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Passive margins generated by rifting and separation of continents are associated with some of the world's thickest sediment accumulations, of which the east coast of North America is a typical example. The sedimentary wedge that has accumulated there is as much as 15 km thick at the seaward edge and represents an almost continuous succession of shallow marine sediments, ranging in age from probable Triassic-Jurassic at the base of the section to Recent at the surface (e.g., Drake and others, 1959; Bally, 1981). The bottommost part of the section may contain rift-valley sequences (Bally, 1981).

The individual postrift strata are thickest on the ocean side of the margin and systematically thin to zero thickness (either by pinch-out or truncation) toward the landward edge. The horizons between strata are quite planar, disrupted occasionally by faulting related to sediment processes only. The back-stripping technique has been developed to remove the effect of sediment loading as a function of time and thus reveal the driving or tectonic subsidence (Sleep, 1971; Watts and Ryan, 1976; Steckler and Watts, 1978; Steckler, 1981). By use of this technique it has been established that the driving subsidence is caused by deep-seated cooling with a thermal cooling constant usually in the range of 50 m.y. to 100 m.y. (Sleep, 1971; Watts and Ryan, 1976; Steckler and Watts, 1978).

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