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.     

Microearthquakes along the back-arc spreading system in the eastern Bismarck Sea

OBS’s were deployed for 26 to 29 days in the eastern Bismarck Sea to investigate the back-arc spreading. Hypocenters of 186 shallow earthquakes were determined using P- and S-waves from at least five stations. In the western survey area, a transform fault zone is marked by a linear micro-earthquake activity striking N65°W and less than 5 km wide. The predominant type of their focal mechanisms is strike-slip. In the eastern area, several intermittent zones of micro-earthquakes and their strike-slip type focal mechanisms suggest the location of short-length transform faults separating en-echelon spreading ridges.     

Kinetic studies of the rate of dissolution of silica and diatom tests in sea water

Kinetic studies have been carried out on the dissolution of 10μm beads of silica and of the siliceous tests of two species of diatoms (Thalassiosira fluviatus andSkeletonema costatum) in sea water at 10–50°C and over the pH range 6–9. At all temperatures dissolution of the biogenous silica occurred most rapidly at pH 8 and most slowly at pH 6. First order kinetics were closely followed when the silica was present in a considerable excess over that required for saturation, the rate being proportional to the area of the silica. Apparent deviation from this type of kinetics occurred when there was insufficient silica to bring about saturation. This was undoubtedly due to the progressive decrease in the surface area which takes place as the particle dissolves. Application of the rather simplistic model developed by Kamataniet al. (1980), which makes allowance for this decrease, gave a close correspondence with the data until ∼ 90% of the silica had dissolved.     

Simplified method of deep-tow seismic profiling

To improve resolution of seismic-reflection profiles in continental slope water depths of 900 to 1500 m, a single hydrophone was towed about 150 m off the bottom to receive reflected signals from a surface-towed sparker sound source. That deep-towed hydrophone data show that valleys which appear V-shaped in records from a surface-towed hydrophone are flat-bottomed, and that subbottom reflections from an erosional unconformity can be much better resolved. The data produced by this technique are very hepful when used in conjunction with records from conventional surface-towed seismic-profiling equipment.     

Submersible structural study of Tamayo transform fault: East Pacific Rise, 23°N (project RITA)

A submersible structural study of Tamayo transform fault, the second field study of an oceanic transform, was conducted with the diving saucer CYANA as part of the international project RITA. On the basis of the surface ship surveys and deep-tow traverses made prior to the diving program, the four successful dives of CYANA established the geometry of the presently active shear zones and demonstrated that the median ridge of the Tamayo transform is tectonically inactive. The dive results require the presence, in an area marked by diapir-like bodies, of an extensional relay zone linking the two offset shear zones which trend about 110°.P. CHOUKROUNE, Laboratoire de Géologie structurale, Université de Rennes I, avenue du Général-Leclerc, 35042-Rennes Cédex, France; P. J. FOX, State University of New York at Alban, Albany, New York, 12222, USA; M. SEGURET, Laboratoire de Géologie structurale Université des Sciences et Techniques du Languedoc, Place Eugéne-Bataillon, 34060, Montpellier Cédex, France; J. FRANCHETEAU, H. D. NEEDHAM, Centre Océanologique de Bretagne, B. P. 337, 29273, Brest Cédex, France; T. JUTEAU, Laboratoire de Minéralogie-Pétrographie, Université Louis-Pasteur, 1 rue Blessig, 67084, Strasbourg, Cédex, France; R. D. BALLARD, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, 02543, USA; W. NORMARK, United States Geological Survey, Pacific-Arctic Branch of Marine Geology, Menlo Park, California, 94025, USA; A. CARRANZA, Centro de Ciencias del Mar y Limnologia, Ciudad Universitaria, Mexico 20 DF, Mexico; D. CORDOBA, J. GUERRERO, Instituto de Geologia, UNAM, Ciudad Universitaria, Mexico 20 DF Mexico; C. RANGIN, UNAM,now at Université de Paris VI, France.     

On the capsize performance of a discus buoy in deep sea breakers

The mode of capsize of a discus buoy in breaking waves is discussed. The results of model tests are given; these show that a judicious choice of mooring design can substantially reduce the chance of capsize. A comparison is made with the performance of the buoy on station.     

Estimation of the leeway drift of small craft

Small craft (<6·4 m) leeway is determined as a function of the wind speed in the range of 5–20 knots (3·6–10·3 m/sec). Leeway is calculated relative to the surface current by measurement of the separation distance of the small craft from a dyed patch of surface water at sea, using time-sequenced aerial photography. Leeway increases linearly with wind speed for small craft equipped with or without a sea anchor in the wind range studied. Leeway for small craft without sea anchor can be calculated from the equation U_L = 0.07 U_W + 0.04 where U_W is the wind speed at 2 m elevation. Leeway for small craft drifted off the be calculated from the equation U_LD = 0·05 U_W − 0·12. The small craft drifted off the downwind direction in about 80% of the experiments. The drift angle is variable and difficult to predict.     

Improved in situ gamma-ray transmission densitometer for marine sediments

The first-generation University of Illinois gamma-ray transmission densitometer, designed for the in situ measurement of sediment bulk density, was modified by incorporating in the detector probe (1) an Americium-241 alpha particle pulser and an anti-walk gain stabilization control to maintain better temperature stability and (2) a small power supply and a IC preamplifier to eliminate the need for a high-voltage coaxial cable between the detector and external signal conditioning electronics package. This second-generation Lehigh University system has been successfully deployed since 1971 in routine use from ships and submersibles in the Atlantic and Pacific Oceans and the Gulf of Mexico. Results are presented of system operations to (1) measure bulk density over the range of 1.2–1.8 Mg/m³ in the Hudson Canyon, (2) penetrate 1.9 m into the seafloor in the San Diego trough and, (3) be lowered to a water depth of 3.6 km in the Gulf of Mexico.