青藏高原东南缘壳-幔变形及其耦合关系的动力学数值模拟研究

During the India/Asia collision and uplift of the Tibetan plateau, large-scale lateral escape of the Tibetan crustal materials occurs to the east and southeast, which is well constrained by the GPS data on the surface. But, the mode of material escape in depth is widely debated, which can be summarized into three end-members: thin viscous sheet, tectonic escape and middle/lower crustal flow. These models all agree with the general observations of crustal extrusion; however, the kinematics and deformation mechanism of the escaped crust, lithospheric mantle and asthenosphere are not well constrained. In the proposed project, we plan to conduct 3-D, high resolution geodynamic numerical models to study the escape mode of the southeastern Tibetan plateau. Firstly, we are going to construct a series of geodynamic models according to the general evolution of the southeastern Tibetan plateau. Secondly, the deformation mechanism of the crust, lithospheric mantle and asthenosphere of the numerical models will be analyzed, including quantitatively calculating the evolution of finite strain ellipsoid (FSE), the crystal preferred orientation (CPO) of the mantle minerals, as well as the SKS shear wave splitting. Thirdly, the numerical models will be compared with the geophysical observations of the southeastern Tibetan plateau, especially with the GPS and shear wave splitting data, in order for better constraining the numerical models. Finally, it is expected to determine the crust-mantle deformation mechanism and the escape mode of the southeastern Tibetan plateau, which will significantly contribute to the better understanding of Tibetan plateau dynamics.

伴随着印度/亚洲碰撞和青藏高原隆升，高原腹地的物质发生大规模的侧向逃逸，但逃逸模式广泛争议，目前主要存在三种端元模型：粘性薄板模型、构造逃逸模型和中/下地壳流模型。三种模型都满足高原地表物质逃逸的基本观测事实，但对于逃逸地体的地壳、岩石圈地幔和软流圈地幔的运动和变形特征，以及三者之间的耦合关系尚存较大差异。针对青藏高原东南缘的物质逃逸模式，本项目拟采用三维、高分辨率的动力学数值模拟技术，（1）构建符合该地区基本演化特征的动力学数值模型；（2）对模型域的地壳、岩石圈地幔和软流圈地幔的变形特征进行分析，包括计算壳-幔中的有限应变椭球演化，以及定量化的模拟地幔矿物晶格优选定向，并合成SKS剪切波分裂；（3）进而与该地区的观测数据（尤其是GPS和地震波各向异性数据）进行直接对比，并对动力学模型进行有效限定；（4）揭示青藏高原东南缘的壳-幔变形机制及物质逃逸模式，从而促进对青藏高原大陆动力学的认识。

印度-亚洲板块的碰撞和青藏高原的隆升是新生代地球上发生的最重要的地质过程之一，也是大陆动力学的核心研究内容。印藏碰撞过程产生巨大的汇聚量，被下盘大印度板块的俯冲及上盘青藏板块的缩短而吸收，而上盘变形又受到青藏岩石圈垂向增厚和侧向挤出的联合控制。因此，印藏碰撞的应变分配和物质平衡问题是青藏动力学的关键科学问题，备受争论。本项目采用动力学数值模拟的手段，系统厘定了青藏高原尤其是东南缘的岩石圈变形特征，获得以下认识：（A）青藏高原的岩石圈垂向增厚及拆沉是吸收印藏汇聚的主要途径，远大于侧向挤出量；（B）青藏高原东南缘挤出地体的壳幔是解耦的，地壳的挤出速率远大于岩石圈地幔；（C）青藏高原中北部之下的岩石圈地幔的减薄过程可能主要由拆沉作用所主导，青藏岩石圈在地体拼合阶段的塑性弱化控制后期的增厚与拆沉；（D）对于50 Myrs的印藏碰撞缩短量（~2900 km），可由上盘青藏板块的缩短（~1700 km）和下盘大印度的俯冲（~1200 km）联合吸收；而如果碰撞达55 Myrs，其巨大的汇聚量难以吸收，需要其它模型。系统性的研究成果对于厘清印藏碰撞演化的一级过程和控制机制具有重要意义，提高了青藏高原大陆动力学的认知水平。