胡立天^1,2,3,,
张培震^1,2,
郝天珧^3,4,5,
徐亚³
1. 中山大学地球科学与工程学院, 广东省地球动力作用与地质灾害重点实验室, 广州 510275
2. 南方海洋科学与工程广东省实验室(珠海), 广东珠海 519082
3. 中国科学院地质与地球物理研究所, 中国科学院油气资源研究重点实验室, 北京 100029
4. 海底科学重点实验室, 自然资源部, 杭州 310012
5. 中国科学院大学, 北京 100049

基金项目: 广东省引进人才创新创业团队（2016ZT06N331）-环南海地质过程与灾害创新团队，中国科学院边缘海与大洋地质重点实验室资助（OMG18-04），中国科学院项目（Y4SL021001），国家自然科学基金（U1505232，41606085，41774049，91628301，91858212，41906049），"全球变化与海气相互作用"专项国际合作项目（GASI-GEOGE-01）联合资助


详细信息

作者简介: 胡立天, 男, 1989年生, 山东东营人, 2017年毕业于中国科学院地质与地球物理研究所, 现为中山大学博士后, 主要从事重力处理与解释方面的研究.E-mail:327319996@qq.com

中图分类号: P718

收稿日期:2018-09-18
修回日期:2018-11-03
上线日期:2019-09-05

The lithospheric structure in the Southwest Subbasin, South China Sea and its geological implication

HU LiTian^1,2,3,,
ZHANG PeiZhen^1,2,
HAO TianYao^3,4,5,
XU Ya³
1. Guangdong Provincial Key Laboratory of Geodynamics and Geohazards, School of Earth Sciences and Engineering, Sun Yat-Sen University, Guangzhou 510275, China
2. Southern Marine Science and Engineering Guangdong Laboratory(Zhuhai), Zhuhai Guangdong 519082, China
3. Key Laboratory of Petroleum Resources Research, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
4. Key Laboratory of Submarine Geosciences, Ministry of Natural Resources, Hangzhou 310012, China
5. University of Chinese Academy of Sciences, Beijing 100049, China


MSC: P718

--> Received Date: 18 September 2018
Revised Date: 03 November 2018
Available Online: 05 September 2019


摘要
摘要:南海地区岩石圈资料稀少，阻碍了其形成演化过程的研究.为此，本次研究结合大地热流、空间重力异常、高程、大地水准面和地震数据，在南海西南次海盆反演了两条2.5维岩石圈剖面.本次计算基于三种假设：岩石圈地幔的密度取决于岩石温度；研究区岩石圈处于热稳定状态；研究区处于重力均衡状态.在剖面A-E中，岩石圈底界面从珠江口盆地的105 km迅速抬升到西沙海槽处的50 km，在西沙海槽、西沙-中沙群岛和西南次海盆变化不大，为50~60 km.在剖面F-I中，岩石圈底界面从西沙群岛-中建地块处的88 km向海盆逐渐抬升，在西南次海盆处为46~50 km，到郑和隆起再逐渐变深至64 km.我们比较了西南次海盆岩石圈的冷却模型和热稳定模型，根据冷却模型由水深和热流数据所推断的西南次海盆年龄比实际年龄差很多，说明冷却模型不适用于西南次海盆.通过对比剖面A-E和剖面F-I，说明了剖面A-E经历了更长时间的拉伸，证明南海西南次海盆在形成演化过程中是从北东向南西逐步打开的渐进式扩张.最后，我们综合分析西南次海盆及其大陆边缘的岩石圈结构、减薄陆壳区范围、碳酸盐台地的分布、下地壳韧性流动、流变结构和沉积层特征等多方面资料，认为西南次海盆在形成演化过程中岩石圈地幔首先破裂而地壳后破裂，属于type Ⅱ型非火山型大陆边缘.
关键词: 西南次海盆/
岩石圈结构/
渐进式扩张/
type Ⅱ型大陆边缘

Abstract:The South China Sea is the largest marginal sea in the west pacific and an important place for the geodynamic research. However,compared with crustal structure,there is little data about the lithospheric structure,which is essential to study continental breakup,seafloor spreading and marginal sea dynamics. So we present two 2.5D lithospheric profiles across the Southwest Subbasin to study its evolution process.
The lithospheric model is based on the free-air gravity,geoid,elevation,heat flow and seismic data,and under three assumptions:the density of the lithosphere mantle dependent on the temperature,thermal steady-state and local isostasy. The model is divided into a number of rectangle bodies with different material properties:density,thermal conductivity and heat production. We establish the crustal model mainly based on the seismic results and calculate initial lithospheric temperature distribution and lithosphere asthenosphere boundary (LAB) by solving the steady-state heat transport equation. Forward results of the initial model are compared with the measured values and the geometry of the lithosphere is tuned until a best-fitting lithospheric structure is found. As the formulas of every geophysical data are not the same,using diverse data can reduce the uncertainty of the final model and obtain a reliable result.
In the profile A-E,the base of the lithosphere varies quickly from 105 km beneath the Pearl River Mouth Basin to 50 km beneath the Xisha Trough. It is 50~60 km deep beneath Xisha Trough,Xisha-Zhongsha Islands and Southwest Subbasin. The shallowest region along the studied profile is 50km beneath the spreading center in the Southwest Subbasin. In the profile F-I,the LAB rises from 88 km beneath Xisha Islands-Zhongjian Massif to 46~50 km beneath the Southwest Subbasin,then deepens to 64km beneath Zhenghe Massif.
The resent deep tow magnetic surveys and IODP results imply the age of the subbasin in the studied profile is 22~16 Ma. According to this age,the heat flow and bathymetry values calculated from the common-used cooling model are 101~114 mW·m^-2 and 3.9~4.1 km,respectively,differ much with the measured values,which are 75~105 mW·m^-2 and 4.3~4.8 km after sediment correction,implying a much older lithosphere. Therefore,the cooling model isn't suitable in the Southwest Subbasin. By comparing the profile A-E with F-I,we conclude the profile A-E stretched for a longer time,proving the Southwest Subbasin experienced propagated rifting from northeast to southwest. Finally,according to the lithospheric structure,the range of thin crust,the distribution of carbonate platform,the lower crust ductile flow,rheological structure and sedimentary characteristics,we argue the Southwest Subbasin shows a two-stage extension. The lithospheric mantle broke up first while the continental crust was still necking during its evolution. Finally,the crust broke up and the seafloor spreading started. It shows a type Ⅱ non-volcanic margin.
Key words:Southwest Subbasin/
Lithospheric structure/
Propagated rifting/
Type II margin



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