Geological and Geophysical Analysis of the Southwestern and Eastern Sub-Basins, South China Sea
Geological and Geophysical Analysis of the Southwestern and Eastern Sub-Basins, South China Sea
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发表时间：
2002
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通讯作者：
Chu Feng-you
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作者：
Chu Feng-you
本文通过对南海东、西南次海盆地质、地球物理特征的对比分析，探讨了南海构造演化历史，认为东次海盆的磁异常和重力异常为东西向，而西南次海盆的磁异常和重力异常为北东向。两个次盆地的布格重力异常中心大于边缘。这表明在盆地中部以下存在高密度物质，这可能与盆地的扩张和地幔高密度物质的侵入密切相关。分析结果表明，西南次海盆磁异常主要受NE-SW方向5 km以上的浅源磁源影响。在5 km以下，磁异常变得模糊，强度降低。东次海盆的磁异常不仅受5 km以上浅源磁源的影响，而且还受深源磁源的影响。他们都在EW方向。两个次级盆地的差异反映了不同的构造机制，南海大部分断裂位于环太平洋构造带和特提斯构造带的交接部位，并受两条构造带的共同影响。南海盆地断裂多为北东向和近东西向。北东向断裂构成了西南次盆地和东部次盆地西部的构造格局。北东向断裂的形成机制和时间与中国东部断裂相同，一般形成于燕山期。喜马拉雅期，由于构造基点东移，中国东南部NE向断裂形成较早，相对稳定，EW向断裂形成较晚，并与NE向断裂交切。由于东西向断裂切割深达岩石圈，常形成地幔的喷溢通道。南海位于太平洋构造带与特提斯构造带的交接部位。其演化受两条构造带的影响。南海与中国东部一样，也受到太平洋构造的影响。南海和中国东部均处于北东-南西向的弧后伸展构造。这些地区也受到特提斯构造的影响。在这一阶段，特提斯洋壳最终俯冲到欧亚板块之下。在强烈的北东-南西向拉张应力作用下，南海形成了北西-南东向的拉张应力。因此，在这两种应力作用下，南海出现了许多北东向的地堑盆地。由于拉张作用较东部其他地区强烈，南海地壳被深部岩浆完全吸收侵入。地壳中的基性-超基性岩是西南次海盆的浅部磁源，始新世（42- 43 Ma）之后，太平洋边界发生了最重要的变化。俯冲方向由北北西向北西西向转变。太平洋在菲律宾海沟俯冲到欧亚大陆之下，在中沙-西沙群岛和南沙群岛之间形成了NEE - EW向的裂谷。根据东南亚的重建，Hall认为西菲律宾海和西苏拉威海的打开，导致了原南海在北波罗-吕宋弧向南俯冲，并导致了华南边缘的南北向伸展和向南的板片拉张力。与此同时，印度人开始了与欧亚大陆的碰撞，导致了西藏的隆升和特提斯的关闭。在持续碰撞作用下，地幔发生大规模的SE向运动，印支高原发生挤压。红河带开始左旋走滑近500 km。在北东东向断裂控制下，
In this paper, we will compare and analyse the geologic and geophysic characters of the Eastern and Southwestern Sub-basins of the South China Sea (SCS) and discuss the SCS' s tectonic evolution history.The results of studies suggest that the magnetic anomalies and the gravity anomalies of Eastern Sub-basin are in EW direction but in NE direction in the Southwestern Sub-basin. In both Sub-basins the Bouguer gravity anomalies in the central part are bigger than in the margins. This indicates that below the central part exists high density matter, which could be closely related to the spreading of the basin and the invasion of high density matter from the mantle.A computational process called the Interpolation Cut Methods was used to analyse the magnetic anomalies of the two Sub-basins. Results of the analyse indicate that the magnetic anomalies of the Southwestern Sub-basin is mainly affected by the shallow magnetic sources above about 5km in NE-SW direction. Below 5km, the magnetic anomalies become blur, and the intensity decreases. The magnetic anomalies of the Eastern Sub-basin is not only affected by the shallow magnetic source above 5km, but also affected by the deep source. They are all in EW direction. The differences between the two Sub-basins reflect the different tectonic mechanisms.Most faults in SCS exist in the conjunctional area of the Circum-Pacific tectonic belt and theTethys tectonic belt and are effected by both belts. The faults in SCS basin is mostly in NE and near EW directions. The NE direction faults formed the structure frame work of the Southwestern Sub-basin and the western part of the Eastern Sub-basin. The formation of the NE direction faults are same in the mechanism and time as the faults in the Eastern China, which were commonly formed in Yanshanian stage. In the Himalayan stage, because the main tectonic point moved to east, the NE direction faults of the Southeast China formed earlier and was relatively stable; the EW direction faults formed later and then cut across the NE direction faults. Because the EW direction faults cut very deep and can even reach the lithosphere, they often formed the gushing channels of earth mantle.The South China Sea is located in the conjunctional areas of the Pacific and the Tethys tectonic belts. Its evolution was affected by both the tectonic belts. The SCS was affected by the Pacific tectonics same as the Eastern China. Both SCS and the eastern China were in back-arc extension tectonics of NE-SW direction. These areas were also affected by the Tetheys tectonics . In this stage, the Tetheys oceanic crust finally subducted under the Eurasia plate. Combined with the strong NE-SW extensional stress, extensional stress in NW-SE direction was formed in the SCS. Therefore, in the action of the two stresses, many graben basins in NE direction appeared in the SCS . As the extension were stronger than in other places of the Eastern China, the SCS crust was completely taken up and intruded by deep magma. Basic-ultrabasic rocks in the crust are the shallow magnetic sources in the Southwestern Sub-basin.After Eocene(42-43Ma) we saw the most important change in the Pacific boundaries. Its subduction direction changed from NNW to NWW. The Pacific subducted at the Philippine trench beneath the Eurasia and formed NEE - EW direction rifts between the Zhongsha-Xisha islands and Nansha Islands. According to the reconstruction of the SE Asia, Hall suggested that the opening of the West Philippine sea and the West Sulawei Sea triggerd the Southward subduction of the Proto-South China Sea at the North Borneo-Luzon Arcs, and caused the N-S orientation extension of the South Chinese margin and the southward slab-pull force. Simultaneously the Indian began its collision with the Eurasia and led the uplift of the Tibet and close of Tetheys. For the continuous collision , the mantle moved in large scale on SE direction and the Indochina plateau extruded. The Red River zone started its left-lateral slip nearly more than 500 kms. In the control of the NEE-EW direction faults in the nor