Marine seismic experiments have revealed that the U.S. east coast rifted margin crustal structure is dominated by voluminous volcanics emplaced during continental breakup. We present results from seismic data recorded onshore during the 1990 EDGE mid-Atlantic onshore/offshore seismic experiment that extend the margin crustal profile westward 180 km across the coastal plain and place the offshore results into a broader tectonic context. The onshore/offshore crustal model defines the crustal thinning profile across the margin, enabling calculation of total extension based on direct measurements of crustal thickness. We find that the crust beneath the coastal plain is largely unextended, has a uniform thickness of ~35 km, shows no evidence for magmatic additions of sufficient volume to affect seismic velocity, and that the total half extension of the crust is 30 to 40 km, less than previous estimates by a factor of 3 to 4. We also present results of depth migration of the offshore vertical incidence and wide-angle seismic data that yield the most complete and accurate image to date of reflectivity across the U.S. east coast margin. We use this image to analyze margin subsidence by exploiting the stratigraphy of the subaerially extruded basalts beneath the sediments. We find that (1) margin subsidence is consistent with a petrogenetic model for initial seafloor-spreading magmatism involving high average melting pressures (3.5--4.0 GPa) and potential temperatures (>1500 ¿C) for a present-day thermal lid thickness of ~225 km; (2) it is difficult to explain margin subsidence with a present-day lid thickness of much less than ~150 km; and (3) rapid subsidence following breakup suggests the initially hot mantle underlying the rifting margin cooled very quickly, implying a limited and finite initial distribution of hot mantle, rather than a continuous upwelling or a ubiquitously warm upper mantle.¿ 1997 American Geophysical Union