ABSTRACT The timing and kinematics of Paleozoic peri-Gondwanan terrane accretion along the southern and central Appalachian margin have long been debated. The Silurian–Devonian Concord plutonic suite intruded the western flank of the Carolina superterrane, suggesting east-dipping subduction of ocean crust beneath the Carolina superterrane just prior to accretion, based on Devonian–Mississippian plutonism and metamorphism in the adjacent Laurentian terranes. Geochemical and isotopic data support a subduction-related origin for the Concord plutonic suite, and our geochronologic data reveal the main pulse of plutonism occurred ca. 405 Ma. Our new sensitive high-resolution ion microprobe (SHRIMP) geochronologic data identify a suite of mafic plutons from the Carolinas to central Georgia that also belong to the Concord suite. These gabbros have U-Pb zircon ages of 372 ± 2 Ma (Gladesville contact aureole), 386 ± 5.7 Ma (Buffalo), 403.8 ± 3.7 Ma (Highway 200), 404.9 ± 6.9 Ma (Mecklenburg), and 416 ± 6.9 Ma (Calhoun Falls). The Ogden Gabbro has a U-Pb age from baddeleyite of 411.91 ± 0.25 Ma. In this study, we identified a previously unrecognized Alleghanian (Pennsylvanian) gabbro suite with U-Pb zircon ages of 308.2 ± 6.2 Ma (Farmington), 311 ± 6.2 Ma (Dutchman’s Creek), and 311 ± 6.5 Ma (Mount Carmel). These gabbros should henceforth not be included in the Concord suite. The ages of Concord suite plutons slightly predate the main phase of plutonism in the Cat Square terrane to the west, which we suggest represents the product of B-type subduction of ocean crust beneath the Carolina superterrane between 415 and 400 Ma. Arc-related magmatism terminated because of the switch to A-type subduction of the eastern Laurentian margin. Prograde upper-amphibolite- to granulite-facies metamorphism, wholesale migmatization, and extensive anatectic plutonism in the eastern Inner Piedmont occurred from Late Devonian into Mississippian time, shortly after cessation of Concord plutonic suite plutonism, which also supports this proposed model. These data, combined with the timing and geometry of foreland clastic wedges, provide compelling support for Devonian–Mississippian accretion of the Carolina superterrane via dextral transpressive obduction above the eastern Laurentian margin.

ABSTRACT The Brevard fault zone is one of the largest faults in the Appalachians, extending from Alabama to Virginia. It had a very complex history of movement and reactivation, with three movement episodes: (1) Acadian-Neoacadian (403–345 Ma) movement accompanying the thermal peak of metamorphism and deformation with dextral, southwest-directed emplacement of the Inner Piedmont; (2) ductile dextral reactivation during the early Alleghanian (~280 Ma) under lower-greenschist-facies conditions; and (3) brittle dip-slip reactivation during the late Alleghanian (260 Ma?). The Brevard is comparable to other large faults with polyphase movement in other orogens worldwide, for example, the Periadriatic line in the Alps. Two types of far-traveled, fault-bounded horses have been identified in the Brevard fault zone in the Carolinas: (1) metasedimentary and granitoid horses located along the southeastern margin of the Alleghanian retrogressive ductile dextral Brevard fault zone in North and South Carolina; and (2) limestone/dolostone horses located along the brittle, late Alleghanian Rosman thrust, the contact between Blue Ridge and Brevard fault zone rocks in North and South Carolina. Field, stratigraphic, petrographic, and Sr-isotope data suggest the carbonate horses may be derived from Valley and Ridge carbonates in the Blue Ridge–Piedmont megathrust sheet footwall. The horses of metasedimentary and granitoid rocks occur along faults that cut klippen of the southwest-directed Inner Piedmont Acadian-Neoacadian Alto (Six Mile) allochthon. New laser ablation– inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb zircon analyses from the metasedimentary mylonite component yield a detrital zircon suite dominated by 600 and 500 Ma zircons, and a second zircon population ranging from 2100 to 1300 Ma, with essentially no Grenvillian zircons, suggesting a peri-Gondwanan provenance. The granitoid component has a sensitive high-resolution ion microprobe (SHRIMP) age of 421 ± 14 Ma, similar to the ~430 Ma plutonic suite in northern Virginia and Maryland—a prominent component of the Cat Square terrane detrital zircon suite in the Carolinas. Peri-Gondwanan Neoproterozoic to Cambrian Avalon–Carolina superterrane rocks are nowhere in contact with the Brevard fault zone at present erosion level. While these far-traveled metasedimentary and granitoid horses may have originated several hundred kilometers farther northeast in the central Appalachians, they could alternatively be remnants of Avalon–Carolina superterrane rocks that once formed the tectonic lid of the southwest-directed Neoacadian–early Alleghanian (Late Devonian–early Mississippian) orogenic channel formed during north-to-south zippered accretion of Avalon–Carolina. The remnant fossil subduction zone survives as the central Piedmont suture. Avalon–Carolina terrane rocks would have once covered the Inner Piedmont (and easternmost Blue Ridge) to depths of >20 km, and have since been eroded. Data from these two suites of horses provide additional insights into the mid- to late Paleozoic history and kinematics of the Brevard fault zone, Inner Piedmont, and Avalon–Carolina superterrane. It was six men of Indostan To learning much inclined, Who went to see the Elephant (Though all of them were blind), That each by observation Might satisfy his mind. … And so these men of Indostan Disputed loud and long, Each in his own opinion Exceeding stiff and strong, Though each was partly in the right, And all were in the wrong. —John Godfrey Saxe (1816–1887) “The Blind Men and the Elephant”

Abstract The Haile gold mine is located in southern Lancaster County, South Carolina, near the town of Kershaw. Gold was discovered at the site in 1827, and four periods of mining have yielded 360,000 ounces of gold. The mine is located between the past producing Ridgeway and Brewer mines that, when all are combined, constitute a significant amount of historical gold production in the southeastern United States. These mines are hosted within Neoproterozoic to lower Cambrian Carolina terrane rocks and are dominated by volcanic and epiclastic units that have experienced greenschist facies metamorphism. Saprolitic weathering is present in the near-surface portions of the deposit and is locally covered by Cretaceous-aged Coastal Plain sediments. The gold mineralization at the Haile mine is hosted within silicified meta-sediments containing fine-grained disseminated pyrite and pyrrhotite and is a replacement type-epithermal deposit. Re-Os ages from molybdenite associated with the mineralization indicate that the deposit formed shortly after major, arc-related volcanic activity. Haile currently has a measured and indicated resource of 4.03 million ounces at an average grade of 1.77 g/t Au with an additional inferred resource of 801,000 ounces at an average grade of 1.24 g/t Au. Included in the resource is a reserve of 2.02 million ounces of gold at an average grade of 2.06 g/t. Mine construction began in May 2015, and gold production is expected by the end of 2016. The construction cost is expected to be US$380 million. Ore will be extracted from eight open pits with mill extraction and the current mine life is 14 years.

Abstract In latest Devonian time, the collision between Avalonia, the New York promontory and Carolina terrane under the impact of Gondwana, generated an orogeny that began in New England and migrated southward in time. Once thought to be the fourth tectophase of the Acadian orogeny, this event is now called the Neoacadian orogeny. Active deformational loading during the event initially produced the Sunbury black-shale basin, whereas subsequent relaxational phases produced the Borden-Grainger-Price-Pocono and Pennington–Mauch Chunk clastic wedges, which largely reflect the dextral transpressional docking of the Carolina terrane against the Virginia promontory and points southward. The Sunbury black-shale basin and the infilling clastic wedges are among the thickest and most extensive in the Appalachian foreland basin. This trip will demonstrate differences in basinal black-shale and deltaic infilling of the foreland basin, both in more active, proximal and in more distal, sediment-starved parts of the basin. In particular, we will examine relationships between sedimentation and tectonism in the Early-Middle Mississippian Sunbury/Borden/Grainger/Fort Payne delta/basin system in the western Appalachian Basin during the Neoacadian Orogeny. We will emphasize the interrelated aspects of delta sedimentation, basin starvation, and mud-mound genesis on and near the ancient Borden-Grainger delta front. Temporal constraints are provided by the underlying Devonian-Mississippian black shales and by the widespread Floyds Knob Bed/zone, a dated glauconite/phosphorite interval that occurs across the distal delta/basin complex.

The Appalachians are a Paleozoic orogen that formed in a complete Wilson cycle along the eastern Laurentian margin following the breakup of supercontinent Rodinia and the coalescence of all of the continents to form supercontinent Pangea. The Appalachian Wilson cycle began by formation of a Neoproterozoic to early Paleozoic rifted margin and platform succession on the southeastern margin of Laurentia. Three orogenies ultimately produced the mountain chain: the Ordovician Taconic orogeny, which involved arc accretion; the Acadian–Neoacadian orogeny, which involved north-to-south, transpressional, zippered, Late Devonian–early Mississippian collision of the Carolina superterrane in the southern-central Appalachians and the Avalon-Gander superterrane in the New England Appalachians, and Silurian collision in the Maritime Appalachians and Newfoundland; and the Alleghanian orogeny, which involved late Mississippian to Permian collision of all previously formed Appalachian components with Gondwana to form supercontinent Pangea. The Alleghanian also involved zippered, north-to-south, transpressional, then head-on collision. All orogenies were diachronous. Similar time-correlative orogenies affected western and central Europe (Variscan events), eastern Europe and western Siberia (Uralian events), and southern Britain and Ireland; only the Caledonide (Grampian–Finnmarkian; Caledonian–Scandian) events affected the rest of Britain and the Scandinavian Caledonides. These different events, coupled with the irregular rifted margin of Laurentia, produced an orogen that contains numerous contrasts and nonthroughgoing elements, but it also contains elements, such as the platform margin and peri-Gondwanan elements, that are recognizable throughout the orogen.

Review of the major post–Middle Ordovician lithotectonic elements of the Appalachian orogen indicates that the middle to late Paleozoic geologic evolution of the Appalachian margin was less uniform than that of the early Paleozoic. Evolutionary divergence between the northern and southern segments of the orogen started in the Late Ordovician to Silurian with staggered accretion of the first peri-Gondwanan elements to reach the Laurentia margin, Carolinia in the south and Ganderia in the north. Divergence was amplified during the Silurian, specifically with respect to the nature of the Laurentian margin and the history of accretion. During this time frame, the northern margin was convergent, whereas the amagmatic southern margin may well have been a transform boundary. In terms of accretion, the Late Silurian–Early Devonian docking of Avalonia was restricted to the northern segment, whereas the southern Appalachians appear to have been largely quiescent during this interval. The evolutionary paths of the two segments of the margin converge on a common history in the Late Devonian during the Famennian event; we suggest that this tectonism was related to the initial marginwide interaction of Laurentia with the peri-Gondwanan blocks of Meguma and Suwanee, providing a uniform tectonic template for margin evolution. The Laurentian-Gondwanan collision is marked by second-order divergences in history. Specifically, during the Carboniferous, the southern segment records a larger component of shortening than the northern Appalachians.

The southern Appalachian crystalline core is composed of lithotectonic assemblages that are largely sedimentary in origin. Sixteen paragneiss samples from the Blue Ridge and Inner Piedmont of North Carolina and Georgia, and one sample of Middle Ordovician rocks from the Sevier-Blountian clastic wedge in the Tennessee Valley and Ridge were sampled for sensitive high-resolution ion microprobe (SHRIMP) U-Pb detrital zircon geochronology, whole-rock geochemistry, and zircon trace-element analyses. Detrital zircon ages range from Archean (~2.7 Ga) to Middle Paleozoic (~430 Ma), with a notable abundance of Mesoproterozoic zircons (1.3–0.9 Ga). Many samples also contain moderate populations of slightly older Mesoproterozoic zircons (1.5–1.3 Ga). Minor populations of Paleoproterozoic (2.3–1.5 Ga) and Neoproterozoic (754–717 and 629–614 Ma) ages occur in several samples; however, Paleozoic detrital zircons (478–435 Ma) are restricted to samples from the Cat Square terrane. Depositional periods of the metasedimentary terranes are bracketed by detrital zircon, metamorphic, and magmatic ages, and include: (1) Mesoproterozoic, (2) Neoproterozoic to early Paleozoic, and (3) middle Paleozoic. A xenolith from the ~1.15 Ga Wiley Gneiss suggests a post–~1.2 Ga period of sedimentation prior to the ~1.15 Ga Grenvillian magmatic event. Detrital zircon populations of Neoproterozoic to Middle Ordovician suggest a mixed Laurentian provenance with Amazonian and peri-Gondwanan sources deposited in divergent and convergent plate settings. Blue Ridge and Inner Piedmont detrital zircon ages, whole-rock geochemistry, lithologic assemblages, and field relationships are compatible with deposition of immature clastic material in a rift and passive-margin setting from the Neoproterozoic to early Paleozoic. Occurrence of 1.3–0.9 Ga, 1.5–1.3 Ga, and 754–717 Ma detrital zircon ages indicate a dominantly Laurentian provenance for the Cartoogechaye, Cowrock, Dahlonega gold belt, Smith River allochthon, and Tugaloo terranes. Minor Paleoproterozoic populations in these terranes suggest input from distal terranes of the Laurentian midcontinent or the Amazonian craton. Transition to a convergent plate margin in the Middle Ordovician resulted in collision of central Blue Ridge and Tugaloo terranes and recycling of material from these terranes into the Mineral Bluff Formation and Sevier Shale. Ordovician and 629–614 Ma detrital zircons from the Cat Square terrane document the first occurrence of peri-Gondwanan material, which was deposited in a convergent setting between the Laurentian margin and the accreting Carolina superterrane during the Late Silurian to Devonian.

Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb ages of more than 400 detrital zircons from the Neoproterozoic–early Paleozoic clastic sequences of Carolinia range from ca. 530 Ma (Early Cambrian) to ca. 2600 (Archean). The majority of analyzed zircon grains are late Neoproterozoic (Ediacaran), with minor amounts of Mesoproterozoic–Paleozoic and accessory Archean grains. The overall distribution of age populations of detrital zircons is consistent with sediment derivation from the Amazonian craton and its peripheral orogenic belts on the margin of west Gondwana. On the basis of the age of the youngest detrital zircon populations (ca. 550 Ma), the Uwharrie, Tillery, Cid, and Yadkin formations are no older than Ediacaran. The minimum depositional ages of the Uwharrie and Cid formations are constrained by ages of contemporaneous volcanism (551 ± 8 and 547 ± 2 Ma, respectively). Thus, all units of the Albemarle sequence were deposited between ca. 550 and 532 Ma. The dominance of Ediacaran and early Paleozoic zircons in the Albemarle Group suggests an underlying local protosource for the sediments. Mesoproterozoic and older detrital grains constitute a minor component and have an age signature that suggests derivation from the underlying continental crust basement. Dated samples from the Albemarle Group yield similar detrital zircon U-Pb age popu lations consistent with a common provenance. The results of this study illustrate that sedimentation in the Albemarle sequence of Carolinia is a manifestation of active tectonics and occurred broadly coeval with felsic magmatism. These relationships suggest that magmatism, tectonism, and deposition were broadly coeval and important regional-scale mechanisms consistent with formation in a late Neoproterozoic–early Paleozoic arc rift to backarc basin tectonic setting.

We present a newly compiled geologic map of the Pine Mountain window based on available 1:24,000 (and smaller) scale geologic maps; this map provides an improved basis to reconcile long-standing issues regarding tectonic evolution. We integrate sensitive high-resolution ion microprobe (SHRIMP) single-grain U-Pb ages of igneous, metamorphic, and detrital zircons from Grenville basement rocks, associated metasedimentary units, and cover rocks to help clarify the pre-Appalachian history and to better delimit the distribution of Laurentian versus peri-Gondwanan and Gondwanan units along the southeast flank of the window. U-Pb results indicate that some units, which earlier had been correlated with Neoproterozoic to Early Cambrian Laurentian rift deposits of the Ocoee Supergroup (i.e., Sparks-Halawaka Schist), actually are supracrustal rocks deposited prior to ~1100 Ma that were intruded and metamorphosed during the Ottawan phase of the Grenville orogeny. Zircons from the Phelps Creek Gneiss are 425 ± 7 Ma and overlap in time with plutons that intruded rocks of the Carolina superterrane during the Silurian (i.e., the Concord-Salisbury suite). The host units to the Phelps Creek Gneiss had also previously been interpreted as Sparks-Halawaka Schist, but field relations combine with the Silurian intrusive age to suggest that they rather belong to the peri-Gondwanan Carolina superterrane, helping to refine the position of the Central Piedmont suture in its most southern exposures. Results suggest that the Pine Mountain window is not framed by a single fault, but by Alleghanian faults of different timing, rheology, and kinematics, some of which were reactivated while others were not. The new map and U-Pb dates reveal that the southwesternmost exposures of the Central Piedmont suture are located farther northwest, so the width of the Pine Mountain window narrows from 22 km wide in central Georgia to only 5 km in Alabama. At its narrowest, the flanks of the Pine Mountain window are marked by two relatively thin normal faults (the Towaliga and Shiloh faults, northwest and southeast, respectively) that have excised the wider, earlier-formed mylonite zones. All of the Alleghanian faults are cut by later high-angle, normal and left- and right-slip brittle faults (Mesozoic?), which also influenced the present configuration of the window.

Recognition of the timing of peak metamorphism in the eastern Blue Ridge (ca. 460 Ma), Inner Piedmont (ca. 360 Ma), and Carolina terrane (ca. 540 Ma) has been critical in discerning the history of the collage of terranes in the hinterland of the southern Appalachian orogen. The Inner Piedmont consists of two terranes: the Tugaloo terrane, which is an Ordovician plutonic arc intruding thinned Laurentian crust and Iapetus, and the Cat Square paragneiss terrane, which is interpreted here as a Silurian basin that formed as the recently accreted (ca. 455 Ma) Carolina terrane rifted from Laurentia and was transferred to an oceanic plate. The recognition of an internal Salinic basin and associated magmatism in the southern Appalachian hinterland agrees with observations in the New England and Maritime Appalachians. Structural analysis in the Tugaloo terrane requires the Inner Piedmont to be restored to its pre-Carboniferous location, near the New York promontory. At this location, the Catskill and Pocono clastic wedges were deposited in the Devonian and Mississippian, respectively. Between the two wedges, an enigmatic formation (Spechty Kopf and its correlative equivalent Rockwell Formation) was deposited. Polymictic diamictites within this unit contain compositionally immature exotic clasts that may prove to have been derived from the Inner Piedmont. Following deposition of the Spechty Kopf and Rockwell Formations, the Laurentian margin became a right-lateral transform plate boundary. This continental-margin transform was subsequently modified and translated northwest above the Alleghanian Appalachian décollement. Thus, several critical recent observations presented here inspire a new model for the Silurian through Mississippian terrane dispersal and orogeny that defines southern Appalachian terrane geometry prior to emplacement of the Blue Ridge–Inner Piedmont–Carolina–other internal terranes as crystalline thrust sheets.