ABSTRACT The southern Appalachian western Blue Ridge preserves a Mesoproterozoic and mid-Paleozoic basement and Neoproterozoic to Ordovician rift-to-drift sequence that is metamorphosed up to sillimanite grade and dissected by northwest-directed thrust faults resulting from several Paleozoic orogenic events. Despite a number of persistent controversies regarding the age of some western Blue Ridge units, and the nature and extent of multiple Paleozoic deformational/metamorphic events, synthesis of several multidisciplinary data sets (detailed geologic mapping, geochronology and thermochronology, stable-isotope chemostratigraphy) suggests that the western Blue Ridge likely records the effects of two discrete orogenic events. The earlier Taconic (470–440 Ma) event involved a progression from open folding and emplacement of the Greenbrier–Rabbit Creek and Dunn Creek thrust sheets as a foreland fold-and-thrust to low-grade hinterland system (D 1A ), followed by deep burial (>31 km), pervasive folding of the earlier-formed fault surfaces, and widespread Barrovian metamorphism (D 1B ). Because this high-grade (D 1B ) metamorphic event is recorded in Ordovician Mineral Bluff Group turbidites, this unit must have been deposited prior to peak orogenesis, possibly as a foreland basin or wedge-top unit in front of and/or above the developing fold-and-thrust belt. The later Alleghanian (325–265 Ma) event involved widespread northwest-directed brittle thrusting and folding related to emplacement of the Great Smoky thrust sheet (D 2 ; hanging wall of the Blue Ridge– Piedmont thrust). Mid-Paleozoic 40 Ar/ 39 Ar muscovite ages from western Blue Ridge samples likely record post-Taconic cooling (hornblende and some muscovite 40 Ar/ 39 Ar ages) and/or Alleghanian thrust-related exhumation and cooling (ca. 325 Ma muscovite 40 Ar/ 39 Ar and 300–270 Ma zircon fission-track ages), as opposed to resulting from a discrete Neoacadian thermal-deformational event. The lack of evidence for a discrete Neoacadian event further implies that all deformation recorded in the Silurian–Mississippian(?) Maggies Mill–Citico Formation must be Alleghanian. We interpret this structurally isolated sequence to have been derived from the footwall of the Great Smoky fault as an orphan slice that was subsequently breached through the Great Smoky hanging wall along the out-of-sequence Maggies Mill thrust.

Abstract Recent field and associated studies in eight 7.5-minute quadrangles near Mount Rogers in Virginia, North Carolina, and Tennessee provide important stratigraphic and structural relationships for the Neoproterozoic Mount Rogers and Konnarock formations, the northeast end of the Mountain City window, the Blue Ridge–Piedmont thrust sheet, and regional faults. Rocks in the northeast end of the Mountain City window constitute an antiformal syncline. Overturned Konnarock and Unicoi formations in the window require a ramp-flat geometry in the hanging wall of the Blue Ridge thrust sheet or stratigraphic pinch-out of the Konnarock Formation. Undulose and ribbon quartz, fractured feldspars, and mylonitic foliations from the Stone Mountain and Catface faults indicate top-to-NW motion, and ductile deformation above ∼300 °C along the base of the Blue Ridge thrust sheet on the southeast side of the window. The Stone Mountain fault was not recognized northeast of Troutdale, Virginia. The Shady Valley thrust sheet is continuous with the Blue Ridge thrust sheet. The ∼750 Ma Mount Rogers Formation occurs in three volcanic centers in the Blue Ridge thrust sheet. Basal clastic rocks of the lower Mount Rogers Formation nonconformably overlie Mesoproterozoic basement in the northeasternmost Razor Ridge volcanic center, but the basal contact in parts of the Mount Rogers and Pond Mountain volcanic centers is strongly tectonized and consistent with a NW-directed, greenschist-facies high-strain zone. The contact between the Mount Rogers Formation and Konnarock Formation is nonconformable, locally faulted. Metarhyolite interbedded with lacustrine and fluvial rocks suggests that volcanism and glaciation were locally coeval, establishing an age of ∼750 Ma for the Konnarock Formation, a pre-Sturtian glaciation. Multiple greenschist-facies, high-strain zones crosscut the Blue Ridge thrust sheet including the Fries high-strain zone (2–11 km wide). Foliations across the Fries and Gossan Lead faults have similar orientations and top-to-NW contractional deformation.

In the Appalachian orogen, the Neoproterozoic(?)–Lower Cambrian Chilhowee Group represents the initial drift-facies deposits along and across the eastern Laurentian continental margin following rifting. In the Southern Appalachians, this group forms thrust sheets along the west flank of the Talladega–Blue Ridge belt. Where the base is unfaulted, it lies depositionally above Ocoee Supergroup rift-facies rocks or Grenville basement. Regionally, the Chilhowee grades up into the Lower Cambrian Shady Dolomite, the initial deposits of the marginwide Cambrian–Ordovician carbonate bank. Sequences more interior to the orogen, including the Kahatchee Mountain Group (Talladega belt), the Nantahala and Brasstown Formations (western Blue Ridge), and the Hollis Quartzite (Pine Mountain belt), are considered to be correlative with the Chilhowee based upon similarities in lithostratigraphic sequence, sequence stratigraphy, sandstone provenance, and paleocurrent studies. Assuming an autochthonous Pine Mountain window, palinspastic restorations of foreland thrusts suggest that the Chilhowee Group restores essentially astride that window, and Chilhowee-equivalent units in the Talladega–Blue Ridge belts, in turn, restore farther southeast. This places the respective sequences southeastward in the order of increasing thickness and depth to basement from the base of the carbonate bank facies, with units restored farthest southeast having the most distal marine characteristics. Retro-deformation of thrust belt structures and the Pine Mountain cover sequence restores the Kahatchee Mountain Group at least to the subsurface position of the Wiggins-Suwannee suture, the southeastern limit of Laurentian continental crust, indicating that this group's basement was subducted beneath Gondwanan or peri-Gondwanan crust, and that the basement of even more outboard Laurentian sequences (e.g., eastern Blue Ridge) was overridden even farther.

Precambrian crystalline basement of the Appalachian Blue Ridge deforms inhomogeneously by developing relatively narrow ductile deformation zones (DDZs). The Paleozoic sedimentary cover develops open to tight folds and penetrative fabrics. A transition between these two styles occurs at the base of the sedimentary cover in the Early Cambrian Chilhowee quartzites of the central Appalachians and in the Late Proterozoic arkosic sandstones of the southern Appalachians. On a mesoscopic scale, the transition zone sediments show tight to isoclinal folds with highly deformed overturned limbs analogous to mesoscopic (1 cm to 10 m wide) DDZs in crystalline basement. Deformation zones in the basement cut across the basement/cover contact and feed into the overturned limbs of tight folds. On a microscopic scale, both arkoses and granitic basement rocks show thin (5 mm) DDZs characterized by grain-size reduction and alteration of feldspars to quartz and mica. The actual style and symmetry of deformation varies with metamorphic grade, proximity to major thrust faults, and amount of tectonic shortening. In the Grandfather Mountain area of the southern Blue Ridge Province, sets of low-dipping DDZs close to major thrust faults approximate a simple shear deformation field. In the central Appalachians of northern Virginia, similar simple shear deformation features are observed close to major thrust faults, but sets of DDZs define a flattening plane perpendicular to tectonic transport direction higher up within the thrust sheets.

The Robertson River pluton is a granitic pluton that intruded Grenville rocks in the Blue Ridge province of western Virginia. Rb-Sr whole-rock analyses show that the pluton was intruded at 570 ± 15 m.y. ago with a relatively high initial 87 Sr/ 86 Sr ratio of 0.712. The Robertson River pluton and the Grenville host rocks are cut by numerous metabasalt dikes that are probably feeder dikes related to the Catoctin Formation, a widespread plateau basalt that once covered this part of the Blue Ridge. The cross-cutting relationship of the metabasalt dikes to the Robertson River pluton considered along with the early Cambrian age of the Chilhowee Group, which overlies the Catoctin Formation, indicate that the Catoctin Formation was extruded at a time close to the Precambrian-Cambrian boundary at about 570 m.y. ago.

Talladega “Series” rock units in Polk and Paulding Counties have been mapped through the Cartersville area along the southeastern fault boundary of the “Corbin granite complex,” and correlated with rock units of the Great Smoky Group in Cherokee County. The Talladega “Series” in this area is bounded on the northwest by the Emerson (Cartersville) fault, along which the Talladega was thrust over (1) Valley and Ridge rocks ranging in age from Early Cambrian (Chilhowee Group) to Mississippian (Fort Payne Chert), (2) the Ocoee Supergroup, and (3) the “Corbin granite complex.” The Emerson fault also overrode the Great Smoky fault, which separates Ocoee from Chilhowee rocks in the Cartersville area. The thrust fault along the southeastern boundary of the Talladega “Series” is only one of numerous closely spaced imbricate faults within and southeast of the Talladega. The presence of closely spaced thrusts within the Talladega, the association of Hillabee Chlorite Schist-“Pumpkinvine Creek” lithologies with many of these faults, and the fact that detailed mapping has not been completed between the Rockmart-Yorkville area and the Alabama state line emphasize the need for caution in projecting correlations and interpretations across this area.