RIFTING, BREAKUP, AND POST-RIFT DEFORMATION ON THE ‘PASSIVE-AGGRESSIVE' MARGIN OF EASTERN NORTH AMERICA (Invited Presentation)

The tectonic evolution of the continental margin of eastern North America was complex, involving rifting, breakup, and post-rift deformation. The onset of rifting, from Florida to the Canadian Grand Banks, was relatively synchronous, occurring by Late Triassic time. The resulting eastern North American (ENAM) rift system was characterized by a broad zone of upper crustal extension in which a few, wide (>100 km), deep (5-10 km), long-lived (>30 m.y.), fault-bounded basins accommodated much of the extension. The border-fault zones (BFZs) of the rift basins were mostly reactivated, pre-existing zones of weakness formed during Paleozoic and older orogenic activity. The cessation of rifting (and presumably the onset of breakup) was diachronous, occurring first in the southeastern United States (latest Triassic), then in the northeastern United States and southeastern Canada (early Early Jurassic), and finally in the Grand Banks (Early Cretaceous). The Central Atlantic Magmatic Province (CAMP) developed simultaneously (latest Triassic / earliest Jurassic, ~200 Ma) throughout eastern North America. Thus, CAMP magmatic activity occurred shortly after rifting in the southeastern United States and during rifting in the northeastern United States and maritime Canada. Much of the current geometry of the ENAM rift system reflects deformation that occurred after rifting, including: 1) reactivation of BFZs and intrabasin faults with reverse and/or strike-slip components of displacement; 2) folding near BFZs and intrabasin faults; and 3) very broad arching whose axial trace coincides with the Appalachian Gravity Gradient. The arching produced regional tilting and uplift, leading to locally >5 km of erosion. This erosion considerably reduced the size (depth and width) of the ENAM rift basins. Although the timing of the post-rift deformation is poorly constrained, a growing body of evidence suggests that some occurred during breakup and/or the early stages of seafloor spreading whereas some occurred during Cenozoic time.