The characteristics of seismic reflection waves in the middle area of the South China Sea and their significance

The middle part of the South China Sea consists of continental slopes,island slopes and a deep-sea basin.Since the 1970s,many seismic surveys have been carried out there.More than 4 000 km 48-channel seismic reflection data from this area have been interpreted.Five seismic interfaces have been distinguished,named T₁,T₂,T₄,T₆ and T_g respectively.Meanwhile,five seismic reflection sequences,named Ⅰ-Ⅴ,have been divided into the ages of Quatemary and Pliocene,Later Miocene,Earlier and Middle Miocene,Oligocene and Pre-Oligocene separately.Sequences Ⅰ-Ⅱ overlie all parts of the area.In the continental slopes and island slopes,the sequences Ⅲ～Ⅴ are mainly found in the grabens.Sequence Ⅲ is found at most profiles of the deep-sea basin (DSB),and the sequence Ⅳ is seen not only at the margins of east subbasin(ESB) but also at the margins of southwest subbabin(SWSB).Strong reflection from Moho is found at most profiles of the DSB.The depth of Moho varies between 10 and 12 km,with a thickness of 6～8 km for the crust.Calculated by age-basement depth correlation formula,the age of basaltic basement in the SWSB is 51～39 Ma.It is indicated that the evolution of the SWSB is simultaneous with or earlier than the ESB rather than later than it.