Adolph Yonkee, Weber State University
Arlo Weil, Bryn Mawr College
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The Sevier and Laramide belts of the North American Cordillera orogenic system provide a long-term record of evolving mountain building styles that developed during changing plate dynamics related to subduction. Primary architecture of the basement and sedimentary cover, which included thick passive margin strata deposited along the continental margin, influenced patterns of subsequent deformation. The Sevier fold-thrust belt formed as a foreland-propagating wedge during Early Cretaceous to Paleogene time and included a western thrust system that carried passive margin strata, and an eastern thrust system that carried thinner strata. Within the Wyoming salient of the Sevier belt, major thrusts display map-view curvature, reflecting a component of primary curvature related to sedimentary prism architecture, followed by ~70% vertical-axis rotation related to curved fault slip and interaction with Laramide arches at the salient ends. Internal deformation in western thrust sheets was limited within shallower levels, whereas deeper levels underwent heterogeneous shear and vertical flattening near a weak basal fault zone, partly reflecting strain softening. Internal deformation in eastern thrust sheets included early layer-parallel shortening (LPS), followed by concentration of slip onto weak fault zones that experienced concentrated fluid influx. 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. Overall E-directed shortening in the Sevier belt partly reflects topographic stresses from the hinterland plateau through a thrust wedge with a weak base. Synorogenic strata were deposited in an evolving foreland basin to the east that formed by flexural loading and regional dynamic subsidence during shallowing subduction. Laramide basement-cored arches developed across part of the foreland 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-oriented, anastomosing network. Limited vertical-axis rotation was localized along obliquely trending arch forelimbs. Internal deformation included limited LPS that refracted across variably trending arches. Laramide deformation was correlated with a region of flat-slab subduction, with overall ENE-oriented shortening at low angles to relative plate motion, reflecting increased plate coupling. An integrated model for evolution of the Sevier and Laramide belts includes: influence of primary basement and sedimentary architecture; enhanced plate coupling during increased plate convergence rates; linkage of upper crustal shortening in the Sevier belt with lower crustal thickening beneath a hinterland plateau; concentrated slip along weak fault zones; redistribution of mass by erosion and deposition of synorogenic strata; and development of Laramide arches above a flat-slab segment; followed by a switch to orogenic collapse during decreased convergence rates and slab removal.

Session

Tectonics of Western North America:Tectonics of Western North America: What's new?
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