84 NORTH AMERICA/Northern Appalachians Figure Tectonostratigraphic relationships pertinent to the convergence between the Baie Verte Oceanic Tract and Dashwoods, and between Dashwoods and the Humber margin East directed subduction started in the Middle Cambrian immediately outboard of the Dashwoods microcontinent and led to underthrusting of the Dashwoods microcontinent beneath the Baie Verte Oceanic Tract (BVOT) The Bay of Islands (BOI) and Thetford Mines (TM) ophiolites formed during local rifting and concurrent seafloor spreading of the allochthonous BVOT after the Dashwoods microcontinent was subducted beneath the BVOT and the east directed subduction zone stepped back into the Humber seaway The Crabb Brook and St Daniel were deposited in piggyback basins formed during obduction of the ophiolites onto the Humber margin BBCL, Baie Verte Brompton Camerons Line to the Boundary Mountain Terrane in New England Spreading between Laurentia and the Dashwoods microcontinent was either very slow or aborted soon after formation, because faunal and palaeomagnetic data suggest that the Dashwoods was separated from Laurentia by only a narrow oceanic seaway (the Humber Seaway) The along-strike extent of the Dashwoods is uncertain Isotopic evidence for the involvement of Laurentian crust in the generation of Early to Middle Ordovician magmatic rocks of the Notre Dame Arc and Laurentian rift-related clastic rocks positioned in an upper plate setting during the Taconic Orogeny (Figures and 3) suggests that this microcontinent extended southward into New England The conversion of the Humber passive margin into a convergent margin is indicated by cratonward migration of a foreland basin formed in response to tectonic loading of the margin by an overriding ophiolite terrane (Figure 3) Foreland-basin development was immediately preceded by uplift and local karst erosion due to the passage of a peripheral bulge across the margin This process started in the Middle to Late Arenig (around 474 Ma) and lasted at least until the Late Caradoc (around 450 Ma) in Newfoundland Loading of the margin started slightly later in Quebec and New England (Figure 3) probably owing to the presence of the Quebec Reentrant–St Lawrence Promontory pair (Figure 1) Dunnage Zone Palaeomagnetic, fossil, and other geological evidence indicates that the oceanic terranes of the Dunnage zone (Figure 1) can be separated into peri-Laurentian (Notre Dame Subzone) and peri-Gondwanan (Exploits Subzone) Detailed geochemical studies have revealed that virtually all the preserved oceanic rocks have compositions typical of suprasubduction zone