南海共轭大陆边缘构造属性的综合地球物理研究

Integrated geophysical research on the tectonic attribute of conjugate continental margin of South China Sea

利用南海最新的重磁资料,在岩石物性分析和全海域分带变倾角化极磁异常反演磁性基底分布的基础上,选择6条典型剖面拟合反演其密度与磁性结构,在此基础上进行了深部结构的对比分析.反演中尽可能以海底地震仪探测数据(OBS)、多道地震等结果作为约束,其中FF'剖面层速度分析是利用"南海大陆边缘动力学及油气资源潜力"973项目2009年最新采集的多道地震数据,并利用了OBS2009南沙段数据对重力数据进行了物性结构拟合反演.根据剖面深部结构与物性分布特征分析认为:南海北部、南部陆缘均应以非火山型构造属性为主.北部陆缘CC'剖面以东,虽有下地壳高密度层存在并伴有活跃的岩浆活动,但缺乏典型向海倾斜反射层(SeawardDipping Reflectors)存在的证据,推断高密度层应与浅表岩浆岩一样,是海底扩张之后陆缘裂解过程中的产物.中沙隆起与礼乐盆地区的磁性基底和深部结构特征十分相似,推断在海盆拉张之前应为同一块体,属于共轭大陆边缘中的"共轭点".西南次海盆南部陆缘地球物理综合分析表明:在陆坡坡脚附近有中生界分布,但层速度计算结果表明可能整条剖面上的中生界并不完全连续.

In this paper, the density and magnetic structure of six profiles in South China Sea was inversed based on the latest potential field data and the rock physical property analysis. The constraint information from the ocean bottom seismometer （OBS） data and multi-channel seismic （MCS） results was used in the inversion process. And the FFI profile used the latest MCS data colleted in 2009 by the 973 project. Based on the MCS data, the interval velocity is computed and the structure model is used for the gravity modeling. Based on the cieep structure anatybls al^u u~ density/magnetic structure of the six profiles in this paper, the tectonic attribution of the northern and the southern margins of the South China Sea are more like non-volcanic continental margin. There exists high density layer in the lower crust accompanied with active magma activity to the east of CCr profile in north margin of SCS. But it is lack of the seaward dipping reflectors in this region. So we deduce that the high density layer in lower crust is the result of splitting of the continental margin after the sea floor spreading, which is same as the magmatic rocks in shallow layer. The magnetic basement and the deep structure of the Zhongsha Uplift and Liyue basin are quiet similar. It was deduced that they were the same block before the pull of sea basin. They are a pair of conjugate points of the northern and southern margins of the South China Sea. The integrated geophysical interpretation of southern margin of southwest sub-sea basin indicated that the Mesozoic group can be found near the base of slope but they may be not continuous.