ABSTRACT The Neoproterozoic to Cambrian rifting history of Laurentia resulted in hyperextension along large segments of its Paleozoic margins, which created a complex paleogeography that included isolated continental fragments and exhumed continental lithospheric mantle. This peri-Laurentian paleogeography had a profound effect on the duration and nature of the Paleozoic collisional history and associated magmatism of Laurentia. During the initial collisions, peri-Laurentia was situated in a lower-plate setting, and there was commonly a significant time lag between the entrance of the leading edge of peri-Laurentia crust in the trench and the arrival of the trailing, coherent Laurentian landmass. The final Cambrian assembly of Gondwana was followed by a global plate reorganization that resulted in Cambrian (515–505 Ma) subduction initiation outboard of Laurentia, West Gondwana, and Baltica. Accretion of infant and mature intra-oceanic arc terranes along the Appalachian-Caledonian margin of the Iapetus Ocean started at the end of the Cambrian during the Taconic-Grampian orogenic cycle and continued until the ca. 430–426 Ma onset of the Scandian-Salinic collision between Laurentia and Baltica, Ganderia, and East Avalonia, which created the Laurussian continent and closed nearly all vestiges of the Iapetus Ocean. Closure of the Iapetus Ocean in the Appalachians was followed by the Devonian Acadian and Neoacadian orogenic cycles, which were due to dextral oblique accretion of West Avalonia, Meguma, and the Suwannee terranes following the Pridolian to Lochkovian closure of the Acadian seaway and subsequent outboard subduction of the Rheic Ocean beneath Laurentia. Continued underthrusting of Baltica and Avalonia beneath Laurentia during the Devonian indicates that convergence continued between Laurentia and Baltica and Avalonia, which, at least in part, may have been related to the motions of Laurentia relative to its converging elements. Cambrian to Ordovician subduction zones formed earlier in the oceanic realm between Laurentia and Baltica and started to enter the Arctic realm of Laurentia by the Late Ordovician, which resulted in sinistral oblique interaction of the Franklinian margin with encroaching terranes of peri-Laurentian, intra-oceanic, and Baltican provenance. Any intervening seaways were closed during the Middle to Late Devonian Ellesmerian orogeny. Exotic terranes such as Pearya and Arctic Alaska became stranded in the Arctic realm of Laurentia, while other terranes such as Alexander and Eastern Klamath were translated further into the Panthalassa Ocean. The Middle/Late Devonian to Mississippian Antler orogeny along the Cordilleran margin of Laurentia records the first interaction with an outboard arc terrane built upon a composite block preserved in the Northern Sierra and Eastern Klamath terranes. The Carboniferous–Permian Alleghanian-Ouachita orogenic cycle was due to closure of the vestiges of the Rheic Ocean and assembly of Pangea. The narrow, continental transform margin of the Ouachita embayment of southern Laurentia had escaped accretion by outboard terranes until the Mississippian, when it collided with an outboard arc terrane.

ABSTRACT The Paleozoic plate boundary zone between Laurussia and Gondwana in western Pangea hosts major magmatic and hydrothermal Sn-W-Ta, Au, and U mineralization. Individual mineral deposits represent the results of the superposition of a series of exogenic and endogenic processes. Exogenic processes controlled (1) the enrichment of the ore elements in sedimentary protoliths via residual enrichment during intense chemical weathering and via climatically or tectonically controlled redox traps, (2) the spatial distribution of fertile protoliths, and, thus, eventually (3) the spatial distribution of mineralization. Endogenic processes resulting in metamorphism and crustal melting controlled the mobilization of Sn-W, Au, and U from these enriched protoliths and, thus, account for the age distribution of Sn-W and Au mineralization and U-fertile granites. It is the sequence of exogenic and endogenic processes that eventually results in the formation of mineralization in particular tectonic zones. Whereas the endogenic processes were controlled by orogenic processes during the assembly of western Pangea itself, the exogenic processes were linked to the formation of suitable source rocks for later mineralization. The contrasting distribution of magmatic and hydrothermal Sn-W-Ta, Au, and U mineralization on the Laurussia and Gondwana sides of the plate boundary zone reflects the contrasting distribution of fertile protoliths and the contrasting tectonic situation on these margins. The Laurussian margin was an active margin during most of the Paleozoic, and the distribution of different mineralization types reflects the distribution of terranes of contrasting provenance. The Gondwanan margin was a passive margin during most of the Paleozoic, and the similar distribution of a wide range of different metals (Sn, W, Ta, Au, and U) reflects the fact that the protoliths for the various metals were diachronously accumulated on the same shelf, before the metals were mobilized during Acadian, Variscan, and Alleghanian orogenic processes.

ABSTRACT The Baie Verte Line in western Newfoundland marks a suture zone between (1) an upper plate represented by suprasubduction zone oceanic crust (Baie Verte oceanic tract) and the trailing continental Notre Dame arc, with related upper-plate rocks built upon the Dashwoods terrane; and (2) a lower plate of Laurentian margin metasedimentary rocks with an adjoining ocean-continent transition zone (Birchy Complex). The Baie Verte oceanic tract formed during closure of the Taconic seaway in a forearc position and started to be obducted onto the Laurentian margin between ca. 485 and 476 Ma (early Taconic event), whereas the Birchy Complex, at the leading edge of the Laurentian margin, was subducted to maximum depths as calculated by pseudosection techniques (6.7–11.2 kbar, 315–560 °C) by ca. 467–460 Ma, during the culmination of the Taconic collision between the trailing Notre Dame arc and Laurentia, and it cooled isobarically to 9.2–10.0 kbar and 360–450 °C by 454–449 Ma (M 1 ). This collisional wedge progressively incorporated upper-plate Baie Verte oceanic tract rocks, with remnants preserved in M 1 high-pressure, low-temperature greenschist-facies rocks (4.8–8.0 kbar, 270–340 °C) recording typical low metamorphic gradients (10–14 °C/km). Subsequently, the early Taconic collisional wedge was redeformed and metamorphosed during the final stages of the Taconic cycle. We relate existing and new 40 Ar/ 39 Ar ages between 454 and 439 Ma to a late Taconic reactivation of the structurally weak suture zone. The Taconic wedge on both sides of the Baie Verte suture zone was subsequently strongly shortened (D 2 ), metamorphosed (M 2 ), and intruded by a voluminous suite of plutons during the Salinic orogenic cycle. Calculated low- to medium-pressure, low-temperature M 2 conditions in the Baie Verte oceanic tract varied at 3.0–5.0 kbar and 275–340 °C, with increased metamorphic gradients of ~17–25 °C/km during activity of the Notre Dame arc, and correlate with M 2 assemblages in the Birchy Complex. These conditions are associated with existing Salinic S 2 white mica 40 Ar/ 39 Ar ages of ca. 432 Ma in a D 2 transpressional shear zone and synkinematic intrusions of comparable age. A third metamorphic event (M 3 ) was recorded during the Devonian with calculated low-pressure, low-temperature conditions of 3.2–3.8 kbar and 315–330 °C under the highest metamorphic gradients (23–30 °C/km) and associated with Devonian–early Carboniferous isotopic ages as young as 356 ± 5 Ma. The youngest ages are related to localized extension associated with a large-scale transtensional zone, which reused parts of the Baie Verte Line suture zone. Extension culminated in the formation of a Middle to Late Devonian Neoacadian metamorphic core complex in upper- and lower-plate rocks by reactivation of Baie Verte Line tectonites formed during the Taconic and Salinic cycles. The Baie Verte Line suture zone is a collisional complex subjected to repeated, episodic structural reactivation during the Late Ordovician Taconic 3, Silurian Salinic, and Early–Late Devonian Acadian/Neoacadian orogenic cycles. Deformation appears to have been progressively localized in major fault zones associated with earlier suturing. This emphasizes the importance of existing zones of structural weakness, where reactivation took place in the hinterland during successive collision events.

ABSTRACT The Baltimore terrane, the Baltimore Mafic Complex (BMC), and the Potomac terrane are telescoped tectonostratigraphic packages of metasedimentary and meta-igneous rocks that record the geologic history of eastern Maryland from 1.2 Ga to 300 Ma. These terranes provide insight into the understanding of the rifting of Rodinia and the initial amalgamation of eastern Laurentia. The oldest of these rocks are exposed as gneiss domes in the Baltimore terrane, with gneissic Grenvillian crust overlain by a metasedimentary cover succession believed to have been deposited during Rodinian rifting and the formation of the Iapetus ocean. These rocks are interpreted to be analogous to the Blue Ridge sequence in western Maryland. Late Cambrian ultramafites and amphibolites of the BMC discordantly overlie the Baltimore terrane to the east and north, and may represent ophiolitic oceanic crust obducted over eastern Laurentia continental rocks as an island-arc collisional event during the Taconian orogeny. To the west, a thick assemblage of schist, graywacke, metadiamictite, and ultramafic bodies comprises the Potomac terrane, a polygenetic mélange that may have formed in an accretionary wedge during Taconian subduction and collision with the Laurentian continental margin. The Pleasant Grove fault zone marks the Taconian suture of these accreted terranes to Laurentian rocks of the central Maryland Piedmont, and preserves evidence of dextral transpression during the Alleghenian orogeny in the Late Pennsylvanian.

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 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.

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”