南海西南次海盆扩张期热液循环与洋壳增生的数值模拟  

Numerical simulation of hydrothermal circulation and oceanic crust accretion during the expansion period of the southwestern sub-basin of the South China Sea

作　　者：张慧慧[1,3] 许鹤华 姚永坚[2,4] 邵佳 纪顺奎 Zhang Huihui;Xu Hehua;Yao Yongjian;Shao Jia;Ji Shunkui(Key Laboratory of Ocean and Marginal Sea Geology,South China Sea Institute of Oceanology,Innovation Academy of South China Sea Ecology and Environmental Engineering,Chinese Academy of Sciences,Guangzhou,Guangdong 511458,China;Southern Marine Science and Engineering Guangdong Laboratory(Guangzhou),Guangzhou,Guangdong 511458,China;University of Chinese Academy of Sciences,Beijing 100049,China;Key Laboratory of Submarine Mineral Resources,Ministry of Natural Resources,Guangzhou Marine Geological Survey,Guangzhou,Guangdong 510760,China;Department of Marine Science and Engineering,Southern University of Science and Technology,Shenzhen,Guangdong 518055,China;China University of Mining and Technology,Beijing 100083,China)

机构地区：[1]中国科学院边缘海与大洋地质重点实验室,南海海洋研究所,南海生态环境工程创新研究院,广东广州510301 [2]南方海洋科学与工程广东省实验室(广州),广东广州511458 [3]中国科学院大学,北京100049 [4]广州海洋地质调查局自然资源部海底矿产资源重点实验室,广东广州510760 [5]南方科技大学海洋科学与工程系,广东深圳518055 [6]中国矿业大学,北京100083

出　　处：《地质学报》2022年第8期2927-2941,共15页Acta Geologica Sinica

基　　金：NSFC-广东联合基金项目(编号U20A20100);中国科学院战略性先导科技专项(A类)(编号XDA13010303)联合资助的成果。

摘　　要：洋壳厚度受多方面因素的影响,前人大多关注地幔温度、地幔源成分等岩石圈深部因素,很少关注岩石圈浅层的热液循环对洋壳厚度的影响。利用基于有限元的数值模拟手段,对扩张期不同背景(洋中脊、拆离断层)、不同扩张速率的热液循环与洋壳增生的关系进行研究。结果表明:洋壳增生达到稳定前,热液循环导致理论洋壳厚度发生阶段性减薄,减薄量随时间改变,并且推迟了上地幔中熔融体出现的时间;当洋壳增生达到稳定后,热液循环下产生的理论洋壳厚度反而比无热液循环的更厚。结合洋壳增生过程中对流热通量的变化分析,在洋壳增生前期的上地幔温度低,驱动热液循环的热源小,产生的对流热通量相对较小且不稳定,热液循环缓慢冷却上地幔顶部的温度,进而推迟上地幔初始熔融的时间,减弱上地幔的熔融,并造成一定时间阶段内的生成理论洋壳比正常理论洋壳厚度更薄;当洋壳增生达到稳定后,对流热通量达到最大并稳定,热液循环持续快速的冷却上地幔顶部温度,导致上地幔深部的热向上地幔顶部补给,反而增大了上地幔顶部的温度和熔融量,进而增大了理论洋壳厚度。随着扩张速率的增大,理论洋壳厚度增大,对流热通量增大,热液循环导致的洋壳阶段性减薄的最大减薄量也增大,阶段性减薄的时间缩短。结合南海西南次海盆的洋壳结构特征分析:两条横跨南海西南次海盆的地震剖面显示,海盆内存在异常薄的洋壳区域,并且两条地震剖面的最薄洋壳厚度相差0.85 km,推测海盆内异常薄洋壳和不同扩张时期的最薄洋壳厚度差异受到扩张期热液循环阶段性减薄洋壳作用的影响。The thickness of the oceanic crust is affected by many factors.Most of the predecessors have paid attention to the deep factors of the lithosphere such as mantle temperature and mantle source composition,and rarely to the influence of hydrothermal circulation in the shallow lithosphere on the thickness of the oceanic crust.The relationship between hydrothermal circulation and oceanic crust accretion under different backgrounds of expansion period(mid-ocean ridge,disassembly faults)and different expansion rates was studied by finite element-based numerical simulations.The results show that before the oceanic crust accretion reaches stability,the hydrothermal circulation leads to a phased thinning of the theoretical oceanic crust thickness.The thinning amount changes with time,and delays the occurrence of melt in the upper mantle;when the oceanic crust accretion reaches stability,the theoretical oceanic crust thickness produced under the hydrothermal circulation is thicker than that with no hydrothermal circulation.Combined with the analysis of the change of convective heat flux in the process of oceanic crust accretion,the upper mantle temperature in the early stage of oceanic crust accretion is low,the heat source driving the hydrothermal circulation is small,the convective heat flux generated is relatively small and unstable,and the hydrothermal circulation slowly cools the temperature of the top of the upper mantle,thereby delaying the initial melting time of the upper mantle,weakening the melting of the upper mantle,and causing the formation of the theoretical ocean crust thinner than the normal theoretical ocean crust within a certain time period.When the oceanic crust accretion reaches stability,the convective heat flux reaches the maximum and stable.The hydrothermal circulation continues to cool the temperature of the top of the upper mantle continuously and rapidly,resulting in the heat of the deep part of the upper mantle being replenished upward to the top of the mantle,which in turn increases the temper

关 键 词：热液循环 洋壳增生 洋壳厚度 数值模拟 南海西南次海盆 扩张速率 地幔熔融 

分 类 号：P736[天文地球—海洋地质]