A tectonic analysis of the south-west Pacific

Abstract

The regional structure of the south-west Pacific is attributed to the spreading and segmentation by sub-crustal convection currents of a Paleozoic—Mesozoic geosynclinal belt marginal to the Australian craton, with migration of the separated sialic fragments northward and eastward into the Pacific Basin. The source of the convection currents presumably lay beneath the mid-ocean rises (Indian-Antarctic and Pacific-Antarctic Ridges) south of Australia and New Zealand. During Cenozoic times Australia has moved northward relative to the New Zealand Plateau, the migration of the latter having a greater eastward component that allowed opening of the Tasman Basin. As a result of these differential movements the geosynclinal belt extending from New Zealand across the north-east margin of the Australian craton has been subjected not only to crustal spreading and stretching in a north-easterly direction with anticlockwise rotation, but also to north-south extension and tensional rifting. The New Guinea section of the geosyncline has been separated by rifting with attenuation, and thrust northward in advance of the Australian craton, vacating the region now occupied by the Coral Sea Basin. The abruptly-terminated northern sections of the Lord Howe Rise and Norfolk Ridge have escaped rotation but have been extended by a system of oblique fractures and dilatational rifts. Independent, convergent movement of the Campbell Plateau - Chatham Rise section of the geosynclinal belt in a north-north-easterly to northerly direction has resulted in a clockwise rotation, through approximately 90°, of the intervening New Zealand segment, accompanied by large-scale flexuring and north-east shear-fracturing. The rotation has swung New Zealand into alignment with volcanic island arcs (the Kermadec-Tonga Ridge and the Macquarie Ridge) forming simultaneously to the north and south. The latter represent zones of convergence and sinking of sub-crustal convection currents, and it follows that the mega-structure of New Zealand has been determined by the intersection of the disrupted geosynclinal belt by such a zone.