南海西北部陆缘构造伸展作用下的深部地壳结构响应

Crustal structure beneath the dispersed blocks and the failed rifts, as controlled by tectonic stretching, contains key information for revealing mechanism of the continental extension along the northwestern continental margin of the South China Sea (SCS). A wide-angle seismic profile (OBS2013-1), which was carried out recently, starts from junction of the Xisha Trough and the Qiongdongnan Basin to the north, passes though the Xisha Block, the Zhongsha Trough and reaches the ocean continent boundary (COB) to the south. It is parallel to seismic profiles of OBS2011-1, OBS2011-2, OBS2006-1 and OBS1996-4, constructing a probing network for comparing of the crustal structure in the northwestern continental margin. Preliminary results of the data processing show that the seismic recordings in OBS2013-1 are of high quality with successive seismic phases from depth of the earth. The research aims to acquire the reliable 2D model of the crustal structure beneath the profile, taking advantage of raytracing method on travel-times and theoretical synthesis on amplitudes and combing with other geophysical methods. By analyzing the seismic velocity, crustal thickness and rock density in the model, we can study the response feature of the crustal structure under continental extension. The rifting pattern as well as the distinct causes and conjugate ways of the continental margin can also be fully discussed. By the above work, this research can provide important scientific foundations for depicting the evolution processes of the SCS.

南海西北部陆缘在构造伸展作用下，形成了离散陆块、夭折裂谷等构造单元，研究其深部地壳结构是揭开陆缘伸展机制的关键线索。最新采集的OBS2013-1测线经西沙海槽与琼东南盆地转折处，穿过西沙地块、中沙海槽，到达洋-陆边界。该测线与OBS2011-1, OBS2011-2,OBS2006-1以及OBH1996-4近乎平行，共同构成了覆盖南海西北部陆缘的探测网络，可提供深部地壳结构的横向对比。初步处理结果表明，OBS2013-1台站数据记录良好，深部震相连续清晰。本项目拟利用射线追踪方法和振幅理论合成方法，并结合其它综合地球物理方法，获得可靠的二维地壳结构剖面。通过对剖面中地震波速度、地壳厚度和岩石密度的分析，研究深部地壳结构在伸展作用下的响应特征，探讨南海西北部陆缘拉张模式及其与南海其它陆缘的差异成因和共轭方式，从而为描述南海形成演化过程提供科学依据。

南海西北陆缘在大陆伸展作用下发生了地壳减薄变形，形成一系列离散陆块、夭折裂谷等构造单元，其深部地壳结构是揭开南海陆缘伸展及裂解的关键线索。本项目共处理OBS2013-1测线中25台OBS数据，拾取折射震相5856个，反射震相3882个，在前期地震数据处理和资料分析、深部探测结果和区域地质资料搜集以及所建立的最佳初始模型基础上，对拾取震相进行了地震波走时正、反演，获得了测线下方的纵波速度结构，对测线邻近地区的天然地震数据进行了接收函数波形模拟，获得了台站下方的一维横波速度结构。对比分析了研究区陆块、裂谷和洋陆转换带地区下方的地壳结构横向变化，获得了以下几点认识：1.陆块下方地壳变形比较均匀，海槽下方地壳变形与深度有关，存在上地壳拆离和中下地壳韧性流变；2. 南海洋陆转换带为超减薄陆壳性质，存在早期洋壳物质混染，指示了快速的大陆裂解和洋壳增生；3.南海共轭陆缘具有地壳结构不对称性，与伸展前俯冲流体作用对原始地壳物质的侵蚀和改造有关；4. 南海陆缘近东西向展布的夭折裂谷或超减薄陆壳未最终演化为成熟洋壳，揭示了大陆裂解过程中的方向转变，其深部动力机制仍需进一步研究。