Characteristics of Tectonic Subsidence of Nansha Area
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摘要: 南海在新生代期间发生多次裂解和离散, 在南海南部陆缘地层中留下了多次构造运动的痕迹.选取南沙海域内覆盖曾母、北康、南薇西、万安盆地的24个模拟井位, 分别进行构造沉降速率的计算, 并分析了自中生代末期以来的4次重要构造运动对南沙海域盆地的形成和构造演化的控制作用.结果显示, 南沙海域构造沉降分为3幕: 第一幕为40.5~30.0Ma, 在西卫运动的影响下, 南沙海域盆地进入断陷阶段, 发生大规模的沉降作用, 构造沉降速率最大达到了160m/Ma; 第二幕为30.0~10.2Ma, 随着南海的张开, 区内盆地相继发生热沉降作用, 进入拗陷期, 由于古南海与婆罗洲的碰撞, 区内盆地地层发生整体抬升, 构造沉降速率达到最小值20~50m/Ma; 第三幕为10.2~0Ma, 晚中新世由于南沙运动影响, 研究区内盆地发生了一次大规模的快速沉降作用, 随着南沙地区一起进入区域沉降阶段.Abstract: Tectonic subsidence histories of Zengmu basin, Beikang basin, Nanweixi basin and Wan'an basin in Nansha area are analyzed with back-stripping technique.1D tectonic subsidence histories of 24 presumed wells are calculated.The results show that Nansha area experienced 3 main subsiding stages in Cenozoic: (1) Nansha area experienced rifting stage in 40.5—30.0 Ma with a wide range of subsidence.In the first subsiding stage, the max tectonic subsidence rate is 160 m/Ma, which was supposed to be controlled by Xiwei movement. (2) In the second stage (30.0—10.2 Ma), Nansha area ended rifting stage and shifted to depression stage in connection with collision between ancient South China Sea and Borneo.The max tectonic subsidence rate decreased to 20—50 m/Ma. (3) For the third stage (10.2—0 Ma), the regional basins experienced a stage of accelerated subsidence in connection with Nasha movement since Late Miocene.Nansha area experienced regional subsiding stage.
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Key words:
- Nansha area /
- tectonics /
- back-stripping technique /
- subsidence rate /
- marine geology
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图 1 南沙海域构造区划与模拟井位置
Fig. 1. The geological frame of the Nansha area and the location of presumed wells
图 2 南沙海域新生代地层系统和构造演化阶段
Fig. 2. Cenozoic tectonic stages and sequence stratigraphy of the Nansha area
图 7 研究区内各盆地平均构造沉降速率
Fig. 7. Average tectonic subsidence rate of four basins in Nansha area
表 1 南沙地区的地层及岩性
Table 1. Proportion of mudstone and sandstone of Nansha area
地震反射层 年代(Ma) 主要岩性 颗粒密度(g·cm-3) 地表孔隙度(%) 压实递减系数(km-1) 海底—T30 0~5.3 砂泥岩 2.69 57.4 0.41 T30—T32 5.3~10.2 碎屑岩(含部分灰岩) 2.67 53.2 0.35 T32—T40 10.2~16.0 碎屑岩(含部分灰岩) 2.67 53.2 0.35 T40—T60 16.0~23.8 砂页岩(夹煤层) 2.69 57.1 0.41 T60—T70 23.8~30.0 砂页岩(夹灰岩、砾岩) 2.69 58.1 0.43 T70—Tg 30~37 (万安为30.0~40.5) 砂页岩(夹灰岩、砾岩) 2.69 58.1 0.43 
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