Abstract The West Antarctic Ice Sheet (WAIS) flows through the volcanically active, late Cenozoic West Antarctic rift system. Active subglacial volcanism and a vast ( > 10 6 km 3 ) extent of subglacial volcanic structures have been interpreted from aerogeophysical surveys over central West Antarctica in the past decade, combined with results from 1960s and 1970s aeromagnetic profiles over the WAIS. Modelling of magnetic anomalies constrained by radar ice sounding shows volcanic sources at the base of the ice throughout large areas, whose subglacially erupted hyaloclastite edifices have been eroded by moving ice, as in Iceland. The 1800 m-high divide of the WAIS is underlain by the 400 km-long volcanic Sinuous Ridge, which rises above sea level; most hyaloclastite edifices there have also been glacially removed, indicating migration of the ice divide through time. Northeast of the divide of the WAIS there is a 400-nT positive magnetic anomaly over the shallowest, most rugged bedrock topography (elevation +380m above sea level), probably comprising subaerially erupted flows erupted when the Sinuous Ridge area was deglaciated. Uplift of the Sinuous Ridge may have forced the advance of the WAIS. Other aspects of the subglacial volcanism in Antarctica can be observed in Iceland and have a direct bearing on our understanding of the subglacial conditions of the WAIS and its dynamics.

Abstract More than 100,000 km of marine multichannel seismic profiles have been acquired over the continental margin of Antarctica since 1976 by scientific research programs of Australia, Brazil, France, Italy, Japan, Norway, Poland, United Kingdom, United States, U.S.S.R. and West Germany. Although scientific results are reported for most of these data, they also are relevant to petroleum resource assessment. Because of the one or two orders of magnitude greater cost of standard land survey techniques in Antarctica compared with marine techniques in areas of open water, there will likely be no great amount of coverage on the interior of the Antarctic ice sheet. Despite this, several countries are experimenting in a research mode using land systems, and deep crustal reflection surveys at carefully selected interior sites will probably be made soon.

Abstract The availability of a high-sensitivity digital aeromagnetic survey over the U.S. Atlantic continental margin and new graphical color-display techniques enabled us to compile residual maps of total magnetic intensity, and of its second-vertical-derivative and wavelength-filtered maps, which show structural elements that are not obvious in the original aeromagnetic maps. A regional tilt in the original total-intensity map was removed by fitting a quadratic surface to the data. The resultant residual map reveals broad positive anomalies over the Baltimore Canyon trough, Carolina trough, and Blake Plateau basin. A regional negative anomaly overlies the Georges Bank basin and Long Island platform. The visual effect of the second-vertical-derivative map is to emphasize the gradients, thereby sharpening and resolving anomalies of small areal extent; the high frequencies are enhanced. The low-frequency anomalies define the major basins that are presently being explored for petroleum resources, such as the Georges Bank basin, Baltimore Canyon trough, Carolina trough, and Blake Plateau basin, as well as a number of smaller basins. The second-vertical-derivative map indicates complex structure associated with the East Coast Magnetic Anomaly (ECMA). For example, two positive lineaments between 36° and 37°30' N suggest that although a simple edge effect associated with the oceanic crust may account for a large part of the total anomaly of 200-600 nT, more complex structures are also present. We compiled a map showing tectonic elements of the continental margin from the second-vertical-derivative map. High-pass filters of 60-, 40-, and 20-km wavelength illustrate successively shallower sources of anomalies and provide a useful first approximation of depth to magnetic basement.

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.

Abstract Since 1972, the U. S. Geological Survey has been carrying on an extensive program of geophysical and geological investigations of the structure of the Atlantic continental margin of the United States. Results of the work in the Baltimore Canyon Trough area are based on profiles covering a distance of about 5,000 km of 24-and 48-channel CDP reflection seismic data, a high-sensitivity aeromagnetic survey, and gravity, and seismic refraction data integrated with subsurface geologic information from the approximately 5-km deep COST hole B-2, several 300-m cores from the USGS Atlantic margin drilling program (1976) and other core information. Sedimentary rock thicknesses as much as 15 km underlie the continental shelf east of New Jersey. Relatively high seismic velocities (3½ to 5 km/sec) increase with depth from 2½ to 15 km. These velocities are associated with rocks marked by moderately low amplitude (−20 mgal) gravity anomalies and indicate the presence of higher density older rocks underlying the continental margin in the area compared with the Gulf of Mexico. Acoustic and magnetic basements appear to be coincident near the coast at a depth of 1 km. They deepen seaward to 10 km about 50 km offshore, further seaward they diverge. In the area of the upper slope, the deepest magnetic horizons (about 6 to 9 km) are shallower than the deepest seismic reflectors, suggesting magnetic contrasts within the sedimentary rock section. There is no evidence in the CDP seismic data for the previously postulated “basement ridge” beneath the outer shelf. The best available recent data suggests the possibility of a reef (?) buried about 5 km beneath the upper slope. The East Coast magnetic anomaly is interpreted as the boundary between oceanic and continental crust. A domal structure (−39°23′N, 73°05′W) 20 to 30 km in diameter is marked by magnetic and gravity anomalies and prominent seismic reflectors. An intrusive body of probable Cretaceous age which has its top at a depth of about 3½ km is the inferred source or these geophysical anomalies.