Roy W. Schlische1, Martha Oliver Withjack1, and Paul E. Olsen 2

1 Department of Geological Sciences, Rutgers University, Piscataway, NJ 08854-8066, U.S.A.
2 Department of Earth and Envirorunental Sciences and Lamont-Doherty Earth Observatory of Columbia University, Palisades, NY 10964-8000, U.S.A.

ABSTRACT

   Short-duration CAMP magmatic activity at ~200 Ma in eastern North America provides a temporal benchmark for assessing the relative timing of rifting, drifting, and basin inversion.  In the southeastern United States, rifting ceased and shortening/inversion began before CAMP magmatism. In the northeastern United States and southeastern Canada, rifting continued during and after CAMP magmatism.  In fact, normal faulting and basin subsidence accelerated during this time. Rifting ceased in the northeastern United States and southeastern Canada by the early Middle Jurassic, after CAMP magmatic activity.  Shortening/inversion occurred in southeastern Canada before or during the Early Cretaceous.  The available geological, geophysical, and geochronological data favor a diachronous rift- drift transition (seafloor spreading began earlier in the south) rather than the traditional synchronous rift-drift transition along the entire central North Atlantic margin. In this scenario, there are two magmatic pulses.  The first includes CAMP and the formation of seaward-dipping reflectors (SDR's) near the continent-ocean boundary during the rift-drift transition along the southern margin segment.  The second, younger magmatic pulse is associated with the formation of SDR's during the rift-drift transition along the northern margin segment.
   We believe that the widespread magmatism and shortening/inversion in eastern North America are related to active asthenospheric upwelling that culminated during the rift-drift transition.  Inversion is a common feature along many volcanic passive margins and is associated with a change in the strain state from extension at a high angle to the margin during rifting to shortening at a high angle to the margin during drifting.  The presence of dikes oriented at a high angle to the trend of the margin (e.g., the dike swarms in the southeastern United States, southeastern Greenland, offshore northwest Europe, and South America) may reflect this change in strain state associated with inversion.