Each Lamont‐Doherty sonobuoy located on well‐dated crust has been carefully analyzed to determine crustal structure down to oceanic layer 3. Results from the Atlantic and the Pacific are compiled separately in order to study crustal structure as a function of plate age in both oceans, since they have very different spreading rates. Layer 2A (refraction velocity about 3.6 km/s) in the North Atlantic is 1.5 km thick at the ridge crest and thins consistently to about 100 m as the crust ages to about 60 m.y. Layer 2A in the east Pacific is 0.7 km thick at the ridge and thins to about 100 m at about 30 m.y. This difference in thickness is probably attributable to the much faster spreading rate in the Pacific. A poorly refractive acoustic basement layer about 200 m thick with similarities to layer 2A but not necessarily composed of the same materials is measured sporadically in the Pacific M‐Series plates and even less consistently in the Atlantic. This layer is not recorded in the Cretaceous or the Jurassic quiet zones. Regressions of refraction velocities in layer 2A as a function of age show that its velocity increases from about 3.3 km/s at the ridge crests to that of layer 2B on crust about 40 m.y. old. There is no corresponding increase of velocity with age in any of the deeper layers. The high resolution of the air gun/sonobuoy records shows that the layer 2B refraction line breaks directly to layer 3 velocities (46 times in the present work) or to a line with a velocity of 6.1 km/s (114 times in the present work), which we call layer 2C. The variance of the velocities in 2C is one fourth that of 2A and 2B, which indicates relative lithological uniformity in 2C. It does not seem likely that layer 2A really thins; what appears to be a thinning of the layer may actually be the result of an increase of its refraction velocity with age. However, the 2A/2B interface is well‐defined by large amplitude refractions from 2B on crust that is younger than about 30 m.y., which seems to rule out a transitional zone at the base of layer 2A where it ‘converts’ to 2B. The seismic observations seem to require a diagenetic process or repeated basaltic intrusions; both processes raise serious objections.

No abstract

No abstract

Cenozoic deep-sea sedimentation in the southwest Pacific area was controlled by large changes in the patterns of bottom-water circulation and erosion. The circulation patterns were largely controlled by

The results of twenty‐eight seismic refraction profiles recorded in the various physiographic provinces of the Philippine Sea as part of the United States and Japan Science Cooperation Program are presented in four schematic structure sections. The basins of the Philippine Sea have fairly normal oceanic crust that includes, between the sea floor and layer 2, a layer of about 3.5‐km/sec velocity controlling the characteristic rough topography. Crustal thickening beneath the Nansei Shoto, Oki‐Daito, Kyushu‐Palau, and the Honshu‐Mariana ridges is associated mainly with an increase in thickness of the 3.5‐km/sec layer and a thick underlying section of material with a velocity between 5.5 and 6.0 km/sec. Beneath the Nansei Shoto trench and the Honshu‐Mariana trench, there is a tendency for layer 2 to increase and layer 3 to decrease in thickness as the trench is approached from the adjacent oceanic sector.

Many new sonobuoy solutions have been combined with published data and incorporated into a revised plot of igneous layer 2A thickness as a function of plate age at the East Pacific Rise. Each solution was redated on the basis of regional magnetic anomaly maps to improve the accuracy of crustal dating. Results show that layer 2A becomes unobservable by seismic methods on crust older than about 15'm.y.Although igneous materials must make up layer 2A at the East Pacific Rise, low-velocity acoustic basement exists elsewhere in the Pacific. Near seamounts and seamount chains, volcanic aprons with the seismic properties of layer 2A thin distally away from eruptive centers and seem to persist indefinitely without being affected by processes that normally increase the sound velocity at the ridge crests. This is apparently due to a lack of fracturing, which prevents hydrothermal solutions from percolating through the volcanic rubble. At active spreading centers, percolation through fractures provides minerals that could fill voids and increase the sound velocity of the basaltic rubble. Statistical studies show that the velocity in layers 2B and 2C is independent of plate age, but layer 3 velocities peak up significantly on the East Pacific Rise. In the older parts of the Pacific, low-velocity acoustic basement is sedimentary or a mixture of sediment with volcanics. An unusually thick sequence (l km) of material that is probably mostly sediment occurs in the Nauru and Mariaha basins. 153 155 155 160 160 170 170 170 J J J J J J 4.25 J 2.99

Seismic profiler data from the continental shelf of the Ross Sea show numerous areas where gently dipping sediments with a total thickness of about 2 km are truncated at the sea floor or a few hundred meters below the sea floor. A series of basement (5.5 km/sec) ‘highs’, which outcrop at the sea floor in the south and lie approximately along the 180th meridian, separates the shelf into two regions. Iselin bank lies on the northern extension of this line. The profiler and sonobuoy data show that the eastern shelf is formed by a sedimentary basin containing up to 4 km of sediments. The western shelf is more complex. Profiler data indicate gentle synclines and anticlines that plunge northward and overlie, in the south, older structures that trend ENE. Sonobuoy data from the shelf show a sharp increase in the sediment thickness in an elongate region parallel to 177°E. The increase in sediment thickness, caused possibly by the presence of N‐S faulting, is not apparent in the profiler data. Over the whole shelf sonobuoy results show a linear increase in sediment velocity with depth. The geological trends seen on the shelf are not consistent with the northwesterly regional strikes mapped in Victoria Land or in western Marie Byrd Land. The continental rise in the east is marked by a sharp seaward increase in the basement depth, from about 4.5 to 6.5 km, at the base of the rise. In contrast, the western sector of the continental rise is more complex with basement ridges, whereas the depth to basement does not drop below about 4.5 km.

The Agulhas Plateau lies 500 km off the Cape of Good Hope in the southwestern Indian Ocean.