THREE DIMENSIONAL KINEMATIC EVOLUTION OF THE WYOMING SALIENT, SEVIER OROGENIC BELT

Paleomagnetic studies, strain and AMS analysis, and construction of balanced cross sections constrain the 3D kinematic history of the Wyoming salient of the Sevier orogenic belt. The salient displays over 90◦ of orogenic curvature, which appears related to a combination of primary thickness variations in the sedimentary wedge, differential shortening to maintain orogenic wedge taper, and interaction with Laramide foreland uplifts. Paleomagnetic data from the Triassic Ankareh Formation record systematic counter-clockwise rotations (~10 to 30◦) in the northern part of the salient, with larger (up to 50◦) rotations in frontal thrust sheets that interacted with the Gros Ventre foreland uplift, and variable (~10 to 40◦) clockwise in the southern part of the salient that interacted with the Uinta uplift. A strike test indicates an average of ~ ¼ primary curvature and ¾ secondary rotation to produce orogenic curvature, with additional complications near oblique ramps and transfer zones. Cleavage at high angles to bedding, minor folds, wedge faults, and cross striking veins in the Ankareh and Jurassic Twin Creek Formations record widespread early LPS and minor tangential extension parallel to structural trends. Principal shortening directions estimated from mesoscopic structures, strain, and AMS fabrics fan about the salient, remaining subperpendicular to structural trends, reflecting a combination of initial curvature and secondary rotation of LPS fabrics during thrusting. Strain analysis of deformed fossils and reduction spots indicates that internal shortening varies from generally <10% in frontal thrust sheets (but locally increases toward salient ends), to 20% in more interior sheets. Rotation and strain patterns, combined with offset of local piercing points, indicate that translation was overall eastward, with locally curved and partly fanned displacement trajectories toward the salient ends. A series of backbone cross sections around the salient indicate ~80 to 120 km of bulk shortening from major folds and faults in the Crawford, Absaroka, and Hogsback thrust systems (decreasing toward salient ends), with an additional ~10 to 30 km of internal shortening from strain. Work is underway to construct 3D restorations of swaths centered about the backbone sections, incorporating strain and rotation data.