南海岩石圈厚度变化特征及其构造意义

Variation of the lithospheric thickness in the South China Sea area and its tectonic significance

地震层析资料表明,南海地区,自红河口向南经南海、苏录海到苏拉威西海,岩石圈速度低,底部横波速度仅4.4km/s,岩石圈厚度在60～80km,为薄岩石圈地区;而软流层的速度也较低,在4.2～4.4km/s,但厚度较大,大于200km。从红河—莺歌海断裂带经南海到苏录海,存在一条北西向宽约200km的上地幔北西向低速带,速度在4.05～4.25km/s(面波速度)。它反映了新生代南海地区上地幔的动力学过程。南海岩石圈厚度变化存在明显差异,南海陆缘,岩石圈厚度在70～80km,而在南海洋盆之下,岩石圈厚度超过100km,岩石圈底部存在高速岩石层,并且洋盆下的岩石圈之厚度比大陆边缘厚,在海盆岩石圈下部的60～80km深度上存在一高速层,纵波速度为8.2～8.3km/s。特别是中央海盆及西北海盆与西南海盆,其下部岩石圈中均存在一高速岩石层,这是非常具有构造意义的。由此笔者提出大陆岩石圈裂离、上地幔因减压而部分熔融所产生的基性岩浆形成南海新生代洋壳的猜想。

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.