KINEMATIC ANALYSIS OF STRUCTURAL FABRICS ASSOCIATED WITH BLACK HILLS OROGENY AND THEIR TECTONIC IMPLICATIONS FOR THE SOUTHEASTERN MARGIN OF THE WYOMING CRATON

The Black Hills Orogeny (~1750-1795 Ma) produced the dominant NNW structural fabric in the Black Hills Precambrian core. The age of this fabric forming event is well constrained by monazite growth at ~ 1750 Ma. This event culminated in emplacement of the Harney Peak Granite that occurred from ~1715-1695 Ma. Careful structural analysis in four different areas including the Lead window, the Stratobowl area, Bear Mountain Dome, and Custer State Park allow the kinematic evolution to be better understood. Strain partition In the Lead window, show a progression of structures related to the Black Hills orogeny, including shallowly to moderately plunging, tight to isoclinal folds with associated NW striking east dipping axial planar foliation. These structures are locally overprinted by steeply east plunging fold and NNW striking axial planar foliation that is very steeply dipping associated with high strain zones. Regionally, these events have been referred to as D2 and D3 deformation events. Kinematic indicators including gash veins and fish structures show both east-side-up and left-lateral shear senses associated with D3. Spectacular examples of D3 high strain zones are observed in the Stratobowl area. These discrete shear zones crosscut D2 structures and have calculated offset of 80 meters. At Bear Mountain, microstructural studies indicate both top-to-the-north and west-side-up shear sense associated with doming or D4 structures in a shallowly east dipping foliation. In Custer state park on the SE margin of the Harney Peak Granite, deformed granite dikes indicate a top-to-the-NNW sense of shear. The kinematic observations when combined with known metamorphic conditions suggest a transpressional orogeny event that involved: 1) Dominantly left-lateral shearing associated with a shallow to moderately plunging stretch. 2) Increased shortening associated vertical stretch and the initiation of doming and decompression. 3) Continued left-lateral shearing and top-to-the-north over during granite and pegmatite intrusions.