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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.