New U-Pb and Rb-Sr isotopic data on the suite of the Crossnore Complex, herein referred to as the Crossnore plutonic suite (CPS), indicate that these plutons crystallized between 680 and 710 m.y. ago; the Crossnore Pluton itself may be as young as approximately 650 m.y. Bulk zircon separates from these rocks contain an older xenocrystic component due to contamination of the CPS magmas by older gneisses. During late Precambrian time, the ancestral Atlantic Ocean basin (Iapetus) formed and separated crustal units which are now part of the Caledonian-Appalachian mountains. These new CPS isotopie data indicate that continental rifting, which preceded the actual formation of the Iapetus Ocean, occurred approximately 690 m.y. ago. The Iapetus Ocean formed 690 to 570 m.y. ago.

The 760 Ma Wilburn Rhyolite Member of the Mount Rogers Formation, in southwestern Virginia, is a mineralogically and compositionally zoned welded ash-flow sheet at least 660 m thick. Compositional zoning, preserved despite greenshist-facies metamorphism, developed in the pre-eruptive magma chamber and was inverted during eruption of the ash-flow sheet. Microphenocrysts of aegerine and riebeckite occur at the base of the sheet, riebeckite alone or with biotite at higher levels, and Fe-rich biotite at the top. Alkali feldspar phenocrysts are more potassic, and riebeckite and biotite exhibit decreasing Mg/Fe toward the top of the ash-flow sheet; F content of biotite increases toward the base. The ash-flow tuff is a high-silica rhyolite (SiO 2 = 76.6 wt%); the basal one-sixth of the sheet is interpreted as originally peralkaline, whereas the remainder is metaluminous. Major- and trace-element zoning within the sheet is similar to other well-documented ash-flow sheets: SiO 2 , Na 2 O, and F increase toward the base of the sheet, whereas Al 2 O 3 , MgO, CaO, K 2 O, and TiO 2 decrease. Concentrations of Be, Rb, Zr, Nb, Sn, Hf, Ta, Th, U, Tb, and Yb increase toward the base; elements more abundant toward the top include Sc, Sr, Ba, La, Ce, Nd, and Eu. These gradients developed in a high-level silicic magma chamber in which peralkaline high-silica rhyolitic magma overlay metaluminous high-silica rhyolite. The 765–740 Ma Crossnore Complex, which includes the Mount Rogers Formation, in the Grenvillian French Broad massif includes A-type granitoids and is interpreted to reflect aborted rifting of Laurentian continental crust. The locus of aborted rifting migrated northeastward to the Shenandoah massif, where A-type magmatism occurred at 735–680 Ma. Continental breakup and opening of the Iapetus Ocean followed Late Neoproterozoic (ca. 572–554 Ma) rifting. Formation of a voluminous high-silica, partly peralkaline magma chamber indicates that the initial pulse of rifting took place in relatively thick continental crust, perhaps explaining why this pulse did not culminate in continental breakup until ∼200 m.y. later.

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.

Abstract Mesoproterozoic basement in the vicinity of Mount Rogers is characterized by considerable lithologic variability, including major map units composed of gneiss, amphibolite, migmatite, meta-quartz monzodiorite and various types of granitoid. SHRIMP U-Pb geochronology and field mapping indicate that basement units define four types of occurrences, including (1) xenoliths of ca. 1.33 to ≥1.18 Ga age, (2) an early magmatic suite including meta-granitoids of ca. 1185–1140 Ma age that enclose or locally intrude the xenoliths, (3) metasedimentary rocks represented by layered granofels and biotite schist whose protoliths were likely deposited on the older meta-granitoids, and (4) a late magmatic suite composed of younger, ca. 1075–1030 Ma intrusive rocks of variable chemical composition that intruded the older rocks. The magmatic protolith of granofels constituting part of a layered, map-scale xenolith crystallized at ca. 1327 Ma, indicating that the lithology represents the oldest, intact crust presently recognized in the southern Appalachians. SHRIMP U-Pb data indicate that periods of regional Mesoproterozoic metamorphism occurred at 1170–1140 and 1070–1020 Ma. The near synchroneity in timing of regional metamorphism and magmatism suggests that magmas were emplaced into crust that was likely at nearsolidus temperatures and that melts might have contributed to the regional heat budget. Much of the area is cut by numerous, generally east- to northeast-striking Paleozoic fault zones characterized by variable degrees of ductile deformation and recrystallization. These high-strain fault zones dismember the terrane, resulting in juxtaposition of units and transformation of basement lithologies to quartz- and mica-rich tectonites with protomylonitic and mylonitic textures. Mineral assemblages developed within such zones indicate that deformation and recrystallization likely occurred at greenschist-facies conditions at ca. 340 Ma.

Abstract The Blue Ridge and Piedmont provinces in the central Virginia Appalachians are underlain by Proterozoic and Paleozoic rocks that record multiple episodes of continental collision and rifting. This trip focuses on rocks and structures formed at the southeastern margin of Laurentia during: (1) the Mesoproterozoic assembly of Rodinia, (2) the Cryogenian to Ediacaran rifting that ultimately created the Iapetus Ocean, and (3) the Paleozoic deformation and metamorphism associated with the closure of the Iapetus Ocean and Appalachian orogenesis. A Neoproterozoic to Early Cambrian cover sequence records the transition from continental rifting to a passive margin, but the character of this sequence is vastly different on the eastern and western limbs of the Blue Ridge anticlinorium, reflecting spatial differences in both the timing and tectonics of the Iapetan rift. Blue Ridge rocks experienced NW-directed contractional deformation during the Neo-Acadian (355-330 Ma), whereas low-grade metasedimentary rocks in the western Piedmont were deformed and cooled prior to ca. 400 Ma. In central Virginia, the boundary between the eastern Blue Ridge and western Piedmont is a 3- to 5-km-wide zone of distributed dextral transpression.

ABSTRACT This field trip examines the results of integrated geologic studies of the 9 August 2020, M w 5.1 earthquake near Sparta, North Carolina, USA. The earthquake generated ~4 km of coseismic surface rupture of the Little River fault and uplifted a surface area of ~11 km 2 . The Little River fault is a thrust fault oriented 110–130°/45–70°SW, and mapped fault segments are en echelon with scarp heights from <5–30 cm. The epicenter is in polydeformed rocks of the Ashe and Alligator Back Metamorphic Suites in the eastern Blue Ridge. Bedrock structure formed during multiple Paleozoic orogenies; the regional foliation strikes NE-SW and dips SE (mean orientation 063°/52°SE). Mapping identified late Paleozoic veins and shear zones, a regional joint set striking 330–340° and 250–240°, and brittle faults that cut the Paleozoic foliation. Brittle faults oriented similar to the Little River fault are mapped up to 4 km along strike from the coseismic rupture along Bledsoe Creek valley, and the combined length of the Little River fault system is ~8 km. Paleoseismic trenches across the Little River fault corroborate the reactivation of an older fault by the 2020 earthquake and reveal two events during late Pleistocene (<50 ka). Surficial mapping identified several terrace deposits, including a deposit along Bledsoe Creek that yielded a 26 Al/ 10 Be isochron burial age of 0.46 ± 0.13 Ma and overlies a brittle fault, thus constraining the timing of movement of the fault at that location. Paleoliquefaction studies document soft-sediment deformation features in alluvium that may represent paleoseismic events. Collectively, these results highlight long-lived paleoseismicity of the Blue Ridge and that the 9 August 2020 earthquake reactivated an older, suitably oriented brittle fault in the bedrock. The Little River fault is an example of a previously unknown but active fault lying outside of known seismic zones with demonstrated recurrence of paleo-ruptures, raising questions about the assumption that damaging earthquakes are limited to areas of ongoing background seismicity, which is counter to seismic hazard assessments in the eastern United States. Bedrock mapping separates eastern Blue Ridge lithostratigraphy of the Lynchburg Group and Ashe and Alligator Back Metamorphic Suites into separate fault-bound packages juxtaposed over various 1.3–1.0 Ga basement rocks of the northern French Broad massif by the Gossan Lead fault.

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.

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.