青藏高原东南缘演化过程的动力学模拟

发布时间：2018-10-08 16:25

【摘要】：青藏高原的大规模隆升起源于～50 Myrs印度板块向欧亚板块的洋陆俯冲转换为陆陆碰撞,之后持续挤压欧亚板块。在这一过程中,青藏高原南部经历了显著的地壳缩短增厚,从而形成了很高的地形,但是在青藏高原其他区域,特别是高原边缘的隆升机制还存在很多争议。在印度板块的挤压下,现在青藏高原中部上地壳在东西方向处于明显的拉伸状态,物质呈现向东的移动,并从东部边缘挤出。青藏高原的东南缘作为物质的主要流出通道,具有三个典型特征。其一:青藏高原物质在东部边缘受到了四川盆地的阻碍,在青藏高原-四川盆地边缘形成了非常陡峭的地形,在100 km以内从～4500 m下降到～500 m。其二:青藏高原物质流在四川盆地分成两支,一支流向华北地区,另一支流向华南地区,流向华南地区的物质绕着喜马拉雅东构造结顺时针旋转。其三:青藏高原东南缘构造活动强烈,断层广泛分布,大地震多发。为了解青藏高原东南缘的地质演化,我们针对青藏高原-四川盆地边缘建立了解析模型和数值模型来模拟龙门山的隆升过程,并确定四川盆地岩石圈粘滞系数分布以及应力和应变率的变化。同时我们正在着手建立川滇地区的三维数值模型,试图探讨青藏高原及其东南缘的浅地表构造对其运动的影响,并建立这一区域的地壳地幔耦合模式。下地壳通道流理论可以很好的解释青藏高原东部边缘的隆起,但是前人的解析模型对于通道流得模拟都存在与事实明显不符的地方,因此我们建立了新的模型,通道厚度随着物质的流入而增加。我们的解析模型结果表明,下地壳粘滞系数越大时,边界上的地形梯度越大,通过实际地形的约束,四川盆地下地壳的粘滞系数为1022Pas,是前人研究结果的10倍。在解析模型的基础上,我们考虑更多可能影响演化过程的因素,建立数值模型。数值模型采用解析模型关于下地壳粘滞系数的结果,进一步确定四川盆地上地壳的粘滞系数为1024Pas,岩石圈地幔粘滞系数为1023Pas。隆升过程中,应力和应变率在隆升前缘很大,在其他区域较小,因为四川盆地下地壳粘滞系数大,物质流动收到阻碍。在隆升过程中,龙门山区域上地壳中的应力状态可能经历了从拉伸到压缩的转变,这一转变可能与龙门山附近多种断层类型相关。另外,青藏高原与川滇地区的各向异性特征不同,快波极化方向的转变可能因为青藏高原和川滇地区的地壳和地幔的形变耦合方式不同。
[Abstract]:The large-scale uplift of the Qinghai-Xizang Plateau originated from the subduction of the Indian plate from 50 Myrs to the oceanic continent of the Eurasian plate into a continental collision, and then sustained compression of the Eurasian plate. During this process, the southern Qinghai-Xizang Plateau experienced significant crustal shortening and thickening, resulting in a very high topography. However, the uplift mechanism in other regions of the Tibetan Plateau, especially the plateau margin, is still controversial. Under the compression of the Indian plate, the upper crust of the central Qinghai-Xizang Plateau is in an obvious stretching state in the east-west direction, with the material moving eastward and extruding from the eastern margin. The southeastern margin of Qinghai-Xizang Plateau, as the main outflow channel of material, has three typical characteristics. Firstly, the matter of Qinghai-Xizang Plateau is blocked by Sichuan Basin on the eastern margin, and a very steep terrain is formed on the margin of Qinghai-Sichuan Basin, which falls from 4500m to 500m within 100 km. Secondly, the material flow in the Qinghai-Xizang Plateau is divided into two branches in Sichuan Basin, one flowing to North China and the other to South China, and the matter flowing to South China rotates clockwise around the eastern Himalayan tectonic junction. Third, the southeast margin of Qinghai-Xizang Plateau has strong tectonic activity, wide distribution of faults and multiple earthquakes. In order to understand the geological evolution of the southeastern margin of the Qinghai-Xizang Plateau, an analytical model and a numerical model were established to simulate the uplift process of the Longmen Mountains in view of the margin of the Qinghai-Xizang Plateau and Sichuan Basin. The distribution of lithospheric viscosity coefficient and the variation of stress and strain rate are determined. At the same time, we are establishing a three-dimensional numerical model of Sichuan-Yunnan region, trying to study the influence of shallow surface structure on the movement of Qinghai-Xizang Plateau and its southeast margin, and to establish the crustal and mantle coupling model in this region. The theory of channel flow in the lower crust can explain the uplift of the eastern edge of the Qinghai-Xizang Plateau well, but the previous analytical models are obviously inconsistent with the facts, so we have established a new model. The channel thickness increases with the inflow of matter. The results of our analytical model show that the larger the viscosity coefficient of the lower crust is, the greater the gradient of topography is at the boundary. The viscosity coefficient of the lower crust in Sichuan Basin is 1022 Pasas through the restriction of the actual terrain, which is 10 times of the previous results. On the basis of the analytical model, we consider more factors that may affect the evolution process, and establish a numerical model. Based on the results of the analytical model on the viscosity coefficient of the lower crust, it is further determined that the viscosity coefficient of the upper crust and the lithospheric mantle of the Sichuan basin is 1024 Pasas and 1023 Pasas respectively. In the uplift process, the stress and strain rates are very large in the front edge of uplift and smaller in other regions because of the large viscosity coefficient of the lower crust of Sichuan basin and the obstruction of material flow. During the uplift, the stress state in the upper crust of the Longmen Mountain region may undergo a transition from tensile to compression, which may be related to many types of faults near Longmen Mountain. In addition, the anisotropic characteristics of Qinghai-Xizang Plateau and Sichuan-Yunnan region are different. The transformation of fast wave polarization direction may be due to the different deformation coupling modes of crust and mantle in Qinghai-Xizang Plateau and Sichuan-Yunnan region.
【学位授予单位】：中国科学技术大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：P542

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