HOW WAS THE IAPETUS OCEAN INFECTED WITH SUBDUCTION?

The history of the Iapetus Ocean is the archetypal "Wilson cycle", but the transition from the opening to closing phase of ocean evolution is poorly understood. In most reconstructions of the Appalachians and Caledonides, the continental blocks (Laurentia, Baltica, and Amazonia – West Africa), which separated to form the Iapetus during the breakup of Rodinia, are the same three continents that subsequently collided during closure, making the Iapetus a test case for models of subduction initiation. It is often assumed that subduction is initiated by subsidence of old, cold ocean floor at passive margins. However, in the best modern analogue, Atlantic margins formed at ~180 Ma are still passive, suggesting that some other mechanism is required to initiate subduction.

During the opening of the Iapetus Ocean, the margin of Laurentia underwent protracted rifting from ~615 Ma to at least 550 Ma, and perhaps later. The ealiest "drift" successions on the Newfoundland margin are as young as ~515 Ma. Subduction, recorded by arc volcanics preserved in the orogen, began relatively early in the history of the new ocean at ~515-505 Ma, and the earliest collisional events are recorded almost simultaneously in peri-Laurentian and peri-Gondwanan microcontinents around 490-480 Ma. However, the stable passive margin of Laurentia survived until after 470 Ma before being converted to an active margin. Closure of the ocean between Avalonia and Laurentia was complete by ~425 Ma.

These relationships are difficult to reconcile with a classic "Wilson cycle" model in which subduction is initiated by inversion of an extensional margin. It is much more likely that closure was initiated at a subduction zone migrating westward into the Iapetus, analogous to the eastward Mesozoic-Cenozoic entry of the Caribbean and Scotia plates into the Atlantic realm. This process was probably initiated at a transform boundary between the "internal" ocean formed during the breakup of Rodinia, and "external" Panthalassan lithosphere, and progressively "infected" the Iapetus with subduction. This model may help to explain: the initiation of subduction and the early closing of the Iapetus; the timing and distribution of the earliest collisional events; the isotopic character of some Iapetan ophiolites; and the distribution of peri-Gondwanan terranes in the orogen.