Abstract: Seismic tomography data obtained in the South China Sea area show that,from the Red River Mouth through the South China Sea and the Sulu Sea to the Celebes Sea,the lithospheric velocity is relatively low,and the lithospheric thickness varies in the range of 60-80 km,suggesting that the Red River-Yinggehai fault is a lithospheric fault.In the continental margins of the South China Sea the lithospheric thickness ranges between 70 and 80 km,whereas in the oceanic basin the lithospheric thickness is over 100 km.In addition,there exists a high-velocity layer with velocities of 8.2-8.4km/s in the depth of 60-80 km under the South China Sea basin.These phenomena are very interesting,hence the authors consider that,when the lithosphere experienced breakup before the seafloor spreading,the lithospheric upper mantle of the Mesozoic South China Sea was subjected to decompression by more than 10 kb.This means that lithospheric upper mantle could produce partial melting if there existed 0.1% water.If there existed 10% partial melting in the upper mantle,the melting basic igneous rock could form an oceanic crust 5 km in thickness.Therefore,the authors held that there existed no mantle plume under the South China Sea area.The authors＇ guess is a scientific guess concerning the formation of the Cenozoic South China Sea basin.