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 Ion microprobe U-Pb zircon rim ages from 39 samples from across the accreted terranes of the central Blue Ridge, eastward across the Inner Piedmont, delimit the timing and spatial extent of superposed metamorphism in the southern Appalachian orogen. Metamorphic zircon rims are 10–40 µm wide, mostly unzoned, and dark gray to black or bright white in cathodoluminescence, and truncate and/or embay interior oscillatory zoning. Black unzoned and rounded or ovoid-shaped metamorphic zircon morphologies also occur. Th/U values range from 0.01 to 1.4, with the majority of ratios less than 0.1. Results of 206 Pb/ 238 U ages, ±2% discordant, range from 481 to 305 Ma. Clustering within these data reveals that the Blue Ridge and Inner Piedmont terranes were affected by three tectonothermal events: (1) 462–448 Ma (Taconic); (2) 395–340 Ma (Acadian and Neoacadian); and (3) 335–322 Ma, related to the early phase of the Alleghanian orogeny. By combining zircon rim ages with metamorphic isograds and other published isotopic ages, we identify the thermal architecture of the southern Appalachian orogen: juxtaposed and superposed metamorphic domains have younger ages to the east related to the marginward addition of terranes, and these domains can serve as a proxy to delimit terrane accretion. Most 462–448 Ma ages occur in the western and central Blue Ridge and define a continuous progression from greenschist to granulite facies that identifies the intact Taconic core. The extent of 462–448 Ma metamorphism indicates that the central Blue Ridge and Tugaloo terranes were accreted to the western Blue Ridge during the Taconic orogeny. Zircon rim ages in the Inner Piedmont span almost 100 m.y., with peaks at 395–385, 376–340, and 335–322 Ma, and delimit the Acadian-Neoacadian and Alleghanian metamorphic core. The timing and distribution of metamorphism in the Inner Piedmont are consistent with the Devonian to Mississippian oblique collision of the Carolina superterrane, followed by an early phase of Alleghanian metamorphism at 335–322 Ma (temperature >500 °C). The eastern Blue Ridge contains evidence of three possible tectonothermal events: ~460 Ma, 376–340 Ma, and ~335 Ma. All of the crystalline terranes of the Blue Ridge–Piedmont megathrust sheet were affected by Alleghanian metamorphism and deformation.

ABSTRACT The Brevard fault zone is one of the largest faults in the Appalachians, extending from Alabama to Virginia. It had a very complex history of movement and reactivation, with three movement episodes: (1) Acadian-Neoacadian (403–345 Ma) movement accompanying the thermal peak of metamorphism and deformation with dextral, southwest-directed emplacement of the Inner Piedmont; (2) ductile dextral reactivation during the early Alleghanian (~280 Ma) under lower-greenschist-facies conditions; and (3) brittle dip-slip reactivation during the late Alleghanian (260 Ma?). The Brevard is comparable to other large faults with polyphase movement in other orogens worldwide, for example, the Periadriatic line in the Alps. Two types of far-traveled, fault-bounded horses have been identified in the Brevard fault zone in the Carolinas: (1) metasedimentary and granitoid horses located along the southeastern margin of the Alleghanian retrogressive ductile dextral Brevard fault zone in North and South Carolina; and (2) limestone/dolostone horses located along the brittle, late Alleghanian Rosman thrust, the contact between Blue Ridge and Brevard fault zone rocks in North and South Carolina. Field, stratigraphic, petrographic, and Sr-isotope data suggest the carbonate horses may be derived from Valley and Ridge carbonates in the Blue Ridge–Piedmont megathrust sheet footwall. The horses of metasedimentary and granitoid rocks occur along faults that cut klippen of the southwest-directed Inner Piedmont Acadian-Neoacadian Alto (Six Mile) allochthon. New laser ablation– inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb zircon analyses from the metasedimentary mylonite component yield a detrital zircon suite dominated by 600 and 500 Ma zircons, and a second zircon population ranging from 2100 to 1300 Ma, with essentially no Grenvillian zircons, suggesting a peri-Gondwanan provenance. The granitoid component has a sensitive high-resolution ion microprobe (SHRIMP) age of 421 ± 14 Ma, similar to the ~430 Ma plutonic suite in northern Virginia and Maryland—a prominent component of the Cat Square terrane detrital zircon suite in the Carolinas. Peri-Gondwanan Neoproterozoic to Cambrian Avalon–Carolina superterrane rocks are nowhere in contact with the Brevard fault zone at present erosion level. While these far-traveled metasedimentary and granitoid horses may have originated several hundred kilometers farther northeast in the central Appalachians, they could alternatively be remnants of Avalon–Carolina superterrane rocks that once formed the tectonic lid of the southwest-directed Neoacadian–early Alleghanian (Late Devonian–early Mississippian) orogenic channel formed during north-to-south zippered accretion of Avalon–Carolina. The remnant fossil subduction zone survives as the central Piedmont suture. Avalon–Carolina terrane rocks would have once covered the Inner Piedmont (and easternmost Blue Ridge) to depths of >20 km, and have since been eroded. Data from these two suites of horses provide additional insights into the mid- to late Paleozoic history and kinematics of the Brevard fault zone, Inner Piedmont, and Avalon–Carolina superterrane. It was six men of Indostan To learning much inclined, Who went to see the Elephant (Though all of them were blind), That each by observation Might satisfy his mind. … And so these men of Indostan Disputed loud and long, Each in his own opinion Exceeding stiff and strong, Though each was partly in the right, And all were in the wrong. —John Godfrey Saxe (1816–1887) “The Blind Men and the Elephant”

Abstract Detailed geologic mapping and new SHRIMP (sensitive high-resolution ion microprobe) U-Pb zircon, Ar/Ar, Lu-Hf, 14 C, luminescence (optically stimulated), thermochronology (fission-track), and palynology reveal the complex Mesoproterozoic to Quaternary geology along the ~350 km length of the Blue Ridge Parkway in Virginia. Traversing the boundary of the central and southern Appalachians, rocks along the parkway showcase the transition from the para-autochthonous Blue Ridge anticlinorium of northern and central Virginia to the allochthonous eastern Blue Ridge in southern Virginia. From mile post (MP) 0 near Waynesboro, Virginia, to ~MP 124 at Roanoke, the parkway crosses the unconformable to faulted boundary between Mesoproterozoic basement in the core of the Blue Ridge anticlinorium and Neoproterozoic to Cambrian metasedimentary and metavolcanic cover rocks on the western limb of the structure. Mesoproterozoic basement rocks comprise two groups based on SHRIMP U-Pb zircon geochronology: Group I rocks (1.2-1.14 Ga) are strongly foliated orthogneisses, and Group II rocks (1.08-1.00 Ga) are granitoids that mostly lack obvious Mesoproterozoic deformational features. Neoproterozoic to Cambrian cover rocks on the west limb of the anticlinorium include the Swift Run and Catoctin Formations, and constituent formations of the Chilhowee Group. These rocks unconformably overlie basement, or abut basement along steep reverse faults. Rocks of the Chilhowee Group are juxtaposed against Cambrian rocks of the Valley and Ridge province along southeast- and northwest-dipping, high-angle reverse faults. South of the James River (MP 64), Chilhowee Group and basement rocks occupy the hanging wall of the nearly flat-lying Blue Ridge thrust fault and associated splays. South of the Red Valley high-strain zone (MP 144.5), the parkway crosses into the wholly allochthonous eastern Blue Ridge, comprising metasedimentary and meta-igneous rocks assigned to the Wills Ridge, Ashe, and Alligator Back Formations. These rocks are bound by numerous faults, including the Rock Castle Creek fault that separates Ashe Formation rocks from Alligator Back Formation rocks in the core of the Ararat River synclinorium. The lack of unequivocal paleontologic or geochronologic ages for any of these rock sequences, combined with fundamental and conflicting differences in tectonogenetic models, compound the problem of regional correlation with Blue Ridge cover rocks to the north. The geologic transition from the central to southern Appalachians is also marked by a profound change in landscape and surficial deposits. In central Virginia, the Blue Ridge consists of narrow ridges that are held up by resistant but contrasting basement and cover lithologies. These ridges have shed eroded material from their crests to the base of the mountain fronts in the form of talus slopes, debris flows, and alluvial-colluvial fans for perhaps 10 m.y. South of Roanoke, however, ridges transition into a broad hilly plateau, flanked on the east by the Blue Ridge escarpment and the eastern Continental Divide. Here, deposits of rounded pebbles, cobbles, and boulders preserve remnants of ancestral west-flowing drainage systems. Both bedrock and surficial geologic processes provide an array of economic deposits along the length of the Blue Ridge Parkway corridor in Virginia, including base and precious metals and industrial minerals. However, common stone was the most important commodity for creating the Blue Ridge Parkway, which yielded building stone for overlooks and tunnels, or crushed stone for road base and pavement.

The creation of new outcrops through construction is an important source of field data for geologists, especially in parts of the Appalachians with limited rock exposure. Users of Google Earth for field research often encounter disparities between the digital topography and the current-day Earth's surface, as newly formed outcrops may not be represented in the topography. Such is the case along sections of the I-26 corridor in Unicoi County, northeastern Tennessee. Twenty-four kilometers of U.S. 23 (future I-26) was widened to four lanes from Sams Gap at the North Carolina–Tennessee line to the Nolichucky River near Erwin, Tennessee, in the early 1990s. The series of outcrops created along the corridor provide an exceptional traverse through Grenvillian-age basement and cover strata which contain numerous stacked Alleghanian thrust sheets and shear zones. Near mile marker 44 along I-26, an ~250 m-long and 65 m-high outcrop was formed as part of the early 1990s construction. Google Earth satellite and Street View images show the outcrop, but the digital terrain in Google Earth does not reflect the approximate 150,000 m 3 of rock removed to form this roadcut. To correct for this, terrain modifications were made with Sketch-Up by copying and virtually excavating the landscape. The SketchUp model was then imported into Google Earth to show the outcrop and interstate as it looks today, with the interstate passing uninterrupted through a ridge rather than draping over hilly topography. This technique can be applied to any area in Google Earth where a mismatch exists between real and virtual topography.