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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 the late Precambrian Boston Basin suggest that Early Cambrian platformal conditions were preceded by rapid extensional or transtensional subsidence within the former magmatic arc. Late Precambrian volcanics in eastern Newfoundland follow an evolutionary path analogous to that of the Basin and Range, and culminate in bimodal and peralkaline activity interpreted to have accompanied terrestrial sedimentation in strike-slip basins prior to the development of shelf conditions during the earliest Paleozoic. In mainland Nova Scotia, wrench-related Cambrian successions have been interpreted to follow strike-slip closure of late Precambrian arc-rift basin(s) that may have formed in response to oblique subduction. In southern New Brunswick, an “Eocambrian” continental rift basalt/red-bed association suggests subduction was replaced by extension within the former magmatic arc.

Avalonian arc-platform transitions that are widely defined as extensional strike-slip features rather than major compressional events suggest that late Precambrian subduction terminated through transform activity rather than collision. The tectonic transitions coincide with the break-up of a late Precambrian supercontinent that was responsible for the inception of the Iapetus cycle and may reflect the major plate reorganizations that would follow such an event.

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