Based on the interpretations of three seismic profiles and one wide-angle seismic profile across the Northwest Sub-basin, South China Sea, stratigraphic sequences, deformation characteristics and an extension model for this sub-basin have been worked out. Three tectonic-stratigraphic units are determined. Detailed analyses of extension show that the event occurred mainly during the Paleogene and resulted in the formation of half-grabens or grabens distributed symmetrically around the spreading center. Sediments are characterized by chaotic and discontinuous reflectors, indicating clastic sediments. Farther to the southwest, the sub-basin features mainly continental rifting instead of sea-floor spreading. The rifting would have been controlled by the shape of the massif and developed just along the northern edge of the Zhongsha-Xisha Block, rather than joined the Xisha Trough. After 25 Ma, a southward ridge jump triggered the opening of the Southwest Sub-basin. The NW-directed stress caused by the sea-floor spreading of the Northwest Sub-basin may have prevented the continuous opening of the sub-basin. After that the Northwest Sub-basin experienced thermal cooling and exhibited broad subsidence. The deep crustal structure shown by the velocity model from a wide-angle seismic profile is also symmetrical * Corresponding author. Tel./fax: þ86 571 81963137, þ86 13588039531 (cellphone).
The Philippine Basin, surrounded by a series of oceanic trenches, is an independent deep ocean basin in the West Pacific Ocean. Its middle part is divided into three marginal sea sub-basins by the Kyushu-Palau and West Mariana Ridges, namely, the West Philippine Basin, the Shikoku and Parece Vela Basins and the Mariana Trough. This paper, through the analysis of the geomorphologic features and gravity and magnetic characteristics of the basin and identification of striped magnetic anomalies, suggests that the entire Philippine Basin developed magnetic lineation of oceanic nature, and therefore, the entire basin is of the nature of oceanic crust. The basin has developed a series of special geomorphic units with different shapes. The KPR runs through the entire Philippine Basin. From the view of geomorphologic features, the KPR is a discontinuous seamount chain (chain-shaped seamounts) and subduction beneath the Japanese Island arc at the Nankai Trough which is the natural boundary between the basin and the Japanese Island arc. At the positions of 25 • N, 24 • N, 23 • N and 18 • N, obvious discontinuity is shown, which belongs to natural topographic discontinuity. Therefore, the KPR is topographically discontinuous.
The Philippine Sea Plate has an extremely special tectonic background. As an oceanic plate, it is almost entirely surrounded by subduction zones with complex internal tectonic features. On the basis of enormous published literature, this paper offers a comprehensive overview of the tectonic and evolution history of the Philippine Basin and the Kyushu-Palau Ridge (KPR) in the Philippine Sea Plate, and discusses the geological features of KPR. Referring to relevant definitions of various "ridges" stipulated in United Nations Convention on the Law of the Sea, so the KPR is believed to be a remnant arc formed during the opening of the Parece Vela and Shikoku Basins in the Philippine Sea Plate. It is a submarine ridge on oceanic plate rather than a submarine elevation. And thus, it is not a natural component of the Japan continental margin.
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