NORTH AMERICA/Northern Appalachians 83 Figure Early Cambrian (around 540 Ma) palaeogeography of the Iapetus Ocean Seafloor spreading that led to the opening of Iapetus started at around 570 Ma Aval., Avalonia; Carol., Carolina; DA, Dashwoods; Gand., Ganderia; Hø´l., Hø´landa; Pc, Precordillera Terrane Meguma was probably situated further east, just outside the field of view, along the north African margin of Gondwana near elements of Armorica Hø´landa and Precordillera are peri Laurentian microcontinents like Dashwoods but ended up in Baltica and the South American part of Gondwana, respectively, after the closure of Iapetus Made with the help of Conall MacNiocaill Zone represents the peripheral part of the Laurentian Craton, which was involved in Appalachian orogenesis The Gander, Avalon, and Meguma zones represent peri-Gondwanan microcontinents that accreted to Laurentia during the Early to Middle Palaeozoic (450–380 Ma) The Dunnage Zone contains the remnants of oceanic terranes that formed within the realm of the Iapetus Ocean, and is subdivided into the Notre Dame and Exploits subzones Orogenesis (the combined effects of spatially and temporally associated deformation, metamorphism, magmatism and sedimentation) was mainly confined to the central portion of the Northern Appalachians, often referred to as the Central Mobile Belt Humber Zone The Humber Zone is mostly underlain by a Late Neoproterozoic–Ordovician rifted passive margin sequence deposited on Laurentian basement that had experienced multiple orogenic events during the Mesoproterozoic (around 1.5–0.95 Ga), culminating in the Grenville orogeny (around 1.2–0.95 Ga) These basement rocks are exposed in several windows from Newfoundland to southern New York state Its Late Neoproterozoic–Palaeozoic cover preserves evidence of rifting, development of a warm-water carbonate shelf and slope, and conversion into a convergent margin Rift-related magmatism took place intermittently during an approximately 70 Ma time-span during the latest Neoproterozoic (620–550 Ma) and was coeval with the deposition of consanguineous clastic sedimentary rocks Exactly when during this time interval the opening of the Iapetus Ocean began is at present unclear, but palaeomagnetic and geological arguments suggest that it occurred at around 570 Ma Several lines of evidence suggest that the final stage of rift magmatism (555–550 Ma) and the subsequent Early Cambrian (around 540 Ma) transition from rift to drift sedimentation preserved in Newfoundland, Quebec, and New England (Figure 3), relates to the departure of a small microcontinent, referred to as Dashwoods This event thus postdates the seafloor spreading associated with the earlier opening of Iapetus Dashwoods equates