青藏高原东缘中下地壳流与地壳变形

Middle-lower crust flow and crustal deformation:insights from a finite element modeling

地壳缩短导致青藏隆升造山是普遍的认识.然而,在青藏东部,越来越多的观测数据和研究支持了中下地壳流与隆升造山的关系.目前,地壳缩短造山机制和中下地壳流造山机制仍然处于争论之中.本文建立了二维黏弹塑性有限元模型,模拟了龙门山断层带的多个地震循环的应变与变形,探讨了无与有中下地壳流情况下,地壳地表的位移、速度与变形的分布和演化;以及有中下地壳流情况下,不同流动范围、速度与黏度对模型结果的影响;并结合地形变观测数据的约束,推测了青藏东缘中下地壳流的流动状态.模拟结果显示,通过对比有和无中下地壳流的模拟结果,发现青藏东部震间的地表垂向速度在变形样式及数值上存在较大差异,即存在地壳流的地表垂向抬升速率显著大于无地壳流;震间在龙门山断层西侧附近产生的垂向凸状隆起随中下地壳流的速度、黏度及通道长度的变化而变化.此外,本文研究结果对青藏其他地区可能存在的地壳流的研究也具有一定的参考意义.

Crustal shortening leading to the uplift of the Tibet Plateau is our basic knowledge.However,in the eastern Tibet,more and more observational data and studies support the relationship between the middle-lower crust flow and uplift orogeny.At present,the orogenic mechanism of crustal shortening and middle-lower crust flow is still in dispute.In this paper,we have established a two-dimensional viscoelastoplastic finite element model to simulate multiple seismic cycles of the Longmen Shan fault zone,so as to discuss the surface distribution and evolution of displacement,velocity and deformation of the crust under the condition of without and with the middle-lower crust flow.We have studied the influence of different range length,velocity and viscosity on model results under the condition of middle-lower crust flow.We have also introduced the constraints of the crustal deformation observation data,and infer that if the lower crust flow exists in the eastern margin of the Tibet Plateau.By comparing the model results of with and without the middle-lower crust flow,we find a big difference in surface vertical velocity and deformation pattern at inter-seismic period in eastern Tibet.The vertical uplift velocity of the crust with crust flow is significantly larger than that of without crust flow.The vertical convex uplift in the vicinity of Longmen Shan fault zone on the left side varies with the change of velocity,viscosity and range length of the middle-lower crust flow.Model results also find that the greater the velocity of the lower crust,the more intense the vertical uplift.While the crustal flow range length is greater,this vertical convex uplift position from the Longmen Shan fault zone closer.The higher the viscosity of the middle-lower crust,the stronger the coupling between the middle-lower crust and the upper crust and mantle.In addition,the results of this study also have some reference significance for the study of possible crustal flow in other areas of the Tibet Plateau.