EVOLUTION OF THE WESTERN CORDILLERA MARGIN: THROUGH THE SEVIER AND INTO THE LARAMIDE

One of the most interesting and oft debated topics in the tectonic evolution of North America is the transition from and interrelations between the thin-skin Sevier fold-thrust belt and thick-skin Laramide foreland. Although decades of high quality structural, geophysical, sedimentologic, and geochronologic studies have led to general consensus on the overall geometry and timing of the Sevier and Laramide systems, geodynamic links between the systems, relations to the ancient Cordilleran margin, and mechanical response of North American lithosphere to inferred flat-slab subduction remain areas of active research. Here we review results of integrated structural, anisotropy of magnetic susceptibility, and paleomagnetic studies across the Sevier and Laramide of Wyoming and develop a conceptual model linking the two belts in terms of primary crustal architecture and evolving subduction dynamics. Within the Sevier belt, shortening directions are subperpendicular to structural trends of systematically curved thrust sheets of the Wyoming salient, reflecting a combination of primary dispersion and secondary rotation during thrusting. Within the Laramide foreland, shortening directions along variably oriented, anastomosing basement-cored arches vary from perpendicular to acute with structural trends, with more complex relations and localized vertical-axis rotations along steep flanks of some arches. Shortening directions vary on average from W-E in the Sevier belt to WSW-ENE in the Laramide foreland. Shortening in the Sevier belt is interpreted to partly reflect stress transmitted from the hinterland through a growing orogenic wedge, topographic stress along the front of the wedge, and local stress deflections related to nature of the primary sedimentary prism. Shortening in the Laramide foreland is interpreted to partly reflect basal traction and hydrodyanamic forces during flat-slab subduction beneath thick cratonic lithosphere, with spatial-temporal variations in paleostress trajectories related to basement heterogeneities and evolving fault systems. With recent advances in resolution of seismic tomography under North America, preserved mantle structures can be linked to ancient plate-boundary evolution, allowing robust integration of geologic observations with geodynamic models.