THE BLACK HILLS OF SOUTH DAKOTA: A KEY TERRAIN FOR CONSTRAINING ASSEMBLY HISTORY OF SOUTHERN LAURENTIA

The Black Hills uplift exposes part of a Neoarchean-Paleoproterozoic complex bounded on three sides by fault zones of uncertain geometry that are mostly buried under Phanerozoic cover. These bounding structures juxtapose the Black Hills domain to the Wyoming craton (west and southwest), to the Yavapai arc terrane (south), and to the Superior craton (east and northeast). In the Black Hills proper, a 2600-2560 Ma gneissic basement is overlain by rift-related suites of sedimentary rocks deposited at 2560-2480 and 2100-1880 Ma. A major unconformity separates the two cover sequences and represents a hiatus of at least 380 Myr (i.e., 2480-2100 Ma). Both sequences are transected by younger, N- to NW-trending faults and shear zones, which may have originated during convergence of the Wyoming and Superior cratons and the intervening Dakota block. Two episodes of Neoarchean deformation in the Black Hills are recognized by gneissic fabrics in the basement granites, the younger of which formed during 2560 Ma emplacement of the Little Elk granite into older country rocks. This basement complex and the cover sequences also experienced multiple episodes of Paleoproterozoic thermotectonism between 2480 Ma (intrusion of the Blue Draw gabbro sill) and 1710 Ma (intrusion of Harney Peak granite). Microchronometry of zircon, monazite, and titanite constrains the ages of discrete foliations related to Laurentia assembly. Thus, Black Hills metamorphic rocks record evidence of: N-directed thrusting at 1775 ± 10 Ma (Yavapai accretion?), tectonic burial ca. 1750 ± 10 Ma (onset of ~E-W-directed Wyoming-Superior collision?), faulting associated with syn-collisional uplift/exhumation at 1730 ± 20 Ma, and syn- to post-collisional granite emplacement at 1710 ± 10 Ma. Other recognized features include a weak, NE-trending foliation of 1680 ± 20 Ma age (Mazatzal accretion?) and older, uplift-bounding faults that were reactivated at 60 Ma. Finally, 1810 ± 10 Ma monazite ages are not well correlated to a specific event during Laurentia assembly, whereas a disturbed zircon population of 2250(?) Ma age may correspond to an axial-planar foliation related to an episode of pre-Laurentia compression. Additional ground truth from the Black Hills, coupled with subsurface imaging of unconformities, fabrics, and faults, will enhance reconstruction of the spatio-temporal assembly of southern Laurentia.