ABSTRACT This one-day field trip will explore the geomorphology, landslide mapping, geochronology, tectonics, meteorology, and geoengineering related to the Blue Ridge Escarpment (BRE), North Carolina, USA. Our aim is to show why it has persisted in the landscape and how it influences landslide frequency and the lives of the western North Carolina people. Some of the work we highlight has been published and some we present for the first time. Landslides pose a frequent geologic hazard to the people of western North Carolina, and they cause losses of road access, property, or, in the worst scenarios, human lives. We will also discuss landslide disaster response and mitigation efforts that required the collaboration of state and local emergency managers with other local, state, and federal agencies and the public. As we traverse the rugged terrain along the BRE in Polk and Rutherford counties, we will examine rockfalls, rockslides, debris flows, and debris slides occurring in late Proterozoic to early Paleozoic metasedimentary and meta-igneous rocks southeast of the Brevard fault zone. Our focus will be steep-walled, topographic reentrants where streams exploit brittle, post-orogenic bedrock structures, incise into the BRE, and produce landforms prone to debris flows and other types of mass wasting, often triggered by extreme rainfall events. The research we present on these extreme historical storms will help illustrate the scope and magnitude of the BRE’s influence on meteorological and hydrological events that lead to landslides and flooding. In addition to ongoing countywide landslide hazard mapping, a complementary research objective is to better understand the influence brittle cross-structures and earlier ductile bedrock structures have on rock slope failures and debris flows in the North Pacolet River valley and Hickory Nut Gorge, two major structurally controlled topographic lineaments.

Abstract The central Piedmont of South Carolina includes two terranes derived from Neoproterozoic peri-Gondwanan arcs and one that preserves the Cambrian Series 2–Series 3 Carolinian Rheic rift-drift sequence. These are the Charlotte, Silverstreet and Kings Mountain terranes. The central Piedmont shear zone juxtaposes each of these terranes against the Late Silurian Cat Square paragneiss terrane. The Kings Mountain terrane is composed of meta-epiclastic rocks with distinctive metaconglomerate horizons, manganiferous formation, meta-sandstones, and dolomitic marbles. One of the lower metaconglomerate horizons yields detrital zircons of latest Middle Cambrian age. This stratigraphy is interpreted to record the Rheic rift-drift sequence on the trailing edge of an Ediacaran-Cambrian arc terrane as it pulled away from the Amazonian craton in Middle Cambrian–Furongian time. The Charlotte terrane records magmatic activity from before 579 ± 4 until ∼535 ± 4 Ma. Mafic-ultramafic zoned intrusive complexes intruded mafic-ultramafic volcanic piles. Ultramafic dikes cut the volcanic rocks and are interpreted as feeders to stratigraphically higher levels of volcanism. These mafic to ultramafic rocks record arc rifting resulting from subduction of a spreading ridge or bathymetric high. These rocks were metamorphosed to amphibolite facies at about the time of the Cambrian–Precambrian transition. The Silverstreet terrane preserves relict medium temperature eclogites and high-pressure granulites in the lower plate (Charlotte terrane) of an arc-arc collision. Relict high-pressure assemblages record 1.4 GPa, 650–730 °C conditions. High-pressure mineralogy and textures are best preserved in the cores of boudins derived from dikes with Ti-V ratios of 20–50 (i.e., MORB). High-pressure metamorphism may have occurred in Ediacaran-Cambrian time, and must have occurred prior to the intrusion of the 414 ± 8 Ma Newberry granite. The Cat Square basin contains detrital zircons as young as 430 Ma, accepted detritus from both Laurentia and Carolinia, and so is interpreted as a successor basin. The Cat Square terrane underwent peak (upper amphibolite-granulite) metamorphic conditions at the time of the Devonian–Mississippian transition while it was at the latitude of the New York Promontory. The peri-Laurentian-Carolinian suture is either buried under the Blue Ridge Piedmont thrust sheet or was thrust up and eroded away. The central Piedmont shear zone is a younger feature, no older than Visean.

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 The Inner Piedmont extends from North Carolina to Alabama and comprises the Neoacadian (360–345 Ma) orogenic core of the southern Appalachian orogen. Bordered to west by the Blue Ridge and the exotic Carolina superterrane to the east, the Inner Piedmont is cored by an extensive region of migmatitic, sillimanite-grade rocks. It is a composite of the peri-Laurentian Tugaloo terrane and mixed Laurentian and peri-Gondwanan affinity Cat Square terrane, which are exposed in several gentle-dipping thrust sheets (nappes). The Cat Square terrane consists of Late Silurian to Early Devonian pelitic schist and metagraywacke intruded by several Devonian to Mississippian peraluminous granitoids, and juxtaposed against the Tugaloo terrane by the Brindle Creek fault. This field trip through the North Carolina Inner Piedmont will examine the lithostratigraphies of the Tugaloo and Cat Square terranes, deformation associated with Brindle Creek fault, Devonian-Mississippian granitoids and charnockite of the Cat Square terrane, pervasive amphibolite-grade Devonian-Mississippian (Neoacadian) deformation and metamorphism throughout the Inner Piedmont, and existence of large crystalline thrust sheets in the Inner Piedmont. Consistent with field observations, geochronology and other data, we have hypothesized that the Carolina superterrane collided obliquely with Laurentia near the Pennsylvania embayment during the Devonian, overrode the Cat Square terrane and Laurentian margin, and squeezed the Inner Piedmont out to the west and southwest as an orogenic channel buttressed against the footwall of the Brevard fault zone.

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.