Structural Analysis of a Larmide-Age, Basement-Involved, Foreland Fault Zone, Rawlins Uplift, South-Central Wyoming
The western border of the Hanna Basin is defined by the Rawlins uplift, a north-northwest–south-southeast-trending, Laramide-age, basement-involved, faulted arch that separates the Hanna Basin on the east from the Great Divide Basin on the west. Detailed geologic mapping and serial cross-section construction, in conjunction with seismic-reflection data, indicate that the breaching faults along the western margin of the Rawlins uplift are not responsible for the net structural relief (~37,000 and ~27,000 vertical feet from the adjacent basins to the east and west, respectively) manifested in this Laramide structural feature. These exposed fault traces are interpreted to be high-angle (~70º) splays off of a shallowly dipping (<30º), blind, master fault zone within Archean granitic rocks of the Wyoming province. A low-dipping duplex structure within these Precambrian basement rocks is inferred to accommodate much of the fault displacement and consequently account for the structural relief that exists between the core of the uplift and the adjacent basins. Displacement along the breaching faults decreases from south to north. Within the map area, bedding attitudes along the west limb (forelimb) of the uplift range from ~30–90º with only local areas of overturned stratigraphic units. In contrast, the backlimb of the uplift dips shallowly (~10–15º) and homoclinally east-northeast into the Hanna Basin. Within the study area, deformation of the basement rock suggests a spectrum of mechanical behavior defined by the end-member processes of broad-scale basement folding and rotation of discrete, fault-bounded, composite blocks of basement and cover rocks.