LAYER PARALLEL SHORTENING ACROSS THE CORDILLERA OF WYOMING: SPATIAL AND TEMPORAL VARIABILITY DURING THE SEVIER AND LARAMIDE OROGENIES

Anisotropy of magnetic susceptibility and detailed structural data were collected in Triassic/Jurassic red beds and limestones from over 400 sites across Wyoming to access the spatial and temporal variations of layer parallel shortening (LPS) fabrics within the western Cordilleran orogen. Sampling was concentrated in the Sevier thrust belt of eastern Idaho, northeastern Utah and western Wyoming, and in multiple Laramide arches of various dimensions and orientations from central and eastern Wyoming. Additionally, paleomagnetic samples were collected from all of the studied sites. Demagnetization results from the red beds are frequently characterized by a near primary remanent magnetization that is used for quantifying vertical-axis rotations. Accordingly, all LPS fabrics are restored to their original orientation, and thus are more readily interpreted in terms of their spatial and temporal variability. Trends of LPS data from the Sevier thrust belt systematically vary around the salient, and are roughly perpendicular to the main structural trend. When restored for secondary rotations, LPS trends are roughly E-W in orientation across the Wyoming salient. Further into the foreland, which is dominated by overall slightly younger basement-cored Laramide uplifts, LPS directions display a more complex pattern. In the simplest structures, shortening directions are generally NE-SW, with localized deflections by as much as 90 degrees associated with clockwise and counter-clockwise wrench-shear rotation along steeper forelimbs of arches. Minor temporal changes in shortening direction are recorded in some structures, which can result from changes in far-field stress directions, or contemporaneous changes in structural trend with a consistently oriented far-field stress. LPS in the Sevier thrust belt is interpreted to partly reflect stresses transmitted from the hinterland through the growing orogenic wedge and topographic stresses along the front of the wedge. LPS in the Laramide foreland is interpreted to partly reflect basal traction during flat slab subduction beneath cratonic lithosphere, with spatial-temporal variations in stress related to basement anisotropy and evolving fault systems.