Supercontinent Breakup and Reassembly in the Appalachian-Caledonide System: Implications for the Wilson Cycle

Abstract:

The Appalachian-Caledonide orogen was the first to be interpreted as a zone of plate-tectonic collision. However, Wilson's original question 'Did the Atlantic close and then reopen?' addresses only part of the supercontinent cycle, spanning the transition from closing oceans, through supercontinent assembly, to breakup and ocean spreading. The transition from separation to convergence was not addressed by Wilson, but the initiation of subduction in new oceans remains a poorly understood part of the supercontinent cycle.

In the Appalachian-Caledonide system, rifting apparently continued to at least ~550 Ma, producing an ocean with numerous hyperextended margins and microcontinental blocks. These include both peri-Laurentian and peri-Gondwanan terranes; the latter have been grouped into domains characterized by platformal Cambrian environments (east and west Avalonia) and deeper-water successions (Ganderia and Megumia). Isotopic work has shown that west Avalonia represents relatively juvenile continental crust, whereas Ganderia is more evolved; east Avalonia may represent continental basement of intermediate isotopic character.

Arcs were present in the developing ocean by 505 Ma, as recorded in parts of the orogen from Maine and Newfoundland, through the British Isles to Scandinavia. Many show juvenile isotopic signatures suggesting that they did not originate on older margins. Some arcs underwent late Cambrian to Early Ordovician interaction with the Laurentian margin, while others were emplaced on peri-Gondwanan crust. Their subsequent histories were complex, involving the opening of back-arc basins within a closing ocean. Paleomagnetic data suggest substantial vertical-axis rotations during closure.

These observations, and Wilson's original comparison with the Atlantic, suggest that spontaneous inversion of passive margins is unlikely to have initiated subduction. Instead arc systems entered the newly formed oceans formed by breakup of Rodinia and initiated ocean closure.