A Giant Submarine Landslide and Its Triggering Mechanisms on the Nansha Trough Margin, South China Sea
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摘要: 大型海底滑坡的研究对认识海底斜坡的稳定性具有重要意义.利用最新的高精度多波束数据和重处理的二维地震资料，识别了南海南沙海槽一处大型海底滑坡，描述了其发育特征，探讨了其可能的形成原因.该滑坡体覆盖面积达6 300 km2，横向最宽50 km，延伸最远140 km.上部源头区外形呈半环形，滑坡后壁的高度落差200~350 m，平均坡度0.7°，发育基底剪切面和掀斜断块.中部滑移区呈拱形，分布于1 600~2 400 m水深段，平均坡度1°~3°，发育基底侵蚀面和大量残余块体.下部堆积区呈扇形，分布于2 400~2 800 m水深段，平均坡度0.1°~1.0°，发育大型碎屑流朵体和逃逸块体.研究表明不断隆升的背斜脊对高供给率沉积物的阻挡是海底斜坡失稳的内在条件，而高通量流体的聚集以及天然气水合物的分解使其变得更加不稳定.Abstract: The research on large-scale submarine landslides is of great significance to understanding the stability of submarine slopes. In this paper, a giant submarine landslide on the Nansha trough margin, South China Sea was identified by using the latest high-precision multibeam bathymetry and reprocessed two-dimensional seismic data, and its development characteristics were described. The possible triggering mechanism was discussed. The landslide covers an area of 6 300 km2, spanning 50 km and extending 140 km at most. The upper semicircular headwall domain has developed a basal shear surface and tilted fault blocks. The headwall scarp is 200-350 m high with an average slope of 0.7°. The middle arch-shaped translational domain has a erosional unconformity and voluminous remnant blocks. It is distributed in the depth section of 1 600-2 400 m with an average slope of 1°-3°. The lower fan-shaped toe domain has a detrital lobe deposit and some out-runner blocks. It is distributed in the water depth section of 2 400-2 800 m with an average slope of 0.1°-1.0°. The study indicates that the barrier of the rising anticline ridge to the sediment with high supply rate is an internal condition, but high flux upward fluids and gas hydrates dissociation may be two key preconditioning factors bringing anticlines closer to failure.
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Key words:
- submarine landslide /
- anticlinal ridge /
- gas hydrate dissociation /
- Nansha trough /
- marine geology
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图 1 研究区地理位置（a）、研究区区域地质简图（b）以及横跨南沙海槽至南海东南陆架的A-A’地质剖面（c）
图a来自http://www.mnr.gov.cn/sj/sjfw/；图b底图据杨胜雄等（2015），剖面位置见图 1a；图c修改自Warren et al.（2010），剖面位置见图 1b
Fig. 1. Regional location map for the study area (a), geological map for the study area (b) and regional 2D geological cross-section of "Line A-A'" through Nansha trough and southeastern shelf of South China Sea (c)
图 4 研究区与海底滑坡有关的地形剖面
图内坡度数据中，蓝色为陡坡最大坡角值，红色为缓坡平均值，位置见图 3
Fig. 4. Topographic profile graghs related to submarine landslide in the study area
图 9 受滑移体影响的3类背斜发育特征
图a、c和e的位置见图 5a；图b、d和f的位置见图 2；其地震剖面来自Morley（2009）并做重新解释
Fig. 9. The three growth patterns of anticlines affected by slides
图 10 背斜生长演化与海底滑坡发生的模式
a. 背斜形成；b. 软弱层形成；c背斜被破坏；d. 软弱层扩大；e滑坡体形成；图中实线BSR为同时期的BSR，虚线BSR为古BSR
Fig. 10. A schematic model of the growth of anticlines ridge and the formation of submarine landslide
表 1 陆架边缘三角洲的演化阶段及其增长样式
Table 1. The evolution stage and its growth styles of shelf-edge deltas in the study area.
期次 定量参数 陆架边缘三角洲类型 相对海平面变化 加积距离（m） 进积距离（km） 轨迹角度（°） 轨迹类型 增长样式 3 129.46 0.98 7.52 高角度上升型 加积型 快速上升 2 93.78 2.42 2.22 低角度上升型 进积与加积混合型 轻微上升 1 37.25 4.29 0.49 平缓或轻微上升型 快速进积型 降低 注：依据重处理地震资料得到的速度谱计算出的层速度和平均速度，进而通过深度（D）和时间（T）散点图得到拟合公式，D=736.8×T+301.5×T2—20.0×T3（考虑水深对速度的影响），其中D的单位是m，T的单位是s. 
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