The frequency distribution of depths in ocean basins has been determined using the most recent American and Russian oceanographic charts as sources and a computer for data processing. Data have been complied for individual ocean basins and marginal seas, and also for nine different physiographic provinces, including, for example, ‘ocean basin,’ ‘rise and ridge’ and ‘continental shelf and slope.’ The summary result differs little from a hypsometric curve drawn by Murray and Hjort in 1912. Considering both land and sea, by far the most common levels are approximately at sea level and at about 5 km, the general depth of ocean floor. The compilation by physiographic provinces leads to a new understanding of the hypsometric curve, even though it does not change the shape. The distribution of depths in the ocean basin province is singly peaked and symmetrical. The distribution in the rise and ridge province is similar, but the mean depth is about 1 km less. This can be interpreted as indicating that one province is merely the elevated equivalent of the other, a conclusion which generalizes the field observation that some localized oceanic rises and ridges are formed by ephemeral bulges in the mantle under a normal oceanic crust. Most occurrences of depths between sea level and the deep sea floor have been produced by the formation of rises and ridges rather than by the deposition of sediment derived from continents. The development and collapse of rises and ridges may have caused substantial fluctuations in sea level during geological time.

The wavelength and amplitude of outer rises seaward of subduction zones and arches surrounding islands and seamounts are used to parameterize flexure profiles in terms of the moment and curvature at the first zero crossing. The data show the clear age dependence in the mechanical thickness of the lithosphere up to 60-100Myr. Saturation of moment at large curvature is interpreted in terms of a depth-dependent yield strength for the lithosphere using relations adopted from laboratory experiments of rock deformation. A comparison of theoretical curves with observed moments indicates that old oceanic lithosphere has no long-term strength below about 40 km depth, with no difference between 100 and 165 Myr old crust. Moderate axial loading forces (+200 MPa) can explain most variations in the moment/curvature observations, except in the case of the Kuril Trench which appears anomalous given the age of the crust. Regional tension causes greater variability in moment as compared to regional compression because of the greater slope in the brittle failure envelope under tension. The observations point to a lithosphere weaker than the prediction from experimental deformation of rocks. Of the possible weakening mechanisms, elevated porefluid pressure on faults does not predict the correct age dependence and is incompatible with earthquake focal mechanisms. Our favoured explanation is that the activation energy, Q, appropriate for ductile flow at geological strain rates is lower than the values derived from laboratory extrapolations of dry olivine data taken at high temperatures. If recent oceanic geotherms are reliable, Q in the lower lithosphere must be lower than lOOkcal mol-'. The method used here is most appropriate for trench profiles with curvatures greater than lO-'rn-'. For lower curvatures, such as along seamount profdes, small errors in the curvature estimate cause large changes in rheological parameters.

Contrary to the expectation that islands should monotonicalIy sink as the aging sea floor thermally contracts, some atolls that were formerly at sea level are now elevated as much as 70 m. A eustatic sea level high cannot explain the magnitude of the larger uplifts or the simultaneous occurrence of elevated and sea level atolls. The observation that recently active volcanoes are found near these raised atolls suggests that they have been tectonically uplifted by the loading effect of the volcanoes. By modeling the oceanic lithosphere as an elastic plate overlying a fluid asthenosphere we can both explain the varying amounts of elevation and bracket the effective flexural rigidity of the oceanic lithosphere between 1.7 and 2.5 ×1022 N m.

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