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A combination of deep seismic imaging and drilling has demonstrated that the ocean-continent transition (OCT) of present-day, magma-poor, rifted continental margins is a zone of hyperextension characterized by extreme thinning of the continental crust that exhumed the lowermost crust and/or serpentinized continental mantle onto the seafloor. The OCT on present-day margins is difficult to sample, and so much of our knowledge on the detailed nature of OCT sequences comes from obducted, magma-poor OCT ophiolites such as those preserved in the upper portions of the Alpine fold-and-thrust belt. Allochthonous, lens-shaped bodies of ultramafic rock are common in many other ancient orogenic belts, such as the Caledonian – Appalachian orogen, yet their origin and tectonic significance remains uncertain. We summarize the occurrences of potential ancient OCTs within this orogen, commencing with Laurentian margin sequences where an OCT has previously been inferred (the Dalradian Supergroup of Scotland and Ireland and the Birchy Complex of Newfoundland). We then speculate on the origin of isolated occurrences of Alpine-type peridotite within Laurentian margin sequences in Quebec – Vermont and Virginia – North Carolina, focusing on rift-related units of Late Neoproterozoic age (so as to eliminate a Taconic ophiolite origin). A combination of poor exposure and pervasive Taconic deformation means that origin and emplacement of many ultramafic bodies in the Appalachians will remain uncertain. Nevertheless, the common occurrence of OCT-like rocks along the whole length of the Appalachian – Caledonian margin of Laurentia suggests that the opening of the Iapetus Ocean may have been accompanied by hyperextension and the formation of magma-poor margins along many segments. SOMMAIRE Des travaux d’imagerie sismique et des forages profonds ont montre que la transition ocean-continent (OCT) de marges continentales de divergence pauvre en magma exposee de nos jours, correspond a une zone d’hyper-etirement tectonique caracterisee par un amincissement extreme de la croute continentale, qui a exhume sur le fond marin, jusqu’a la tranche la plus profonde de la croute continentale, voire du manteau continental serpentinise.  Parce qu’on peut difficilement echantillonner l’OCT sur les marges actuelles, une grande partie de notre comprehension des details de la nature de l’OCT provient d’ophiolites pauvres en magma d’une OCT obduite, comme celles preservees dans les portions superieures de la bande plissee alpine.  Des masses lenticulaires de roches ultramafiques allochtones sont communes dans de nombreuses autres bandes orogeniques anciennes, comme l’orogene Caledonienne-Appalaches, mais leur origine et signification tectonique reste incertaine.  Nous presentons un sommaire des occurrences d’OCT potentielles anciennes de cet orogene, en commencant par des sequences de la marge laurentienne, ou la presence d’OCT a deja ete deduites (le Supergroupe Dalradien d’Ecosse et d'Irlande, et le complexe de Birchy de Terre-Neuve).  Nous speculons ensuite sur l'origine de cas isoles de peridotite de type alpin dans des sequences de marge des Laurentides du Quebec-Vermont et de la Virginie-Caroline du Nord, en nous concentrant sur les unites de rift d'âge neoproterozoique tardif (pour eviter les ophiolites du Taconique).  La conjonction d’affleurements de pietre qualite et de la deformation taconique omnipresente, signifie que l'origine et la mise en place de nombreuses masses ultramafiques dans les Appalaches demeureront incertaines.  Neanmoins, la presence frequente de roches de type OCT tout le long de la marge Caledonnienne-Appalaches de Laurentia suggere que l'ouverture de l'ocean Iapetus peut avoir ete accompagnee d’hyper-etirement et de la formation de marges pauvres en magma le long de nombreux segments.

geoscience canada

The Ocean – Continent Transition Zones Along the Appalachian – Caledonian Margin of Laurentia: Examples of Large-Scale Hyperextension During the Opening of the Iapetus Ocean