Rifted or passive margins have formerly been classified into non-volcanic and volcanic margins, based upon the volume of surficial volcanism or volcanic outpourings imaged in seismic reflection profiles as seaward dipping reflections. Recent seismic studies reviewed here show that this distinction is not useful [ Mutter, 1993], however, because the magmatic input to the crust during continental rifting is extraordinarily variable both in time and space. A number of recent studies have also addressed the role of the upper mantle in the extension process.
High seismic velocities revealed on refraction profiles from the east coast of North America, in the Carolina Trough and in the mid-Atlantic region, and from southwest Greenland provide evidence for significant volumes of extrusive volcanism and underplated or intruded mafic rock in the lower crust [ Gohl and Smithson, 1993; Holbrook and Kelemen, 1993; Sheridan et al., 1993; Holbrook et al., 1994a, b 1994b]. The joint inversion of P- and S-wave travel time data from Greenland indicates a basal high-velocity (7.4 and 7.8 km/s) layer that could represent an underplated, metamorphosed garnet-pyroxene-granulite assemblage. Wide-angle migration of ocean-bottom seismometer and ocean bottom hydrophone refraction data by Holbrook et al. [1992] yielded a direct image of the crust/mantle boundary (Moho) in the Carolina Trough region, confirming previous seismic models indicating that the Moho is substantially deeper than inferred from gravity modeling, which Holbrook et al. [1992] postulated to be a result of the large volumes of magmatic underplating. The discovery of these large volumes of magmatism indicates that the eastern coast of North America constitutes a large-igneous province, lacking an obvious hotspot source in contrast to the Norwegian-Greenland margins [ Holbrook and Kelemen, 1993]. The huge volumes of underplated or magmatically intruded crust found along the mid-Atlantic margin and elsewhere suggest that underplating or intrusion during continental rifting is an important mechanism for crustal growth whose contribution may have been previously under appreciated [ Reymer and Schubert, 1984].
In contrast to the mid-Atlantic and southwest Greenland margins, seismic velocity models for the eastern Canadian margin at the Grand Banks provide evidence for a 60- to 70-km-wide zone of very thin crust over highly serpentinized mantle, indicating that there was almost no magma up welling during the initial continental breakup and separation [ Reid, 1993; 1994]. Reid's [1993; 1994] results based on ocean-bottom seismometer data are fully consistent with the conjugate Iberian margin, where Ocean Drilling Program results and seismic work also find serpentinites and thin crust [ Sibuet, 1992]. Chian and Louden [1992, 1994] report that amagmatic extension of the continental margin of southwest Greenland (south of the area investigated by Gohl and Smithson [1993]) has produced a crust that is essentially oceanic in thickness, comprised of thinned continental rocks and serpentinized upper mantle rocks. Assuming that this thinned crust can be subducted, this amagmatic crustal thinning constitutes a previously unrecognized means of reducing the total volume of the continents.
Processes of crustal extension were also investigated in the western Woodlark Basin, where the geometry of extensional faults in the extended continental crust of Papua New Guinea was studied by reflection methods [ Silver et al., 1991; Mutter et al., 1993]. Low-angle reflections imaged on these records were interpreted as seismogenic normal faults based on earthquake hypocenters and mechanisms [ Mutter et al., 1993].
The role of the upper mantle during continental extension was likewise investigated in a number of studies. A grid of reflection lines acquired using modern marine multichannel acquisition methods in the southeastern Gulf of Guinea, western Africa, yielded images of the crust/mantle boundary (Moho) across the margin from oceanic to continental crust [ Rosendahl et al., 1991; 1993]. Moho was traced more or less continuously from oceanic to continental crust in many of these profiles. These profiles indicate that the transition from oceanic to continental crust is highly spatially variable leading these workers to conclude that the transition is highly variable both in space and time. This study demonstrates that a single line across a continental margin is not likely to be representative of the margin as a whole. The uplift of mountain chains adjacent to rifted margins across the Transantarctic Mountains in Antarctica may result from extension in the upper mantle and associated thermal buoyancy in the mantle during extension [ Beaudoin et al., 1992; ten Brink et al., 1993; Tréhu et al., 1993]. These studies all show, however, that the crust under the Ross Sea has been thinned to about 20 km with magmatic underplating or intrusion confined to narrow basins within this wide rifted province.