Sea-floor spreading between Australia and Antarctica

Abstract

In the southeast Indian ocean, geoid and gravity maps derived from the Geos3 and Seasat satellites data show clear evidence of a lineated pattern located between the southeast Indian ridge and the southern branch of the Ninetyeast Ridge.We have carried out two-dimensional spectral analyses of the gravity data over four regions, each of 10° sx 10° dimension, three being located over the Indo-Australian plate, one over the Antarctica plate. We have also analysed 13 Seasat geoid height profiles roughly parallel to the southeast Indian ridge, covering both sides of the ridge.Power spectra of the gravity and geoid data over the Indo-Australian plate confirm the existence of undulations oriented N40°E of ∼ 200 km wavelength, and of ∼ 10 mGal and < 1 m amplitude, respectively. These undulations appear at plate ages of ∼ 10 Ma, are well developed between 10 and 40 Ma, then vanish beyond ages > 40 Ma. The wavelength remains constant with increasing plate age. Such undulations are not seen west of the southeast Indian ridge, i.e. over the Antarctica plate. Where observed, the geoid undulations have a sinusoidal signature quite different from the typical step-like signature of fracture zones.The lineations observed in the southeast Indian ocean have characteristics comparable to those reported by Haxby and Weissel in the central Pacific and interpreted as evidence of small-scale convection occurring in a low viscosity layer just beneath the lithosphere.