DIFFERENTIAL DEFORMATIONAL HISTORY OF FAULT-BOUNDED BLOCKS: "SOUTHERN TRANS-HUDSON" OROGEN, BLACK HILLS, SOUTH DAKOTA

The cratonic core of North America contains a network of Paleoproterozoic orogenic belts that records the 1.91-1.76 Ga Laurentian assembly. These belts include the deformed remnants of accreted island arc terranes, associated intra-oceanic deposits, and the margins of formerly independent microcontinents. The Trans-Hudson orogeny (THO) is considered the culminating event of Paleoproterozoic Laurentian assembly. However, the nature and timing of THO events in southern Laurentia is poorly understood relative to the exposed northern segments in Canada. The Black Hills offer the only surface exposure of the deformed and metamorphosed belt of Paleoproterozoic continental margin rocks in the collisional zone between the Archean Wyoming and Superior provinces. Based on geophysical evidence, this zone has been broadly interpreted to be the southern extension of the THO that was truncated by the ~1680 Central Plains orogen.

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.