FOLIATION TRANSPOSITION AND VORTICITY IN A LATE PALEOZOIC OROGEN-PARALLEL SHEAR ZONE IN THE SOUTHERN APPALACHIAN MOUNTAINS

The Brevard fault zone is the westernmost orogen-parallel shear zone in the Southern Appalachian Mountains. Late Paleozoic vorticity indicators in the fault zone imply dextral shear in transpression. Stretching lineations, shear bands, folds and asymmetric porphyroclasts are widespread in the fault zone. Some of these indicators are the result of finite strain over multiple deformational events, but others formed during incremental deformation accompanied by partial retrograde metamorphism. Mylonite foliations and compositional layering are mesoscopically parallel, yet in hand specimens the mylonites have discontinuous flaser layering, augen structures, and asymmetric crenulations in schists. Microscopically, nodular masses within the foliation envelope are remnant closures of disarticulated isoclinal folds. These masses include lenticular quartz mosaics, fine white mica lenticles, internally foliated garnets, and muscovite porphyroclasts. Intensity of transposition and superposition of layer-parallel extension via asymmetric shear bands coincident with retrograde metamorphism distinguish mylonites from the wall rock of the Brevard fault zone. Shear bands provide an estimate of vorticity, at least for the later stages of deformation, with dextral asymmetry and band-parallel displacement. The maximum asymmetry vector is approximately parallel to strike of the shear zone. The shear bands are distinctive micro-shear zones of finite thickness in which quartz and white mica lenses are progressively recrystallized and into which the chloritic tails of rotated retrograded garnets curve. Microlithon stretching lineations similarly curve into the deformation bands, but extended and neo-mineralized grains within shear bands also lie in the same foliation-normal plane. This vorticity plane is coincident with the strike of the fault zone, and is consistent with dextral transpression. Intercalated gneiss and micaceous mylonites express an affine vorticity sense with asymmetric boudins of weaker layering. Overall, structural elements in the Brevard fault zone agree with a finite sense of nearly horizontal dextral motion where latter stages of shear involved extension of the foliation and maximum incremental vorticity in agreement with dextral transpression.