THE RELATIONSHIP BETWEEN GRAIN SHAPE AND CRYSTALLOGRAPHIC ORIENTATION OF QUARTZ PORPHYROCLASTS FROM A MID-CRUSTAL SHEAR ZONE, ROCKFISH VALLEY DEFORMATION ZONE, VIRGINIA

The Rockfish Valley Deformation Zone (RVDZ) in the Blue Ridge Mountains in central Virginia exposes protomylonites and mylonites deformed at lower to upper greenschist facies conditions, and therefore represents an ideal natural laboratory to investigate deformational processes in the mid-crust. The RVDZ is a 1-3 km thick NE-SW striking ductile shear zone that dips moderately to the SE. Well-developed kinematic indicators, such as asymmetric porphyroclasts and S-C’ structures, consistently suggest top-NW shear, and regional relationships suggest it formed during late Paleozoic contraction. Although the mylonitic rocks are lithogically heterogeneous, some samples show isolated quartz porphyroclasts that record varying degrees of strain. Axial ratios of these grains are highly variable, ranging from near 1 up to 4.2 and beyond. We used electron backscatter diffraction to investigate the relationship between crystallographic orientation and deformation in quartz. Analysis of 352 porphyroclasts reveals an asymmetric type 1 cross girdle crystallographic fabric. Misorientations across subgrain boundaries suggest activity of many slip systems in quartz, including basal<a>, prism<a>, and rhomb<a>, as well as prism<c+a> and prism<c>. However, comparison of the orientations of high versus low axial ratio grains indicates that basal<a> was most important for porphyroclast deformation. Quartz c-axes from the relatively undistorted low axial ratio grains plot as a single girdle oriented perpendicular to the macroscopic foliation. In contrast, the c-axes in the high axial ratio grains define small circles around the pole to foliation. These more deformed crystals are ideally oriented for activity the basal <a>, supporting the idea that this slip system tends to most readily accommodate quartz deformation in greenschist facies conditions.