No abstract

No abstract

Summary A regional GLORIA survey in the southern Cape Verde Basin identified highly variable patterns of backscattering intensity that were interpreted as resulting from seabed roughness and small-scale slope changes. It was inferred that their origin was linked to turbidity-current pathways across the continental rise. Farther north, a survey of the Saharan Continental Rise SE of Madeira identified even more spectacular changes in backscattering. Follow-up 3.5 kHz echo-sounder surveys together with core-sampling and bottom-camera studies allowed an assignment of acoustic facies to sediment type. In this area turbidity-current channels and deposits have been overrun by long-range debris flows resulting from the late-Quaternary Saharan sediment slide. Preliminary results of a further GLORIA survey that links the previously studied areas with a single swath along the continental rise are reported in this paper. The resulting sonographs show details of submarine-slide-debris-flow complexes on this margin that had previously been mapped from 3.5 kHz records. They extend across the lower continental rise to the plain areas. Others do not extend as far as the lower rise. Debris flows can no longer be thought of as unequivocal indicators of proximality in the geological record.

A new bathymetric map, based mainly on Seabeam data, has been established for the Goban Spur area during a postcruise survey of DSDP Leg 80. Using both Seabeam data and a new series of eight seismic profiles obtained perpendicular to the continental margin, we have constructed a new detailed structural map of the Goban Spur continental margin which clearly reveals Caledonian and Variscan trends. Both the thinning of the Goban Spur continental crust, from Early Cretaceous (late Cimmerian phase) to middle Albian time, and subsequent widening of the adjacent oceanic domain, from middle Albian to Campanian time, resulted from tensional movements in a N70° direction which followed Caledonian trends. During the rifting phase, the tops of the tilted fault blocks remained close to sea level. The rapid subsidence of the margin seems to have occurred in the early Albian during the last stage of rifting.Eocene intraplate deformation affected the whole Goban Spur continental margin, but is particularly evident at the Pastouret Ridge, a reactivated oceanic fracture zone. The oceanic domain underwent a slight intraplate compression, which fractured the old oceanic crust through its entire thickness, probably along previous zones of weakness, such as fracture zones.

A new bathymetric chart has been compiled from conventional wide-beam echo-sounder profiles and recently acquired swath bathymetric data for the Lau Basin between 18°30' to 21°S and 175°30' and 178°W. Long-range sidescan sonar data has been interpreted for the same area, and together the two data sets provide powerful imagery revealing the morphological and textural setting for the Ocean Drilling Program Sites 834 through 839. Our interpretation of these data suggests that the Lau Basin may be subdivided into two principal morphotectonic domains on the basis of significantly different topography and structure. The recognition of a fundamental contrast in the crustal character between the earliest formed basin and the most recently formed (1 Ma) seafloor has far-reaching implications for understanding the early geological evolution of all marginal basins, particularly those in backarc settings.Major bathymetric compilations of the Lau Basin have been made by a number of workers, including Hawkins (1974) and Chase (1985); modifications to these base maps have been incorporated into other published reviews of the morphology and geology of the region, the most notable among them being Kroenke (1984). The principal source of data used in the preparation of these charts has been single-track, echo-sounder profiles acquired by survey vessels. The seafloor topography between these lines of primary data has then been interpolated using the subjective skill and judgment of the compiler. Most recently, cruises by French, German, and American survey vessels fitted with multibeam bathymetric swath mapping (SeaBeam) systems have accelerated the capacity to produce more detailed and accurate bathymetric charts, and these latter data are in various stages of publication. The SEAPSO program (Foucher and Shipboard Party, 1986) covered the southern portion of the backarc axis on and near the Valu Fa Ridge, between 20° and 23°S. These data were followed by published results from the Sonne 35 and 48 cruises (von Stackleberg, 1990), who in addition surveyed some of the central Lau Basin. Scripps Institution of Oceanography PAPATUA expedition Leg 04 (Hawkins, 1988) and Leg 05 (Craig and Pareda, 1987) surveyed extensively throughout the area, and ROUNDABOUT expeditions Legs 14 and 15 (Hawkins, 1989) provided additional coverage between 15° and 19°S. Almost all of these data have been acquired since the last major bathymetric revision, and we present here the first major compilation of bathymetry to include all of these new data.As well as including all of the data recorded using swath multibeam bathymetric techniques, we have included the most recent soundings acquired by conventional echo-sounding methods. These include data accessed by means of the National Geophysical Data Center (NGDC), Boulder, CO. The chart presented as Figure 1 (back-pocket foldout) forms a small subset of a regional compilation of the bathymetry of the Lau Basin made at a scale 1:375000, which is in the final stages of preparation (Parson et al., unpu...