Deformation Mechanism of the Eastern Tibetan Plateau： Insights from Numerical Models

详细信息   

• 责任者：Yao Qi (National Center of Active Fault Studies ; Institute of Geology ; China Earthquake Administration ; Beijing 100083 ; China) ; Xu Xiwei
• 刊名：Chinese Journal of Geophysics
• 出版年：2012
• 卷期：55(3)
• 关键词：隆起 ; uplifts ; 构造变形 ; tectonic deformation ; 青藏高原 ; Qinghai-Tibetan Plateau
• 页码：p.863-874
• 图表：5 illus., 37 refs.
• 分类号：0300
• issnNo：ISSN0001-5733
• isbnNo：CN11-2074/P

摘要

The crustal structure of the eastern Tibetan plateau plays a key role in studying the uplift mechanism of Tibetan.Ductile flow in middle-lower crust was considered to be the main driving force in the uplifting of eastern Tibetan plateau.However,both the discovery of the low resistivity and low velocity layer between the mid-upper crust and its correspondence with the 2008 Ms8.0 Wenchuan earthquake show the possibility of southeastward splaying of coherent lithospheric blocks in eastern Tibetan plateau.Unfortunately,popular FEM models of eastern Tibetan plateau and Longmenshan fault zone in recent years were established with viscous flow of the crust.And the low resistivity and low velocity layer was set as a fault plane or a factor weakening the crust in these models.Few researchers have addressed the decoupling problem of the low resistivity and low velocity layer.The interaction of faults was ignored in previous FEM models either,for continuous thickening and deformation.In this paper,we present a coherent block model with elasto-plastic material,containing Longmenshan fault and Longriba fault which parallel to each other,setting a thin soft layer under the upper crust as the decoupling layer.A non-linear rate-independent finite element method（including the frictional contact） was carried out using R-minimum strategy to limit the step size,to simulate the block deformation of upper crust and fault behavior of the eastern Tibetan plateau.The results show that the low resistivity and low velocity layer controls the distribution of horizontal and vertical deformation on surface.The continuous lateral extrusion pressure causes quick and huge uplift of the Bayan Har block which is under the control of decoupling layer,while little uplift of the Sichuan Basin which is not controlled by decoupling layer.The difference of uplifts makes stress accumulating on the Longmenshan fault,which is the border of the two blocks,and results in large earthquake on the fault,as Wenchuan earthquake.The Longriba fault moves southeastward with the two rigid subblocks.While its shear deformation absorbs most of the strike-slip displacement added on the model boundary,and strongly reduces the strike-slip displacement in the adjacent subblock and Longmenshan fault.Therefore,the Longmenshan fault shows dominantly dip-slip reverse faulting,with a little right-lateral oblique faulting.This paper introduces deformations of coherent blocks and relative fault activities only controlled by the decoupling of the low resistivity and low velocity layer,without any other influence from ductile flow in middle-lower crust.It should be possible,therefore,to oblique stepwise rise in the eastern Tibetan plateau.This could eventually lead to more discussion about the growth of the Tibetan.