VORTICITY, KINEMATICS AND PARTITIONING IN PORPHYROCLASTIC GNEISS

Porphyroclastic augen gneiss, mylonite gneiss and mylonite compose the dominant tectonite fabric in Mesoproterozoic or early Neoproterozoic Blowing Rock Gneiss in the eastern Blue Ridge province of the Southern Appalachian Mountains in North Carolina. The protolith of these tectonites was a porphyritic granitic plutonic rock. Mid- to late-Paleozoic deformation produced a strong mylonitic schistosity and moderate stretching lineation in the rocks. The massif containing Blowing Rock Gneiss experienced pervasive ductile deformation in an early compressional event and then superimposed non-pervasive ductile to brittle deformation during late Paleozoic Alleghanian thrusting. The gneiss in its various stages of deformation contains a wide range of kinematic indicators including rotated porphyroclasts, deformed porphyroclasts, shear bands, foliation boundins, asymmetric folds and domino porphyroclasts. All of these structures are components of the pervasive tectonite fabric and reflect what appears to be the same dextral shear sense that when aligned to the vorticity vector predicts top-to-northwest shear and displacement. Strain is, however, partitioned among coarse augen gneiss with rigidly rotated clasts, strongly foliated porphyroclastic gneiss with sigma and delta clasts, flaser laminated porphyroclastic gneiss with domino or lenticle porphyroclasts, and strongly laminated mylonite or phyllonite. Sectional kinematic vorticity estimates in the coarser works yields vorticity numbers between 0.6 and 0.8, but these values are influenced by spawned neoporphyroclasts from domino feldspar grains. These cleavage fragments partially reset the vorticity gauge and may result in differential estimates of the vorticity number depending on what strain patch is being analyzed. Nonetheless, the cumulative analytical result of strain in these rocks is that of prevalent sub-simple shearing with localized zones of higher strain and flattening.