EXCEPTIONS TO UNIFORM RIGID AUGEN ROTATION AND VORTICITY IN A BLUE RIDGE CRYSTALLINE MASSIF

Initial vorticity and kinematic analyses of porphyroclasts in augen gneiss of the Blowing Rock Gneiss within the Grandfather Mountain window, North Carolina Blue Ridge, give kinematic vorticity numbers between 0.6 and 0.8. These are sectional flow values in the vorticity normal surface (VNS), that in present orientation is an approximately north-south trending vertical plane. Sub-simple shearing is confirmed by shear bands, asymmetric passive folds, and non-rigid core-mantle asymmetries in porphyroclasts. Shear sense is sinistral around the vorticity vector resulting in top-to-north displacement schistosity. The porphyroclast array in these rocks contains many intact potassium feldspar grains, often single crystals, but there are also semi-rigid rotated skeletal feldspar porphyroclast stacks (dominoes) and sigma or delta clasts. Some intact clasts are cleavage fragments detached from progenitor grains. Chains of spawned fragments may be connected by quartz or quartz-feldspar umbilicals. Minimally separated domino porphyroclasts are evolutionarily between progenitor grains and fully separated spawn. Progenitor decomposition begins along cleavage planes at a high angle to the stretched and/or rotated long axes of these grains. Those axes lie at a low angle to schistosity. At some critical value of separation (which may be an indirect measure of vorticity and/or change in rheology) the rigid fragments are displaced and rotated within the enclosing ductile schistosity. Given the takeoff orientations of spawned grains, their orientations relative to schistosity may be "reset" to a high angle, thus reducing the local vorticity number. Rather than fully separate, some domino clasts form millipedes - asymmetrically folded dominoes, where the kinematic sense mimics that of other sinistral indicators in the augen gneiss. Consequently, aggregated kinematic vorticity determinations in these rocks may produce an erroneously low number, that is, underestimate the simple shearing component of finite strain.