基于离散元法的龙门山构造带隆升变形主控因素研究

Main Controlling Factors of Uplift Deformation of Longmenshan Structural Belt:Insight from Discrete Element Method

为探明龙门山构造带隆升变形的主要控制因素,基于龙门山构造带东西两侧下地壳物质层属性差异巨大的特征,进行3组PFC2D离散元数值模拟对比实验,将深度扩大至下地壳,记录颗粒运动状态,实现定量化分析。实验得到的变形结果及模型颗粒运动矢量图显示,在下地壳物质属性无明显差异的条件下,板块碰撞挤压应力及地壳厚度的差异不会在龙门山构造带形成巨大的地形高差。当下地壳黏度系数存在明显差异时,软弱下地壳物质层颗粒相对运动速率为1.5~2.94 m/s,平均运动速率为1.62 m/s,大约是坚硬下地壳层颗粒平均运动速率的54倍。模型中部(龙门山构造带)出现隆升变形,纵向影响范围为94.74%,隆升幅度为19.85%。软弱下地壳上覆的中地壳和上地壳颗粒具有较大的向上速度分量,上地壳物质层上涌趋势明显。巴颜喀拉块体和四川盆地地壳存在20 km的厚度差异,使得龙门山构造带隆升幅度由14.79%增至19.85%。综合分析3组离散元模拟实验结果,得出巴颜喀拉地块下地壳物质层与四川盆地下地块物质层的黏度差异是龙门山构造带垂向隆升变形最关键控制因素的结论,在下地壳黏度结构存在明显差异的前提下,巴颜喀拉块体和四川盆地的地壳厚度差异对龙门山构造带纵向上逆冲隆升幅度有明显的促进作用。

In order to explore the main controlling factors of uplift deformation of Longmenshan structural belt,based on the differences in the properties of lower crust material layer between the east and west sides of Longmenshan structural belt,three groups of PFC2D discrete element numerical simulation are carried out to realize quantitative analysis.The experimental deformation results and the model particle motion vector map show that under the condition of no obvious difference in the material properties of the lower crust,the existence of plate collision and compression stress and crustal thickness difference will not form a huge topographic elevation difference in the Longmenshan structural belt.When there are obvious differences in the viscosity coefficient of the lower crust,the relative value of the particle movement rate of the weak lower crust material layer is 1.5-2.94 m/s,and the average movement rate is 1.62 m/s,which is about 54 times of the average movement rate of the particles of the hard lower crust layer.Uplift deformation occurs in the middle of the model(Longmenshan structural belt),with a vertical influence range of 94.74%and a uplift amplitude of 19.85%.The particles of the middle crust and upper crust overlying the weak lower crust have a large upward velocity component,and the upward trend of the material layer of the upper crust is obvious.There is a 20km thickness difference between Bayankala block and the crust of Sichuan Basin,which increases the uplift amplitude of Longmenshan structural belt from 14.79%to 19.85%.Based on the comprehensive analysis of three discrete element simulation experiments,it is concluded that the viscosity difference between the material layer of the lower crust of Bayan Kara block and the material layer of the underground block of Sichuan Basin is the most key control factor for the vertical uplift deformation of Longmenshan structural belt.On the premise that there are obvious differences in the viscosity structure of the lower crust,the crustal thickness differences between the Bayan Kara block and the Sichuan Basin significantly promote the vertical thrust uplift amplitude of the Longmenshan structural belt.