A REFINED MODEL FOR TECTONIC EVOLUTION OF THE SEVIER AND LARAMIDE BELTS

One of the most interesting topics in the evolution of the North American Cordillera concerns connections between thin-skin Sevier and thick-skin Laramide deformation. Herein multiple data sets are integrated to develop a refined model for tectonic evolution of the Sevier to Laramide belts, including influences of primary crustal architecture, changing plate dynamics, stress transfer, and topography. The Sevier belt comprises a western thrust system with large, thick thrust sheets developed in passive margin strata, and an eastern system with closer spaced thrusts developed in thinner strata. Laramide arches formed in cratonic basement farther east. The western system experienced early fluid influx and internal deformation from ~140-120 Ma, possibly related to initial hinterland thickening, with large-scale thrusting and deposition of synorogenic strata in a flexural foredeep from ~125-90 Ma. Internal deformation was limited at upper levels, whereas lower levels underwent vertical thinning and simple shear above a weak basal fault zone with high fluid pressure. The eastern system experienced overall W-E shortening from ~110-85 Ma, favored by increased burial, fluid flow, and stress near the advancing orogenic front. Deformation then became concentrated along eastward propagating thrusts from ~85-50 Ma, synchronous with deposition of synorogenic strata in the foreland basin. Major thrust/fold traces display >90° of curvature related to the primary sedimentary prism and ~75% secondary rotation during thrusting. Thin-skin shortening was balanced by lower crustal thickening in the hinterland that likely developed into a broad plateau. The Laramide foreland experienced overall WSW-ENE shortening, with local refraction related to crustal weaknesses. Deformation became concentrated along faults with uplift of anastomosing basement-cored arches from ~65-50 Ma. Within the Sevier belt, stress transmitted from the hinterland through the thrust wedge combined with topographic stress along the wedge front to favor curved thrust slip and rotation. Within the Laramide foreland, stress was partly related to basal traction during flat-slab subduction beneath thick cratonic lithosphere, with variations in paleostress trajectories related to basement heterogeneities and evolving fault systems.