Reconstructing the kinematic evolution of curved mountain belts: Internal strain patterns in the Wyoming salient, Sevier thrust belt, U.S.A.

Abstract

Analyses of mesoscopic structural and strain patterns in red beds of the Triassic Ankareh Formation and limestones of the Jurassic Twin Creek Formation, in concert with complementary paleomagnetic studies, constrain the three-dimensional kinematic evolution of curved fold-and-thrust systems in the Wyoming salient of the Sevier thrust belt. Spaced cleavage, fracture and vein networks, minor folds, and minor faults in limestones and red beds accommodated early layer-parallel shortening (LPS) concentrated in front of the growing thrust wedge, along with minor strike-parallel extension and wrench shear related to the development of orogenic curvature. Strain, estimated using mass balance relations for cleavage seams, crinoid ossicles in bioclastic limestone, and reduction spots in red beds, displays systematic regional patterns. Principal shortening directions are subperpendicular to structural trend around the salient, reflecting a combination of primary curvature and secondary rotation of early LPS fabrics. LPS magnitudes vary from <5% in central parts of the frontal Hogsback thrust system, where cleavage is absent, to 10%–30% in the more interior Crawford thrust system, where cleavage intensity is moderate to strong; strain also increases toward the salient ends. Internal strain is a significant component of total deformation and should be considered when restoring cross sections. Strain patterns are consistent with a kinematic model involving curved fault slip and differential shortening that produced progressive secondary curvature during thrusting.