南海西南次海盆疑似蛇纹石泥火山研究-以龙门海山为例

Longmen Seamount is located in the median valley of the southwest sub-basin of the South China Sea (SWSB). This seamount was considered as the late Cenozoic basaltic seamount. This seamount with a size of 20*20 *1.5 km, is characterized by chaotic seismic reflections, weak magnetic anomalies and negative peak on free-air gravity anomaly map. Preliminary research shows that the Longmen seamount is a low-density seamount, which is up to 8 km thick. These features and phenomena are diametrically opposed to those normal basaltic seamounts, which should be characterized by a strong positive peak on free-air gravity anomaly map and high density. Meanwhile, bottom seawater samples show significant high methane concentrations, lots of iron nodules, calcite, pyrite and deep-water benthic organisms are found in the seamount, and no basalt found. These features and phenomena are similar to those serpentine mud volcanoes. We speculate that Longmen Seamount is a large active serpentine mud volcano. To confirm the conjectures, it is planned to carry out the NSFC-supported cruise surveys to collect sub-bottom profile and ocean-bottom seismic, and to dredge samples from the seabed. It is designed to incorporate new technologies, such as seismic velocity imaging with coincident reflection and refraction, density imaging, and deep-seated hydrothermal solution analysis, to characterize the deep structures, source and geodynamic origin of the Longmen Seamount. If it is confirmed that Longmen Seamount is an active serpentine mud volcano related to methane gas leakage, this indicates the SWSB has been created by tectonic dominated spreading. This study will provide new clues for crustal properties of the SWSB. Meanwhile, known serpentine mud volcanoes in the world mainly develop in plate subduction zone. If the conjecture is true, it indicates that the distribution of serpentine mud volcanoes will be extended from plate subduction zone to mid-oceanic ridge.

南海西南次海盆的形成是岩浆主导还是构造拉张主导认识不清。地球物理调查显示，龙门海山是西南次海盆中脊区的大型海山（宽20km、高1.5km），其自由空间重力异常表现为大规模、高幅值波谷异常，对应于巨厚的低密度物质，这与正常玄武岩海山应具有的强波峰重力异常和高密度特征相反，暗示着尚未认识的新型海山。该海山海底沉积物富含铁结核、方解石、黄铁矿和生物碎屑，未见玄武岩，底层海水溶解甲烷浓度高。这些特征与低密度蛇纹石泥火山的相似，推测龙门海山是正在活动的、地幔橄榄岩蛇纹岩化形成的蛇纹石泥火山。为验证这个推测，拟以已有地球物理和沉积取样资料为基础，在龙门海山补充浅地层剖面和海底地震观测，采集海底沉积物和海水样品；通过应用反射/折射地震联合速度成像、密度成像和地球化学技术，深化对该海山深部结构、物质来源和地质成因的认识。若得到证实，将是南海首次发现的蛇纹石泥火山，说明西南次海盆的形成为构造拉张主导型。

南海西南海盆的形成是岩浆主导还是构造拉张主导认识不清。龙门海山是位于南海西南海盆洋中脊裂谷区的大型海山（直径达20 km，高约1.5 km），地球物理资料显示，龙门海山的自由空气重力异常的相对变化微弱（~19mGal），明显低于周围相近规模的玄武岩海山，这与全球现已发现的洋中脊海山所具有的显著高重力异常特征截然相反，暗示着尚未认识的新型海山。为揭示龙门海山地质成因之谜，解决西南海盆的形成是岩浆主导还是构造拉张主导的问题，本项目横跨龙门海山采集了两条重力和多波束数据，通过电视抓斗采集了海底岩石样品；通过多道地震叠前深度偏移成像和三维重力模拟，研究了龙门海山的构造形变、地震速度结构、密度结构及其空间变化特征；通过研究龙门海山海底岩石的矿物成份、组成特征，分析岩石矿物与地幔橄榄岩蛇纹岩化的关联性；通过龙门海山的构造形变、地震速度结构、密度物结构、矿物成份和组成特征，结合周围海区地壳结构、断裂特征和地质背景，研究龙门海山地质成因。研究结果表明，龙门海山具有泥火山形貌，其地震速度（1.6–3.2 km/s）和体密度（ ~2.1 g/cm3）偏低，且与未固结深海沉积物的相当，海山坡度较低，40%的区域海底坡度低于10度。由于，龙门海山位于慢速扩张的残留中脊裂谷区，地壳薄（~1.6 km）；北西向转换断裂和广泛发育的北东向伸展断裂在此交汇，地壳破碎；海水易于渗入地壳至地幔，导致上地幔岩石的高度蛇纹岩化和蛇纹石泥流的形成。基于上述结果，推测龙门海山是蛇纹石泥流喷发堆积形成的蛇纹石泥火山。西南海盆洋中脊蛇纹石泥火山的出现证实了它的形成为构造拉张主导型。.全球现已发现的蛇纹石泥火山都位于俯冲增生带，而本项目发现的蛇纹石泥火山位于慢速扩张中脊裂谷内，不仅代表了一种新型蛇纹石泥火山，扩张域裂谷型蛇纹石泥火山，还代表了新的海山形成动力机制。新型泥火山的发现对全球其它慢速扩张中心蛇纹石泥火山的发现具有重要意义。