青藏高原岩石圈各向异性的动力学内涵及成因约束: 来自组构分析-原位弹性波测量-流变学实验研究的启示

Anisotropy is one of the most important parameters in lithosphere dynamics. The anisotropy behavior observed in Tibetan Plateau is quite complicated and changeable. There is a lot of uncertainty and still a debate in the interpretation of lithosphere seismic anisotropy in Tibet and in the understanding of its formation and its dynamic implications since we lack the necessary data in rock fabric and elastic anisotropy of Tibetan lithosphere mantle and deep crust. The scientific problems have to be settled urgently are followed. What are the basic features of the rock fabric and its anisotropy? How dose melt distribute in rocks under strain and the distribution affect on lattice preferred orientation of minerals? How can we calculate the anisotropy formed by the co-action of melt preferred orientation (MPO) and LPO？What lithosphere dynamic processes could be inferred from the complicated anisotropy observed in Tibetan Plateau? As the research targets to obtain the quantitative data of rock fabric and its anisotropy from Tibetan lithosphere, the definite relationship between partial melting and deformation and the response of fabric to melt, we propose doing (1) rheological experiments, (2) LPO measurements by EBSD technique and anisotropy calculation based on the LPO data, (3) the measurement of elastic anisotropy of samples collected from Tibetan Plateau lithosphere in-situ. It is expected that the experiments and measurements could provide systematic data and essential constraints on the interpretation of the formation mechanism of the complicated anisotropy observed in Tibetan Plateau and the understanding of Tibetan lithosphere dynamic processes.

各向异性是揭示岩石圈动力学过程的重要参数。青藏高原岩石圈的各向异性非常复杂。由于缺乏其组构和弹性波各向异性的基础资料，致使在解释其各向异性测深数据，分析其成因和理解其动力学内涵方面依旧存在不确定性和争论。亟待回答的科学问题包括，青藏高原岩石圈组构和各向异性的基本特征？应力作用下熔体定向分布(MPO)规律及其对矿物晶格优选方向(LPO)的影响？如何计算MPO和LPO共同引起岩石各向异性？复杂的各向异性所揭示的深部动力学过程？项目拟以获取青藏高原岩石圈组构及各向异性的定量数据，获取熔体强化变形及组构响应的定量关系为目标，开展(1)高温高压流变学实验(MPO规律和LPO类型与变形关联)，(2)岩石组构测量及各向异性计算(组构特征与各向异性关系)，(3)原位条件下的弹性波各向异性测量，并将实验和测量结果与地震测深研究结合，为青藏高原岩石圈各向异性成因解释及复杂多样的动力学过程分析提供约束和启示。

在项目组成员4年的共同努力下，顺利完成了自然科学基金任务书所提出的研究计划和任务，并超额完成论文发表等任务，尽管受到2020年新冠疫情的严重干扰。各向异性是揭示岩石圈动力学过程的重要参数。青藏高原岩石圈的各向异性非常复杂。由于缺乏其组构和弹性波各向异性的基础资料，致使在解释其各向异性测深数据，分析其成因和理解其动力学内涵方面依旧存在不确定性和争论。针对项目申请书中提出的科学问题，即青藏高原岩石圈各向异性成因及其动力学内涵是什么？构造变形如何影响或控制软介质/熔体的形态分布及其如何强化岩石的各向异性等？项目组从多个方面开展了深入的研究并取得了重要研究进展。1. 通过地幔包体的EBSD测量，我们首次获得了青藏高原及其周边新生代以来岩石圈地幔的组构数据并依据该数据计算了其各向异性参数，模拟了其岩石圈地幔弹性性质随深度的变化，这不仅为合理解释青藏高原SKS/SKKS资料提供了定量约束，也为认识青藏高原岩石圈地幔变形格架提供了新见解。2. 实验研究了花岗质岩浆的电性特征，揭示了喜马拉雅壳内高导层与地壳深熔的关系。指出当今喜马拉雅壳内正在发生着部分熔融作用并形成力学上的软弱层，该软弱带对壳内的解耦起着关键作用。3. 在大位移剪切变形条件下，实验研究了软介质(如熔体)的分布形态和各向异性对变形过程的响应。该研究为模拟弱相介质在大变形条件下的分布行为及其引起的岩石物性变化提供了新的研究思路和技术方案。