Bifurcating and wandering ocean ridges: A progress report

First conclusions of a study of the fault pattern in the North Atlantic ocean are briefly outlined. Correlation of fracture zones in deep ocean basins, even over short distances (30 miles) is difficult when using only topographic and seismic profiler data, and requires the additional criterion that the magnetic pattern is shifted along the fracture zones. This implies that each track must be paralleled by at least one more track at close distance to detect breaks in the magnetic pattern.Transform faults are regarded as complementary to the median rift, which, as a tension fault, should be normal to the direction of maximum strain according to laboratory experiments by Gramberg. Median rift segments and transform faults together form an orthogonal system approximating an originally irregular but continuous fissure.Mantle convection currents are thought to be the agents of continental drift. In view of the dimensions of convection cells, a direct coupling between ridges (or ridge segments) and the upwelling branch of a convection current seems impossible. Thus the known bifurcations of mid-ocean ridges in the Indian and in the Pacific Ocean may be interpreted as the result of a divergence of the horizontal branch of a convection cell or a divergence of the movements of crustal plates through collision of continents. Secondary fissures would lead to the development of secondary ridges that cannot be distinguished from primary ridges. In this concept mid-ocean ridges can partake in the horizontal movement by asymmetrical spreading. This might explain the disappearance of the East Pacific Rise under the North American continent.The resulting convection current system is simple: one upwelling and one downgoing current as two complementary helices winding round the earth. Ways are indicated to check several aspects of the proposed hypotheses.     

Particle transport from the West African shelves of Liberia and Sierra Leone to the deep sea: A chemical approach

The chemical and mineralogical composition of suspended particle suites has been used to assess off-shelf transport on the West African shelves of Liberia and Sierra Leone. Using the ratios of Si/Al, Fe/Al, Mg/Al and Mn/Al as tracers, it was possible to detect shelf-derived materials in slope waters. In the majority of cases, these inputs could not have been detected using particle mass or light scattering measurements and could only be measured by using the chemical signatures of the particulate matter. At the time of sampling, the suspensate which had been moved seaward over the slope was detected adjacent to submarine canyons and highly turbid areas on the outer shelf. The chemical and mineralogical composition of the suspensates, and sediments in the adjacent eastern Atlantic basin are similar to those found in the water column seaward of the West African continental shelves and yet distinct from the Sahelian dust which is considered the major source of sedimentary material for the tropical and semi-tropical areas of the deep eastern basin. These findings suggest that materials from this shelf area could be a more important input to the deep sea than was previously realised.     

Bathymetry of three deep-sea channels in the northeast and central Tufts Abyssal Plain

The drainage pattern in the northeast and central Tufts Abyssal Plain is described in detail. Satellite navigation on the systematic survey has allowed precise location of the major channel systems of the northeast Pacific Ocean. Two hundred channel profiles were collected from the echograms showing the Moresby-Scott, Mukluk, and Horizon Channel Systems trending in either a southwestward or westward direction across this section of the Tufts Plain. The channel profiles illustrate the prominence of the higher and wider right-hand levee (facing downstream). The Moresby-Scott Channel System disperses in the form of several distributaries throughout the area studied, and is probably responsible for much of the sediment deposits. Unlike the Moresby-Scott, the Mukluk extends throughout the survey area as a solitary channel with one minor branch. The Horizon Channel crosses the Sedna Fracture Zone east of the Sedna Seamount and terminates in distributary fashion in the central portion of the Tufts Plain. The Moresby-Scott, Mukluk, and Horizon Channels form one major system which encompasses the entire northeast and central Tufts Abyssal Plain.     

Beach accretion and erosion, Burullus-Gamasa coast, Egypt

Several areas of erosion and accretion were observed along the Burullus-Gamasa beach. The accreted sands are coarser and less sorted than the eroded ones. Differentiation between them could be achieved on the basis of their grain-size fractions, shape of grain-size distribution curves and statistical grain-size parameter relationships.     

Plate boundaries and evolution of the Solomon Sea region

The Solomon Sea Plate was widely developed during late Oligocene, separating the proto-West Melanesian Arc from the proto-Trobriand Arc. Spreading in the Bismarck Sea and in the Woodlark Basin resulted from interaction between the Pacific and Australian Plates, specifically from the collision of the proto-West Melanesian Arc with north New Guinea, which occurred after arc reversal. This model explains the extensive Miocene, Pliocene, and Quaternary volcanism of the Papua New Guinea mainland as it related to southward subduction of the Trobriand Trough. Our interpreted plate motions are concordant with the geological evidence onshore and also with complex tectonic features in the Solomon Sea Basin Region.