Plate tectonics background to biogeographic development in the southwest Pacific over the last 100 million years
Abstract About 170 million years ago Greater India, an unknown bit of continent, Greater Australia, Greater New Zealand (includes submarine New Zealand Plateau) an Antarctica were all of a piece and part of Gondwana. The lines of pending dismemberment were present as long-established rift zones. The northern margin of this part of Gondwana (New Guinea), the eastern one (New Caledonia—Norfolk Ridge) and southeastern one (New Zealand—New Zealand Plateau) were active margins with considerable volcanism and both shallow- and deep-water sedimentation. At about 165 m.y. the unknown piece of continent was carried off with the creation of the Wharton Basin. In the Early Cretaceous (140-100 m.y.). Greater India departed to the northwest, a brief marine invasion occurred along the line of the present south margin of Australia and there was a great spread of epeiric seas over the remainder of Greater Australia and possibly also Greater New Zealand. The northern, New Guinea edge, was a volcanic island arc setting; a bordering arc system along the New Caledonia—New Zealand eastern edge made up the Inner Melanesian Arcs. The Rangitata Orogeny culminated in the Early Cretaceous, and was followed by a time of uplift, metamorphism, and regression, over much of Greater New Zealand. Regression of the seas was also a feature of Australia in the Late Cretaceous (100-65 m.y.). The major event of the Late Cretaceous was the creation of the Tasman Sea (78-56 m.y.). The the separation of Greater New Zealand from Antarctica. A large continental block, Tasmantis (Lord Howe and New Caledonia Ridges, New Zealand), was thus detached from Austro-Antarctica. This was itself split by the formation of the New Caledonia Basin at about 65 m.y. The Coral Sea was opened by spreading and sinistral strike-slip of a part of New Guinea north of the Papuan Mobile Belt at roughly the same time. The Inner Melanesian Arc System was disrupted. An Outer Melanesian Arc system now developed from New Guinea to Tonga and a young marginal sea began to form on the east side of the New Caledonia—Norfolk Ridge. The major event, however, was the separation of Australia from Antarctica. Obduction of seafloor and crust took place at the New Guinea and New Caledonia margins. Greater New Zealand was split by a dextral fracture system. The New Zealand Plateau remained on the Pacific Plate. Greatly increased convergence between Australia and Pacific Plates began at about 40 m.y. By mid-Oligocene (30 m.y.) the circum-Antarctic Current had begun its action. At the same time the New Hebrides—South Fiji marginal sea grew very large. By 6 m.y. (end of the Miocene) the northern boundary between Indo-Australian and Pacific Plates (the Melanesian Borderlands) became increasingly a zone of intense sinistral strike-slip and many island blocks moved WNW. Strong vertical movements have been a feature of the Borderlands. The Fiji platform rotated and the New Hebrides arc back-tracked to its present position. The old marginal sea was one-half consumed, the North Fiji Basin created by complex local spreading. The Lau Ridge became a remnant arc with the creation of the Lau—Havre interarc basin. New Zealand was joined to Tonga by way of the Kermadec Islands and subduction zone. Movement along the Alpine Fault has continued, conveying a strike-slip component to Kermadec—Tonga subduction.