GEOLOGY AND STRUCTURAL HISTORY OF THE BLUE RIDGE BASEMENT COMPLEX IN ALBEMARLE COUNTY, VIRGINIA

The Mesoproterozoic (1,180-1,030 Ma) basement complex in the Virginia Blue Ridge consists of several mineralogically and texturally heterogeneous units. Near Crozet, Virginia, five compositionally distinct rock types occur and include: 1) layered gneiss, 2) charnockititc gneiss, 3) biotite granitoid gneiss, 4) leucogranite, and 5) megacrystic granitoid. Layered gneiss consists mainly of orthogneiss with minor inliers of quartzitic paragneiss. Charnockitic gneiss and layered gneiss have a weakly to moderately developed high-temperature foliation that formed under upper amphibolite to granulite facies metamorphism during Grenvillian orogenesis. Hypersthene-bearing charnockitic gneiss grades into biotite granitoid gneiss across ~100 m transitional zones containing hypersthene, uralite and biotite. Leucogranite and megacrystic granitoid intrude these older units and lack the high-temperature foliation.

Most basement units have a Paleozoic greenschist facies foliation with a SE plunging mineral elongation lineation. Foliation strikes NE-SW and dips moderately to the SE, commonly overprinting the high-temperature fabric in the older units. Several NE-SW trending high-strain zones, including the correlated Rockfish Valley and White Hall high-strain zones, show top to the NW movement. Granitic and gneissic protoliths are transformed into protomylonite, mylonite and ultramylonite across 1-3 km wide zones of increased deformation intensity. Lenses of relatively less deformed rock occur within the anastomosing zones of heterogeneous strain. R_f/Φ strain analysis of mylonites yields moderate to high strain ratios (R_s>5) in both the lineation parallel/foliation normal and lineation normal/foliation normal planes consistent with flattening strain. Kinematic vorticity (W_m) analysis reveals general shear and asymmetries on the lineation normal plane suggest a component of triclinic strain symmetry. Characterization of the relationship between deformation intensity and strain enables the construction of a retro-deformable cross section with which to quantify the displacement and throw across the basement core of the Blue Ridge. Estimates suggest displacement of 0.5-4 km with throws less than 3 km, placing an upper limit on ductile displacement across Paleozoic high strain zones.