Crustal Structure of the Northwest Sub-Basin of the South China Sea and Its Tectonic Implication
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摘要: 西北次海盆的深部地壳结构蕴含着南海北部陆缘拉张过程的重要信息.广角反射/折射测线(OBS2006-2)长386 km，是目前唯一的一条沿NEE向穿过西沙地块、并平行于西北次海盆扩张脊的深地震测线.通过射线追踪与走时模拟方法(RAYINVR)，获得了OBS2006-2测线下方的速度结构.结果表明：西沙地块的沉积层厚度约为1~2 km，而西北次海盆的沉积层厚度大约为2~3 km；Moho界面从西沙地块的27 km逐步抬升到西北次海盆的12 km，Moho界面下方的速度为7.8~8.0 km/s；未发现壳内高速层和低速层.在西沙地块和西北次海盆的过渡区，有着较大量的岩浆活动信息，推测与西北次海盆的初始扩张有关.OBS2006-2测线中114.5°E以西的地区为减薄的陆壳，而114.5°E以东的地区为洋壳，莫霍面在陆壳与洋壳的结合处剧烈抬升，地壳厚度明显减薄.西北次海盆的扩张脊下方可能有残余岩浆的存在.Abstract: Deep crustal structure in the northwest sub-basin contains the key information of the rifting process of the northern continental margin of the South China Sea (SCS). A 386 km-long seismic profile (OBS2006-2) with wide-angle reflective and refractive ocean bottom seismometer (OBS) data was acquired across Xisha block and the Northwest sub-basin (NW sub-basin) of the SCS in NEE direction, which is the only profile hitherto parallel to the expanding ridge of the NW sub-basin. The crustal structure along OBS2006-2 has been obtained by using ray-tracing simulation method (RAYINVR), which yields information on the velocities, tectonics, and geophysical properties. Our velocity structure indicates that sediment thickness increases from Xisha block (about 1-2 km) to NW sub-basin (about 2-3 km). Moho interface changes from typical continent margin of South China (~27 km) to oceanic crust (~12 km). The velocity at the top of upper mantle varies from 7.8 to 8.0 km/s. No high velocity layer and low velocity layer have been identified. Voluminous magmatism in transition zone between Xisha block and NW sub-basin, is possibly related to the initial extending of the NW sub-basin. The part of model in the domain of 220-370 km would be of oceanic crust while the western part in the domain of 0-220 km may be of thinned continental crust. Residual magma may exist under the speading ridge of the NW sub-basin deduced by the anomaly of Pn seismic phase.
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Key words:
- NW sub-basin /
- crustal structure /
- magmatism /
- douple-peak seamount /
- ocean bottom seismometer (OBS) /
- sedimentology /
- geophysics
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图 1 研究区水深及相邻地震测线布设位置(磁条带数据来自于Briais et al., 1993)
Fig. 1. Bathymetry of the research area and surrounding deployed seismic lines
图 2 OBS2006-2测线与MCS97303多道地震剖面双程走时对比
a.MCS97303多道地震剖面，据Li et al., 2008修改；b.OBS2006-2测线与MCS97303测线位置与界面对比；AA'和BB'分别对应OBS2006-2测线和MCS97303测线
Fig. 2. Comparison of twice way time of MCS97303 and OBS2006
图 3 OBS5台站地震记录剖面及射线追踪
a.OBS5台站垂直分量综合地震记录剖面，折合速度为6.0 km/s；b.实际观测走时与理论计算走时拟合结果；c.速度模型与射线追踪
Fig. 3. OBS5 seismic recording section and ray tracing simulation
图 4 OBS2台站地震记录剖面及射线追踪
a.OBS2台站垂直分量综合地震记录剖面，折合速度为6.0 km/s；b.实际观测走时与理论计算走时拟合结果；c.速度模型与射线追踪
Fig. 4. OBS2 seismic recording section and ray tracing simulation
图 5 OBS11台站地震记录剖面及射线追踪
a.OBS11台站垂直分量综合地震记录剖面，折合速度为6.0 km/s；b.实际观测走时与理论计算走时拟合结果；c.速度模型与射线追踪
Fig. 5. OBS11 seismic recording section and ray tracing simulation
图 6 OBS2006-2测线最终速度结构及自由空间重力异常
a.OBS2006-2的重力异常；b.OBS2006-2最终速度模型；c.一维速度结构
Fig. 6. The final velocity model of OBS2006-2 and its corresponding free-air gravity anomaly
图 7 OBS2006-2测线所有台站PmP及Pn震相的走时模拟
a.PmP和Pn震相覆盖情况；b.PmP和Pn震相走时模拟
Fig. 7. Ray tracing simulation for all PmP and Pn seismic phases
图 8 OBS2006-2测线速度结构的检测板测试
a.输入模型(加入±0.25 km/s的速度扰动)；b.输出模型(加入了射线覆盖)
Fig. 8. Checker board test for the final velocity model of OBS2006-2
图 9 SO49-25多道地震剖面(据丁巍伟等, 2009, SO49-25测线位置见图 1)
Fig. 9. Multi-channel seismic profile of SO49-25
表 1 OBS2006-2测线12个台站所有震相的射线追踪结果
Table 1. Ray tracing for all seismic phases recorded by total 12 OBSs
震相 拾取点数 追踪点数 百分比(%) RMS(s) χ2 Pdw 344 342 98.5 0.046 0.862 Pg 929 892 96.0 0.087 3.044 PmP 340 337 99.2 0.077 2.633 Pn 184 178 96.7 0.096 3.075 总计 1796 1739 96.8 0.079 2.529 注：该统计结果不包括OBS10号台站的Pn震相(41个点)；RMS为计算走时和观测走时残差的均方根值；χ2为卡方分布检测值，当χ2接近于1时拟合程度最好. 
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