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.

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.

Abstract This field guide covers a two-day east-to-west transect of the Blue Ridge and Valley and Ridge provinces of northwestern Virginia and eastern West Virginia, in the context of an integrated approach to teaching stratigraphy, structural analysis, and regional tectonics. Holistic, systems-based approaches to these topics incorporate both deductive (stratigraphic, structural, and tectonic theoretical models) and inductive (field observations and data collection) perspectives. Discussions of these pedagogic approaches are integral to this field trip. Day 1 of the field trip focuses on Mesoproterozoic granitoid basement (associated with the Grenville orogeny) and overlying Neoproterozoic to Early Cambrian cover rocks (Iapetan rifting) of the greater Blue Ridge province. These units collectively form a basement-cored anticlinorium that was thrust over Paleozoic strata of the Valley and Ridge province during Alleghanian contractional tectonics. Day 2 traverses a foreland thrust belt that consists of Cambrian to Ordovician carbonates (Iapetan divergent continental margin), Middle to Upper Ordovician immature clastics (associated with the Taconic orogeny), Silurian to Lower Devonian quartz arenites and carbonates (inter-orogenic tectonic calm), and Upper Devonian to Lower Mississippian clastic rocks (associated with the Acadian orogeny). Alleghanian structural features include the Little North Mountain thrust, Cacapon Mountain anticlinorium, Broad Top synclinorium, and Wills Mountain anticlinorium. Within the road log of this field guide we include both planned and optional stops, so that readers can explore the pedagogic concepts discussed herein in more detail, if desired.

Large bodies of eclogite in the Eastern Blue Ridge Province of western North Carolina crop out immediately southeast of the Burnsville fault zone, an Acadian dextral strike-slip fault that separates Laurentian Mesoproterozoic basement and Neoproterozoic to early Paleozoic units (Western Blue Ridge) from an inferred accretionary wedge complex (Eastern Blue Ridge). The peak metamorphic assemblage in eclogite is omphacite (Jd 27–35 ) + garnet (Alm 48 Prp 30 Grs 22 ) + quartz + rutile ± zoisite ± zircon ± apatite ± sulfides ± Fe-Ti oxides; evidence of an amphibolite-facies overprint is regionally widespread but variably developed. Geochemical and isotopic characteristics of the eclogites and some surrounding amphibolites are consistent with their derivation from mid-ocean-ridge basalt protoliths. Zircon from the least-altered eclogite yielded a U-Pb, isotope dilution–thermal ionization mass spectrometry age of 459.0 +1.5/−0.6 Ma. Multimineral plus whole-rock Sm-Nd isotopic data indicate that Sm-Nd mineral systematics were disturbed, likely during amphibolite-facies metamorphism. Partly amphibolitized eclogite contains titanite with a U-Pb age of 394 ± 4 Ma; titanite from another sample shows disturbed U-Pb systematics with apparent ages between 448 Ma and 417 Ma. Both eclogite and partly amphibolitized eclogite contain rutile with a U-Pb age of ca. 334–340 Ma. These ages correspond broadly to the time of the Taconian, Acadian, and Alleghanian orogenesis, respectively, and match the timing of metamorphic events and pluton emplacement in the Eastern Blue Ridge Province. The Ordovician geodynamic setting in which the eclogite formed was possibly a complex plate arrangement of island arcs, accretionary complexes, rift basins, and rifted microcontinental blocks, perhaps similar to the Australia-Pacific plate boundary between New Zealand and Papua New Guinea. Taconian collisional orogenesis was either synchronous with or closely followed high-pressure metamorphism, and both Acadian and Alleghanian events completely to partially reset titanite and rutile chronometers.

There is no completely accepted definition of a mélange, and the papers in this volume reflect this fact. In our regional assessment, the term mélange is principally used for a technically fragmented and mixed body of rock. A different term, olistostrome, is used for a chaotic and mixed rock body that formed by sedimentary processes such as slumping or gravity sliding. The term olistostromal mélange is used here if sedimentary processes and tectonic deformation were both involved in the fragmentation and mixing. In some cases there is evidence that these were effectively concurrent. Four main belts of Paleozoic mélanges and olistostromes have been recorded in the Appalachians of the northeastern United States and mainland Canada. These include: (1) olistostromes and olistostromal mélanges along Logan’s line and the Taconic allochthons, which are related to thrusting during the Taconian orogeny; (2) mélanges associated with ophiolite fragments along the Baie Verte–Brompton line, which are thought to represent a Taconian suture; (3) mélanges containing ophiolite fragments along the Hurricane Mountain mélange belt (Boone and Boudette, this volume), which are thought to represent a Penobscottian terrane boundary; and (4) Acadian mélanges and olistostromes such as the Silurian Deadman Harbour mélange, an olistostromal mélange that probably formed at the front of an Acadian overthrust. In addition, Precambrian olistostromes have been recognized in southeastern New England and in the Green Head Formation of New Brunswick. These may have originated along normal faults on the rifted continental shelf of Gondwanaland, or they may relate to an earlier Pan-African cycle. These Precambrian olistostromes, therefore, accompany the extensional development of the Iapetus Ocean, whereas the Paleozoic olistostromes and mélanges mark its progressive closure. Five main sets of Paleozoic mélanges and olistostromes have been recorded in the central and southern Appalachians. These are: (1) a composite mélange-olistostrome belt in the Piedmont of Maryland and Virginia that includes olistostromes such as the Sykesville Formation (of unknown age) and mélanges such as those of the Morgan Run Formation (of debated age) and Mine Run Complex, some of which contain possible ophiolite fragments; (2) mélanges in the Blue Ridge Province from Virginia to Alabama, which contain possible ophiolite fragments; (3) Silurian or Early Devonian olistostromes of the Lay Dam Formation in the Talladega slate belt of Alabama; (4) mélanges, including the Falls Lake and Juliette mélanges in the Carolinas and Georgia, that border the Carolina terrane as defined by Secor and others (1983); and (5) broken formations and mélanges along major faults such as the Pulaski and Brevard, which are mostly related to Alleghanian thrusting. In addition, there is good evidence in the Ocoee Supergroup of the Blue Ridge Province for Late Proterozoic olistostromes related to the initiation of grabens prior to the opening of the Iapetus Ocean.

Abstract The Scottsville Basin in the central Virginia Piedmont forms one of the westernmost Mesozoic sedimentary basins in eastern North America. This small basin has received limited scientific attention during the past 50 years; this field trip focuses on recent stratigraphic and structural research concerning the Scottsville Basin and surrounding region. The ∼110 km 2 Scottsville Basin and adjacent ∼5 km 2 Midway Mills Sub-basin formed astride the boundary between the eastern Blue Ridge and western Piedmont. The Scottsville Basin is a half-graben, bound on its northwest margin by a segmented normal fault that places Neoproterozoic to early Paleozoic metamorphic rocks in the footwall against Triassic strata in the hanging wall. Basin strata dip to the northwest toward the boundary fault, and dip angles increase from west to east. The southeastern basin boundary, previously interpreted as a small displacement normal fault, is an unconformity with phyllitic rocks of the western Piedmont. Strata within the basin include 2–3 km of boulder to pebble conglomerate, breccia, arkosic sandstone, and siltstone. Sedimentary rocks in the Scottsville Basin were sourced primarily from Proterozoic rocks in the Blue Ridge province to the west of the basin. The age of Triassic strata in the Scottsville Basin is poorly constrained. The Midway Mills Sub-basin was originally contiguous with the Scottsville Basin, but now forms an erosional outlier. A suite of north-northwest–striking Jurassic diabase dikes crosscuts Triassic sedimentary rocks and is subparallel to the dominant extensional fracture set in basin sedimentary rocks.