ARC-MICROCONTINENT INTERACTION IN THE EARLY HISTORY OF THE IAPETUS OCEAN

In the final stages of the breakup of Rodinia, rifting apparently continued until ~550 Ma, producing an ocean with numerous hyperextended margins and microcontinental blocks. These include: peri-Laurentian terranes such as the Chain Lakes, Dashwoods, Grampian, and Helgeland terranes; and peri-Gondwanan terranes with affinities to Amazonia and/or west Africa. The peri-Gondwanan terranes have been grouped into domains characterized by platformal Cambrian environments (e.g. Avalonia) and deeper-water successions (e.g. Ganderia and Megumia). Isotopic work has shown that W. Avalonia represents relatively juvenile continental crust, whereas Ganderia is much more evolved; E. Avalonia may be of intermediate isotopic character.

Volcanic arcs were also present in the developing ocean system by late Cambrian (Furongian) time, as recorded in numerous parts of the orogen, from Maine and Newfoundland, through Ireland and Great Britain, to Scandinavia. Many show juvenile isotopic signatures suggesting that they did not originate on older continental margins. Some of these arcs underwent late Cambrian to Early Ordovician interaction with peri-Laurentian blocks in early phases of the Taconian orogeny. However, others were emplaced upon peri-Gondwanan terranes during roughly the same interval in the Penobscot event. Kinematic indications of the direction of thrusting in these early ophiolite emplacement events are rare. However, in coastal Maine, Penobscot deformation appears to have been northwest-directed, suggesting that arcs impinged on Ganderia from a still more outboard position (relative to Laurentia). The subsequent histories of these arc-microcontinent systems were complex, involving the opening of back-arc basins within an ocean that was progressively closing. Paleomagnetic data suggest substantial vertical-axis rotations during ocean closure.

Traditional models for the Appalachian-Caledonide system invoke subduction initiation by spontaneous inversion of passive margins. Our observations support an alternative model, in which arc systems entered the Iapetan realm from an external ocean, in a process similar to the entry of the modern Caribbean and Scotia plates into the Atlantic, eventually incorporating both peri-Gondwanan and peri-Laurentian microcontinents into a complex orogen.