Tectonics of the Acadian Orogeny in New England and Adjacent Canada

Abstract

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 the other side of the ocean. An identical geometry, with an Avalonian accretionary prism overriding the convergent North American margin, is proposed for the Acadian Orogeny in New England to explain these aspects of the regional geology: (1) early west vergent structures in the Merrimack Trough in Maine and New Hampshire, related here to subduction beneath Avalonia; (2) east vergent structures at a deeper structural level in the trough in Connecticut, related to subduction beneath North America (Rodgers 1981); (3) rapid subsidence of the Piscataquis Volcanic Arc beneath a thick pile of east-derived flysch in Devonian; (4) subsequent deep tectonic burial and high grade metamorphism of parts of this belt beneath west-vergent nappes; and (5) only minor Acadian deformation and metamorphism on the Avalonian side of the trough.