Were parts of the Laurentian margin of Iapetus hyper-extended?

The time and nature of the opening of the Iapetus Ocean along the Laurentian Humber margin has been a contentious issue for a considerable time. Rift-related magmatism in eastern Laurentia spanned ca. 200 my (750-550 Ma), but in most places was not voluminous. Opening of the Iapetus Ocean is generally thought to have taken place between 580 and 530 Ma, but the exact rift-drift transition varies significantly (40-50 my) depending whether one follows paleomagnetic-data based arguments or whether one relies solely on the timing of thermal subsidence of the margin. Part of this controversy was previously explained by invoking distinct pulses of rifting and formation of isolated peri-continental ribbons, such as Dashwoods, separated from the margin by narrow oceanic seaways (e.g. Taconic seaway). The main supporting evidence for the existence of the peri-continental ribbons is found in the basement to the continental Early Ordovician Taconic arc, which supposedly formed by subduction of the Humber margin and attached oceanic lithosphere beneath it. Another problem concerns the apparent lack of a Taconic metamorphic imprint in slices of west-verging allochthonous Humber margin rocks in many places, whereas relatively narrow belts of adjacent, probably originally more outboard positioned rocks were locally subducted to considerable depths during the Taconic. Mainly based on our evidence from Newfoundland, we propose that segments of the Humber margin were hyper-extended, which produced several allochthonous ribbons of extended continental crust separated by exhumed lithospheric mantle during one progressive long-lasting phase of rifting, analogous to the formation of the Galician margin. Evidence for this model is provided by lenses of ultramafic rocks tectonically interleaved with Laurentian-derived meta-sedimentary rocks (e.g. Fleur de Lys Supergroup) and associated, small volumes of ca. 558 Ma MORB-like rift-related mafic rocks (e.g. Birchy Complex), which we interpret as ocean-continent transitional rocks. We propose that progressive closure of this highly extended margin and intervening seaways led to several phases of accretion/collision, while a thick sediment blanket may have delayed cooling and hence initiation of thermal subsidence of the Humber margin deep into the Early Cambrian.