final radial pattern of LPS directions. Fluids promoted vein formation during episodes of high fluid pressures and alteration concentrated along weak fault zones. Laramide arches form an anastomosing network about a NW-SE structural grain and accommodated ∼50 km (10%) regional SW-NE shortening. The foreland experienced limited (<5%) LPS with development of minor faults and refraction of paleostress directions along variably trending arches. Differences in shortening directions, paleo-topography, and geographic distribution of the Sevier and Laramide belts suggest differences in origins and transmission of paleostress. Topographic stresses were likely important within the Sevier belt that formed a low-taper wedge tied to the hinterland with W-E shortening subperpendicular to the prior passive margin. Stress transfer from a flat slab was likely important for the Laramide belt, with SW-ENE shortening subparallel to the direction of relative plate motion. Although integrated studies have improved our understanding of Cordillera-style systems, key questions remain with future research areas including geodynamic modeling of oblique convergence, importance of terrane accretion, nature of fluid pressure transients and fault strengths, and modulation of orogenic systems by internal feedbacks.