Unmasking a Terrane Triple Point in Central Virginia

Abstract

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 peri-Gondwanan or distal Laurentian regional scale thrust sheet of metaclastic and meta-igneous rocks. 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. This study investigates these terranes at the presumed 'terrane triple point' in central Virginia. The eastern BR cover sequence includes Neoproterozoic Lynchburg Group, Ediacaran Catoctin Formation, and a wide belt of the Ediacaran-Early Cambrian Evington Group. 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 of the Spears Mountain terrane (SMT) occur in the southeastern part of the study area. A small inlier (<2 km 2) of Grenvillian basement is in tectonic contact with the SMT. The basement inlier requires a previously unrecognized thrust fault at the Blue Ridge-Piedmont boundary in central Virginia. Structural evidence is incompatible with a rootless SRA 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. The Alleghanian Caskie fault truncates the transition zone and emplaces SMT lithologies against the BR and WP.