The eastern Laurentian margin in northeastern North America is marked by promontories and embayments that are defined by northeast-striking rift zones offset by northwest-striking transform faults. The complete history of the northeastern margin, from the initiation of continental rifting to the onset of passive-margin thermal subsidence, is preserved in a dynamic stratigraphic succession and in anorogenic magmatic suites. Late Neoproterozoic–Early Cambrian clastic and volcanic deposits overlie ca. 1.0 Ga and older Laurentian basement and define multiphase continental extension that rifted Laurentia out of Rodinia, opening the Iapetus Ocean as well as the more marginal Humber Seaway. Continental extension is also...