NORTH AMERICA/Northern Appalachians 89 to Early Devonian shallow-marine dominantly siliciclastic sedimentary rocks, which are locally interlayered with rift-related bimodal Early Silurian (442–438 Ma) volcanic rocks The Meguma Zone experienced intense orogenesis during the late Early Devonian to Early Carboniferous (395–350 Ma; Neoacadian orogenesis) Fossil evidence (fish and crinoid) suggests that, during the Late Silurian, Meguma was close to Avalonia and/or Baltica, and separated from Gondwana When combined with the magmatic evidence for Late Ordovician rifting and Neoacadian orogenesis, this suggests that Meguma was a microcontinent or part thereof (perhaps Armorica) during at least the Silurian and Devonian, as Gondwana was not accreted to Meguma until the Carboniferous–Permian Alleghanian Orogeny Overview and Summary of the Tectonic Evolution of the Northern Appalachians The oldest orogenic events associated with the closure of Iapetus are the Late Cambrian–Early Ordovician (495–484 Ma) accretion of the Baie Verte Oceanic Tract to the Dashwoods and the Penobscot complex to Ganderia, with the Baie Verte Oceanic Tract and the Penobscot complex both in upper-plate settings (Figures and 4) Exposed, inferred Ganderian, basement locally contains evidence of older Early Cambrian orogenesis, but this is probably related to peripheral subduction around the Gondwanan continent outside the realm of Iapetus The accretions of the Baie Verte Oceanic Tract and the Penobscot complex were nearly coeval, but took place on opposite sides of the Iapetus Ocean, when the ocean was at its widest Because these events took place shortly after the initiation of subduction on either side of the ocean, they signal a major plate reorganization that changed Iapetus from an Atlantic-type ocean into a Pacific-type ocean Time constraints provided by stitching plutons indicate that both accretionary events were short-lived, forming me´ langes and ductile–brittle deformation, and did not lead to any significant regional metamorphism The first major orogenic event recorded in the Northern Appalachians was the Early to Middle Ordovician (474–455 Ma) Taconic Orogeny, which principally resulted from closure of the Humber seaway and accretion of the Dashwoods microcontinent to Laurentia Closure of the Humber Seaway initiated loading of the Humber margin, initially by emplacement of oceanic terranes, some of which are younger than the Baie Verte Oceanic Tract (484–479 Ma) These younger oceanic terranes, which include the Bay of Islands and Thetford Mines ophiolites in Newfoundland and Quebec, respectively, formed either during the initiation of subduction in the Humber Seaway or as a result of localized pericratonic seafloor spreading in pull-aparts or above embayments in the downgoing plate The Taconic Orogeny culminated in imbrication, folding, and locally intense Barrovian metamorphism, mainly in upper-plate rocks associated with the Dashwoods Age constraints on Taconic metamorphism (470–455 Ma) are remarkably consistent along the length of the orogen Docking of Dashwoods was rapidly followed by accretion of the Annieopsquotch accretionary tract between 470 and 460 Ma and the Popelogan–Victoria Arc at 455–450 Ma to composite Laurentia along its ocean-facing eastern margin The effects of these accretionary events, which are centred on the Red Indian Line, are difficult to separate from those of the orogenesis accompanying the Dashwoods– Laurentia collision to the west Since separation of these events on the basis of kinematic arguments is often impossible outside Newfoundland, these events are included in the Taconic orogeny Structural relationships and the presence of young suprasubduction-zone oceanic rocks (480–464 Ma) in the Annieopsquotch accretionary tract indicate that a new west-dipping subduction zone had formed in the Arenig to the east of Dashwoods Accretion involved south-east-verging duplex-style thrust complexes, folding, me´ lange development, and a sinistral oblique reverse shear zones The Popelogan–Victoria Arc was formed in the Early Arenig (at about 478 Ma) on the leading edge of Ganderia after the Penobscot orogeny It was active until the Late Caradoc and formed above an east-dipping subduction zone Upper-plate extension and rifting as a result of slab rollback dispersed the Popelogan–Victoria Arc into Iapetus and formed a wide (up to 1000 km) Japan Sea-style back-arc basin (Tetagouche–Exploits back-arc) that was partly underlain by Llanvirn oceanic crust The opposite subduction polarities associated with the Annieopsquotch accretionary tract (west-dipping) and the Popelogan–Victoria Arc (east-dipping) require that the Red Indian Line marks a collision between two arc terranes (Figure 4) Structural and seismicreflection data in central Newfoundland indicate that the Popelogan–Victoria Arc was partly subducted beneath the Annieopsquotch accretionary tract Palaeomagnetic evidence suggests that between 480 and 450 Ma these two subduction zones consumed approximately 3000 km of ocean, implying a minimum closure rate of around 10 cm yr–1 in the main Iapetus tract The remaining tracts of Iapetus (Humber Seaway, Tetagouche–Exploits back-arc, and the ocean