Subsidence in Late Paleozoic basins in the northern Appalachians

During the interval between continental collision in the Devonian and continental breakup in the Triassic the northern Appalachians became the site of a wide plate boundary zone of dominantly right‐lateral strike slip. As is typical of intracontinental transforms, tectonism was both diachronous and rapidly variable along strike through regimes of ‘pure’ strike slip, transpressional deformation, and rapid subsidence of extensional basins. Up to 9 km of mainly nonmarine, clastic sediments accumulated in these local depocenters, which subsided episodically in two stages: (1) an initial phase of stretching and thinning of the lithosphere, when subsidence was rapid, fault controlled, and often accompanied by volcanism and (2) a subsequent phase of gradual thermal subsidence, during which the depositional basins expanded to bury the earlier border faults and progressively younger sedimentary units onlapped basement. The largest depocenter, the Magdalen Basin, opened as a pull‐apart between strike slip faults in Newfoundland and New Brunswick from late Devonian to early Carboniferous. Subsequent thermal subsidence affected a large area during medial and late Carboniferous, a phenomenon that is well recorded to the north and west, where no later tectonism occurred. In areas to the south and east of the basin, strike slip on other faults continued into the time of thermal subsidence, introducing complications such as localized transpressional deformation and rapid subsidence in smaller pull‐aparts.