褶皱冲断带时空演化动力学过程的数值模拟

Numerical simulation of the dynamic process of spatial and temporal evolution for fold-thrust belts

利用库仑临界锥角理论和沙箱物理模拟进行褶皱冲断带的研究时,通常忽略了楔形体介质的内聚力.岩石力学实验结果表明,岩石内聚力通常在几到几十兆帕范围内变化.楔形体介质强度的变化是否会影响褶皱冲断带的时空演化?针对这一问题,我们建立了岩石内聚力分别是10MPa、20MPa和50MPa的3个二维弹塑性有限元模型,模型包含了楔形体介质的弹塑性材料非线性和底部滑脱带的接触非线性.该模型考虑了不同介质强度、底部滑脱带摩擦、重力和边界构造加载的影响,更为符合实际.计算结果表明,岩石内聚力为10MPa时,楔形体内的断坡首先在楔形体近端产生,从近端依次向远端发展;岩石内聚力是20MPa时,断坡开始在楔形体近端产生,随后在远端也开始形成,最后由两端向中间汇聚;岩石内聚力是50MPa时,断坡先从楔形体远端形成,从远端向近端依次发展.我们还讨论了底部滑脱层倾角对褶皱冲断带演化的影响,结果表明较低的底部滑脱面倾角易产生由近端向远端演化的样式,中等滑脱面倾角易产生两端向中间演化的样式,较高滑脱面倾角易产生由远端向近端演化的样式.我们得到了三种不同的褶皱冲断带时空演化的模式,其结果可以用来解释青藏高原东北缘依次向北东方向发展的海原-六盘山断层、天景山断层、烟筒山断层系统的时空演化.

The effect of cohesion is usually neglected in the study of deformation of fold and thrust belts by using Coulomb critical wedge theory and sandbox physical simulation. Rock mechanic experiments show that the cohesion of rocks varies from several to several dozen MPa. Will the change of the strength of a wedge medium affect the temporal and spatial evolution of fold and thrust belts? To address this issue,we construct three two-dimensional elastoplastic finite element models containing rock cohesion of 10 MPa,20 MPa and 50 MPa,respectively. The models include nonlinear elastoplastic material of a wedge-shaped medium and contact nonlinear nature of the bottom detachment zone. The effects of different medium strength,bottom detachment zone,gravity,and boundary tectonic loading are considered in the models,which can mimic realistic geology. The results show that when the rock cohesion is 10 MPa,a thrust ramp in the wedge first appears at the near end of the wedge and develops from the near end to the distant end in turn. When the rock cohesion is 20 MPa,the thrust ramp begins to appear at the near end of the wedge body,then begins to form at the distant end,and finally converges from the two ends to the middle. When the rock cohesion is 50 MPa,the ramp begins to form from the distant end of the wedge,and develops from the distant end to the near end. We also examine the influence of the dip angle of the basal detachment on the evolution of fold-thrust belts. The results show that the lower dip angle of the basal detachment tends to evolve from the near end to the distant,the middle dip angle of the basal detachment tends to evolve from the two ends to the middle,and the higher dip angle of the basal detachment tends to evolve from the distant to the near end. We have obtained three different models of temporal and spatial evolution of fold-thrust belts and examined the influence of wedge-shaped medium strength on the evolution of thrust ramps. The results can be used to explain the temporal and spatial evolution of the Haiyuan-Liupanshan,Tianjingshan and Yantongshan fault systems in the northeastern margin of the Tibet plateau,which are developing northward and eastward in turn.