VORTICITY AND KINEMATICS FROM FELDSPAR PORPHYROCLASTS IN THE BLOWING ROCK GNEISS, GRANDFATHER MOUNTAIN WINDOW, NORTH CAROLINA

Massifs of tectonized augen gneiss form large parts of the Middle Proterozoic Blowing Rock Gneiss within the Grandfather Mountain window in the Blue Ridge of North Carolina. The window is an Alleghanian structure, but the pervasive tectonite fabric in augen gneiss is a consequence of mid-Paleozoic events superposed on Grenville fabrics. Sections of the augen gneiss contain multiple Late Proterozoic dikes that are now greenstone metadiabases exhibiting strong cleavage contemporaneous with primary schistosity in augen gneiss wall rocks. We have initiated analyses of the kinematics in these rocks by documenting feldspar porphyroclast core-mantle asymmetries, semi-rigid rotated skeletal feldspar porphyroclast stacks, flow path plots of rotated augen, and mesoscopic fold vergence. Our kinematic finite strain reference frame is the augen gneiss primary schistosity and mineral stretching lineation. The schistosity has a uniform NE-SW strike and SE dip; the lineation is southeasterly. Delta porphyroclasts are the predominate core-mantle particles, although sigma shapes are also present. Both are strongly biased toward sinistral rotations directed up dip of the reference plane, that is to the northwest. Domino-like porphyroclast stacks are remarkable in some sections, greatly extended into the schistosity, and uniformly also show vorticity senses up dip of the foliation. Flow path analysis of several hundred rotated augen in multiple sample sections indicate a kinematic vorticity number of approximately 0.8. There are localized shear gradients within the augen gneiss across which the vorticity number varies, but overall these data are consistent with other kinematic indicators. Tight, highly asymmetric folds in veins similarly verge up dip and to the northwest. We have yet to examine the three dimensional aspects of vorticity in these rocks, but the general sense is of, in present coordinates, flow toward the northwest during a thoroughly pervasive Paleozoic tectonic event.