南海中部地震反射波特征及其地质解释

20世纪70年代以来,在南海中部海区开展了各种地震调查,为研究盖层和基底发育、断裂和岩浆活动、海盆成生演化提供了重要依据。在对南海中部海区4112km48道反射地震资料解释的基础上,识别出了T₁,T₂,T₄,T₆,T_g等五个反射界面;识别出了I～V五套地震反射层组,推测时代分别为上新世-第四纪、中新世晚期、中新世早-中期、渐新世和前渐新世。层组I～Ⅱ全区广布。在陆坡、岛坡区,层组Ⅲ以下层组主要见于断陷中;在深海盆,层组Ⅲ分布仍较广,除了在深海盆北段见到层组Ⅳ外,在西南次海盆剖面两缘也见到该层组。在东部次海盆剖面中还不同程度见到了双程反射时间为8.4～8.7s的莫霍面反射,埋深为10～12km,地壳厚度为6～8km.西南次海盆水深和新生界基底埋深均比深海盆北段除外的东部次海盆深,分别为4000-4300和5200～5500m.根据年龄和基底深度关系经验公式,计算西南次海盆基底年龄为距今51～39Ma.地震反射层组解释和年龄一基底深度关系计算表明,西南次海盆形成并非晚于东部次海盆,而是同时或早于东部次海盆。

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.