Abstract

The Black Hills and Fanny Peak monoclines, which intersect near Newcastle, Wyoming, separate the Black Hills uplift from the eastern Powder River Basin. These forced monoclines are developed over basement faults which have dominantly dip slip. Assemblages of geologic structures associated with the basement deformation zones, however, suggest that components of strike slip are also present.

Between Moorcroft and Newcastle, structures associated with the Black Hills monocline have map aspects consistent with left slip. Pedro and Newcastle terrace are in-line lenses which have releasing and restraining bends within the deformation zone. South of Newcastle, the Fanny Peak monocline has structures suggesting right slip. The assemblage includes parallel forced folds, oblique folds, oblique normal faults, restraining junctions, and joint sets which may represent extensional fractures. North of Newcastle, the Fanny Peak monocline displays a left-slip component introduced by a scissors-like movement. Oblique folds, a parallel forced fold, and an in-line lens constitute evidence for left slip. The Dewey fault zone also has left slip generated by a scissors movement; evidence includes oblique folds, oblique normal faults, and an anastomosing map aspect.

Paleotectonic activity on the principal deformation zones is recorded in Lower Cretaceous rocks which have discrete lenses of sediment coinciding with younger structural terraces. Interpretations of strike slip on the deformation zones imply that regional horizontal compression postulated to produce Laramide structures in the Wyoming foreland may have influenced uplift of the Black Hills. In addition, strike slip interpretations predict the existence of local structural traps and possible fluid migration patterns.