Abstract The geologic strip-map for Transect E-l cuts a swath from the Thousand Islands region on the New York-Ontario border to the Atlantic Ocean floor off Georges Bank (see Fig. 1). It includes portions of New York, Ontario and of all of the New England states. The western part, mainly in New York, belongs to the North American craton. The remainder of the onland portion, east of Logan's Line, belongs to the Appalachian Orogen. Southeastward from Logan's Line the transect crosses a series of distinctive terranes. Several of these terranes are believed to be exotic, and to have been accreted to the North American craton during the Paleozoic. Superposed on these are several grabens and half-grabens containing early Mesozoic sediments and mafic volcanics. There are also Mesozoic eruptive complexes of an alkalic nature cutting across the Appalachian Orogen from southern Quebec, across New England, and continuing as a chain of seamounts offshore. Cenozoic rocks are limited to a small, but significant occurrence near Brandon, Vermont (BL on Fig. 2) and a few occurrences in the Cape Cod region and on the adjacent islands in southeastern Massachusetts. Offshore the corridor passes over the Gulf of Maine and Long Island Platforms, thence across Georges Bank and into the North Atlantic Basin. The Gulf of Maine and Long Island Platforms (Fig. 2) are underlain by Paleozoic metamorphic and plutonic rocks and early Mesozoic grabens, as in the adjacent onland regions, but are partially covered offshore by a 1-3 km section of late Mesozoic and

Abstract 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).

Abstract Understanding the geology of the U.S. Atlantic margin is based on a sparse set of drilling and dredge data integrated with a much larger base of geophysical data. Geophysical data provide the primary basis for interpreting structural features and extrapolating from the sparse geological control. The purpose of this chapter is to present the scope of geophysical data coverage of the Atlantic continental margin in a general review format. The discussion will touch on the kinds of data produced by different technologies, the historical reasons for the use of various methods, and the improvement of methods. There are two basic classes of geophysical data used in margin studies: seismic data and potential field data. Seismic techniques utilize acoustic energy sources and receivers to examine seismic-wave propagation characteristics of the subsurface rocks. This information includes variation in acoustic reflectivity and velocity within the rock. Near-vertical incidence reflection profiling is used to map structures on a regional basis. Wide-angle seismic-reflection and seismic-refraction profiling are used to examine seismic-velocity structures. Magnetic and gravity potential field anomaly data sets complement the seismic information, providing a means for extrapolating it over broad regions. The comparatively low cost of magnetic and gravity surveying has resulted in the acquisition of fairly dense regional data sets (2 to 10 km line spacing) compared with more coarse regional seismic grids (20 to 30 km line spacing).

Abstract Georges Bank is a shallow part of the Atlantic Continental Shelf southeast of New England (Emery and Uchupi, 1972, 1984). This bank, however, is merely the upper surface of several sedimentary basins overlying a block-faulted basement of igneous and metamorphic crystalline rock (Figs. 1 and 2). Sedimentary rock forms a seaward-thickening cover that has accumulated in one main depocenter and several ancillary depressions, adjacent to shallow basement platforms of Paleozoic and older crystalline rock. Georges Bank basin contains a thickness of sedimentary rock greater than 10 km, whereas the basement platforms that flank the basin are areas of thin sediment accumulation (less than 5 km). We will discuss the structural, stratigraphic, and tectonic framework of the Georges Bank area, presenting a synthesis of geophysical and geologic data, much of which has been collected over the past decade (Austin and others, 1980; Klitgord and others, 1982; Schlee and Fritsch, 1982; Klitgord and Schlee, 1986). In particular, we shall be concerned with the crustal foundations of Georges Bank, the main stages of sedimentary buildup, and the forces that we think have influenced the evolution of the basin. A general outline of the geology of Georges Bank basin has been given by Drake and others (1959), Maher (1971), Emery and Uchupi (1972), Schultz and Grover (1974), Mattick and others (1974), Ballard and Uchupi (1975), and Schlee and Klitgord (1982). More detailed studies of the character and distribution of basement structures have been given by Klitgord and Behrendt (1979), Austin and others (1980), Klitgord and others (1982), and Klitgord and Schlee (1986). The character and deposition patterns of the overlying Mesozoic and Cenozoic sedimentary wedge have been studied in detail by Schlee (1978), Valentine (1981), Poag (1982a, 1982b), Klitgord and others (1982), and Schlee and Fritsch (1982). Quaternary evolution of the bank has been described by Emery and Uchupi (1972), Oldale and others (1974), Lewis and Sylwester (1976), and Schlee and Fritsch (1982).

Abstract Opening of the central Atlantic Ocean basin during the past 200 Ma separated North America from Africa and created a classic example of plate tectonic divergent motion and associated geologic features (LePichon, 1968; Morgan, 1968). The entire history of relative motion of these two plates is preserved in the fabric of sea-floor spreading (SFS) recorded by magnetic lineation and fracture zone (FZ) patterns (Fig. 1) (Vine and Matthews, 1963; Heezen and Tharp, 1965) on both flanks of the Mid-Atlantic Ridge. Age calibration of the SFS magnetic anomaly pattern (Cox, 1973; Harland and others, 1982; Kent and Gradstein, this volume) enables us to treat SFS lineations as isochrons of sea-floor crustal ages. FZs mark the path of spreading center offsets (transform faults) through time, providing an approximate flowline trace of the motions that separated the North American and African plates. Reconstruction poles of rotation and stage poles of motion can be determined from the SFS lineation and FZ data sets (Bullard and others, 1965; McKenzie and Parker, 1967; McKenzie and Sclater, 1971; Harrison, 1972). The kinematic history described by these poles provides a framework for examining major tectonic events, anomalous plate behavior, geologic phenomena, paleooceanographic events, etc. (e.g., Vogt and others, 1969; Tarling and Runcorn, 1973; Dewey and others, 1973; Vail and others, 1977; Sclater and others, 1977; Pitman, 1978; Rona and Richardson, 1978; Schwan, 1980; Kerr and Fergusson, 1981;

Abstract The Carolina Trough is a long, linear, continental margin basin off eastern North America. Salt domes along the trough’s seaward side show evidence of active diapirism and a normal growth fault along its landward side has been continually active at least since the end of the Jurassic. This steep fault extends to a strong reflection event at about 11 km depth that may represent the top of a salt layer. We infer that faulting is caused by seaward flow of salt from the deep part of the trough into domes, thereby removing support for the overlying block of sedimentary rock. Diapirs off eastern North America seem to be concentrated in the Carolina Trough and Scotian Basin, where basement seems to be thinner than in other basins off eastern North America, south of Newfoundland. Thinner basement, probably due to greater stretching during rifing, may have resulted in earlier subsidence below sea level, a longer life for the salt evaporating pans in these basins, and thus a thicker salt layer, which would be more conducive to diapirism.

Abstract A detailed magnetic study of the U.S. Atlantic continental margin north of Cape Hatteras delineates the pattern of basins and platforms that form the basement structure. A 185,000-km, high-sensitivity aeromagnetic survey acquired in 1975 over the entire U.S. Atlantic continental margin forms the basis of this study. Magnetic depth-to-source estimates were calculated for the entire survey using a Werner "deconvolution" type method. These depth-to-basement estimates are integrated with multichannel seismic reflection profiles to interpolate basement structures between seismic profiles. The deep sediment-filled basins along the margin are bounded on their landward sides by blockfaulted continental crust; their seaward sides are marked by the East Coast magnetic anomaly. The trends of the landward sides of these basins vary from 030° in the south to 040° in the north, consistent with a common pole of opening for all of the basins. The ends of these basins are controlled by sharp offsets in the continental crust that underlie the various platforms. These offsets are the result of the initial breakup of North America and Africa and are preserved as fracture zones under the continental rise. The regions west of the various basins are comprised of platforms of Paleozoic and older crust and embayments of Triassic-Jurassic age. The Long Island platform is a series of ridges and troughs. These troughs are oriented northeastward, parallel with the Baltimore Canyon trough and the Georges Bank trough. The Connecticut Valley Triassic basin has a broad magnetic low associated with it that can be traced across Long Island. A similar magnetic signature is associated with the trough between Martha's Vineyard and Nantucket Island, suggesting that it also may be a Triassic basin. The Salsbury Embayment with its Triassic-Jurassic age sediments lies just west of the Baltimore Canyon trough while the Carolina platform, which has a few smaller Triassic basins within predominantly Paleozoic and older crust, lies landward of the Carolina trough. The area around Charleston is another major embayment of Triassic-Jurassic age, and west of the Blake Plateau is the Florida platform with Paleozoic and older crust. A magnetic basement high associated with the East Coast magnetic anomaly separates oceanic crust from the deep sediment-filled troughs. The minimum depth of this high ranges from 6 to 8 km and the susceptibility contrast suggests that it is more likely an uptilted block of oceanic crust than a massive intrusive body. The magnetic anomaly probably is produced by a combination of a basement high and an "edge effect," where the edge is between the uptilted block and flat-lying, nonmagnetic sediments to the west.