TECTONIC EVOLUTION OF THE LARAMIDE AND SEVIER BELTS, NORTH AMERICAN CORDILLERA: VARYING RESPONSES OF LITHOSPHERE TO CHANGING SUBDUCTION PATTERNS

First-order questions persist regarding continental deformation along Cordilleran-style plate margins, including relations between thin-skin and thick-skin shortening, influence of primary crustal architecture, and relations to subduction dynamics. The thin-skin Sevier and thick-skin Laramide belts of the North American Cordillera preserve a long-term record of intraplate shortening within different types of lithosphere during changing subduction patterns. The Sevier belt formed in thick passive margin to transitional strata deposited above previously rifted lithosphere, whereas the Laramide belt formed in cratonic lithosphere with a thin sedimentary cover above strong basement. The Sevier belt developed as a forward-propagating (west to east) fold-thrust wedge from Early Cretaceous to Paleogene time during increased relative plate motion and absolute W-motion of the overriding North American plate toward the plate margin. The Laramide belt developed as an anastomosing network of basement-cored arches and broad basins during Late Cretaceous to Paleogene SW-NE subduction of a low-angle to flat-slab segment. Paleostress-strain fields, estimated from minor fault, cleavage, and AMS data, were different for the two belts, ranging from regional W-E shortening in the Sevier belt, interpreted to reflect stresses transmitted via a topographically high hinterland, to regional WSW-ENE shortening at low angles to relative plate motion in the Laramide belt, interpreted to reflect increased coupling between a lithospheric keel and flat-slab segment. Refraction of shortening directions and regional vertical-axis rotations in the Sevier belt appear partly related to curved topographic slopes, whereas refraction and local rotations along oblique arches in the Laramide appear related to basement weaknesses. The stress-strain field changed during termination of low-angle to flat-slab subduction with southward rollback, enhanced magmatism, and extensional collapse. A conceptual model for tectonic evolution of the Sevier and Laramide belts is presented, but questions remain concerning the nature of stress transfer via end loading, basal traction, and hydrodynamic forces, and feedbacks between hinterland thickening, foreland shortening, lithosphere removal, and subsidence/uplift histories.