【作者】 吴振利；

【导师】 李家彪；

【作者基本信息】 中国科学院研究生院（海洋研究所） ， 海洋地质， 2010， 博士

【摘要】 南海西北次海盆东接东部次海盆,南靠中沙群岛,西临西沙海槽,北依南海北部陆坡,东北宽西南窄,是南海三个次海盆中面积最小的一个海盆,但其位置独特,构造演化复杂。西北次海盆作为南海地区的一个重要地质构造单元,记录了从大陆张裂到海盆扩张的丰富信息,西北次海盆附近区域则涵盖了裂谷、海盆等多种地质构造现象以及陆壳、过渡壳和洋壳等多种构造单元。而且由于西北次海盆尺度小,南北陆缘距离较近,成为研究大陆边缘初始张裂过程、共轭属性和区域重要地质事件的有利地区。本论文基于2006年西北次海盆海底地震仪(OBS)广角反射/折射地震调查所获取的两条高质量剖面数据,结合多道地震剖面资料,通过精细数据处理、射线追踪正反演技术,反演得到了西北次海盆纵向和横向的深部地壳结构模型。并结合重力数据,进行了密度反演计算,获取了海盆下方的密度分布模型。综合研究区内搜集到的地球物理数据,参考前人的研究结果,对西北次海盆的深部地壳结构特点进行了深入的分析与对比。对西北次海盆的陆缘张裂特征、莫霍面形态特点以及海盆地壳性质进行了重点剖析,以南海北部边缘张裂演化史和南海海盆构造演化史为主线,对西北次海盆北部陆缘性质,海盆地貌形态、深部地壳结构特征,构造成因及其形成、演化历史等进行了广泛讨论,获得了多项有益的研究成果,主要有以下几个方面:1)首次利用OBS广角地震数据建立了西北次海盆纵向和横向深部地壳结构模型,获取了详细的莫霍面形态和结构特征;通过重力反演获取了西北次海盆地壳深部密度分布信息。2)西北次海盆地壳结构表明,海盆下方Moho面呈隆起状,埋深约11km,两侧形态近似对称,中央稍深为12.0km,且与多道地震剖面发现的中央火山隆起区有镜像对称关系。地壳厚度也从陆坡厚约21km,至海盆减薄为最小7.7km,发生了强烈减薄,西北次海盆的张裂模式属于纯剪切模式。3)OBS2006-1、OBS2006-2两条剖面地壳结构中均不存在下地壳高速层,进一步证实了南海北部陆缘西部的非火山型性质。OBS2006-1地壳剖面的分布位置为划分南海北部地壳性质分界线提供了新的重要信息。4)西北次海盆地壳性质属新生洋壳性质。西北次海盆已经进入海底扩张阶段,但由于其规模小,从初始扩张到稳态的海底扩张时间短,记录不到清晰磁异常条带,故无明显磁异常条带表现。而西北次海盆经历了层2熔岩的不对称溢流,进一步又模糊和干涉了原有的磁条带。5)地貌、沉积物和构造分析表明,西沙海槽并非简单的西北次海盆向西扩展产物。南海北部地区西沙海槽,西北次海盆和东部次海盆在动力学特征方面更多表现出差异性,反映不同的演化过程和内在联系。更多还原

【Abstract】 Located at the northwestern part of the South China Sea(SCS) among the east subbasin, the Zhongsha Islands(Maccelesfield Bank) and the Xisha Islands(Paracel) , the NW subbasin is the smallest ocean basin in all the three subbasins of SCS. It presents all major stages from continental rifting in the late Oligocene to initial seafloor spreading in the mid-Miocene, thus provides an ideal place to study therifting and initial seafloor spreading process in great detail. And because of its small scale and short distance between the northern and southern continental margin, theNW subbasin is a good place to study the continental rifting process, the conjugatenature and local geology events.Based on two high quality Ocean Bottom Seismometer(OBS) wide anglereflection/refraction profiles which were obtained in the northwest subbasin SCS,in 2006, through precious data processing, 2D ray tracing method and inversion technique, we calculated the velocity distribution under the NW subbasin and get the deep crust structure model. At the same time, cooperated with gravity data density model was calculated. Synthesize other geophysics data collected in the research area and read other scholars’research results for reference, analysis and contrast were done to the characteristic of the deep crust structure of the NW subbasin. The key analysis was conducted to the rifting characteristic, Moho surface geometry and crust nature of the NW subbasin in this paper, combined with the rifting evolution history of the northern continental margin and tectonic evolution history of the SCS, we have widely discussion on the continental margin nature, the morphologic shape, deep crust characteristic and tectonic evolution history,many beneficial results have been obtained which are mainly showed in the follow,1)For the 1st time the deep crust structure was built with OBS seismic data in the NW subbasin, detailed Moho geometry information and layered structure characteristic were got. Density model was calculated through gravity inversion in the NW subbasin.2)Data inversion and analysis show that, the crust thickness under the continental slope decreases from 21km to 11km, the crust thickness of the NW subbasin is about 7.7km, and the moho depth ascends from 21km to 11km. The tectonic geometry and velocity structure of the NW subbasin and its margins on both sides shows symmetrical and conjugate and indicates pure shear mode in continental margin rifting mechanism.3)We don’t find clear seismic signals from high velocity layer under the lower crust of the continental margin in the northern of the NW subbasin, which supplies new evidence for the viewpoint that western part of the northern continental margin of South China Sea is non-volcano crust.4)The crust of the NW subbasin similar to the east subbasin in structure is confirmed to be oceanic crust, with thicker layer 1(sedimentary layer) and thinner layer 2 , which is some different from typical oceanic crust structure. In addition, because the seafloor spreading period of the NW subbasin was short, the layer 2 experienced asymmetric basalt magma flow, which blurred the magnetic anomaly belt of the NW subbasin.5)Analysis on the geomorphic feature, sediment and tectonic showed that the Xisha Trough was not simply the extension result of the NW subbasin. The Xisha Trough, NW subbasin and east subbasin had more difference in dynamic characteristic which implied different evolution history and potential relationship.更多还原