Tectonic evolution of the Sevier and Laramide belts within the North American Cordillera orogenic system
The thin-skin Sevier and thick-skin Laramide belts of the North American Cordillera provide a long-term record of the interrelations between evolving styles of mountain building and plate dynamics over a complete tectonic cycle, from onset of rapid subduction, to protracted growth of a composite orogenic system, to final collapse. Primary architecture of basement and sedimentary cover rocks, which included a thick passive margin section deposited along the western continental margin, influenced patterns of subsequent deformation. The Cordilleran orogenic system, comprised of an interrelated forearc accretionary complex, magmatic arc, retroarc hinterland, Sevier fold-thrust belt, and foreland basin locally deformed by Laramide arches, developed during Jurassic to Paleogene Andean-style subduction and terrane accretion. The Sevier belt formed as a foreland-propagating (west to east) wedge mostly during Cretaceous to Paleogene time, and included a western thrust system with aerially extensive thrust sheets that carried thick passive margin strata, and an eastern thrust system that carried thinner strata. Within the Wyoming salient of the Sevier belt, major thrust and fold traces display systematic map-view curvature about an average N-S structural trend, reflecting a component of primary curvature related to sedimentary prism architecture, followed by 60-80% vertical-axis rotation of thrust sheets related to curved fault slip and interaction with Laramide arches at the salient ends. Internal deformation in the western thrust sheets was limited within strong shallower levels, whereas deeper levels underwent shear and vertical flattening near a weak basal fault zone. Internal deformation in the eastern thrust sheets included widespread early layer-parallel shortening (LPS), followed by concentration of slip onto weak fault zones. Approximately 200 km of thin-skin shortening in the Sevier belt was transferred into lower crustal thickening and uplift of an orogenic plateau in the hinterland to the west. Synorogenic strata were deposited in an evolving foreland basin to the east that formed during flexural loading of thrust sheets and regional dynamic subsidence during subduction. Overall E-oriented shortening in the Sevier belt is interpreted to reflect increased gravitational potential energy and evolving topographic slopes from a hinterland plateau through a growing thrust wedge. Laramide basement-cored arches and intervening basins developed during later Cretaceous to Paleogene time, overlapping with younger stages of Sevier deformation. Arches and associated reverse faults display a wide range of trends within an overall NW-SE oriented, anastomosing network, partly reflecting primary basement heterogeneities. Limited vertical-axis rotations were localized along obliquely trending arch forelimbs and near arch intersections. Internal deformation in the foreland included limited LPS that refracted near variably trending arches. Laramide deformation was spatially and temporally correlated with a region of flat-slab subduction, recorded by changes in magmatic and subsidence patterns. Overall ENE-oriented shortening in the Laramide belt was at low angles to relative plate motion, likely reflecting increased plate coupling near a cratonic lithosphere keel. An integrated model for tectonic evolution of the Sevier and Laramide belts includes: influence of primary sedimentary architecture and basement weaknesses; enhanced plate coupling from increased overriding plate motion, fast convergence rates, and development of a flat-slab segment; linkage of upper crustal shortening in the Sevier belt with lower crustal thickening and uplift of a hinterland plateau; interaction between frontal thrusts in the Sevier belt and Laramide arches with differently oriented stress fields; concentrated slip along weak fault zones; redistribution of mass by erosion and deposition of synorogenic strata; and a switch to orogenic collapse during decreased convergence rates and slab removal.