The Avalon terrane of the Northern Appalachians is best defined by (1) the presence of latest Precambrian (c. 600 Ma) volcanic-sedimentary successions and cogenetic granitoid bodies widely attributed to the development of ensialic arc(s), and (2) early Paleozoic platformal sequences bearing Acado-Baltic fauna believed to define the “European” margin of the Iapetus Ocean. The tectonic transition from magmatic arc to stable platform lacks evidence for a major collisional event and is recorded in the development of arc-related successor basins taken to reflect the transform termination of oblique subduction.

In southeastern New England, within-plate mafic volcanism and thick marine clastics of...