NEW INSIGHTS ON THE KINEMATIC EVOLUTION OF THE WYOMING SALIENT, SEVIER THRUST BELT

Integrated analysis of internal strain, paleomagnetism, and regional structural relations constrain the 3-D kinematic evolution of the Wyoming salient of the Sevier thrust belt. Limestones of the Jurassic Twin Creek Formation and redbeds of the Triassic Ankareh Formation display spaced cleavage, vein and fracture networks, minor folds, ane minor faults that accommodated early layer-parallel shortening (LPS), minor strike-parallel extension, and local shear. Grain-scale strain estimated from deformed fossils and reduction spots displays systematic regional variations. Principal shortening directions are subperpendicular to structural trends around the salient. This radial pattern reflects a combination of initial curvature and vertical-axis rotation of LPS fabrics during thrusting. Principal shortening values vary from <5% in the frontal Hogsback thrust system, to ~20% in the more interior Crawford system; internal strain also increases toward the salient ends. Redbeds display a near-primary Triassic (Tr) paleomagnetic component carried by hematite and a Cretaceous (K) chemical remagnetization carried partly by magnetite. Restored site means for Tr and K components have well clustered inclinations, but display variable declinations related to systematic vertical-axis rotations. Statistical analysis of both components indicates that ~75% of present-day salient curvature resulted from secondary rotation, with ~25% primary curvature related to sedimentary basin architecture. Analysis of individual thrust sheets indicates only slightly greater rotation in more internal sheets, suggesting that rotations were concentrated near the leading edge of the propagating thrust wedge. Transfer zones and oblique ramps display locally more complex patterns. Regional cross sections through the salient indicate a decrease in fold-thrust shortening toward the southern part of the salient, and displacement transfer between frontal thrusts that interacted with foreland uplifts toward the northern part of the salient. Strain, paleomagnetic, and regional structural relations are consistent with a kinematic model involving curved thrust slip and minor wrench shear that produced progressive secondary curvature during growth of the thrust wedge.