ON THE TECTONIC EVOLUTION OF MARGINAL BASINS IN NORTHERN MELANESIA AND THE SOUTH CHINA SEA

Magnetic, bathymetric and seismological data are used to define segments of active seafloor spreading and associated magnetic lineations trending ENE in two marginal basins of northern Melanesia. In the Bismarck Sea, asymmetric back arc spreading behind the New Britain arc-trench system formed the Manus Basin during the past 3.5 m.y. at a total opening rate of approximately 13 cm/yr. In the non back arc Woodlark Basin, seafloor spreading began prior to 3.5 m.y. in the east and propagated westwards, splitting the Woodlark Rise from the Pocklington Rise. The Woodlark spreading center is being obliquely subducted beneath the Solomon arc-trench system to the east. Determining the opening history of the Woodlark Basin and Bismarck Sea has allowed us to quantitatively reconstruct the tectonic evolution of northern Melanesia back to mid-Pliocene time, and to account for the unusual setting of several volcanic suites.; The South China Basin is an "Atlantic-type" marginal basin bounded by passive continental margins to the north and south, and a transform margin to the west. Rifting of the proto-China margin began in the latest Cretaceous or Paleocene. North-south opening of the basin moved microcontinental blocks including southwest Mindoro, northern Palawan, and Reed Bank, from their Paleogene position adjacent to the China mainland. East trending magnetic lineations 5D to 11 which we identify in generated at a southwest trending spreading center in the southwest of the basin than at the east trending spreading center to the east. We speculate that a majority of the opening of the western half of the basin occurred by crustal stretching of microcontinental blocks. The landward boundary of normal oceanic crust in the South China Basin and other Atlantic-type basins may be characterised by a positive density contrast between oceanic and continental crust. The margins of the South China Sea record a regional mid-Oligocene unconformity which we interpret as caused by the superposition of breakup and sealevel effects. Seafloor spreading in the basin ended slightly before the late Middle Miocene cessation of subduction at the Palawan subduction zone to the south.