南海西南海盆磁性层结构及初始扩张研究

The formation and sea floor spreading process of the Southwest basin in the South China Sea (SCS) are very complex. By now, there have been some problems about the magnetic anomaly studies to reveal the spreading process of the Southwest basin in the SCS. Except for the limitation of the magnetic anomaly data themselves, the most important factor is the simplicity of the traditional single-magnetic-layer model of the oceanic crust. The model is difficult to reflect the impact of the magnetic layer of the deep oceanic crust which increases the difficulties of recognizing the magnetic anomaly stripes. The traditional model is relying solely on the comparison of the magnetic anomaly profiles, rather than the implementation of the constraints of the horizontal and vertical structure of the magnetic layer of the oceanic crust. This also caused the uncertainty of the recognition and comparison of the magnetic anomaly stripes. Therefore, in this proposal, the impact of deep magnetic structure in the vertical dimension and lateral variability of the magnetic layer in the horizontal dimension are considered. Based on double-magnetic-layer model, we will analyze the newly obtained magnetic data, combined with other geophysical data to study the magnetic structure of the Southwest basin, determine the magnetic anomalies distribution pattern of the Southwest Basin, the crustal structure and nature of the southwest and marginal Southwest basin in the SCS. Furtherly, it contribute to study the boundary fault between the Southwest and East basin, rifting and spreading process on the margin of the Southwest basin in the SCS, and understand well the initial spreading characteristic in the Southwest basin.

南海西南海盆的形成环境和扩张过程复杂，用磁异常研究其扩张过程时，采用传统的单层二维板模型过于简单，难以真实反映洋壳磁性层，更增加了磁异常条带识别难度。本项目拟利用西南海盆积累的丰富磁异常剖面，以及地震、重力等精细的地球物理数据，采用更加合理真实的双磁性层结构模型为基础，在横向上，加入洋壳磁性层横向变化的约束信息，在垂向上，考虑磁性层厚度变化的影响，并结合综合地球物理资料的约束处理，重点分析西南海盆磁性层结构及磁条带空白区的磁条带分布及年龄，研究西南海盆南北边缘及西南部等关键构造部位的地壳结构和性质，进而确定西南海盆陆缘破裂特征和西南部地壳演化阶段，西南海盆与东部海盆的边界断裂特征，揭示西南海盆初始扩张过程。

通过穿过西南海盆西南端及南北陆缘的地震剖面可以看到，西南海盆南北陆缘结构不均一和南北两侧的洋盆厚度不对称。北侧上、中地壳较为均一的磁性，其性质接近的统一整体，而南侧上、中地壳磁性差异明显，上地壳具有与北侧相似的低磁性特征，而下地壳则磁性强，且横向变化明显，在断裂处存在明强烈磁化率强弱变化。.西南海盆以中建南盆地和郑和隆起为界东西两段的洋盆具有明显的东西向差异。西段残留扩张中心没有明显的裂谷肩，沉积最厚，由扩张中心往两侧，基底逐渐抬升，沉积层减薄；东段则相反，残留扩张中心基本被火山占据，扩张中心处沉积很薄，往两侧逐渐加厚，这种差异可能体现了东西两段在扩张过程的岩浆供给的不同，东段岩浆供应充足，以岩浆增生的稳定扩张为主，西段岩浆供应不足，构造作用占主导。.鉴于南西海盆处于低纬度地区，斜磁化及磁化倾角变化的影响明显，为此，利用了归一化磁源强度、解析信号振幅的各向异性标准差等，以及直接反演等方法加强了磁异常条带的信号的识别。.基于南海北部的地震解释剖面结果，利用磁异常特征分析了南海“两次跳跃”模型与最早扩张磁条带。结果显示，扩张中心第一次在28.3Ma向南跳跃20km，第二次在23.85Ma向南跳跃20km。