Origin and history of the South China Sea basin

Magnetic anomaly data acquired on R/V Vema cruises in the South China Sea in 1979 have allowed us to refine the previously identified pattern of seafloor spreading in the South China Basin. East trending magnetic lineations 5D to 11 identified in the eastern half of the basin date seafloor spreading as mid-Oligocene through early Miocene (32-17 m.y. B.P.). Heat flow data are consistent with these ages. Half spreading rates varied between 2.2 and 3.0 cm/yr. An east trending chain of seamounts occurs near the center of symmetry of the magnetic lineations. Basalts dredged from these and other seamounts in the basin have major element chemistries which are alkalic or transitional between tholeiitic and alkalic. Thick sediments (1-2.5 km) and relatively smooth oceanic basement characterize the older portions of the basin, whereas thinner sediments (300 m to 1 km) and a blocky basement fabric characterize the younger central part of the basin. A broad basement arch topped by fault-bounded tilted blocks occurs in the east-central area of the basin. The South China Basin is an `Atlantic-type' marginal basin, bounded by passive continental margins to the north and south. Opening of the basin moved microcontinental blocks including northern Palawan and Reed Bank from their Paleogene position adjacent to the China mainland. Fracture zones in the eastern half of the basin and the inferred transform margin east of Vietnam indicate that the South China Basin opened in a north-south direction and require a distant pole to describe the opening. However, the spreading fabrics of the western and eastern halves of the basin are significantly different. Free air gravity and limited seismic reflection data delineate a southwest trending relict spreading center in the southwest of the South China Basin. Much less oceanic crust was generated at this relict spreading center than at the east trending spreading center farther east. We speculate that opening of the western half of the basin was dominantly by crustal stretching of the complex of microcontinental blocks inferred to occupy a large part of the area. The landward boundary of normal oceanic crust in the South China Basin is marked by a change in basement structure and magnetic anomaly signature, by a free air gravity anomaly low, and by a steep landward gradient in the isostatic gravity anomaly. Analysis of the gravity data suggests that the boundary is associated with a density contrast at crustal depths, which is compensated below the Moho. Comparison with other Atlantic-type continental margins suggests that a positive density contrast between oceanic and continental crust may be a common feature of continent-ocean boundary zones. The rifting of the proto-China margin which preceded seafloor spreading in the South China Basin probably began in the latest Cretaceous or Paleocene (∼65±10 m.y.). The rifting was localized along a former Andean-type arc terrain at which volcanism had ceased by approximately 85 m.y. B.P. North Palawan and Reed Bank are inferred to have been forearc areas in the Mesozoic. The margins of the South China Sea record a regional mid-Oligocene unconformity which we interpret as caused by the superposition of breakup and sea level effects. Seafloor spreading in the basin ended slightly before the late middle Miocene cessation of subduction at the Palawan subduction zone to the south.