Terrane history of the Iapetus Ocean as preserved in the northern Appalachians and western Caledonides

The Iapetus Ocean was the first ancient ocean to be identified following the development of plate tectonics; its history has been fundamental in relating orogenesis and plate motion. The ocean probably formed following 3-way rifting between Laurentia, Baltica, and Amazonia – West Africa (a block that became incorporated in Gondwana). Closure of the ocean trapped numerous terranes during the development of the Appalachian–Caledonide Orogen. Subsequent deformation, including late Paleozoic strike slip, transpression, and transtension, and Mesozoic stretching during Pangea breakup, must be taken into account in models for orogen development. Traditional analyses of Iapetan terranes have focussed on Cambrian sedimentary successions, and on isotopic criteria, to classify terranes into larger domains: Ganderia, Avalonia and Megumia. Detrital zircon data show that these domains did not cross the Iapetus as single entities, while paleomagnetic data reveal significant vertical-axis terrane rotations. We here review and interpret 17 paleomagnetic poles and >350 published detrital zircon data sets from the northern Appalachians and western Caledonides, using consistent and rigorous criteria for the selection and presentation of data. We place these data on an integrated stratigraphic chart to show timing relations and to seek constraints on the provenance and travel of terranes in the Iapetus Ocean. We distinguish groups of terranes that likely travelled together as terrane assemblages. In the Taconian/Grampian Orogeny, Furongian to Katian continent–arc collision involved off-margin blocks along the hyperextended Laurentian margin. In New England, early Taconian collision by 475 Ma involved the Gondwana-derived Moretown assemblage. An assemblage of the Bronson and Popelogan arc terranes probably arrived at the main Laurentian margin 25-30 Myr later. Subduction polarity reversal then led to the progressive accretion of additional terrane assemblages (Salinian Orogeny). The Miramichi–Victoria assemblage arrived close to the Ordovician–Silurian boundary. The Miramichi terrane underwent partial subduction in the Québec re-entrant, whereas the Victoria terrane was juxtaposed with the Newfoundland promontory without major metamorphism. In mid-Silurian time, an assemblage including the Gander terrane of Newfoundland and related portions of Britain and Ireland was accreted to Laurentia, along with Baltica (Scandian Orogeny). The St. Croix – La Poile assemblage may have been accreted slightly later, but is distinguished by the development of a Silurian arc–backarc system (coastal igneous belt) above a northwest-dipping subduction zone. The Avalon–Brookville assemblage encountered this system in Přídolí to Middle Devonian time (Acadian Orogeny), leading to the collapse of the backarc basin and northwest-vergent thrust emplacement onto Laurentia during sinistral transpression in the Appalachian Orogen. Acadian deformation involved mainly sinistral strike slip in Britain and Ireland. Several of the terranes that were accreted to the Laurentian margin carried internal records of earlier deformation that took place near Amazonia – West Africa in Early Ordovician time and earlier (Monian/Penobscottian Orogeny). The Iapetus Ocean thus contained a complex array of terranes, small ocean basins, arcs, and previously emplaced ophiolites analogous to modern southeast Asia. It closed to form a complex array of sutures in an orogen within which no single Iapetus suture can be clearly identified