SOUTHERN BLUE RIDGE TERRANES AND PROBLEMS WITH ROCK UNITS, AGES, AND TIMING OF EVENTS: READ THE DETAILED GEOLOGIC MAPS

The western Blue Ridge (WBR) contains pre-Grenville (1.6 Ga, NC) to Early Devonian (AL) rocks that record Appalachian evolution from rifted margin to stable Laurentian platform (Shady Valley syncline). The Talladega belt-Murphy Group could contain Middle Ordovician (or even younger) rocks and the Middle Ordovician unconformity. Validity of reported mid-Paleozoic fossils from the WBR (SE TN) is questioned because some sample localities occur in well-mapped (1/12,000-scale) sequences conformably overlain by fossiliferous Lower Cambrian Chilhowee Group rocks. The central Blue Ridge contains rocks associated with particular thrust sheets, suggesting several new small terranes could be defined. The Cowrock terrane contains predominantly Coweeta Group rocks and is bounded by the Hayesville and Soque River thrusts (W, SW, SE), and the structurally higher Chunky Gal Mountain-Shope Fork (CGSF) sheet (NE); the Cartoogechaye terrane contains a mixture of possible Tallulah Falls and Coweeta Group rocks, Grenville basement fragments, the largest and most abundant mafic-ultramafic complexes, and the 470 Ma Taconian granulite facies metamorphic core; it is bounded by the Hayesville fault (NW), the CGSF fault (SW, S), and the Soque River fault (SE). These terranes occupy higher grade thrust sheets structurally above the lower grade Dahlonega gold belt (DGB), like the higher grade Chattahoochee thrust sheet (CTS, Tugaloo terrane) to the SE. The Rabun Granodiorite contains both magma flow and tectonic foliations, is concordant with the synmetamorphic S2 foliation, is truncated by the later-folded CTS, and has U-Pb ages of 374 and 335 Ma. ~350 Ma metamorphic ages (Ar/Ar, monazite electron microprobe, and U-Pb ion microprobe zircon rims) in the WBR and EBR suggest a major Late Devonian-early Mississippian thermal-deformational event that is compatible with map, documented fossil, and fabric relationships from VA to AL. Post-350 Ma ages are consistent with uplift accompanying emplacement of the Blue Ridge-Piedmont megathrust sheet.