Origin of High Velocity Layer in Lower Crust of Southwestern Nansha Block
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摘要: 高速层成因的争议限制了对南海深部结构、构造演化以及南海完整演化历史的认识.运用Oasis Montaj软件对穿越南沙西南部的最新地震测线进行重震联合反演，分析莫霍面起伏、地壳厚度及高速层的分布，计算全壳伸展因子和现今高速层的温度、识别火山时代，并探讨高速层的成因.南薇西盆地和礼乐盆地区伸展因子为1.5~4.0，未达到蛇纹石发育条件；南沙海槽区伸展因子大，最大为11.2，海水可通过深大断裂下渗与橄榄岩反应生成蛇纹石，高速层处温度低于蛇纹石稳定温度；通过地震剖面确定火山在南海停止扩张之后形成.研究结果表明，南沙西南部高速层按成因分为两类，南薇西盆地和礼乐盆地区为南海扩张停止后火山喷发残余的岩浆，而南沙海槽盆地区为早期橄榄岩的蛇纹石化与南海停止扩张后岩浆的混合体.Abstract: The controversy over the genesis of the high velocity layer limits the understanding of the deep structure, tectonic evolution and complete evolution of the South China Sea.In order to understand the origin of high velocity layer in the lower crust of the southwestern Nansha block, the joint inversion of gravity and seismic data was performed by using Oasis Montaj software in this study.The seismic profile is across the southwestern Nansha Block.The Moho undulation, crustal thickness and the distribution of the high velocity layer are recorded.The regional extension factor and the temperature at the high layer depth are calculated, and the age of volcanic eruption is determined by seismic features.The whole crustal stretching factor of the Nanweixi Basin and the Liyue Basin area is 1.5-4.0, which is impossible to develop serpentine.The whole crustal stretching factor is up to 11.2 in the Nansha trough.The serpentine is formed by the reaction of peridotite and sea water which enters the mantle through the fractures.The temperature near the high velocity layer is lower than the stable temperature of the serpentine.The volcanic eruption was found in the same place in the seismic profile, and the volcano formed after the cessation of the spreading of the South China Sea.The results indicate that the high velocity layer of the southwestern Nansha Block can be divided into two types.The high velocity layer in the Nanweixi and the Liyue basins is formed by volcanic eruption after the cessation of the spreading of the South China Sea; and the other section in the Nansha trough basin is a mixture of serpentinized peridotite and late magma.
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Key words:
- Nansha /
- deep crustal structure /
- high velocity layer /
- volcano /
- stretch factor /
- serpentinization /
- marine geology
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图 2 南沙西南L1地震剖面
VE为剖面纵横比，下同
Fig. 2. Multichannel seismic profile L1 across the southwestern Nansha Block
图 3 南沙西南部深部异常反射
位置见图 2
Fig. 3. Fragmentary but distinct reflector elements of the deeper parts of southwestern Nansha
图 4 基于Oasis Montaj软件的L1测线重震联合反演结果
黑色实线为测量重力异常数据；绿色实线为模拟重力异常数据；红色实线为模拟误差.位置见图 1
Fig. 4. Results of gravity modeling based on Oasis Montaj software
图 6 南沙西南部L1测线全壳伸展因子
Fig. 6. Curve showing the whole crustal stretching factor of L1 profile in southwestern Nansha
图 7 南沙西南部火山形态与时代判别图
c和d为时间域剖面；图c据Vijayan et al.(2013)；位置见图 1
Fig. 7. Volcano morphology and age of southwestern Nansha
图 9 南沙地块深部结构与高速层成因示意图
据Reston(2009)和Dong et al.(2016)修编
Fig. 9. Dynamic evolution diagram of deep crustal structure and high velocity layer of the Nansha Block
表 1 重力模拟参考密度值
Table 1. Density parameters used in the gravity modeling
分层 密度(g/cm3) 海水 1.03 沉积层 2.40 陆壳 2.70 洋壳 2.83 高速层 2.97 地幔 3.20 注：据郝天珧等(2011)、 McIntosh et al.(2014) 和高金尉(2015).
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