ABSTRACT This field guide presents a one-day trip across the northern Virginia Blue Ridge geologic province and highlights published geologic mapping of Mesoproterozoic rocks that constitute the core of the Blue Ridge anticlinorium and Neoproterozoic cover-sequence rocks on the fold limbs. The guide presents zircon SHRIMP (sensitive high-resolution ion microprobe) U-Pb crystallization ages of granitoid rocks and discusses the tectonic and petrologic evolution of basement rocks during the Mesoproterozoic. U-Pb data show more of a continuum for Blue Ridge Mesoproterozoic magmatic events, from ca. 1.18–1.05 Ga, than previous U-Pb TIMS (thermal ionization mass spectrometry)-based models that had three distinct episodes of plutonic intrusion. All of the younger dated rocks are found west of the N-S–elongate batholith of the Neoproterozoic Robertson River Igneous Suite, suggesting that the batholith occupies a fundamental Mesoproterozoic crustal boundary that was likely a fault. Narrow belts of paragneiss may represent remnants of pre-intrusive country rock, but some were deposited close to 1 Ga according to detrital zircon U-Pb ages. For late Neoproterozoic geology, the guide focuses on lithologies and structures associated with early rifting of the Rodinia supercontinent, including small rift basins preserved on the eastern limb of the anticlinorium. These basins have locally thickened packages of clastic metasedimentary rocks that strike into and truncate abruptly against Mesoproterozoic basement along apparent steep normal faults. Both basement and cover were intruded by NE-SE–striking and steeply dipping, few-m-wide diabase dikes that were feeders to late Neoproterozoic Catoctin Formation metabasalt that overlies the rift sediments. The relatively weak dikes facilitated the deformation that led to the formation of the Blue Ridge anticlinorium during the middle to late Paleozoic as the vertical dikes were transposed and rotated during formation of the penetrative cleavage. This field trip generally follows the GSA guide published in GSA Field Guide 57 (available at https://pubs.geoscienceworld.org/gsa ): Burton, W.C., and Schindler, J.S., 2020, Proterozoic and Paleozoic evolution of the Blue Ridge geologic province in northern Virginia, USA, in Swezey, C.S., and Carter, M.W., eds., Geology Field Trips in and around the U.S. Capital: Geological Society of America Field Guide 57, p. 1–19, https://doi.org/10.1130/2020.0057(01) .

ABSTRACT The Appalachian Mountains expose one of the most-studied orogenic belts in the world. However, metamorphic pressure-temperature-time ( P-T-t ) paths for reconstructing the tectonic history are largely lacking for the southernmost end of the orogen. In this contribution, we describe select field locations in a rough transect across the orogen from Ducktown, Tennessee, to Goldville, Alabama. Metamorphic rocks from nine locations are described and analyzed in order to construct quantitative P-T-t paths, utilizing isochemical phase diagram sections and garnet Sm-Nd ages. P-T-t paths and garnet Sm-Nd ages for migmatitic garnet sillimanite schist document high-grade 460–411 Ma metamorphism extending south from Winding Stair Gap to Standing Indian in the Blue Ridge of North Carolina. In the Alabama Blue Ridge, Wedowee Group rocks were metamorphosed at biotite to staurolite zone, with only local areas of higher-temperature metamorphism. The Wedowee Group is flanked by higher-grade rocks of the Ashland Supergroup and Emuckfaw Group to the northwest and southeast, respectively. Garnet ages between ca. 357 and 319 Ma indicate that garnet growth was Neoacadian to early Alleghanian in the Blue Ridge of Alabama. The P-T-t paths for these rocks are compatible with crustal thickening during garnet growth.

ABSTRACT This field guide presents a one-day trip across the northern Virginia Blue Ridge geologic province and highlights published geologic mapping of Mesoproterozoic rocks that constitute the core of the Blue Ridge anticlinorium and Neoproterozoic cover-sequence rocks on the fold limbs. The guide presents zircon SHRIMP (sensitive high-resolution ion microprobe) U-Pb crystallization ages of granitoid rocks and discusses the tectonic and petrologic evolution of basement rocks during the Mesoproterozoic. U-Pb data show more of a continuum for Blue Ridge Mesoproterozoic magmatic events, from ca. 1.18–1.05 Ga, than previous U-Pb TIMS (thermal ionization mass spectrometry)-based models that had three distinct episodes of plutonic intrusion. All of the younger dated rocks are found west of the N-S–elongate batholith of the Neoproterozoic Robertson River Igneous Suite, suggesting that the batholith occupies a fundamental Mesoproterozoic crustal boundary that was likely a fault. Narrow belts of paragneiss may represent remnants of pre-intrusive country rock, but some were deposited close to 1 Ga according to detrital zircon U-Pb ages. For late Neoproterozoic geology, the guide focuses on lithologies and structures associated with early rifting of the Rodinia supercontinent, including small rift basins preserved on the eastern limb of the anticlinorium. These basins have locally thickened packages of clastic metasedimentary rocks that strike into and truncate abruptly against Mesoproterozoic basement along apparent steep normal faults. Both basement and cover were intruded by NE-SE–striking and steeply dipping, few-m-wide diabase dikes that were feeders to late Neoproterozoic Catoctin Formation metabasalt that overlies the rift sediments. The relatively weak dikes facilitated the deformation that led to the formation of the Blue Ridge anticlinorium during the middle to late Paleozoic as the vertical dikes were transposed and rotated during formation of the penetrative cleavage.

Prepared in conjunction with the GSA Southeastern and Northeastern Sections Joint Meeting in Reston, Virginia, the four field trips in this guide explore various locations in Virginia, Maryland, and West Virginia. The physiographic provinces include the Piedmont, the Blue Ridge, the Valley and Ridge, and the Allegheny Plateau of the Appalachian Basin. The sites exhibit a wide range of igneous, metamorphic, and sedimentary rocks, as well as rocks with a wide range of geologic ages from the Mesoproterozoic to the Paleozoic. One of the trips is to a well-known cave system in West Virginia. We hope that this guidebook provides new motivation for geologists to examine rocks in situ and to discuss ideas with colleagues in the field.

ABSTRACT The geology of Great Smoky Mountains National Park (GRSM) in Tennessee and North Carolina is dominated by siliciclastics and metamorphic strata. However, in the western portion of GRSM, a series of carbonate fensters (windows) expose the Lower Ordovician–age section of the Knox Group, a series of dolomite and limestone units that are partially marbleized as a result of contact metamorphism from the Great Smoky fault. The fensters create opportunities for allogenic recharge to occur at points along the contact of the surrounding insoluble strata with the underlying soluble carbonates. The combination of chemically aggressive surface recharge and vertical relief has resulted in the formation of deep caves, many of which have active streams and water resources. Though the karst is limited in extent and the number of caves is fairly small, the significance of the resources is substantial, with several of the caves in the area over 150 m in depth and at least two being major bat hibernacula. In 2017, the U.S. Geological Survey (USGS) began a study to better understand the hydrologic behavior of these karst systems through hydrologic and geochemical monitoring, groundwater tracing using fluorescent dyes, and seepage runs. Stage and water-quality instrumentation was installed in two caves in GRSM, the main stream of Bull Cave, and in a sump pool in Whiteoak Blowhole, at 173 m and 70 m below land surface, respectively. Following setup of the cave sites, dye injections were conducted to determine discharge points for four of the deep cave systems on Rich Mountain and Turkeypen ridge. Results show water in these systems has an extremely rapid travel time, with tracers detected from caves to springs in less than 24 h for each of the systems. This field guide describes the complex geology, regional hydrogeology, and unique landscape characterized by high-gradient subterranean streams, carbonate fensters, and deep caves of the GRSM karst.

ABSTRACT The eastern Great Smoky Mountains basement complex consists of the following components: (1) ca. 1350–1325 Ma orthogneiss and mafic xenoliths that represent some of the oldest crust in Appalachian Grenville massifs (similar to “pre-Grenville” basement components in the Adirondack, Green Mountain, Hudson Highland, and Shenandoah massifs); (2) ca. 1150 Ma augen orthogneisses and granitic orthogneisses correlating with the Shawinigan phase of Grenville magmatism; and (3) paragneisses (cover rocks) that have either pre- or syn-Grenville (i.e., Mesoproterozoic) versus post-Grenville (Neoproterozoic) depositional ages, and that experienced Taconian metamorphism and migmatization. Mesoproterozoic paragneisses contain major zircon age modes that require a component of Proterozoic crust in the source region. The Neoproterozoic paragneisses exhibit the archetypical “Grenville doublet” in detrital zircon age distributions that matches the age distribution of Ottawan and Shawinigan magmatic/metamorphic events in eastern Laurentia. Most zircon U-Pb age systematics exhibit variable lead loss interpreted to result from high-grade Taconian (ca. 450 Ma) regional metamorphism and migmatization. Neodymium mantle model ages (T DM ) for ortho- and paragneisses range from 1.8 to 1.6 Ga, indicating that all rocks were derived from recycling of Proterozoic crust (i.e., they are not juvenile), which is consistent with Proterozoic detrital zircon ages in pre- to syn-Grenville paragneisses. Lead isotope compositions confirm the presence of an exotic (Amazonian) crustal component in the source region for the protoliths of the pre-Grenville orthogneisses and xenoliths, and that this exotic component was incorporated to varying degrees in the evolution of the basement complex. The oldest age component may represent an Amazonian pre-Grenville analog to the ca. 1.35 Ga native Laurentian crust present in Adirondack and northern Appalachian basement massifs.

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 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 The central Appalachians form a classic orogen whose structural architecture developed during episodes of contractional, extensional, and transpressional deformation from the Proterozoic to the Mesozoic. These episodes include components of the Grenville orogenic cycle, the eastern breakup of Rodinia, Appalachian orogenic cycles, the breakup of Pangea, and the opening of the Atlantic Ocean basin. This field trip examines an array of rocks deformed via both ductile and brittle processes from the deep crust to the near-surface environment, and from the Mesoproterozoic to the present day. The trip commences in suspect terranes of the eastern Piedmont in central Virginia, and traverses northwestward across the Appalachian orogen through the thick-skinned Blue Ridge basement terrane, and into the thin-skinned fold-and-thrust belt of the Valley and Ridge geologic province. The traverse covers a range of deformation styles that developed over a vast span of geologic time: from high-grade metamorphic rocks deformed deep within the orogenic hinterland, to sedimentary rocks of the foreland that were folded, faulted, and cleaved in the late Paleozoic, to brittle extensional structures that overprint many of these rocks. Stops include: the damage zone of a major Mesozoic normal fault, composite fabrics in gneiss domes, transpressional mylonites that accommodated orogen-parallel elongation, contractional high-strain zones, and overpressured breccia zones in the Blue Ridge, as well as folds, thrusts, and back thrusts of the Alleghanian foreland.