DIFFERENTIAL DEFORMATIONAL HISTORY OF FAULT-BOUNDED BLOCKS: "SOUTHERN TRANS-HUDSON" OROGEN, BLACK HILLS, SOUTH DAKOTA
Our work in the Mount Rushmore 7.5-minute quadrangle in the Black Hills has revealed a key structural discontinuity along the Keystone fault, which trends NNW through most of the Paleoproterozoic core. Rocks NE of the fault have as many as 5 deformational fabrics, with a dominant S2 fabric that is axial-planar to steeply plunging isoclinal folds. Rocks SW of the fault have a markedly different structural history dominated by the metamorphic and structural overprint of the late syn- to post-orogenic intrusion of the 1.715 Ga Harney Peak granite (D3;HPG). No evidence for development of a correlative S2 fabric SW of the fault has been observed. Rocks in close proximity but not immediately adjacent to the HPG have a strong D3 overprint suggesting a similar reorientation of all earlier fabrics in response to the HPG emplacement.
Reported 40Ar/39Ar closure ages from rocks N of the study area exhibit markedly different ages across the inferred trace of the discontinuity. Rocks NE of the discontinuity have 1286±7 to 1457±7 Ma ages, while rocks SW have 1787±8 to 1714±10 Ma ages. Taken together, the structures and ages are consistent with movement of blocks to the E of the discontinuity along a transpressional boundary. Fault-bounded blocks E of the discontinuity may represent terranes moved N as a result of Cheyenne Belt (Dakota Block?) collision following the intrusion of the HPG but prior to the terminal collision between the Wyoming and Superior provinces.