Crustal Structure and Features in the Conjugate Margins of South China Sea
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摘要: 为了分析南海共轭大陆边缘的地壳结构, 在收集南海地区多次海底地震仪探测、海陆地震联测以及重、磁探测等成果资料的基础上, 首先构建了南海北部陆缘3条由东向西横贯海陆的深部地壳结构剖面图, 并以中地壳低速层和下地壳高速体的分布特征为基础, 推测滨海断裂带可能为华南正常陆壳与南海减薄陆壳的分界断裂; 以地壳减薄程度和下地壳高速层的尖灭为标志, 圈定了洋陆壳转换带, 发现南海北部陆缘从东到西地壳拉张减薄的程度和模式具有较大差异性.然后以南沙地块水深、磁力和重力异常数据为基础, 揭示了礼乐滩洋陆壳转换带表现为水深、磁力和重力异常变化的陡峭梯度带, 莫霍面深度从礼乐滩的~24km急剧抬升至洋盆区的~11km; 礼乐滩以西的洋陆壳转换带从东到西逐渐变得平缓; 南沙海槽两侧的水深和重力异常具有对称性, 为“U”形结构; 并将水深、莫霍面和重力急剧变化的梯度带拟定为南沙地块洋陆壳转换带的分界标志.最后构建了穿越南海共轭陆缘的地壳结构剖面, 并将其划分为减薄陆壳、洋陆壳转换带、洋壳及拉张裂谷等类型.Abstract: In order to analyze crustal structures and features in the conjugate margins of South China Sea (SCS), we collected large numbers of studied results from the ocean bottom seismometers experiments, onshore-offshore seismic experiments, gravity and magnetism anomalies.This paper firstly constructs three land-sea super cross-sections from east to west in the northern margin of SCS.The results show the onshore-offshore transitional zone is a border separating the unstretched and the stretched continental crust.The low velocity layer (LVL) in the middle crust was widely imaged in the unstretched continental crust.However, the high velocity layer (HVL) in the lower crust was detected in the stretched continental crust.By analyzing the mechanisms of the LVL in the middle crust and HVL in the base of crust, we believe the crustal structures had distinctly different attributes in the continental South China and in the northern SCS, which indicates that the littoral fault zone (LFZ) could be the boundary fault between them.Then, we reveal the crustal features in the Liyue Bank based on water depth, gravity and magnetism anomalies.The Moho depth thins from about 24 km in the Liyne bank to 11 km in the oceanic basin.The ocean-continent transitional zone generally widens from east to west in the southern margin.Finally, we devide the crustal structure in the conjugated margin of SCS into four types, including the unstretched continental crust, the stretched continental crust, the ocean-continent transitional zone, and the ancient ocean crust.
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图 1 南海共轭陆缘深地震探测测线分布
黑实线为海底地震仪探测测线(OBS、OBH); 灰实线为海陆地震联测测线(OOS); 黑色点线为双船扩展剖面(ESP)
Fig. 1. Distribution of deep seismic sections in the conjugated margin of SCS
图 3 南海南、北陆缘水深、磁力和重力异常剖面
图中黑色箭头所示为推测的洋陆壳转折点; 图中黑线和黑色坐标值代表水深, 红线和红色坐标值代表磁异常, 绿线和绿色坐标值为自由空间重力异常; 在右下脚插图中从东到西依次为L1-L13
Fig. 3. Sections of water depth, gravity and magnetism anomalies in the conjugated margin of SCS
图 4 南海水深地形与莫霍面深度分布(莫霍面深度据苏达权等, 2004修改)
彩色为水深图; 等值线为莫霍面深度图(m)
Fig. 4. Distribution of water depth and moho in the SCS
图 5 南海自由空间重力异常与莫霍面深度分布(莫霍面深度据苏达权等, 2004修改)
彩色为重力异常图, 等值线为莫霍面深度(m)
Fig. 5. Distribution of gravity anomalies and Moho in the SCS
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