VORTICITY AND STRAIN PARTITIONING BETWEEN AUGEN GNEISS AND HIGH STRAIN ZONES IN THE BLOWING ROCK GNEISS, NORTH CAROLINA BLUE RIDGE

Large parts of the late Mesoproterozoic or early Neoproterozoic Blowing Rock Gneiss inside the Grandfather Mountain window in the eastern Blue Ridge of North Carolina are highly tectonized porphyroclastic augen gneiss, mylonite gneiss, and mylonite. Initial vorticity estimates for the main mylonite foliation based on porphyroclast rotation sense and shape provide sectional kinematic vorticity numbers between 0.6 and 0.8. Shear sense indicators (shear bands, sigma porphyroclasts, asymmetric layer and foliation boudins, asymmetric crenulations, domino porphyroclasts) predict dextral shear within the southerly dipping mylonite foliation reference frame with flow directed generally to the north. The vorticity sense and conditions of metamorphism are consistent with mid- to late-Paleozoic tectonic displacement and flow in the eastern Blue Ridge of North Carolina. Though regionally consistent, locally strain and vorticity are partitioned between the bulk of the Blowing Rock Gneiss massif and narrow high strain zones from a few meters to many meters thick. High strain zone boundaries are continuous transitions within spatially narrow distances. Mylonites within the high strain zones are finer grained than surrounding augen gneiss, strongly laminated, and contain vorticity indicators that are more symmetrical with respect to the enclosing mylonite foliation than those in the gneiss. White mica and chlorite are more abundant in the high strain zones and may form lenticular phyllonite bodies. Feldspar porphyroclasts within the high strain zone are present as symmetrical winged shapes or, more commonly, highly elongate domino stacks with boundary fractures orthogonal to the mylonite foliation. Where they deviate from orthogonal, the domino fragments may show dextral or sinistral shear, particularly near the tips of the domino assemblage. Material continuity across high strain zone boundaries and parallel orientations of dominant foliations and stretching lineations between the zones and augen gneiss (and therefore apparent alignment of vorticity vectors) suggest that flow within the Blowing Rock Gneiss was partitioned between regionally prevalent sub-simple shearing and localized deformation with a higher component of flattening.