Magnetic Anomalies, Bathymetry and the Tectonic Evolution of the South Pacific since the Late Cretaceous

Abstract

Summary 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.