Geosat profiles_ of the Exact Repeat Mission have been averaged over a 1-year period and high-pass-filtered using inverse method techniques. The geoid surface constructed with both ascending and descending profiles shows at medium wavelengths band-shaped anomalies preferentially elongated in the east-west direction. These anomalies have an average amplitude of ~30 cm and dominant wavelengths of 750 krn and 1100 km. We have performed numerous tests to show that the lineations are not artefacts created by the filtering process. Moreover, two-dimensional (2-D) filtering with the inverse method applied on a regional basis over the Pacific gives essentially similar results, indicating that the filtered geoid is not affected by directional bias. Seafloor topography in the Pacific filtered by 2-D inverse method also shows east-west trending depth anomalies positively correlated to medium-wavelength geoid lineations. Along the East Pacific Rise, there is a clear correlation between geoid lineations and regional variations in axial depth. Cross-spectral analysis carried_out on geoid and topography data over the Pacific area gives coherence maxima at 750-km and 1100-km wavelengths and admittance values of 2-3 m/km. Observed admittance matches the magnitude and shape of admittance predicted by lithospheric cooling models, suggesting that the lineations are related to regional variations in the plate cooling process. Convection models produce much higher admittances than observed unless a low-viscosity layer is assumed so that dynamical support cannot be completely discarded. In most instances, however, the position and direction of the lineations seem to be controlled by major fractures zones which is in favor of a lithospheric origin. In the south central Pacific where the lineations appear parallel to absolute plate motion, there may be a combination of both lithospheric and sublithospheric processes.

Short wavelength anomalies in the geoid, derived from Geos 3 satellite radar altimeter data, have been investigated in the southern Indian Ocean and in French Polynesia. A number of these anomalies, associated with sea-mounts or volcanic islands, can be explained by regional isostatic compensation models, yielding estimates of the flexural rigidity and the effective elastic thickness H of the oceanic lithosphere. This thickness appears to be about one third of the seismic estimates for this thickness. The Halso indicates some age dependence, compatible with the models of the thermal lithosphere. These results are in agreement with the results of previous studies on the flexure of the lithosphere.

Seafloor subsidence effects due to cooling of the oceanic lithosphere have been removed from bathymetry data. The corrected ocean floor topography presents long wavelength highs, in particular over western Pacific. We show in this study that this long wavelength residual topography can be interpreted as either a term of seafloor flattening at old ages or a dynamic response to large‐scale convection. Whatever its origin, this long wavelength residual topography is dominated by a degree 2 pattern highly correlated with geoid, lower mantle heterogeneities and plate age.

Convection under certain continents should be strongly affected by the presence of a neighboring subduction zone. The cold subducting oceanic lithosphere is removing heat from the continental mantle. Model cavities with lateral cooling are investigated to quantify the physics of such situations. Convection cells with a large aspect ratio are generated, and a sizable portion, typically 50% or more, of the heat produced under continents is transferred to the oceanic lithosphere in the models. This leads to geophysical speculations concerning the global heat balance in the earth and the capability of large subcontinental cells to drive the continental plates towards the trenches. Other points are discussed, particularly the influence of the chosen rheology (constant viscosity, Herring‐Nabarro or Weertman creep) and the reduced heat flow profiles.

Geoid height anomalies derived from SEASAT altimeter data have been analyzed across fracture zones and over ocean ridges in two limited regions of the South Pacific (Eltanin fracture zone system; East Pacific Rise) and southeast Indian Ocean. Observed geoid height‐age and geoid height derivative (with respect to age)‐age relationships have been established. Comparison with theoretical relationships computed for the plate model of lithospheric cooling permits an estimation of the thermal parameters entering into the model. Two quantities can be derived: the product ακTm (α, thermal diffusivity; κ, volume coefficient of thermal expansion; Tm, bottom boundary temperature) and the thickness H of the plate. The best fitting values for the South Pacific and the southeast Indian Ocean are ακTm = 0.52×10−3 cm2 s−1 and H in the range 50–70 km for ages less than 30 m.y., whereas for larger ages (≥30 m.y.), geoid observations are better explained by a larger H value, in the range 70–90 km. The depth‐age relationship observed in the South Pacific region is consistent with these parameters. On the other hand, data for the northeast Pacific argue rather for a lithospheric thickness larger than 100 km. This suggests that local and regional variations exist in the thermal properties of the oceanic lithosphere.