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.

The deformed wedge of Paleozoic sedimentary rocks in the southern Appalachian foreland fold-thrust belt is defined by the configurations of the undeformed basement surface below and the base of the Blue Ridge–Piedmont megathrust sheet above, together with the topographic free surface above the thrust belt. The base of the Blue Ridge–Piedmont sheet and undeformed basement surface have been contoured using industry, academic, and U.S. and state geological survey seismic-reflection and surface geologic data. These data reveal that the basement surface dips gently SE in the Tennessee embayment from Virginia to Georgia, and it contains several previously unrecognized normal faults and an increase in dip on the basement surface, which produces a topographic gradient. The basement surface is broken by many normal faults beneath the exposed southern Appalachian foreland fold-thrust belt in western Georgia and Alabama closer to the margin and beneath the Blue Ridge–Piedmont sheet in Georgia and the Carolinas. Our reconstructions indicate that small-displacement normal faults form beheaded basins over which thrust sheets were not deflected, whereas large-displacement normal faults (e.g., Tusquittee fault) localized regional facies changes in the early Paleozoic section and major Alleghanian (Permian) structures. These basement structures correlate with major changes in southern Appalachian foreland fold-thrust belt structural style from Virginia to Alabama. Several previously unrecognized structures along the base of the Blue Ridge–Piedmont sheet have been interpreted from our reconstructions. Large frontal duplexes composed of rifted-margin clastic and platform rocks obliquely overridden along the leading edge of the Blue Ridge–Piedmont sheet are traceable for many kilometers beneath the sheet. Several domes within the Blue Ridge–Piedmont sheet also likely formed by footwall duplexing of platform sedimentary rocks beneath, which then arched the overlying thrust sheet. The thickness and westward limit of the Blue Ridge–Piedmont sheet were estimated from the distribution of low-grade foot-wall metamorphic rocks, which were observed in reentrants in Georgia and southwestern Virginia, but are not present in simple windows in Tennessee. These indicate that the original extent of the sheet is near its present-day trace, whereas in Georgia, it may have extended some 30 km farther west. The southern Appalachian foreland fold-thrust belt consists mostly of a stack of westward-vergent, mostly thin-skinned thrusts that propagated westward into progressively younger units as the Blue Ridge–Piedmont sheet advanced westward as a rigid indenter, while a few in northeastern Tennessee and southwestern Virginia involved basement. Additional boundary conditions include temperatures <300 °C and pressures <300 MPa over most of the belt. The southern Appalachian foreland fold-thrust belt thrusts, including the Blue Ridge–Piedmont megathrust sheet, reach >350 km displacement in Tennessee and decrease both in displacement and numbers to the SW and NE. Much of the Neoproterozoic to Early Cambrian rifted-margin succession was deformed and metamorphosed during the Taconic orogeny, and it is considered part of the rigid indenter. Only the westernmost rocks of the rifted-margin succession exhibit ideal thin-skinned behavior and thus are part of the southern Appa-lachian foreland fold-thrust belt. Palinspastic reconstructions, unequal thrust displacements, and curved particle trajectories suggest that deformation of the belt did not occur by plane strain in an orogen that curves through a 30° arc from northern Georgia to SW Virginia. Despite the balance of many two-dimensional cross sections, the absence of plane strain diminishes their usefulness in quantifying particle trajectories. Coulomb behavior characterizes most individual faults, but Chapple's perfectly plastic rheology for the entire thrust belt better addresses the particle trajectory problem. Neither, however, addresses problems such as the mechanics of fault localization, out-of-sequence thrusts, duplex formation, three-dimensional transport, and other southern Appalachian foreland fold-thrust belt attributes.

ABSTRACT The southern Appalachian orogen is a Paleozoic accretionary-collisional orogen that formed as the result of three Paleozoic orogenies, Taconic, Acadian and Neoacadian, and Alleghanian orogenies. The Blue Ridge–Piedmont megathrust sheet exposes various crystalline terranes of the Blue Ridge and Inner Piedmont that record the different effects of these orogenies. The western Blue Ridge is the Neoproterozoic to Ordovician Laurentian margin. Constructed on Mesoproterozoic basement, 1.2–1.0 Ga, the western Blue Ridge transitions from two rifting events at ca. 750 Ma and ca. 565 Ma to an Early Cambrian passive margin and then carbonate bank. The Hayesville fault marks the Taconic suture and separates the western Blue Ridge from distal peri-Laurentian terranes of the central and eastern Blue Ridge, which are the Cartoogechaye, Cowrock, Dahlonega gold belt, and Tugaloo terranes. The central and eastern Blue Ridge terranes are dominantly clastic in composition, intruded by Ordovician to Mississippian granitoids, and contain ultramafic and mafic rocks, suggesting deposition on oceanic crust. These terranes accreted to the western Blue Ridge during the Taconic orogeny at 462–448 Ma, resulting in metamorphism dated with SHRIMP (sensitive high-resolution ion microprobe) U-Pb ages of metamorphic zircon. The Inner Piedmont, which is separated from the Blue Ridge by the Brevard fault zone, experienced upper amphibolite, sillimanite I and higher-grade ­metamorphism during the Acadian and Neoacadian orogenies, 395–345 Ma. These events also affected the eastern Blue Ridge, and parts of the western Blue Ridge. The Acadian and Neoacadian orogeny is the result of the oblique collision and accretion of the peri-Gondwanan Carolina superterrane overriding the Inner Piedmont. During this collision, the Inner Piedmont was a forced mid-crustal orogenic channel that flowed NW-, W-, and SW-directed from underneath the Carolina superterrane. The Alleghanian orogeny thrust these terranes northwestward as part of the Blue Ridge–Piedmont megathrust sheet during the collision of Gondwana (Africa) and the formation of Pangea.