南海北部陆缘裂后异常沉降与下地壳流的关系的动力学数值模拟研究

There is anomalous tectonic subsidence in the tension of the north continental margin of South China Sea. Lower crust flow theory is often used to explain the phenomenon of depth-dependent stretching, but the lower crust flow theory is still controversial. Although there are some research results recently, most of which are quantified or half-quantified or on the base of the kinematics model. The aim of this project is to quantitatively simulate the problem that is the relationship between lower crustal flow and anomalous post-rift subsidence along the northern South China Sea caused by the differences between upper and lower crust. The project selects continental marginal along the northern South China Sea , the continental blocks across the South China, continent to ocean transition zone to ocean basin of South China Sea(including Pearl River Mouth Basin and Beibuwan Basin and depression belt) as the study objects, then build the dynamical numerical model combined the earth' s surface process (mountain erosion,basin deposition process) and rifting process, through the numerical analysis,we study whether the continental marginal along the northern South China Sea is possible to have the condition of lower crustal flow during the rifting process in new era. If possible,what is the time or flow direction of lower crustal flow? The purpose of this work is to explore the cause of formation of passive continental marginal sea and the evolution of the lithosphere , it can provide important information about the process of from continental splitting to seafloor spreading and reliable basis to geological structure, seismic hazard assessment and resource development and utilization of oil and gas and hydrate.

南海北部陆缘拉张演化过程中出现了裂后异常构造沉降现象，下地壳流经常被用来解释地壳随深度的差异拉张现象，但下地壳是否发育下地壳流一直是目前人们所关注的焦点。近年来已有一些研究成果，但前人的工作是偏于定性、半定量或者为运动学模型的基础上的。本项目针对南海北部陆缘是否发育下地壳流进行定量的动力学数值模拟研究。选择南海北部陆缘，横穿华南大陆地块、大陆向海洋过渡带直到南海海盆（包括珠江口盆地和北部湾盆地及其凹陷带）为研究对象，建立结合地表过程（包括地表侵蚀、盆地沉积过程）和张裂过程的动力学数值模型，通过数值模拟分析，研究南海北部大陆边缘在新生代构造演化过程中是否具备发育下地壳流的条件。如果发育下地壳流，发生的时间和流动的方向如何？从而探索被动大陆边缘海成因和岩石圈演化的动力学机制，揭示南海北部大陆张裂到南海海底扩张的重要信息。并为地质构造、地震危险性评估和油气与水合物资源的开发利用提供可靠的依

下地壳流经常被用来解释地壳随深度的差异拉张现象，但下地壳是否发育下地壳流,一直是人们所关注的焦点。通过本项目采用粘-弹性、弹-塑性和蠕变三种模型模拟的研究，对这一问题有了初步的认识。得到了32Ma以来，地形受沉降作用、地壳拉张减薄和地幔上涌作用的演化过程，特别是下地壳在增厚的两翼受挤压变形的塑性行为的明显变化,说明了下地壳流动的局部行为,表明裂后异常沉降现象不能完全取决于下地壳流失。为了研究南海特别是北部陆缘裂后异常沉降现象,我们根据原来积累和新增的热流数据,进行了南海岩石圈三维热结构数值模拟工作。根据前人多波束测深、重磁数据分析和地震Vs波反演等结果,得到水深、基底、莫霍面和地震岩石圈底界数据,建立了三维稳态模型，通过试错法得到了南海三维“热”岩石圈厚度。通过对比, 发现我们计算的“热”岩石圈厚度在海盆区与洋陆转换带,比前人通过地震Vs波反演所得到的岩石圈厚度，平均埋藏浅10-20km,说明裂后在洋中脊和个别热点依然处于热状态。与此同时,我们也定量计算了南海及其北部陆缘主要地震剖面的重力均衡情况和密度分布，发现重量增加，地壳被抬升。通过以上三个方面的研究，认为南海特别是南海北部陆缘裂后异常沉降现象发生的原因,可能与地壳拉张减薄过程中的下地壳流失、裂后重力不均衡和裂后热冷却过程有关。