A STRUCTURAL ANALYSIS OF PRECAMBRIAN SHEAR ZONES AND THEIR INFLUENCE ON LARAMIDE AGE FAULTING: STRATOBOWL AREA, BLACK HILLS, SOUTH DAKOTA, U.S.A

The Black Hills uplift of South Dakota, the easternmost Laramide age (80 – 35 Ma) basement-cored domal uplift, is composed of Paleoproterozoic metamorphosed sediments and Harney Peak Granite. The dominant structural fabric in the metasediments is related to folding about a NNW-trending axes with a steeply east dipping axial planar foliation formed during the Black Hills Orogeny (1,750 Ma). In the Stratobowl area, crosscutting Laramide faults have a similar orientation to ductile shear zones. Two identified zones in the area have an en-echelon pattern with a strike of 340° and steep dips. The rock type in the shear zones is phyllonite containing muscovite, biotite, quartz, and chlorite. Typically these rocks form under plastic conditions at depths between 15 and 25 kilometers. Kinematic indicators in the shear zones include tension gashes, drag folds and quartz fish. The southernmost shear zone exhibits a steep prominent drag fold normal to the direction of shear. The drag fold located on the east side of the shear zone occurs in foliated rocks and has a trend and plunge of ≈310°, 70° and is truncated by the 340° shear. Opposite the drag fold are quartz filled tension gashes formed during rotation with terminal ends approximately 45° to shear. Drag folds and tension gashes indicate left-lateral movement. Quartz mica fish observed in thin sections suggest the southern shear zone also has a minor vertical component of motion (east-side up). Offset of NNW- trending folds indicate a minimum displacement of 200 meters in the NNW – SSE direction. In addition to shear zones, the phyllites are locally brecciated and filled with vein quartz suggesting a Proterozoic age for brecciation.

East of the basement shear zones the Stratobowl fault, a steeply dipping Laramide age structure measuring nearly three kilometers in length has a vertical separation of ≈20 meters. Here, the Stratobowl fault crosscuts Paleoproterozic metasediments and early to middle Paleozoic sedimentary rocks. The Stratobowl fault also intersects a brecciated Proterozic fault following a trend of 353°. North of this intersection, the Stratobowl fault begins crosscutting the NNW trending shear zones and becomes parallel to shear. The fault’s change in trend to 340° suggests a control by the observed shear zones within the highly foliated Precambrian basement.