LATE PRE-CAMBRIAN CONTINENTAL RIFTING IN THE SOUTHERN BLUE RIDGE, VA-NC-TN, REVISITED: HONORING 50 YEARS OF DOUG RANKIN’S CONTRIBUTIONS TO APPALACHIAN GEOLOGY

Field mapping begun by Doug Rankin 50 years ago in the southern Appalachian Blue Ridge, VA–NC–TN, identified calc-alkaline granites, and peraluminous rhyolites and related clastic rocks as evidence for the opening of the Iapetus ocean. Those studies form the foundation for recent geochronology and field studies that provide modern insight into stratigraphic and structural relationships during Proterozoic and Paleozoic Appalachian orogenesis. The previous interpretation of a single rift event is now established as two distinct events: 780–700 Ma and 575–555 Ma. Clastic rocks, basalt, and rhyolite of the Mount Rogers Formation mark the earlier event; these are dispersed in three volcanic centers (Pond Mountain, Mount Rogers, and Razor Ridge) within separate Paleozoic thrust sheets (Catface, Stone Mountain, Shady Valley). The basal contact of the Mount Rogers Formation with 1.3–1.06 Ga Mesoproterozoic basement rocks is a major unconformity that had considerable topographic relief, and is locally strongly deformed along the southeast margin. Detrital zircon populations between 780–730 Ma, and rhyolite clasts suggest initiation of rifting by 780 Ma followed by cannibalization of older volcanic deposits. The oldest rhyolites dated are in the Catface thrust sheet, which consists of a ~760 Ma coarse-grained rhyolite porphyry surrounded by clastic rocks and tuffs. At least four younger rhyolite members, 755–749 Ma, comprise the Mount Rogers center that overthrusts the Catface sheet along the Stone Mountain fault. The occurrence of rhyolite interbedded with glaciogenic lacustrine (some containing dropstones) and fluvial rocks in the Shady Valley sheet suggests that volcanism and glaciation were locally contemporaneous at ~750 Ma. The underlying Mountain City window ends in the northeast as an antiformal syncline (20°, N 65° E). Mylonites along the Catface thrust contain porphyroclastic k-feldspar phenocrysts and a strong muscovite-chlorite fabric, and along with kinematic indicators record top-to-the-NW lower-greenschist facies ductile deformation related to Alleghanian emplacement of the Blue Ridge thrust sheet. These recent studies may provide new links with other Neoproterozoic rift-related rocks in the Blue Ridge and other cratons.