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The U.S. Atlantic continental margin (Plate 2C, Fig. 1) is one of the best studied passive (Atlantic-type) continental mar- gins. As the U.S. margin evolved through compressional, exten- sional (rifting), and vertical (subsidence) tectonic phases, a distinctive set of deep crustal structures, basement stractures, and sedimentary features was created. A series of Paleozoic orogenies created the Appalachian mountains and formed large thrust faults, terrane boundaries, and magmatic structures that would control the locus of crustal fracturing during the subsequent ex- tensional phase. During the rifting phase, as the African píate started to break away from the North American píate, the margin was an active píate boundary. Only during the subsidence phase is an Atlantic-type margin actually a passive continental margin (i.e., not an active píate boundary). The very thick sedimentary wedge that overlies crystalline basement on the margin limits our knowledge of basement and underlying crustal structures, but it nevertheless provides a detailed record of the subsidence phase of margin evolution. Distinctive magnetic-anomaly and gravity- anomaly lineations, discontinuities, and characteristic pattems also developed during the evolution of this margin. These geo- physical anoomalies provide the basis for inferring crustal structures and crustal types in lieu of more direct seismic or sample information.

Models for the evolution of Atlantic-type continental mar- gins have been developed from studies of other extensional- tectonic regimes, such as the Southern Australian margin (Falvey and Mutter, 1981), Biscay margin of France (Montadert and others, 1979; LePichon and Barbier, 1987), the North Sea (Sclater and Christie, 1980), the Red Sea (Cochran and others, 1986), the Ligurian Tethys (Lemoine and others, 1986), and the Basin and Range Province (Anderson and others, 1983; Wer- nicke, 1985). Applications of subsidence models to the U.S. Atlantic margin (Watts, 1982; Steckler and Watts, 1982; Sawyer and others, 1983; Steckler and others, this volume) have en- hanced our ability to interpret the sedimentary record of the postrift evolution of the margin. Models for the mechanical de- formation of the crust during rifting and early subsidence phases are just now being more closely examined (McKenzie, 1978; Bally, 1981; LePichon and Sibuet, 1981; Falvey and Middleton, 1981; Beaumont and others, 1982; Foucher and others, 1982; Hellinger and Sclater, 1983; Wemicke, 1985; Lister and others, 1986; LePichon and Barbier, 1987).

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