Structural geology and tectonics of the paleoproterozoic rocks of the Mount Rushmore Quadangle, Black Hills, Souh Dakota

Abstract

The Paleoproterozoic assembly of Laurentia occurred between ̃1.96 to 1.70 Ga, and continued with the accretion of island arc terranes until about 1.63 Ga. The Black Hills, South Dakota lie along the eastern margin of the Wyoming Archean craton and the western edge of the southern projection of the Trans-Hudson orogen (THO). Precambrian rocks of the Black Hills uplift record multiple deformational and metamorphic events related to three orogenic events between 1̃780 and 1690 Ma. Previous workers ascribed these events to several orogenies based on maxima in distribution of ages. There is a lack of agreement, however, about the relationship of the age maxima to specific deformations, and it is unclear whether or not several could have been accommodated within the relatively short time period. Rocks of the Mount Rushmore quadrangle underwent a ductile folding, intrusion of the 1̃715 Ma Harney Peak granite (HPG), and two periods of metamorphism during this time. Relationships among these processes provide information on the tectonics forces acting along this part of the Wyoming craton's margin. A major structural discontinuity defined by the NNW-trending Keystone and Empire Mine faults separates different structural domains. Domains to the east of the discontinuity are interpreted as "tectonic slivers" that moved unknown distances along the eastern margin of the Archean Wyoming craton. Structural features mapped in the study area and the overall structural trends of the east-central Black Hills may be best explained by an oblique convergence model.