STRUCTURAL ANALYSIS OF AN EARLY PROTEROZOIC SHEAR ZONE AND ITS TECTONIC IMPLICATIONS FOR THE EARLY PROTEROZOIC EVOLUTION OF THE WYOMING PROVINCE, HOMESTAKE MINE AREA, BLACK HILLS, SD USA

The Pre-Cambrian core of the Black Hills Uplift is a window into the structural and tectonic evolution of the eastern margin of the Wyoming craton. In this study, we focus on a shear zone localized on the west limb of the Lead anticlinorium. The anticlinorium is a moderately SE-plunging fold with a steeply east-dipping foliation (local D1 and regional D2). The west limb of the anticlinorium is overprinted by a high strain deformation (local D2). In the higher strain west, local D2 is associated steeply plunging NE-trending reclined folds and lineation. Folds are tight to isoclinal and locally have sheath fold geometries. Toward the east, in lower strain zones, linear structural elements have steep SE plunges that finally give way to earlier moderately southeast plunges. In the shear zone, all linear features lie in the high strain zone(s) that is steeply dipping and strikes 015°. The kinematic indicators in the shear zone are left-lateral and east-side-up indicating sinistral-oblique motion. These results agree with previous structural studies in the Homestake mine ~2.5 km vertically below this study area. The shear zone is coincident with a vertical N-S striking garnet isograd, and garnet-biotite geothermometry of metapelites sampled across the anticlinorium indicates a steep metamorphic field gradient of 150 °C/km (east side warmer). The shear zone deformation is associated with peak metamorphic to retrograde metamorphic assemblages and the age is constrained by PbSL analysis monazite inclusions in garnet that revealed a 207Pb/206Pb age of 1746±10 Ma, which is an upper limit for the local D2 deformation. A lower limit is inferred from an overprinting crenulation cleavage (local D3) that has been previously interpreted to be a distal effect of emplacement of Harney Peak age (1.715±3 Ma) S-type granite emplaced NE of the anticlinorium. These ages are similar to those obtained form total-Pb age from the EMP and EMP Ultrachron. The origin of the steep metamorphic gradient may be explained by juxtaposition of warmer rocks in the east against cooler rock and/or advective heating by fluids along the shear zone. Zones of apparent offset occur in other areas of the Pre-Cambrian core and may have significant vertical and perhaps lateral displacements that occurred in the early stages of doming and emplacement of S-type granite.