Geophysical evidence for a serpentine mud volcano in the relict slow-spreading center of the South China Sea
Geophysical evidence for a serpentine mud volcano in the relict slow-spreading center of the South China Sea
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DOI：
10.1016/j.tecto.2022.229665
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发表时间：
2023-01
期刊：
影响因子：
2.9
通讯作者：
Yanlin Wang;P. Yan;Junhui Yu;Guanghong Tu;Jun Wang;Y. Qiu;Changliang Chen
中科院分区：
文献类型：
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作者：
Yanlin Wang;P. Yan;Junhui Yu;Guanghong Tu;Jun Wang;Y. Qiu;Changliang Chen
扩张中心形成的海山为玄武岩，重力峰突出，密度高。而位于南中国海残留缓慢扩张中心的龙门海山，高1.5公里，∼20公里，显示出一个弱重力峰，重力变化幅度小(∼19mgAl)。为了揭示异常海山的地质成因，根据反射和折射地震资料的约束条件，对龙门海山进行了叠前深度偏移成像和三维重力正演模拟。结果表明，龙门海山的容重和地震速度分别低至∼2.1g/cm~3和1.6~3.2m/S，相当于弱固结沉积物的容重和地震速度。此外，在Lm海山之下还观察到了泥火山系统的典型结构元素，如反射空白区、向上弯曲和向下弯曲反射。由于LM海山以下地壳较薄(1.6公里)，多次清晰的不连续反射揭示了地壳的高度断裂，海水很容易渗入薄的地壳，导致上地幔岩石高度蛇纹岩。与体积密度低、地震速度低的蛇纹状泥火山相结合，龙门海山很可能是一座位于缓慢扩张的大洋中部的蛇纹状泥火山。由于已发现的蛇纹石泥火山都局限于收敛的板块边缘，这一研究将对蛇纹石泥火山作用的机制有一个新的认识。
The seamounts formed in spreading centers are of basaltic with prominent gravity peaks and high density. But Longmen Seamount, 1.5 km high and ∼ 20 km wide, located in the relict slow-spreading center of the South China Sea, shows a weak gravity peak with a low-amplitude gravity variation (∼19 mGal). To reveal the geological origin of the unusual seamount, we conducted pre-stack depth-migrated image and 3D forward gravity modeling for Longmen Seamount based on the constraint from reflection and refraction seismic data. Our results show that bulk density and seismic velocity of Longmen Seamount is as low as ∼2.1 g/cm3, and 1.6–3.2 km/s, respectively, equivalent to that of poorly consolidated sediments. Furthermore, typical structural elements of the mud volcano system, such as a reflection blank zone, up-bent and downward bending reflections, are observed beneath LM Seamount. For the crust beneath LM Seamount is thin (1.6 km) and highly faulted revealed by numerous clear discontinuous reflections, it is easy for seawater to infiltrate through the thin crust and leads to a high degree of serpentinization of the upper mantle rocks. Combined with serpentine mud volcanoes featuring low bulk density and low seismic velocity, Longmen Seamount is most probably a serpentine mud volcano in a slow-spreading mid-ocean center. For the discovered serpentine mud volcanoes are all restricted to the convergent plate margins, this study will lead to a new understanding of the mechanism of serpentine mud volcanism.