UNMASKING A TERRANE TRIPLE POINT IN CENTRAL VIRGINIA

The central and southern Appalachian Piedmont contains terranes with both Laurentian and Gondwanan affinities that were amalgamated during a series of Paleozoic orogenies. In the southwestern Virginia Piedmont, the Smith River Allochthon (SRA) is interpreted as a regional scale thrust sheet of metaclastic and meta-igneous rocks variously interpreted as either peri-Gondwanan or distal Laurentian by different workers. The SRA was first identified in southern Virginia, and regional-scale maps extend the SRA northeast into the central Virginia Piedmont along the boundary between the Blue Ridge (BR) and western Piedmont (WP). In southern Virginia, the Bowens Creek fault separates the BR from the WP, and regional maps project this fault into central Virginia as a ~1 km wide mylonite zone. We investigate these terranes at the presumed ‘terrane triple point’ in central Virginia.

The eastern BR cover sequence includes arkosic rocks of the uppermost Lynchburg Group, greenstones of the Ediacaran Catoctin Formation, and a wide belt of the Evington Group, primarily a pelitic sequence of metasedimentary rocks interlayered with quartzite, carbonate, and greenstone. Evington Group lithologies crop out in a repeated set of NE-SW striking belts. A monotonous sequence of low-grade quartzose phyllite and metagraywacke characterizes the WP, while higher grade mica schists and amphibolites occur in the southeastern part of the study area. A small inlier (<2 km²) of Grenvillian basement is in tectonic contact with the Evington Group to the northwest, and is unconformably overlain by greenstones to the southeast. The basement inlier requires a significant and previously unrecognized thrust fault at the Blue Ridge-Piedmont boundary in central Virginia. This isolated basement inlier, overlain by greenstones, may have originally developed during hyper-extension along the distal Laurentian continental margin during the opening of the Iapetus. Structural evidence is incompatible with a rootless SRA at this latitude in central Virginia. The BR-WP boundary forms a 3-5 km wide, NE-trending transition zone that separates NW-SE contractional structures of Neoacadian age in the BR from younger dextrally transpressive structures in the WP.