T18 Regional Tectonics of Basement-Cored Foreland Shortening: Integrating Geological and Geophysical Insights from Laramide and Analogous Orogens

Sunday, 28 October 2007
8:00 AM - 12:00 PM

DISTRIBUTED BASEMENT DEFORMATION IN LARAMIDE DOMAL GENESIS, EASTERN BLACK HILLS UPLIFT, SOUTH DAKOTA, USA

Paper number: T18-7
Program: Technical Sessions
Time: 9:20 AM – 9:35 AM

Authors

Abstract

The eastern flank of the Black Hills uplift extends 300 km from southwestern Montana to southwest South Dakota as a broad half-dome. Outcrop patterns of Phanerozoic strata from Montana to the latitude of Rapid City generally trend N.40W.: South of Rapid City across a distance of 50 km, the strike changes to N.30E. forming a broad reverse-C pattern. Greatest structural relief across the uplift (~2,100 m) occurs in the area of the curving flank where exposures of the Proterozoic Trans-Hudson Province are exposed in the topographic Black Hills. The basement fabric of faults and multiply deformed metasedimentary and metaigneous rocks parallel gravity survey high and low contours and SSE-plunging anticline-syncline pairs in the Phanerozoic strata, suggesting a basement control for later folding. With few exceptions, the multiple Laramide folds of the uplift are west-vergent forming asymmetric anticlines on the east side and monoclines on the west, including those separating the uplift from the adjoining Powder River basin.

Recent 1:24,000-scale geologic mapping in the Cretaceous shale section exposed in the prairies southward from Rapid City shows that the strata do not curve in a continuous arch. Rather, a mosaic of SE-plunging anticline-syncline pairs (to 40 km long) and east-plunging, south-facing monoclines bound triangular, gently east-dipping, planar segments (to 10 km on a side). North-trending structure contour lines in each segment show a consistent right-step across such folds and structural relief to 100 m. The half dome does not form a continuous arch, therefore, but results from integration of adjustments on multiple basement-cored blocks.