Linking the Central and Southern Appalachian Blue Ridge: What We Know and Don’t Know about Stratigraphy, Structure, Tectonism, and Regional Correlation in the Eastern Blue Ridge of Virginia

Abstract

The transition from Neoproterozoic Lynchburg Group rocks on the eastern limb of the para-autochthonous Blue Ridge anticlinorium in central Virginia to the fault-bounded Ashe Formation and Alligator Back Formation in southern Virginia has been a source of intense debate and speculation for decades. There are fundamental differences in the tectonogenetic interpretation for these rock packages, despite many similarities in lithology. This problem is compounded by insufficient detailed mapping (1:24,000-scale) in critical key areas of this transition. Lynchburg Group rocks unconformably overlie Mesoproterozoic meta-igneous rocks and underlie Catoctin greenstone on the east limb of the anticlinorium in central Virginia. In southern Virginia, similar metasedimentary lithologies – metasandstone (meta-feldspathic greywacke, meta-quartz arenite, meta-quartz wacke), graphitic schist, and pebble metaconglomerate – and mafic to ultramafic rocks of the Ashe Formation and Alligator Back Formation are faulted onto Mesoproterozoic meta-igneous rocks along the Gossan Lead and Red Valley faults. Internal to the eastern Blue Ridge of southern Virginia, the Rock Castle Creek fault separates Ashe Formation, consisting of Lynchburg-like lithologies, from polydeformed Alligator Back Formation rocks, which crop out in the core of the Ararat River synclinorium. Regional reconnaissance suggests Ashe Formation rocks re-emerge on the eastern limb of the synclinorium. Fundamental and conflicting differences in tectonogenetic models for these rocks compound the problem. The Neoproterozoic Lynchburg Group has long been thought to represent Laurentian margin rift-related rocks, with intrusive mantle-derived dikes and sills of mafic and ultramafic character. In contrast, the Ashe Formation and Alligator Back Formation in southern Virginia and northwestern North Carolina are possibly in part younger, and interpreted to be part of a distal margin accretionary wedge with entrained and tectonically emplaced dismembered ophiolite fragments of mafic and ultramafic rocks. Only detailed mapping in critical areas, coupled with new and emerging geochemical and geochronologic analyses will solve the persistent questions about the various units.