Abstract

Integration of new data with existing information indicates that the tectonic development of the passive margin of eastern North America between the Carolina Trough and Scotian Basin was considerably more complex than the classic two-stage, rift-drift model. First, the transition from rifting to drifting was diachronous. In the southeastern United States, the rift-drift transition occurred after the Late Triassic synrift deposition and before eastern North America magmatism in the earliest Jurassic ( approximately 200 Ma). In maritime Canada, the rift-drift transition occurred after eastern North America magmatic activity and synrift deposition in the Early Jurassic and before postrift deposition in the early Middle Jurassic ( approximately 185 Ma). Second, the deformational regime changed substantially after rifting on both the southern and northern segments of the margin. Generally, northwest-southeast postrift shortening replaced northwest-southeast synrift extension. Northeast-striking reverse faults formed, and many of the rift-basin boundary faults had reverse displacements. In the southeastern United States, the change in the deformational regime occurred in the Late Triassic-Early Jurassic during the rift-drift transition. Simultaneously, diabase sills and dikes, many striking nearly perpendicular to the trend of the rift basins, intruded the continental crust; and a massive wedge of volcanic or volcaniclastic rocks developed near the continent-ocean boundary. In maritime Canada, the change in the deformational regime occurred during or after the Early Jurassic and before or during the Early Cretaceous; that is, during the rift-drift transition or early stages of sea-floor spreading.

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