WHY WAS THE IAPETUS OCEAN SO SHORT-LIVED?

Subduction of the Paleozoic Iapetus Ocean began relatively soon after its opening during the last stages in the break-up of Rodinia. Although estimates of the timing of break-up have varied, the earliest sedimentary rocks in the post-rift succession on the Laurentian margin date from the Bonnia-Olenellus zone at the top of proposed Cambrian series 2, and are younger than 515 Ma. The earliest indications of subduction are recorded in arc rocks preserved in ophiolites less than 10 million years younger than the end of Iapetan rifting. Vestiges of the Iapetan oceanic lithosphere are preserved as supra-subduction zone ophiolites and related mafic complexes in the Appalachian-Caledonian orogen. Available Sm-Nd isotopic data indicate that the mantle source of these ophiolites was highly depleted as a result of a history of magmatism that occurred before the Iapetus Ocean existed. The earliest collisional events are recorded almost simultaneously in elements of the Laurentian and peri-Gondwanan margins during the Early Ordovician, between 490 and 480 Ma, and led to the development of active continental margins on both sides of the young ocean, in most reconstructions. Closure of the ocean between Avalonia and Laurentia was complete by about 425 Ma. Compared with the modern Atlantic, the Iapetus Ocean was very short-lived.

We propose that subduction of the Iapetus Ocean was initiated early in its opening history at a subduction zone that migrated from the adjacent external 'Paleopacific' ocean in a manner analogous to the proposed Mesozoic-Cenozoic “capture” of the Caribbean plate in the Atlantic realm. This hypothesis may help to explain: the initiation of subduction and the premature closing of the Iapetus Ocean; the timing of the earliest collisional events; the isotopic character of Iapetan ophiolites; and the distribution of peri-Gondwanan terranes in the Appalachian-Caledonian orogen.