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.

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The oceanic crust in the eastern Pacific between 7°N and 30°N and east of 127°W contains a fairly complete history of the spreading centers associated with the East Pacific Rise since 25 m.y. B.P. (late Oligocene). In this paper, we have summarized the seafloor spreading magnetic‐anomaly data and the bathymetric data that reflect the record of this tectonic history. The well‐defined magnetic lineations north of the Clarion fracture zone, in the mouth of the Gulf of California, and on the east flank of the East Pacific Rise (EPR) are carefully examined and used to provide a guide for interpreting the spreading pattern between the Clarion and Clipperton fracture zones, southward of the Rivera fracture zone over the Mathematician Ridge, and over the entire EPR east of the Mathematician Ridge between the Rivera and Siqueiros fracture zones. The bathymetric data provide a trace of the fracture zone pattern in each of the above mentioned areas. The fracture zone bathymetry and the seafloor spreading magnetic lineations on the EPR south of the Rivera fracture zone have a distinctive fanning pattern caused by close poles of rotation and plate boundary reorganizations. All these data provide a good record of the plate reorganizations in the middle Miocene at magnetic anomaly 5 A time (12.5 to 11 m.y. B.P.), in the late Miocene at magnetic anomaly 3′−4 time (6.5 m.y. B.P.), and in the Pliocene at magnetic anomaly 2′−3 time (3.5 m.y. B.P.). Several abandoned spreading centers, including the Mathematician Ridge, were left behind as a result of these reorganizations. The Mathematician Ridge is shown to be a set of ridges and trough whose origin is related to the tectonic activity associated with each of the above mentioned reorganizations since anomaly 5A.

The northeast Pacific topography and magnetic lineations (25 m.y. B.P. to the Present) record the traces of three major spreading reorganizations. Only one spreading center is observed today, but there is evidence for several ephemeral episodes of twin spreading accompanying the evolution from an extensive Pacific‐Guadalupe plate boundary to a much shortened Pacific‐Cocos and Pacific‐Rivera plate boundary. The 25 m.y. B.P. plate reorganization culminated with the formation of the Guadalupe plate, bound by the Murray fracture zone to the north and the Cocos‐Nazca spreading ridge to the south. Between 25 and 12.5 m.y. B.P. spreading continued while the plate retained the same general outline. The 12.5‐11 m.y. B.P. reorganization resulted in the creation of a much shortened Pacific‐Cocos plate boundary located in its early stages over the Mathematician seamounts and a much reduced Cocos‐plate. The last reorganizaton (6.5–3.5 m.y. B.P.) resulted in the abandonment of the Mathematician spreading ridge as a Pacific‐Cocos plate boundary in favor of the East Pacific Rise.

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