南海共轭陆缘构造不对称性研究及其成因的数值模拟

Based on the multi-beam echo sounder, multichannel seismic data, gravity and magnetic field, borehole data and Ocean Bottom Seismometer (OBS) data, structural asymmetry of conjugate margins of the South China Sea (SCS) in both North-South direction and West-Easr direction, will be investigated. Numerical modelling will be used to discover the formation mechanism and the controlling factors of the asymmetric conjugate margins. The stratigraphic structure and the deep faults will be revealed by the explanation of multichannel seismic data. Joint inversion of gravity, magnetic and seismic data, combined with OBS data, will be carried out on the typical seismic sections to acquire the density and velocity structure of the crust, which are used to investigate the variation of crustal structure of the conjugate margins. Forward and inverse modeling based on the flexural cantilever model will be applied to the rifting-subsidence process. Rifted topography can be restored with the backstripping method incorprating the fault geomorphy, isostatic subsidence and geothermal perturbation. Two-dimension tectonic-stratigraphic model will be established with reference to typical seismic cross-sections, and used to predict the tectonic and heat flow history. The results are taken as the basis of kinematic constrast between the conjugate margins. Rheological and initial fault model for the rifted marginal lithosphere of the SCS will be established based on the study of structural asymmetry associated with petrophysical data. Non-Newtonian viscous flow equation will be solved by the finite element method, calculating the response of the lower crust and mantle lithosphere to the brittle faulting, and simulate the strain softing, which will probably explain the confusing observation that upper crustal brittle faulting can not fit the observed overall crustal thining. Intergrated data will be used finally to explain the dynamic process of the asymmetric conjugate margins formation and its controlling factors.

本项目综合利用多波束、重磁震、钻井和海底地震仪（OBS）等数据开展南海共轭陆缘的构造不对称性研究，包括南北向和东西向的差异，运用数值模拟方法探讨不对称陆缘的形成机制及控制因素。通过地球物理综合解释,揭示沉积地层结构和深部断裂发育及展布特征。对重点测线开展重磁震联合反演，结合OBS数据，获得地壳的的密度和速度结构，分析地壳深部的结构特征及差异。以挠曲悬臂梁模型正反演陆缘张裂-沉降过程,考虑断裂、均衡沉降及热扰动，利用回剥法恢复地层的裂陷期地形；参考典型地震剖面建立二维构造-地层模型，正演预测构造史和热流史，对比分析陆缘的运动学差异。在构造研究的基础上，结合岩石物性资料，建立南海张裂陆缘岩石圈的初始断裂和流变学模型，采用有限元方法求解非牛顿粘性流方程，计算下地壳和岩石圈地幔对断裂作用的响应，模拟应变软化过程，解释脆性断裂诱发地壳强烈减薄的机制，阐明不对称陆缘形成的动力学过程及其控制因素。

南海陆缘的不对称性研究对于深刻揭示陆缘演化的成因机制具有重要意义，因此日益成为研究的热点内容，但总体研究程度不高。因此，本项目综合利用重力、多道地震和海底地震仪（OBS）等数据开展南海共轭陆缘的不对称性研究，主要内容包括结构的差异和运动学特征的对比，既考虑南北差异也兼顾东西变化，利用数值模拟手段进一步探讨其动力学成因。地震剖面揭示的上地壳单剪变形存在东西差异，东部次海盆的北部陆缘相对南部发育了更加完整的陆架陆坡体系。重力反演地壳厚度结果表明，南海北部陆缘不同单元之间同样存在构造差异，沉积盆地内的上地壳减薄程度大于下地壳，而远端陆缘则相反。研究认为该现象受控于下地壳流流动方向的不同，岩浆活动是反映构造活动的良好指标，基于地震剖面研究了南海北部陆缘新生代岩浆岩的时空分布规律。结果表明，岩浆活动在海底扩张结束之后最为强烈，且伴生有热液活动，而裂陷期非常微弱，研究支持南海北部为贫岩浆型陆缘的观点。基于体积守恒的全吻合重建原理，将南海共轭陆缘恢复到晚白垩世未发生伸展时的形态，结果显示西部陆缘相比东部发生了更大程度的伸展。基于黏弹塑性模型模拟了上下地壳厚度比、拉张速率、沉积速率和下地壳流变强度等对南海陆缘结构的控制机理。结果表明，上地壳增厚使得岩石圈温度增高，强度降低；更快的拉张速率趋向于形成更加不对称性的陆缘；恒定沉积速率更容易发育分散式断裂而形成较宽陆缘，而变沉积速率可导致地壳较早发生破裂；下地壳流变强度对陆缘结构控制作用显著，可形成四种不同构造样式。结合南海地质与模拟结果，弱的下地壳模型可能控制了南海的张裂，但仍存在东强西弱的特征。本项目利用观测和模拟相结合的手段，综合研究了南海共轭陆缘的构造不对称性，探讨了不同陆缘结构的形成机制及控制因素，研究结果为深刻揭示被动大陆边缘演化规律提供了较好的研究案例，具有重要的科学意义。