Bathymetric and magnetic anomaly data indicate that the South Pacific Ocean floor between New Zealand and Antarctica formed since late Cretaceous time by sea-floor spreading at the Pacific-Antarctic Rise and the southern portion of the East Pacific Rise. Following the initial breaking apart of the Campbell plateau from Antarctica, the Pacific plate moved away from the Antarctic plate at a fast rate of about 90 mm/yr. Between approximately 60 and 40 My ago, (anomalies 25 and 13) after spreading within the Tasman Sea had stopped, and during which Australia began to drift away from Antarctica, the direction of relative plate motion in the South Pacific changed and the average rate decreased to 40 mm/yr. During the re-alignment inany of the minor transform faults disappeared and the two that form the Eltanin fracture zone system changed in direction, offset and spacing in a predictable way. The pole of rotation moved nearer to the region. The topography of the sea-floor created during this interval was rough, presumably because of the slow rate of spreading. Since this time, the pole of rotation has been moving farther away and the spreading rate has been increasing. The present relative velocity of the plates ranges from about 60 mm/yr in the south-west to about 100 mm/yr in the north-east.Using reconstructions of Australia and Antarctica and preliminary evidence of the sea-floor spreading history of the Tasman Sea, we reconstructed the surrounding continental fragments at various times since the late Cretaceous. Bending of the ' New Zealand Geosyncline ' appears to have occurred entirely after about 40 My ago, and at the same time the southern portion of the Tasman Sea floor probably formed by slow spreading near the present Macquarie Rise, between the Pacific and Indian plates. Relative motion between East and West Antarctica of about 500 km seems to be required between about 80 and 40 My ago.

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 linear response function technique is used to analyze two 1300‐km tracks of SEASAT altimeter data and corresponding bathymetry in the Musician Seamounts region north of Hawaii. Bathymetry and geoid height are highly correlated in the 50‐ to 300‐km wavelength range. A predictive filter is developed which can operate on SEASAT altimetry in poorly surveyed oceanic regions to indicate the presence of major bathymétrie anomalies. Modeling of the bathymetry‐geoid correlation in the Musician region is attempted using the elastic plate model. The flexural rigidity D of the plate is not well constrained by our data but appears to lie in the range 5×1021 N m ‐ 5×1022 N m at the time of loading. Since the Musician Seamounts and the crust on which they lie are both Late Cretaceous in age, this value represents the effective flexural rigidity of very young lithosphere that was ‘frozen in’ at the time of voicanism, The modeling indicates that the general form of a predictive filter will strongly depend on various geologic parameters, especially the effective flexural rigidity. Hence, some a priori geologic constraints are necessary to estimate successfully the bathymetry from the altimeter data. Alternately, if high‐quality bathymetry is available, a crude estimate of the age of loading (i.e., voicanism) can be made from the altimeter data.

Ocean-floor spreading tore southern Baja California from mainland Mexico 4 million years ago and has subsequently rafted it 260 kilometers to the northwest along the Tamayo Fracture Zone. Magnetic-anomaly profiles indicate spreading at the mouth of the gulf at 3.0 centimeters per year and a rise-crest offset of 75 kilometers inside the gulf across the Tamayo Fracture Zone.

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