The U.S. Atlantic continental margin contains substantial thicknesses of generally seaward dipping Mesozoic to Cenozoic sediments. Multichannel seismic reflection profiling show that these sediments reach thicknesses >14 km under the continental shelf off New Jersey and >7 km under the slope and rise. Commercial exploration wells and the Continental Offshore Stratigraphic Test (COST) No. B-2 and B-3 wells in the shelf and slope off New Jersey show that most of the sediments were deposited in continental or shallow water (marine) environments. Such large thicknesses of shallow water sediments cannot be caused by sedimentary loading alone and other factors must be involved. A useful approach to the problem is to isolate these factors by "backstripping" sediment as wel as water loads for different intervals of time during margin evolution. "Backstripping" studies in which the effects of compaction, paleobathymetry, sea-level changes and the flexural strength of the basement are included have been carried out using downhole geological and geophysical logs from the COST B-2 and B-3 wells. The resulting tectonic subsidence at the wells can be satisfactorily explained by a simple thermal model in which the lithosphere undergoes a passive extension at the time of rifting. In this model, extension causes stretching or thinning of the lithosphere, which subsequently cools and subsides with time. The model assumes isostatic equilibrium during and after the extension so that there is an initial subsidence which depends on the initial crustal thickness, followe by a thermal subsidence determined by the amount of heating. Based on the model the best fitting estimates of the amounts of extension at the margin are between 250 and 300% (ß =3.5 to 4.0). Simple mechanical models have been constructed, using these estimates of ß and a flexural strength of the basement which increases with time, in which sediments infill the subsiding margin. The thermal and mechanical modelling imply an equal proportion of sediments accumulating during the period of initial subsidence (pre- and syn-rift sediments) and the period of thermal subsidence (post-rift sediments). Unfortunately, there is presently too little seismic and

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lithologic information on the actual proportion of pre- and syn-rift to post-rift sediments off New Jersey to constrain these models. The models predict a coastal plain sequence in which younger sediments progressively overstep older sediments due to the increase in flexural strength of the basement with age. The coastal onlap, which is observed at many margins, may therefore be due at least partly to tectonic rather than eustatic effects. The tectonic subsidence at the COST B-2 and B-3 wells off New Jersey has been used to estimate the paleotemperature in the sediments and the amount of crustal thinning that has occurred at the margin. The well data indicate that up to about 20 km of crustal thinning has occurred beneath the outer part of the shelf. This estimate is in general agreem nt with gravity and geoid data across the margin but is poorly constrained by heat flow and seismic data. Future studies based on multichannel seismic reflection profiling and continental shelf and slope drilling should provide better observational constraints on both the extent and amount of crustal thinning that has occurred off New Jersey. These studies, in conjunction with the development of more refined thermal and mechanical models, offer the most promise of determining the origin and evolution of U.S. margin during the next decade.