南海西沙海域东岛北峡谷体系形态、演化及其成因机制

Morphology,evolution and genetic mechanism of the Dongdaobei canyon system in the Xisha sea area of South China Sea

海底峡谷是全球大陆边缘分布较广泛的地貌单元,是地形地貌、深水沉积和海洋地质灾害领域研究的重要内容。本研究基于高分辨率多波束测深和二维地震资料,以南海西沙东部海域为研究区,深入剖析了东岛北海底峡谷体系。东岛北海底峡谷分布在水深1000~3150 m范围,长度137 km,下切深度70~400 m,表现出西高东低的地形特征,总体由一条主峡谷和4条分支峡谷构成。峡谷上游有东岛西北部Ⅰ区和永兴海台东部Ⅱ区的沉积物供给,峡谷中游加入了东岛东北部Ⅲ区供给的沉积物。3个物源区的沉积物供应以线状的峡谷、水道和面状的块体流沉积类型为主。主峡谷北坡周缘分布有大量的海底麻坑,侧壁呈阶梯状不断后撤垮塌,因重力驱动作用和水流侵蚀,使峡谷壁外缘发育呈不规则小型“枝杈”状水道;主峡谷南岸因浊流作用,发育沉积物波。NE走向主断裂,控制着主干峡谷NE方向延伸,而峡谷南岸分布海山和海丘地形、岩浆底辟,影响主峡谷各分段的转向;同时峡谷和周缘下部地层发育的断层,控制峡谷侧壁向谷底呈阶梯状下降。

Dendritic canyon system is the most widely distributed type of submarine canyon on the global continental margin.It is important in understanding the field of deep-water sedimentation and marine geological disasters.Based on high resolution multi-beam sounding and two-dimensional seismic profile data,taking the Xisha sea area of the South China Sea as the study area,the dendritic canyon system in the north of Dongdao is studied.The Dongdaobei submarine canyon is distributed in the range of 1000~3150 m water depth,which is generally composed of one main canyon and four branch canyons.The sediment supply at the upstream of the canyon comes from the northwest of Dongdao(area I)and the east of the Yongxing Plateau(area II).Sediments from zone III in the northeast of Dongdao are injected into the middle reaches of the canyon.Sediments are transported to deep water through canyon system and mass-transport deposits system.A large number of submarine pockmarks are distributed around the north slope of the main canyon.The side wall of the canyon collapsed backward in a ladder shape.Sediment waves are developed on the south slope of the main Canyon,which are affected and transformed by tides and waves.The main canyon with overall SW-NE strike is controlled by the strike of the main fault,while seamounts are distributed on the south bank of the canyon,controlling the turning of each section of the main canyon.At the same time,the faults developed in the lower stratum of the canyon and its periphery control the side wall of the canyon to descend in a ladder shape to the valley bottom.