SINISTRAL SHEARING OF ARCHEAN BASEMENT IN LITTLE ELK CANYON, BLACK HILLS, SD; AN ALTERNATIVE HYPOTHESIS

The easternmost flank of the Wyoming Province crops out in Little Elk Canyon, Black Hills, SD as one of only two Archean basement exposures in the uplift. The basement rocks exposed in the canyon have been subdivided into two lithologies, the Little Elk Gneissic Granite (LEG) in the eastern portion of the exposure and the Biotite Feldspar Gneiss (BFG) in the western portion. The coarse-grained LEG is a muscovite-biotite granite that exhibits a moderately developed, NW-striking foliation with local 5–10 cm wide shear zones parallel to this foliation. This fabric becomes increasingly more penetrative as the contact with the BFG is approached from the east. The BFG is compositionally similar to the LEG except for being finer grained and having a penetrative, NW-striking, mylonitic fabric defined by biotite and quartz banding. In both units the NW-striking fabrics are locally rotated to NE-striking. Shear-sense indicators in both units include sigma and delta grains (mm to cm in diameter) which support left-lateral offset. Recent interpretations call for intrusion of LEG (2559 ± 6 Ma) into the BFG (2563 ± 6 Ma); however, the overlapping ages allow for an alternative hypothesis. Any model proposed must include the following field relationships; 1) they have similar composition, except for grain size which can be explained by grain-size reduction during mylonitization, 2) parallel shear fabrics with similar shear sense and 3) the increase in fabric development in the LEG as the BFG is approached from the east. We propose exposures of Archean basement in Little Elk Canyon were a single granitic intrusion later cut by a wide, left-lateral shear system partitioned primarily into the BFG portion of the exposure, giving it the gneissic fabric characteristic of this unit. The shear deformation and related fabric dissipates to the east into the less strongly foliated LEG. This shear fabric continues into the overlying supracrustal rocks requiring the shearing event to be younger than deposition of these rocks, and presumably associated with ca 1780-1715 Ma deformation of supracrustal rocks across the Black Hills uplift. Furthermore, if the newly-recognized mylonitic shear zone near Rockerville, SD, 30 km along strike to the southeast, is related to the shearing described above, then these rocks may record a significant tectonic event.