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Ordovician and Siluro-Lower Devonian magmatic rocks in the northern Appalachians south of the Iapetus suture are currently interpreted as distinct belts composed of multiple, small, peri-Gondwanan terranes that amalgamated during the sequential closures of Iapetus (latest Ordovician), the Tetagouche backarc basin (early Silurian), the Acadian seaway (Siluro-Devonian), and the Rheic Ocean (Devono-Carbonferous) (multiple terrane model). Here, the Siluro-Lower Devonian magmatic belts are shown to have slab failure affinities and together with the Ordovician arcs form paired belts parallel to the Iapetus suture, which suggests that they were emplaced along the common, peri-Avalonian margin during pre- and post-collisional processes. The Iapetan suture and the paired belts are inferred to repeat in Atlantic Canada due to dextral, strike-slip processes of mid-Late Devonian or younger age (terrane wreck model). In Newfoundland, the repetition is inferred to be the result of oblique, dextral offset of ca. 250 km. In the Quebec Embayment, the Iapetan paired magmatic belts are repeated twice in the limbs of a Z-shaped orocline related to oblique, dextral offsets of ca. 1200 km of the southern limb. Limited Siluro-Devonian paleomagnetic data indicate no paleolatitudinal differences across the Iapetus suture, however ca. 100[degrees] post-mid Silurian clockwise rotation is indicated for the middle fold limb; these data favour the terrane wreck model. The terrane wreck model results in a simple tectonic scenario of southerly subduction of Iapetus beneath a single ribbon continent (Avalonia sensu lato) that was subsequently deformed. Key words: Appalachians, Ordovician arcs, Silurian--Lower Devonian slab failure, palinspastics, oroclines, Ganderia, terrane wreck. Desrochesmagmatiques d'age ordovicien et silurien-devonien inferieur dans le nord des Appalaches au sud de la suture de Iapetus sont actuellement interpretees comme representant des ceintures distinctes composees de multiples petits terranes perigondwaniens qui se sont amalgames durant les fermetures successives de Iapetus (Ordovicien terminal), du bassin d'arriere-arc de Tetagouche (Silurien precoce), du bras de mer acadien (Silurien--Devonien) et de l'ocean Rheique (Devonien--Carbonifere) (le << modele des terranes multiples >>). Nous demontrons que les ceintures magmatiques d'age silurien--devonien inferieur presentent des affinites de rupture de plaque et que, avec les arcs ordoviciens, elles forment des ceintures jumelees paralleles a la suture de Iapetus, ce qui indiquerait qu'elles ont ete mises en place le long de la marge periavalonienne commune durant des processus pre- et post-collision. Nous inferons que la suture de Iapetus et les ceintures jumelees se repetent au Canada atlantique en raison de processus de decrochement dextre actifs au milieu du Devonien tardif ou apres (le << modele d'empilement de terranes >>). A Terre-Neuve, nous inferons que la repetition est le resultat d'une translation dextre oblique sur environ 250 km. Dans le rentrant de Quebec, les ceintures magmatiques jumelees iapetennes sont repetees deux fois dans les flancs d'un oroclinal en Z associe a des translations dextres obliques d'environ 1200 km du flanc sud. Si des donnees paleomagnetiques limitees pour le Silurien--Devonien ne revelent aucune difference de paleolatitude de part et d'autre de la suture de Iapetus, quelque 100[degrees] de rotation horaire apres le Silurien moyen sont indiques pour le flanc median du pli; ces donnees favorisent le modele d'empilement de terranes. Ce modele se traduit par un scenario tectonique simple de subduction vers le sud de Iapetus sous un seul continental rubane (l'Avalonie au sens large) qui a par la suite ete deforme. [Traduit par la Redaction] Mots-cles : Appalaches, arcs ordoviciens, rupture de plaque au Silurien--Devonien inferieur, palinspatique, oroclinaux, Ganderie, empilement de terranes., Introduction The formation and deformation of orogenic belts is fundamental to plate tectonics and informs our evolving understanding of convergent margin processes. A convergent margin preserves a combination of evidence [...]

Current interpretations of the geology of Cape Breton Island suggest that it exposes a complete cross-section of the Appalachians from Laurentia across Iapetan vestiges to Avalonia. Crucial to this view is the presence of ca. 1 Ga plutons, including anorthosites, which have been regarded as correlatives of Grenvillian basement, a correlation that overlooks the fact that Avalonia is also underlain by a ca. 1 Ga basement. We analyzed zircons from the Red River anorthosite (Blair River Complex, northwestern Cape Breton Island) previously dated as ca. 1.1 Ga: they yielded 421 ± 3 Ma intrusive ages with older ages between 865 ± 18 Ma and 1044 ± 20 Ma inferred to be either xenocrysts derived from the country rock or from the source. Implications of these data suggest that the accompanying low pressure granulite-amphibolite facies metamorphism of the Blair River Complex is either the root of a 440–410 Ma, magmatic belt produced during slab break-off or relict ca. 1 Ga basement. The Blair River Complex occurs in a NNE-SSW, sinistral positive flower structure that progresses upwards from a Neoproterozoic rifted arc through a low grade upper Ordovician-Silurian overstep sequence to amphibolite facies fault slices, capped by the low-pressure, granulite facies rocks (Blair River Complex). The correlation of Neoproterozoic, rifted arc units across most of Cape Breton Island suggests it represents the deformed northwestern margin of Avalonia intruded by a Silurian-Lower Devonian magmatic belt. As the geological record in the Blair River Complex is similar to both Grenvillian and Avalonian basements, its provenance is equivocal, however Pb isotopic data suggest the Blair River Complex has Amazonian (≈Avalonia) affinities. Thus, Cape Breton Island, rather than representing a complete cross-section of the Appalachian orogen, is part of pristine—deformed Avalonia with a positive flower structure exposing a cross-section of Avalonian crust. Unlabelled Image • 420 Ma intrusive age for anorthosite supercedes previous 1 Ga age • Cape Breton Highlands is a +ve flower structure in Silurian magmatic belt. • Pb isotopes confirm Avalonian affinity of entire Cape Breton Island. [ABSTRACT FROM AUTHOR]