illustration of the upper layers of the earth showing an ocean basin with mid-ocean ridge
The continents moved away from one another as parts of different tectonic plates. The transitions from thick continental to thin oceanic crust are passive continental margins because they are far away from the current plate boundary at the Mid-Atlantic Ridge. Thick sedimentary deposits (yellow) accumulated as the continental margins cooled and subsided.
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Modified from “Parks and Plates: The Geology of our National Parks, Monuments and Seashores,” by Robert J. Lillie, New York, W. W. Norton and Company, 298 pp., 2005, www.amazon.com/dp/0134905172.