ABSTRACT The ophiolite of the Buck Creek and Chunky Gal Mountain areas (North Carolina, USA) consist of very high-temperature mafic and ultramafic units. These mafic and ultramafic rocks were once part of the oceanic lithosphere before being subducted and then thrust into continental rock by the Taconic orogeny, which took place during the Paleozoic era. The thrusting of these mafic and ultramafic rocks into nearby metasedimentary rock was controlled by the Chunky Gal thrust fault. Studies on this ophiolite were mainly petrological, geochemical, and structural in nature. The main rocks of interest within this area are dunite, metatroctolite, and amphibolite. The dunite mostly consists of olivine and minor chromite spinels. Hydrous altered dunite may also consist of antigorite, chlorite, talc, and anthophyllite. This dunite is associated with minor scattered lenses of metatroctolite. The metatroctolite is dominated by plagioclase and olivine, which have symplectites and/or coronas of prograde minerals such as pyroxene, spinel, and sapphirine. Some of this metatroctolite was altered via hydration into chlorite schist and edenite-margarite schist, some of which is corundum-bearing. The dunite and metatroctolite are separated from the surrounding metasedimentary units by the surrounding Chunky Gal Amphibolite. This amphibolite, whose protolith is basaltic/gabbroic, is dominated by hornblende and plagioclase.

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 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.

The three field guides in this volume explore facets of the geology of western North Carolina, USA. The trip to Buck Creek and Chunky Gal Mountain examines high-temperature mafic and ultramafic rocks interpreted to be part of a dismembered ophiolite thrust onto Laurentian crust during the Taconic orogeny. The second trip traverses rugged terrain along the Blue Ridge Escarpment from near the South Carolina state line to Hickory Nut Gorge just southeast of Asheville to examine rockfalls, rockslides, debris flows, and debris slides triggered by extreme rainfall events. The trip to Sparta, North Carolina, highlights the results of interdisciplinary studies among collaborators following the 2020 M w 5.1 earthquake.

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 New two-dimensional (2-D) thermomechanical finite-element models are used to test whether thrust advection, particularly at normal (10–20 km m.y. ‒1 ) to high (>50 km m.y. ‒1 ) horizontal slip rates, can substantially influence relatively high metamorphic heating rates (150–250 °C m.y. ‒1 ). Simple beam models that involve a single thrust with a dip of ~30° and geothermal gradients that are initially equal in the hanging wall and footwall yield maximum footwall heating rates of 15, 32, 75, and 150 °C m.y. ‒1 for imposed thrust rates of 5, 20, 50, and 100 km m.y. ‒1 (5–100 mm yr ‒1 ), respectively. Thrust rates were chosen to represent the possible range of rates interpreted in ancient collisional systems and observed in modern systems. More complex tapered wedge models, which include an elevated geothermal gradient in the hanging wall (with respect to the footwall), are intended to approximate the compressed isotherm sequences resulting from thrust-related hanging-wall exhumation predicted in previously published coupled thermomechanical models that include a strain continuum. In those models, thrust rates of 50 and 80 km m.y. ‒1 yield maximum footwall heating rates of 112 °C m.y. ‒1 and 170 °C m.y. ‒1 , respectively. In the immediate footwall of the regional-scale Ben Hope thrust in northwest Scotland, diffusion modeling of quartz inclusions in garnet yields heating rates of ~150–250 °C m.y. ‒1 . Although advective heating due to mass transfer at relatively high thrust rates cannot account for heating rates as high as those obtained from diffusion models (in Scotland and other orogens), the conduction-advection thrust models presented here suggest that thrust emplacement at relatively high rates (50–80 km m.y. ‒1 ) can contribute substantially to the total heating budget in the footwall of major thrusts. Additionally, the distribution of both footwall heating and hanging-wall cooling due to advective heat transfer along faults may have implications for the distribution of prograde and retrograde metamorphic assemblages in thrust belts. Other mechanisms that may substantially influence the thermal budget near crustal-scale faults may include shear heating, particularly at high rates of movement on thrusts, and pre- to synorogenic magma emplacement.

ABSTRACT Fold-and-thrust belts and their adjacent foreland basins provide a wealth of information about crustal shortening and mountain-building processes in convergent orogens. Erosion of the hanging walls of these structures is often thought to be synchronous with deformation and results in the exhumation and cooling of rocks exposed at the surface. Applications of low-temperature thermochronology and balanced cross sections in fold-and-thrust belts have linked the record of rock cooling with the timing of deformation and exhumation. The goal of these applications is to quantify the kinematic and thermal history of fold-and-thrust belts. In this review, we discuss different styles of deformation preserved in fold-and-thrust belts, and the ways in which these structural differences result in different rock cooling histories as rocks are exhumed to the surface. Our emphasis is on the way in which different numerical modeling approaches can be combined with low-temperature thermochronometry and balanced cross sections to resolve questions surrounding the age, rate, geometry, and kinematics of orogenesis.

ABSTRACT The structure and geology of former rifted continental margins can exert significant influence on their subsequent incorporation into collisional orogens. While thinned continental crust attached to the subducting mantle lithosphere may be incorporated into subduction channels, the weakly rifted parts of the margin are likely to resist subduction and thus deform ahead of the main orogenic front. This expectation is corroborated by a case study from the external western Alps. The former Dauphinois basins have inverted to form external basement massifs. Much of the deformation was widely distributed, with few localized thrust structures. Existing models that invoke distinct deformation events separated in time by a major (late Eocene, “Nummulitic”) unconformity are abandoned here in favor of crustal shortening that was continuous in time. Integrated stratigraphic, paleothermal, and geochronological data reveal that basin inversion was protracted over 6–10 m.y., coeval with deformation in the more internal parts of the chain. The notion of continuous, rather than episodic, deformation raises issues for the ways in which rates and tectonic activity may be evaluated within ancient orogens.

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 The Berkshire massif in western Massachusetts is one of several external basement massifs in the New England Appalachians. The Day Mountain thrust is a segment of the western frontal thrust of the Berkshire massif that carried Mesoproterozoic basement gneisses and unconformably overlying cover rocks of the Neoproterozoic (?) Dalton Formation and Cambrian Cheshire Quartzite over the Cambrian to Ordovician Stockbridge Formation. The basal unit of the Dalton Formation is a distinctive deformed quartz-pebble conglomerate. We made 27 strain estimates at 18 locations using the deformed conglomerate to investigate the strain field in the Day Mountain thrust sheet and test the plane-strain model of thrust emplacement. Although the strain ellipsoids vary from prolate to oblate shapes over distances as small as 200 m, and the orientations of the principal directions of strain range widely, a remarkably simple strain pattern, broadly consistent with simple shear, emerges when the strain data are plotted on contoured stereograms. The preferred orientation of the maximum elongation direction plunges gently and approximately coincides with the west-northwest transport direction of the thrust sheet, the preferred orientation of the intermediate principal strain axis is nearly horizontal and perpendicular to the transport direction, and the preferred orientation of the short axis plunges steeply. Most of the strain ellipsoids fall in the prolate field, which is indicative of constrictive flow, especially in the northern part of the thrust sheet. We suggest that the steep gradients in three-dimensional strain type were caused by flow of the more ductile conglomerate over an irregular surface of relatively rigid basement rocks, which were little affected by Paleozoic deformation. The constrictive flow conditions that dominate the strain field in the northern part of the thrust sheet may reflect the irregular paleotopography of the unconformity surface and/or a lateral ramp oriented at an oblique angle to the transport direction that impeded west-northwest–directed thrusting.

ABSTRACT Final closure of the Neotethys Ocean basin along the Eurasian margin in southeastern Europe during Eocene–Oligocene time was accompanied by upper-crustal extension expressed as a series of low-angle detachments, basins bounded by normal faults, and volcanism. This extensional belt spanned the southern Balkan Peninsula from the Albanides along the southern Adriatic coast in the west to western Anatolia in the east. Despite the widespread occurrence of this phenomenon within the southern Balkan region, similar extension has not previously been observed in association with the Neotethys closure in the Dinarides, which form the western geographic continuation of this orogenic belt, ending in the Austrian Alps in the northwest. The Mid- Bosnian Schist Mountains are a fault-bounded body of greenschist-facies metamorphic rocks located along the paleogeographic margin of the present-day Adria continental block in the Internal Dinarides. We combine low-temperature thermochronometric ages with field observations of kinematic shear sense indicators and demonstrate that the Mid- Bosnian Schist Mountains were exhumed along a normal fault between 43 and 27.5 Ma. The most rapid cooling occurred between ca. 35 and 27 Ma, coincident with a regional-scale magmatic event. These data constitute the first evidence for major extension in the Dinarides contemporaneous with collision between Adria and the Eurasian margin, and they are consistent with removal of a subducting slab during the transition between oceanic subduction and continental collision.

ABSTRACT Midcrustal channel flow has been hypothesized to be responsible both for the Greater and Lesser Himalayan Sequences (the Miocene Himalayan channel theory), and for the present eastward and northward movement and extension of the Tibetan upper crust (the Tibetan middle-crustal channel flow theory). Because processes within the crust cannot be directly observed, various studies have attempted to validate midcrustal channel flow theory by using indirect approaches, including outcrop patterns and other field data from the Himalayas, Tibet, and exposed older orogenic roots. The results have been highly debated, because arguments can be made that the internal structure of a channel and, therefore, the outcrop patterns of a paleomidcrustal channel are not unique. This paper investigates the types of structural patterns that may be produced within a midcrustal channel and discusses why they can be difficult, if not impossible, to distinguish from outcrop patterns produced by other mechanisms. A new example from the exposed middle crust of southern Finland is also discussed in this context. While outcrop structural patterns must indeed agree with other potential results that may imply a midcrustal channel, the inverse is not necessarily true: One cannot infer a midcrustal channel based on outcrop patterns alone, due to the nonunique nature of the patterns.