STRUCTURAL ANALYSIS OF A LARAMIDE-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 roughly northwest–southeast-trending, Laramide, basement-involved, faulted arch that separates the Hanna Basin from the Green River Basin. The Rawlins uplift is a macroscale, asymmetric, fault-propagation fold that verged southwest. Detailed, new geologic mapping and serial cross sections combined with seismic-reflection data indicate that the surface-breaching faults, exposed along the western flank of the Rawlins uplift, are not responsible for the net structural relief (in excess of 27,000 feet) manifested in this Laramide uplift. These surface fault traces are probably splays off a shallowly dipping, blind, master fault zone in the Precambrian basement rocks (chiefly Archean granitic rocks of the Wyoming province). The surface-breaching faults reduce in displacement from south to north within the map area. The west limb (forelimb) of the fold is moderately to steeply dipping with attitudes ranging from 30–90°. Within the map area, only local areas of overturned strata exist. However, to the southwest of the map area, broad regions of overturned strata were mapped by Barlow (1953, Ph.D. dissert., Univ. of Wyoming). The backlimb is a shallow, northeast-dipping homocline. Numerous "parasitic" structural features are superimposed upon the large-scale fold of the uplift. The dramatic eastward bends at the southern and northern ends of the uplift are significant in that they: (1) are roughly orthogonal to the general strike of the uplift; (2) imply some component of strike-slip displacement along the southern and northern ends of the uplift; and (3) are inferred to be localized along pre-existing basement weaknesses or discontinuities, which are responsible for the abrupt change in strike of the Laramide fault trace(s). Although the timing of the Rawlins uplift is still poorly bracketed, evidence from the adjacent Hanna Basin suggests that the structural evolution of the Rawlins uplift was chiefly during the late Paleocene. The Rawlins uplift thus is an important tectonic feature in the progressive structural partitioning of the greater Green River Basin during the latter part of the Laramide orogeny.