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Paleogeographic analysis of post-Taconian rocks in New England and adjacent parts of Canada has revealed the existence of two volcanic arcs which shut off at the time of the Acadian Orogeny. One was built on arc basement previously accreted to North America during the Taconic Orogeny, the other on Precambrian continental basement of Avalonia. In the intervening Merrimack-Fredericton Trough, metamorphosed and poly deformed turbidites and black shales record deep water conditions in Silurian. Following McKerrow and Ziegler (1971), this tract is interpreted as the site of an ocean which closed in Siluro-Devonian by simultaneous subduction beneath both continental margins. In the Molucca Sea in Indonesia, a comparable arc-arc collision is in an early stage of development; Moore et al. (1982) suggested that an accretionary prism built against one arc is overthrusting its counterpart, which developed on …