KINEMATIC ANALYSIS OF THE GARTH RUN SHEAR ZONE IN THE BLUE RIDGE PROVINCE OF VIRGINIA

Ductile shear zones are commonly developed in the Appalachian mountain belts. In the Canadian Appalachians (Newfoundland, Nova Scotia), a lot of these shear zones exhibit strain geometries suggesting triclinic transpressional deformation paths. We have investigated a well-exposed ductile shear zone in the Blue Ridge province of Virginia, the Garth Run shear zone, in order to reveal its kinematic history. The Garth Run shear zone is developed in the Livingston Group consisting of leucogranite, charnokinte, metabasalt and pegmatites. The shear zone strikes nearly north-south and dips east at about 40 degrees. Mylonitic C-foliation is well developed throughout the zone, and there is a compositional layering parallel to the C-foliation defined by transposed and boudinaged pegmatite layers, flattened pegmatite boudins with tails, and flattened metabasalt. There is a down-dip stretching lineation on the mylonitic C-foliation. Fabric asymmetry is observed both on the section parallel to the lineation and perpendicular to the C-foliation (down-dip section) and perpendicular to the stretching lineation (along-strike section), producing a triclinic symmetry. On the down-dip section, the asymmetry is defined by an S-C fabric indicating a top to west thrusting sense of shear, while on the along-strike section the asymmetry is defined by pegmatite boudins with tails suggesting a sinistral sense of shear. A C'-foliation is visible on the along-strike section, but the sense of shear varies between samples along strike of the zone. It is possible the sinistral shear is an overprinting on the thrusting sense of shear. We thus conclude that the Garth Run shear zone has a thrusting-dominated sense of movement overprinted by a sense of movement with more sinistral strike-slip component. Whether the earlier thrusting-dominated sense of movement has a significant strike-slip component and therefore has produced a triclinic fabric in the mylonite is being investigated by microstructural analysis. We also propose a tectonic model for the kinematic history of the Garth Run shear zone.