OI.R (1986) 33 (12) D. Submarine Geology and Geophysics 1019

Two-dimensional numerical models with retrograde slab migration (opposite to the direction of motion of the plates to which the slabs are attached) show that migrating slabs are at an angle to streamlines in the surrounding mantle. Laterally migrating slabs do not separate streamlines that turn in different directions at depth, and do not turn backward beneath the plates to which they are attached. Instead they become parts of the circulations beneath the overriding plates, and even at depth continue to move away from their oceanic ridge sources. Migrating slabs do not separate mantle convection cells. The models also show that slab migration slows the flow under the attached plate by diverting some of it to the circulation beneath the overriding plate, increasing the magnitude of the basal drag and the length of the overriding plate subject to a trench-directed drag. This drag on the overriding plate is the source of the 'trench suction' force found in models of plate driving mechanisms. Dept. of Geol., Hebrew Univ., Givat-Ram, Jeru- salem 91904, Israel.

86:6959 Kawasaki, Ichiro, 1986. Azimuthally anisotropic

model of the oceanic upper mantle. Phys. Earth planet. Interiors, 43(1):1-21.

A mixed form of forward and backward modelings of azimuthally anisotropic oceanic upper mantle is attempted for a data set of surface-wave velocities of periods 30-100 s. One of the basic configurations of the anisotropy postulated in the LID and LVZ is axial symmetry of elastic properties with a horizontal axis parallel to the spreading direction as found in ultramafic rocks from ophiolite complexes of the Bay of Islands and Oman. Basic features of the resulting model are a thin lithosphere of ~45 kin, a decrease of S-wave velocities, and an increase of P-wave velocities and of partial melting/dislocation relaxation with depth in the LVZ. This single model explains the overall features of anisotropy obser- vations in the Pacific Ocean. CIRES, Univ. of Colorado, Box 449, Boulder, CO 80309, USA.

86:6960 Kondopoulou, D.P., G.A. Papadopoulos and S.B.

Pavlides, 1985. A study of the deep seismotec- tonics in the Hellenic Arc. Boll. Geofis. teor. appl., 27(107): 197-207.

Reliable data for intermediate depth shocks verify the existence of a clearly formed Benioff zone dipping at an angle of 38 ° beneath the South Aegean area from SSW to NNE. Thirty-three reliable focal mechanism solutions lead to the following obser- vations: (1) A-axes agree with the direction of the sinking slab; (2) P-axes are almost horizontal, while

T-axes are almost vertical. These observations favour the explanation that the intermediate depth shocks in the Hellenic Arc are caused by shear zones within the descending slab. Geophys. Lab., Univ. of Thessaloniki, 54006, Thessaloniki, Greece.

86:6961 Michard, A., R. Montigny and R. Schlich, 1986.

Geochemistry of the mantle beneath the Rodri- guez Triple Junction and the South-East Indian Ridge. Earth planet. Sci. Letts, 78(1):104-114.

REE abundances and Sr, Nd, Pb isotope compo- sitions were measured on zero-age dredge samples from the Rodriguez Triple Junction (RTJ) and the South-East Indian Ridge (SEIR). Along the SEIR, the data may be fairly well accounted for by a binary mixing between an Indian MORB-type component and the plume-type St. Paul component. The RTJ has distinctive geochemical properties: low 2°6pb/2°4Pb, high 2°spb/2°4pb and 87Sr/S6Sr ratios, isotopic homogeneity, and variable LREE/HREE fractionation. The geochemical properties of the Indian Ocean basalts were examined using a three- component mantle model. Centre de Reck Petro- graph. Geochim., BP 220, 54501 Vandoeuvre-le- Nancy, France.

86:6962 Olson, Peter and I.S. Nam, 1986. Formation of

seafloor swells by mantle plumes. J. geophys. Res., 91(B7):7181-7191.

In this paper, results of laboratory experiments on development of surface topography above ascend- ing, low-viscosity plumes are presented. The objec- tives are first to provide some insight into the processes governing the early stages of swell for- mation and second to apply scaling laws derived from the experiments to estimate the physical dimensions of mantle plumes required to support seafloor swells. The most significant qualitative results of the experiment are that swells formed by interaction of low-viscosity mantle plumes and oceanic lithosphere can be characterized by axial symmetry, rapid uplift, and shallow compensation. Dept. of Earth and Planet. Sci., Johns Hopkins Univ., Baltimore, MD 21218, USA.

86:6963 Papadimitriou, E.E. and B.C. Papazachos, 1985.

Seismieity gaps in the Aegean and surrounding