Subduction initiation in the Appalachian-Caledonide orogen
John Waldron, University of Alberta
David Schofield, British Geological Survey
Brendan Murphy, St. Francis Xavier University
The Appalachian-Caledonide orogen was the first to be interpreted as a zone of plate-tectonic collision in a landmark paper by Tuzo Wilson 50 years ago. 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 representing final stages in the breakup of Rodinia 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. Arcs were present in the developing ocean by 505 Ma. In the development of the orogen, the earliest stages of deformation of peri-Laurentian and peri-Gondwanan rocks took place concurrently from latest Cambrian into Early Ordovician. Deformation close to the Laurentian margin is characterized as Taconian (in N. America) or Grampian (in Europe) whereas deformation of the Gondwanan margin, is characterized as Penobscottian (in N. America) or Monian (in Europe). Most models for these deformation episodes require that subduction was initiated along former passive margins on both sides of the young Iapetus Ocean. However, the record in Mesozoic-Cenozoic oceans suggests that spontaneous inversion of passive margins is tectonically unlikely.
In an alternative model, convergence was localized at arc systems, which entered the Iapetan realm from an external ocean, a process similar to the entry of the modern Caribbean and Scotia plates into the Atlantic. Thus the Taconian and Penobscot deformation episodes may have occurred at different points on a single sinuous subduction system. Closure of the Iapetus eventually incorporated both peri-Gondwanan and peri-Laurentian microcontinents, with their records of early deformation, into a complex orogen. A number of lines of evidence suggest that arcs were not generated by inversion of the passive margin of Iapetus. Many show juvenile signatures suggesting that they did not originate on older margins, nor in the new lithosphere formed by rifting of Rodinia.
These observations, and Wilson's original comparison with the Atlantic, suggest that spontaneous inversion of passive margins is unlikely to have localized subduction. Instead arc systems entered the newly formed oceans formed by breakup of Rodinia and initiated ocean closure.