James P. Hibbard

The Carolina Zone: overview of Neoproterozoic to Early Paleozoic peri-Gondwanan terranes along the eastern Flank of the southern Appalachians

Abstract The Carolina Zone is an amalgamation of mainly Neoproterozoic–Early Paleozoic metaigneous-dominated terranes that are clustered along the eastern flank of the southern Appalachians. These terranes are distinguished from other divisions of the orogen by a commonality in gross geologic content and by their close spatial association. They are considered exotic relative to Laurentia on the basis of stratigraphic and tectonic evolution, paleontology, and position in the orogen analogous to that of exotic terranes in the northern Appalachians. They are probably peri-Gondwanan in origin. Within this first-order identity, the terranes exhibit remarkable heterogeneity, with respect to deposition, magmatism, and tectonothermal overprint. The depositional–magmatic history of the zone is viewed in three broad stages, including: (I) pre-600 Ma, (II) ca. 590–560 Ma, and (III) younger than ca. 550 Ma. Although each stage records significant felsic volcanism, there are few compelling stratigraphic linkages between terranes. Stage III plutonism may form a link between the two largest terranes in the zone. The isotopic evolution of the zone reflects the stratigraphic heterogeneity; isotopically juvenile magmatism in some terranes is coeval with more crustally evolved magmatism in others. The tectonothermal history of the zone is heterogeneous, producing a patchwork of suprastructural and infrastructural elements of different ages. Major tectonothermal events responsible for this pattern span the Neoproterozoic–earliest Cambrian, the Late Ordovician–Silurian, and the late Paleozoic. Evidence for regionally extensive events in the zone is sparse and such a fundamental concept as its time of accretion to Laurentia is speculative. The central Piedmont shear zone, a late Paleozoic ductile thrust that defines the western limit of the Carolina Zone, marks the final emplacement of the zone against Laurentian elements.

A comparative analysis of pre-Silurian crustal building blocks of the northern and the southern Appalachian orogen

The New York promontory serves as the divide between the northern and southern segments of the Appalachian orogen. Antiquated subdivisions, distinct for each segment, implied that they had lithotectonic histories that were independent of each other. Using new lithotectonic subdivisions we compare first order features of the pre-Silurian orogenic ’building blocks’ in order to test the validity of the implication of independent lithotectonic histories for the two segments. Three lithotectonic divisions, termed here the Laurentian, Iapetan, and the peri-Gondwanan realms, characterize the entire orogen. The Laurentian realm, composed of native North American rocks, is remarkably uniform for the length of the orogen. It records the multistage Neoproterozoic-early Paleozoic rift-drift history of the Appalachian passive margin, formation of a Taconic Seaway, and the ultimate demise of both in the Middle Ordovician. The Iapetan realm encompasses mainly oceanic and magmatic arc tracts that once lay within the Iapetus Ocean, between Laurentia and Gondwana. In the northern segment, the realm is divisible on the basis of stratigraphy and faunal provinciality into peri-Laurentian and peri-Gondwanan tracts that were amalgamated in the Late Ordovician. South of New York, stratigraphic and faunal controls decrease markedly; rock associations are not inconsistent with those of the northern Appalachians, although second-order differences exist. Exposed exotic crustal blocks of the peri-Gondwanan realm include Ganderia, Avalonia, and Meguma in the north, and Carolinia in the south. Carolinia most closely resembles Ganderia, both in early evolution and Late Ordovician-Silurian docking to Laurentia. Our comparison indicates that, to a first order, the pre-Silurian Appalachian orogen developed uniformly, starting with complex rifting and a subsequent drift phase to form the Appalachian margin, followed by the consolidation of Iapetan components and ending with accretion of the peri-Gonwanan Ganderia and Carolinia. This deduction implies that any first-order differences between northern and southern segments post-date Late Ordovician consolidation of a large portion of the orogen.

Docking Carolina: Mid-Paleozoic accretion in the southern Appalachians

Accretion of the extensive Carolina zone to Laurentia is one of the most critical unresolved problems in southern Appalachian tectonics. Synthesis of southern Appalachian tectonism reveals three lines of evidence from across the orogen that indicate a Late Ordovician to Silurian time of accretion. A tectonic unconformity in native Laurentian rocks, an extensive magmatic pulse in the Piedmont zone of the orogen, and active uplift in the Carolina zone are all related to this accretionary event. Structural geometries in the Carolina zone suggest that accretion involved sinistral transpression. This new model for Carolina accretion is consistent with models for the accretion of northern Appalachian peri-Gondwanan terranes; collectively their timing and mode of accretion to Laurentia place tectonic constraints on Paleozoic global reconstructions.

Alternative plate model for the early Miocene evolution of the southwest Japan margin

The Neogene-Holocene tectonics of southwest Japan have been controlled by the interaction of the Eurasian, Philippine Sea, and Pacific plates, which now meet at the central Japan trench-trench-trench (TTT) triple junction. Previous plate models have depicted the Shikoku back-arc basin as opening at the eastern margin of the Philippine Sea plate, parallel to and against the Japan margin. These models consider the TTT triple junction as predating the Shikoku Basin, but they predict neither the evolution of this junction nor the nature of the southwest Japan plate boundary during basin opening. Our recent work on the Oligocene-Miocene strata at Cape Muroto, Shikoku Island, has allowed us to locate the approximate position of the Shikoku Basin spreading ridge at 15 Ma. Combined with a compilation of the geology of the early Miocene southwest Japan margin, this determination leads us to propose that the Shikoku Basin was separated from the Japanese margin by an extension of the Pacific plate during basin opening. The basin subsequently collided with southwest Japan at ca. 15 Ma; this model implies that the central Japan triple junction was generated during this collision.

Early Ordovician rifting of Avalonia and birth of the Rheic Ocean: U-Pb detrital zircon constraints from Newfoundland

Abstract: Avalonia is the largest accreted crustal block in the Appalachian orogen and comprises a collection of late Neoproterozoic volcano-sedimentary sequences that are overlain by a Palaeozoic platformal sedimentary succession. Detrital zircons from the Conception Group are dominated by 570–620 Ma ages and contain a significant component generated by erosion of coeval igneous arc-volcanic rocks. Overlying samples from the Cuckold and Crown Hill formations are dominated by Neoproterozoic populations with ages between 600 and 650 Ma and are interpreted to be derived from the underlying calc-alkaline arc-plutonic rocks. Early Palaeozoic platform units are dominated by c. 620 Ma zircons with lesser Mesoproterozoic and Palaeoproterozoic zircons. The range of detrital zircon ages is inconsistent with a West African provenance and suggests that Avalonia originated along the Gondwanan margin of the Amazon craton. The influx of Mesoproterozoic and Palaeoproterozoic detritus in the Avalonian platform suggests a major change in tectonic regime. The prominent change in provenance is interpreted to be related to separation of Avalonia from Gondwana during the Early Ordovician opening of the Rheic Ocean. The Redmans Formation is interpreted to represent the rift–drift transition of the Rheic Ocean, which imposes important constraints on the palaeotectonic evolution of Avalonia. Supplementary material: U–Pb isotopic data of LA-ICP-MS analysis of detrital zircons are available at http://www.geolsoc.org.uk/SUP18346.

Precise U‐Pb Zircon Constraints on the Earliest Magmatic History of the Carolina Terrane

The early magmatic and tectonic history of the Carolina terrane and its possible affinities with other Neoproterozoic circum‐Atlantic arc terranes have been poorly understood, in large part because of a lack of reliable geochronological data. Precise U‐Pb zircon dates for the Virgilina sequence, the oldest exposed part, constrain the timing of the earliest known stage of magmatism in the terrane and of the Virgilina orogeny. A flow‐banded rhyolite sampled from a metavolcanic sequence near Chapel Hill, North Carolina, yielded a U‐Pb zircon date of 632.9 +2.6/−1.9 Ma. A granitic unit of the Chapel Hill pluton, which intrudes the metavolcanic sequence, yielded a nearly identical U‐Pb zircon date of 633 +2/−1.5 Ma, interpreted as its crystallization age. A felsic gneiss and a dacitic tuff from the Hyco Formation yielded U‐Pb zircon dates of 619.9 +4.5/−3 Ma and 615.7 +3.7/−1.9 Ma, respectively. Diorite and granite of the Flat River complex have indistinguishable U‐Pb upper‐intercept dates of 613.9 +1.6/−1.5 Ma and 613.4 +2.8/−2 Ma. The Osmond biotite‐granite gneiss, which intruded the Hyco Formation before the Virgilina orogeny, crystallized at 612.4 +5.2/−1.7 Ma. Granite of the Roxboro pluton, an intrusion that postdated the Virgilina orogeny, yielded a U‐Pb upper intercept date of 546.5 +3.0/−2.4 Ma, interpreted as the time of its crystallization. These new dates both provide the first reliable estimates of the age of the Virgilina sequence and document that the earliest known stage of magmatism in the Carolina terrane had begun by 633 +2/−1.5 Ma and continued at least until 612.4 +5.2/−1.7 Ma, an interval of ∼25 m.yr. Timing of the Virgilina orogeny is bracketed between 612.4 +5.2/−1.7 Ma and \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Late Paleozoic strike-slip faults in Maritime Canada and their role in the reconfiguration of the northern Appalachian orogen

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Smith River allochthon: A southern Appalachian peri-gondwanan terrane emplaced directly on laurentia?

\end{document} Ma (reported age of the upper Uwharrie Formation). The U‐Pb systematics of all units studied in the Virgilina sequence are simple and lack any evidence of an older xenocrystic zircon component, which would indicate the presence of a continental‐type basement. This observation, together with the juvenile Nd isotopic character of the Virgilina volcanic arc sequence, suggests that the oldest part of the Carolina terrane was built on oceanic crust away from a continental crustal influence.