STRUCTURAL GEOMETRY OF THE EASTERN BLUE RIDGE PROVINCE, CENTRAL AND NORTHERN VIRGINIA

The contact between Mesoproterozoic granitoid gneisses and metasedimentary rocks of the Neoproterozoic Lynchburg Group on the southeastern flank of the Blue Ridge anticlinorium in central and northern Virginia has long been interpreted as a southeast dipping unconformity. The Lynchburg Group consists of metamorphosed arkosic sandstone, graywacke, siltstone, and minor cobble conglomerate deposited in a subaqueous rift setting. We mapped 100 km2 in the eastern Blue Ridge at locations in Nelson and Madison Counties. In Nelson County the basement-cover contact dips to the northwest with basement rocks structurally above the Lynchburg Group. At the contact bedding in metasiltstones and fine-grained metasandstones dips northwest and sedimentary structures indicate that beds are upright. Greenschist facies mylonite zones within the basement complex also dip northwest, have down-dip mineral elongations, and display top-to-the southeast (reverse) kinematic indicators. These observations are consistent with a northwest dipping reverse fault, not an unconformity. The overall structure of the Lynchburg Group in Nelson County is a broad anticline. In Madison County the basement-cover contact dips to the northwest and bedding in Lynchburg metasiltstones near the contact is upright and dips to the northwest. At a few locations blocks of basement granitoids are imbricated with cover rocks. The basement-cover contact interpreted to be a northwest dipping reverse fault in Madison County. At the map-scale the Lynchburg Group is folded into a sequence of tight to open anticlines and synclines that plunge to the gently northeast. The steeply dipping reverse fault at the basement-cover contact in the eastern Virginia Blue Ridge is interpreted to be a structure that accommodated northwest-directed shortening during the Paleozoic and may be related to the movement of Blue Ridge basement over ramps.