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.