Deep Structures and Lithospheric Breakup Processes at Northern Continent-Ocean Transition Zone of the South China Sea
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摘要: 洋-陆过渡带是理解大陆岩石圈破裂和海底初始扩张的关键位置，但是在南海北部地区仍然存在关于相关地质过程的诸多疑问.通过近年开展的国际大洋发现计划航次以及深部地质地球物理探测，取得以下4个方面的认识.（1）南海北部的洋-陆边界一般与自由空间重力异常的正-负值过渡位置对应，而更加准确地限定需要结合反射、折射地震资料.稳定大洋岩石圈生成与大陆岩石圈最终破裂之间的洋-陆过渡边界的位置比以往认为的还应往深海盆方向移动.（2）洋-陆过渡带代表了远端带构造作用减弱和岩浆作用逐渐增强的区域.陆坡地壳发育扩张后岩浆底侵、洋-陆过渡带发育同破裂期岩浆喷出结构和侵入反射体.（3）在中生代的古俯冲带弧前区域，新生代的断裂沿着早期的构造开始活动，岩石圈多处发生强烈的共轭韧性剪切作用.随着大陆岩石圈的进一步拉伸减薄，部分靠陆一侧的裂谷中心停止张裂，成为夭折裂谷，以台西南盆地南部凹陷、白云凹陷、西沙海槽为代表，而南海陆缘异常伸展和最终破裂的地方集中在南侧裂谷中心.夭折裂谷下亦发现地幔蛇纹石化，进一步反映了较弱的同破裂岩浆活动.（4）南海初始洋壳的增生沿着大陆边缘走向具有显著的变化，南海东北部洋-陆过渡带下伏地幔明显抬升和部分蛇纹石化，地震纵、横波速度以及折射波衰减特征都支持此观点，反映南海东北部是一个贫岩浆型大陆边缘.未来，南海北部洋-陆过渡带有望成为南海“莫霍钻”的理想备选钻探区.Abstract: Continent-ocean transition zone is a key position to understand the breakup of continental lithosphere and the initial seafloor spreading processes, but some questions about related geological processes still remain controversial in the northern margin of the South China Sea (SCS) today. Four new perspectives have been acquired by International Ocean Discovery Program (IODP) and deep geological and geophysical surveys in recent years. (1) Spatially, the continent-ocean boundary generally corresponds to positive-negative transition zone of the free-air gravity anomaly, but more accurate delimitation needs to be calibrated with seismic reflection and refraction data. True continent-ocean boundary between steady oceanic lithosphere and final breakup point of continental lithosphere should be located further toward the oceanic basin by about 20 km on average than previous definitions. (2) The continent-ocean transition zone represents a region with gradually weakened tectonism and strengthened magmatism in the distal margin, where magma underplated the lower crust of the continental slope after cessation of seafloor spreading. (3) In forearc areas of Mesozoic paleo-subduction zone, Cenozoic faulting inherited from early structures was reactivated to generate strong conjugated ductile shear deformation in the lithosphere. With further thinning of the lithosphere, rift center on the continental side stopped stretching and evolved into failed rift, represented by the southern sag of the Taixinan basin, the Baiyun sag and the Xisha trough. The hyper-extended crust and final continental breakup focused on the rift axis in the southern branch of the two conjugated rifts. Serpentinized mantle is found beneath failed rift, further indicating relatively weak syn-rifting magmatism. (4) The accretion pattern of initial oceanic crust in the SCS has significant variations along the marginal trend. Evidenced by seismic P-and S-wave velocities and attenuation characteristics, deep mantle upwelling and serpentinization, along with syn-rifting eruptive magma and intrusive reflectors occurred in the continent-ocean boundary of the northeastern SCS. This indicates a magma-poor northeastern continental margin. The continent-ocean boundary of the SCS could be an ideal candidate site of Moho drilling in the future with its elevated Moho and extremely thinned crust.
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Key words:
- South China Sea /
- continent-ocean transition /
- deep structure /
- continental breakup /
- seafloor spreading /
- seismic survey /
- tectonics
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图 1 南海北部洋‒陆边界位置
圆圈数字1~5为标记的不同洋‒陆边界，分别据Taylor and Hayes, 1983；Briais et al., 1993；McIntosh et al., 2014；Song et al., 2019；Wen et al., 2021
Fig. 1. The distribution of continent-ocean boundary in the northern SCS
图 2 南海东北部洋‒陆过渡带的深部折射、反射成像结构
a~c.OBS2016-2折射剖面，修改自Hou et al., 2019；Wan et al., 2019；V1-3代表下地壳高速异常体；d.T1反射剖面，修改自Wen et al., 2021；位置见图 1
Fig. 2. The deep seismic refraction/reflection structure in the northeastern continent-ocean boundary
图 3 南海北部远端带岩石圈变形及夭折裂谷演化机制
a.中‒上地壳的韧性剪切变形，修改自Peng et al., 2022；b.地震解释及P波速度投影，修改自Liu et al., 2021；c.夭折裂谷的同张裂竞争型演化模式，修改自Li et al., 2020；位置见图 1
Fig. 3. The deformation of continental lithosphere in the distal domain of the northern SCS and the evolution mechanism of failed rifts
图 4 南海北部洋壳初始扩张沿走向的变化示意图
a.南海东北部蛇纹岩地幔代表的初始洋壳，修改自Wen et al., 2021；b，c.南海中北部岩浆侵入构造代表的初始洋壳，分别修改自Ding et al., 2020；Zhang et al., 2021；位置见图 1
Fig. 4. The along-strike variation of initial oceanic crust in the northern SCS margin
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