Basement-cover relations, Sevier orogenic belt, northern Utah

Abstract

Precambrian crystalline basement and sedimentary cover rocks were deformed within a regional anticline in the Wasatch Range of northern Utah during the Cretaceous to early Tertiary Sevier orogeny. This regional fold had a complex uplift and deformation history and is an imbricated ramp anticline that marks the transition to basement-involved deformation. Large-scale thrusting and folding within the basement-cored anticline produced about 60% shortening, and most of the slip on thrusts within the anticline was transferred into slip on frontal thrusts of the Idaho-Utah-Wyoming thrust belt. A network of ductile deformation zones (DDZ9s) accommodated heterogeneous internal deformation of the basement, and cleavage, minor folds, and vein arrays accommodated internal deformation of the sedimentary cover. Kinematics of internal deformation are consistent between basement and cover rocks in most areas and vary systematically with structural position, recording early layer-parallel shortening, localized layer-parallel shear and minor detachment of the cover, and later fanning of structures during large-scale folding. DDZ sets with opposite shear senses accommodated bulk coaxial flattening perpendicular to cleavage away from major thrust faults, and DDZ9s with consistent shear sense produced simple shear near major thrusts. Cleavage intensifies and refracts and minor folds tighten in the cover near major thrusts and glide horizons. Vein arrays record principal extension down the dip of cleavage, minor extension parallel to the regional fold axis, and differential displacements at high angles to fold axes. Slip on basement DDZ9s produced about 5% to 20% bulk shortening subparallel to the basement-cover contact. Shortening in the cover ranges from about 5% to 30% in most areas but reaches 65% near imbricate thrusts and along glide horizons, reflecting heterogeneous simple shear. Principal extension is down the dip of cleavage, and minor extension parallels local fold axes, consistent with the geometry of mesoscopic structures.