The basins of the Sulu and Celebes Seas are located within a complex geodynamic zone between Pacific, Indian-Australian, Eurasian and Philippine Sea Plates. Three models have been proposed for the tectonic origin of these basins; trapped fragments from a larger oceanic plate, back-arc spreading, and rifting from the Southeast Asia margin. The basins areseparated by the shallow partly emerged Sulu volcanic Ridge extending from Western Mindano to Northeast Borneo. The Sulu Volcanic Ridge is bounded on the northwest by the Sulu Trench with evidence for subduction of the Sulu Sea beneath the Sulu Ridge. The nature of the southeast margin of the Sulu Ridge is difficult to assess because of a thick sedimentary cover and the lack of reflection or refraction seismic data. Hamilton (1979) and Rangin (1989) proposed flipping of subduction along the Sulu Arc from its southeast margin (subduction of the Celebes Sea under the Sulu Arc) to its northwest margin (subduction of the Sulu Sea under the Sulu Arc). Gravity observations are modeled across the Sulu Arc and adjacent basinal structures using existing seismic data. The gravity modeling is consistent with the concept that subduction zones exist along both flanks of the Sulu Arc and also that extinct spreading centers exist in both the Sulu and Celebes Seas.
The Southeastern Asian region where four prominent plate boundeies converge is an area of unique complexity marked by active arc-arc or arc-continent collision zones, and subduction zones. The Sulu and Celebes Seas are located within this region between the Pacific, Indian Australian, Eurasian, and Philippine Sea Plates (Fig. 1).
The Sulu Basin is a small marginal basin, elongated northeast southwest, bounded by Palawan, Negros, Negros Trench, Zamboanga, Sulu Ridge, and Borneo (Fig. 1). The Sulu Sea is about 400 km wide and is divided by a bathymetric high, the Cagayan Ridge. The Cagayan Ridge is an elongated submarine volcanic ridge parallel to the Palawan Trench which divides the Sulu Sea into two sub-basins, the Northwest Sulu Basin and the Southeast Sulu Basin. The Cagayan Ridge has been interpreted as an island arc associated with the Palawan Trench (Holloway, 1982) and also a remnant volcanic arc for the Sulu Arc. Palawan Island, located on theNorthwest Sulu Basin margin is a part of an older Eocene subduction complex associated with the extinct subduction atthe Palawan Trench (Fig. 1) (Holloway, 1982). The Northwest Sulu Basin is filled by relatively undeformed sediment disconformably overlying the deformed strata. The Southeast Sulu Basin is filled by 1–2 km thick sediments overlaying the acoustic basement ranging 4500 to SWOrn in depth (Rangin and Silver, 1990).
The Celebes Sea is underlain by oceanic crust and is bordered by the Cotabato Trench in the northeast and North Sulawesi Trench in the south. The sedimentary thickness in most of the Celebes Basin is about 1 km and increase to 2 km along its Southern margin (Hamilton, 1979).