Characteristics of seismic reflections in central region of the South China Sea and their geological significance

More than 4 000 km 48-channel seismic reflection data from the central region of the South China Sea have been interpreted.Five seismic interfaces have been distinguished,named T₁,T₂,T₄,T₆ and T₈ respectively.Meanwhile,five seismic sequences numbered Ⅰ-Ⅴ have been divided with the ages of Quaternary and Pliocene, Later Miocene,Earlier and Middle Miocene, Oligocene and PreOligocene separately.Sequences Ⅰ-Ⅱ overlie all parts of the area.In the continental slope and island slope,Sequences Ⅲ-Ⅴ are mainly found in the grabens.Sequence Ⅲ is found at most profiles of the deep-sea basin,and Sequence Ⅳ is seen not only at the margins of the east subbasin but also at the margins of the southwest subbasin.Strong reflection from Moho is found at most profiles of the deep-sea basin.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 southwest subbasin is 51-39 Ma.It is indicated that the evolution of the southwest subbasin is simultaneous with or earlier than that of the east subbasin.