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Abstract

The Sevier fold-thrust belt and Laramide foreland comprise two interrelated mountain systems that formed during subduction-related orogenesis along the Cordillera margin of western North America. This field trip integrates field observations from across the two mountain systems with results of recent detailed structural and paleomagnetic studies to develop a tectonic model for evolution of these two classic belts and their relations to plate dynamics. Within the Sevier belt, regional structural relations, synorogenic sedimentation, patterns of internal strain in limestones, and paleomagnetically determined vertical-axis rotations and anisotropy of magnetic susceptibility (AMS) in red beds are examined to better understand processes that lead to systematic curvature in thrust belts. Widespread early layer-parallel shortening (LPS) was accommodated by spaced cleavage, fracture sets, minor folds, and minor faults. LPS directions are subperpendicular to structural trends of systematically curved, thin-skin thrust sheets of the Wyoming salient, reflecting a combination of primary dispersion about an average E-W direction and secondary rotation during thrusting. Rotation was concentrated along the front of a forward propagating wedge, where tectonic stress transmitted from the hinterland, topographic-related stresses, and along strike variations in sedimentary thickness, lithology, and fault strength led to curved thrust slip and differential shortening. Within the Laramide foreland, structural styles of basement-cored arches, sedimentation in basins, paleostress/strain patterns, and combined AMS and paleomagnetism of red beds are examined to test models of foreland deformation and relations to flat slab subduction. Limited LPS was accommodated mostly by minor faults with conjugate wedge and strike-slip geometries. Estimated paleostress directions have a regional WSW-ENE average, but vary from perpendicular to acute to variably trending, thick-skin basement-cored arches. Steep forelimbs display more complex relations, including younger fault sets that developed during evolving stress states and localized, limited vertical-axis rotations. Variations in arch trends and LPS directions are interpreted to partly reflect basal traction during flat-slab subduction beneath thick cratonic lithosphere, combined with spatial-temporal variations in stress/strain fields related to basement heterogeneities and evolving fault systems.

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