Vorticity Assessment in Non-Uniformly Deforming Augen Gneiss

Kinematic vorticity assessments from feldspar porphyroclasts in augen gneiss depend on several key assumptions. Chief among these are steady state flow and clast rigid stability. Many naturally occurring tectonites, on first approximation, appear to justify these assumptions. For example, many porphyroclasts are individually intact or have minimal deformation rims. Other tectonites are mixtures of clasts with features suggesting rigid behavior and clasts with incipient segmentation or recrystallization tails. Overall bulk strain in the latter suggests rather uniform flow and internally consistent shear sense, yet the range of porhyroclast morphologies indicate a more complex path. Augen gneiss from Proterozoic Blowing Rock Gneiss bodies in the Southern Appalachian Blue Ridge demonstrate these complex morphologies. Kinematic analysis of porphyroclast core-mantle asymmetries, semi-rigid rotated skeletal feldspar porphyroclast stacks, flow path plots of rotated augen, and mesoscopic fold vergence determine kinematic vorticities between 0.6 and 0.8 in these rocks. Shear sense from all kinematic indicators is sinistral, that translates here into top to the northwest movement. There are localized shear gradients within the augen gneiss - ductile shear zones - across which the vorticity number varies, but overall these data are consistent with other kinematic indicators. The wide range of porphyroclast morphologies (sigma, delta clasts; domino stacks; rigid grains) is partly the result of an inherited fabric in the augen gneiss from Grenville deformation. In context, however, many feldspar clasts while appearing structurally intact clearly originated as segments from larger porphyroclasts at some earlier stage of what is inferred to be a single continuous deformation. Some angular clasts separated by narrow quartz-feldspar umbilicals can be reassembled into protolith grains. Strings of rotated augen in some cases are disarticulated members of pegmatitic veins. The emerging kinematic picture is one of semi-rigid feldspar rotation in a sub-simple shearing regime, but also one in which at least some older porphyroclasts spawned new ones during flow. In this case, numerical estimates of kinematic vorticity could be misleading. Rotation values for spawned porphyroclasts are dependent on the flight angle and orientation at the time of initiation, and on the bulk kinematic flow parameters.