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 global distribution, setting, and dynamic implications of Ordovician orogenesis are reviewed. Evidence for true Ordovician continent-continent collision is absent. Orogenesis is principally due to accretion of arc terranes and/or ribbon microcontinents. Most arc terranes are ensialic and separated from the adjacent continents by backarc or marginal basins, the episodic closure of which commonly was responsible for orogenesis. Little evidence is preserved for true intra-oceanic juvenile arcs during the Early to Middle Ordovician. Instead, subduction appears to have been localized near the margins of Laurentia, Gondwana, Baltica, and Siberia, forming extensive linear orogenic belts during relatively short periods when the upper plate switched from extension to compression. Such tectonic switching appears to have taken place along the entire length of the Pacific and Iapetan margins of Gondwana (>10,000 km) from Middle–Late Cambrian to Early Ordovician time. The onset of orogenesis along Gondwana's Pacific margin during the end of the Early Cambrian (ca. 513 Ma) coincided with subduction initiation along both margins of the Iapetus Ocean. Orogenesis and subduction initiation are causally related to a global-scale plate reorganization, probably induced by terminal amalgamation of Gondwana. During the Paleozoic, Laurentia's Iapetan margin steadily grew in size and expanded southward owing to continuous accretion of suprasubduction zone oceanic crust, peri-Gondwanan arc terranes, and ribbon microcontinents. In contrast, the Pacific, Iapetan, and Rheic margins of Gondwana saw little addition of new, allochthonous crust. Accretion mainly involves reattachment of previously rifted-off arc terranes and small slivers of the intervening marginal basin crust.

Abstract During the Early to Middle Palaeozoic, prior to formation of Pangaea, the Canadian and adjacent New England Appalachians evolved as an accretionary orogen. Episodic orogenesis mainly resulted from accretion of four microcontinents or crustal ribbons: Dashwoods, Ganderia, Avalonia and Meguma. Dashwoods is peri-Laurentian, whereas Ganderia, Avalonia and Meguma have Gondwanan provenance. Accretion led to a progressive eastwards (present co-ordinates) migration of the onset of collision-related deformation, metamorphism and magmatism. Voluminous, syn-collisional felsic granitoid-dominated pulses are explained as products of slab-breakoff rather than contemporaneous slab subduction. The four phases of orogenesis associated with accretion of these microcontinents are known as the Taconic, Salinic, Acadian and Neoacadian orogenies, respectively. The Ordovician Taconic orogeny was a composite event comprising three different phases, due to involvement of three peri-Laurentian oceanic and continental terranes. The Taconic orogeny was terminated with an arc–arc collision due to the docking of the active leading edge of Ganderia, the Popelogan–Victoria arc, to an active Laurentian margin (Red Indian Lake arc) during the Late Ordovician (460–450 Ma). The Salinic orogeny was due to Late Ordovician–Early Silurian (450–423 Ma) closure of the Tetagouche–Exploits backarc basin, which separated the active leading edge of Ganderia from its trailing passive edge, the Gander margin. Salinic closure was initiated following accretion of the active leading edge of Ganderia to Laurentia and stepping back of the west-directed subduction zone behind the accreted Popelogan–Victoria arc. The Salinic orogeny was immediately followed by Late Silurian–Early Devonian accretion of Avalonia (421–400 Ma) and Middle Devonian–Early Carboniferous accretion of Meguma (395–350 Ma), which led to the Acadian and Neoacadian orogenies, respectively. Each accretion took place after stepping-back of the west-dipping subduction zone behind an earlier accreted crustal ribbon, which led to progressive outboard growth of Laurentia. The Acadian orogeny was characterized by a flat-slab setting after the onset of collision, which coincided with rapid southerly palaeolatitudinal motion of Laurentia. Acadian orogenesis preferentially started in the hot and hence, weak backarc region. Subsequently it was characterized by a time-transgressive, hinterland migrating fold-and-thrust belt antithetic to the west-dipping A–subduction zone. The Acadian deformation front appears to have been closely tracked in space by migration of the Acadian magmatic front. Syn-orogenic, Acadian magmatism is interpreted to mainly represent partial melting of subducted fore-arc material and pockets of fluid-fluxed asthenosphere above the flat-slab, in areas where Ganderian's lithosphere was thinned by extension during Silurian subduction of the Acadian oceanic slab. Final Acadian magmatism from 395– c . 375 Ma is tentatively attributed to slab-breakoff. Neoacadian accretion of Meguma was accommodated by wedging of the leading edge of Laurentia, which at this time was represented by Avalonia. The Neoacadian was devoid of any accompanying arc magmatism, probably because it was characterized by a flat-slab setting throughout its history.

The Appalachian-Caledonian orogen records a complex history of the closure of the Cambrian-Ordovician Iapetus Ocean. The Dunnage Zone of Newfoundland preserves evidence of an Ordovician arc-arc collision between the Red Indian Lake Arc, which forms part of the peri-Laurentian Annieopsquotch accretionary tract (ca. 480–460 Ma), and the peri-Gondwanan Victoria Arc (ca. 473–453 Ma). Despite the similarity in age, the coeval arc systems can be differentiated on the basis of the contrasts that are apparent across the suture zone, the Red Indian Line. These contrasts include structural and tectonic history, stratigraphy, basement characteristics, radiogenic lead in mineral deposits, and fauna. The arc-arc collision is considered in terms of modern analogues (Molucca and Solomon Seas) in the southwest Pacific, and the timing is constrained by stratigraphic relations in the two arc systems. The Victoria Arc occupied a lower-plate setting during the collision and underwent subsidence during the collision, similar to the Australian active margin and Halmahera arcs in the southwest Pacific. The timing of the subsidence is constrained by three new ages of volcanic rocks in the Victoria Arc (457 ± 2; 456.8 ± 3.1; 457 ± 3.6 Ma) that immediately predate or are coeval with deposition of the Caradoc black shale. In contrast the Red Indian Lake Arc contains a sub-Silurian unconformity and a distinct lack of Caradoc black shale, suggesting uplift during the collision. The emergent peri-Laurentian terranes provided detritus into the newly created basin above the Victoria Arc. The evidence of this basin is preserved in the Badger Group, which stratigraphically overlies the peri-Gondwanan Victoria Arc but incorporated peri-Laurentian detritus. Thus the Badger Group forms a successor basin(s) over the Red Indian Line. Following the collision, subduction stepped back into an outboard basin, the Exploits-Tetagouche backarc, closing the Iapetus Ocean along the Dog Bay Line in the Silurian. Correlative tracts in the Northern Appalachians and British Caledonides support the Ordovician arc-arc collision; however, the evidence is less obvious than in Newfoundland.

The eastern flank of the Appalachian orogen is composed of extensive Neoproterozoic–early Paleozoic crustal blocks that originated in a peri-Gondwanan setting. Three of these blocks record the evolution of Neoproterozoic magmatic-arc systems, including Carolinia in the southern Appalachians and Ganderia and Avalonia in the northern Appalachians. Relationships among these three crustal blocks are important for understanding both the accretionary history of the orogen and the evolution of the Iapetus and Rheic Oceans, first-order geographic features of the Paleozoic globe. Traditionally, Carolinia and Avalonia have been considered to represent a single microcontinental magmatic arc that accreted to Laurentia in the middle to late Paleozoic. The early lithotectonic history (ca. 680–570 Ma) of the two blocks is obscure; however, their latest Neoproterozoic-Paleozoic histories are distinct. This disparity is manifest in the first-order features of (1) timing and style of magmatic-arc cessation and (2) the nature of their Paleozoic lithotectonic records. Magmatic arc activity ceased in Avalonia in the late Neoproterozoic (ca. 570 Ma), succeeded by extension-related magmatism and sedimentation that was transitional into a robust latest Neoproterozoic–Silurian platformal clastic sedimentary sequence. This platform was tectonically unperturbed until the Late Silurian–Early Devonian. In contrast, Carolinia records late Neo-proterozoic tectonothermal events coeval with arc magmatism, which extended into the Cambrian; a relatively thin Middle Cambrian shallow-marine clastic sequence is preserved unconformably atop the Carolinia arc sequences. Subsequently, Carolinia experienced widespread Late Ordovician–Silurian deformation and metamorphism. However, we note striking similarities between Carolinia and Ganderia; specifically, in Ganderia, like Carolinia, late Neoproterozoic tectonism was accompanied by arc magmatism that extended into the Cambrian. Ganderian arc rocks are capped unconformably by a Middle Cambrian to Early Ordovician clastic sequence, and they were tectonized in the Late Ordovician–Silurian, similar to relations in Carolinia. Independent studies indicate that the Late Ordovician–Silurian tectonism in both blocks was related to their accretion to Laurentia. Thus, Carolinia and Ganderia show parallel development of first-order lithotectonic characteristics for two endpoints in their global strain path, i.e., their Gondwanan source region and their accretion to Laurentia. Consequently, we posit that Carolinia appears to be more closely affiliated with Ganderia than with Avalonia. The recognition of this linkage between Appalachian peri-Gondwanan realm crustal blocks in light of paleomagnetic and isotopic data leads to a unified model for the accretion of these blocks to the eastern margin of Laurentia.