KINEMATIC HISTORY OF GRANITIC GNEISSES AT THE FALLS OF THE NOTTOWAY IN THE CAROLINA TERRANE, EASTERN PIEDMONT, VIRGINIA

The Falls of the Nottoway River form a major knick zone in the southern Virginia Piedmont and exposes a set of large bedrock outcrops. We studied these exposures to better understand the structural geometry and kinematic history of deformation near the boundary between the Carolina and Raleigh Terranes. Rocks exposed at the Falls of the Nottoway include a variety of granitic gneisses with biotite-rich gneissic inclusions, and two-sets of late-stage K-feldspar pegmatite dikes. Horton et al. (1995) report a U-Pb zircon age of 295 to 315 Ma from these rocks, making them part of the widespread suite of Alleghanian granitic plutons in the southern Appalachians. Two sub-vertical sets of N- and NNE-striking pegmatite dikes intrude the older granitic rocks.

Gneissic rocks are L>S tectonites with a N- to NE-trending subhorizontal penetrative lineation, and a weak N- to NE-striking subvertical foliation. The lineation is defined by rod-like quartz and feldspar aggregates with aligned biotite and muscovite. Fabric analysis of quartz aggregates yields K-values of 1.2 - 2.5 (apparent constriction) and D-values between 0.3 - 1.2. Microstructures preserved in the granitic gneisses are consistent with deformation at the amphibolite facies (500 - 600˚ C). These rocks experienced general shear with a dextral asymmetry, and record NW-SE contraction and strike-parallel (orogen-parallel) elongation.

Pegmatite dikes are cut by a plethora of quartz veins and fractures. Quartz precipitated within dilational shear fractures indicating that these features are contemporaneous. The majority of the fractures, including all right-lateral fractures, are predominantly E-striking, while those with left-lateral kinematics are N-striking. There are two sets of quartz veins with N- and E-strikes. Shear fractures and quartz vein orientations are consistent with a NW-SE oriented σ₁ during brittle deformation.