TESTING MODELS OF LARAMIDE FORELAND DEFORMATION: CONSTRAINTS FROM STRUCTURAL AND PALEOMAGNETIC ANALYSIS OF THE TRIASSIC CHUGWATER GROUP, WYOMING

First-order questions regarding foreland deformation center on geodynamic processes that form basement arches and relationships to plate margin stresses. Ongoing structural, anisotropy of magnetic susceptibility (AMS), and paleomagnetic studies of Triassic redbeds around fold structures along the flanks of the Wind River, Owl Creek, Big Horn, and Sweetwater arches in Wyoming constrain models for origins of diversely trending Laramide foreland structures. Minor fault systems in structurally simple settings include conjugate contraction faults with dip slip and conjugate tear faults with strike slip, which maintain consistent geometries with respect to bedding around folds, indicating development during early layer parallel shortening (LPS). Kinematic analysis of early minor faults and AMS fabrics along gentle backlimbs yield LPS directions ranging from 030 to 090 degrees, recording minor spatial dispersion about a regional 060 shortening direction; this dispersion is partly related to underlying basement anisotropy. Steep forelimbs display more complex patterns, including later reverse and strike-slip faults that overprint early LPS structures. Early LPS directions and paleomagnetic declinations are rotated clockwise/ counter-clockwise respectively along more northerly/ easterly trending forelimbs, recording localized wrench shear. Shortening directions for later faults that have not been rotated trend NE. In contrast LPS directions in the thin-skin Sevier fold thrust belt trend overall E after corrections for rotation. A model for Laramide foreland deformation combines spatial and minor temporal variations in stress directions, with localized wrench shear along variably oriented arches. Regional shortening directions are consistent with stresses partly originating from relative motion between the North American and low-angle subducted oceanic plates, with local modification by basement anisotropy and stress concentrations along propagating faults.