Physical Modeling Of The Longmen Shan Fold And Thrust Belt

As the front of growing Tibetan Plateau toward the Sichuan basin,the Longmen Shan fold and thrust belt has undergone regional shortening since the late Cenozoic,forming the current steep topography.Its deforming processes,possessing key implications for deformation propagation and transition between the Tibet Plateau and the adjacent Sichuan Basin,have attracted widespread concerns.Based on the critical taper theory,series of physical experiments were constructed in the sandbox to simulate the evolution of thrust wedges,which is analogue to the outgrowth of the eastern margin of the Tibet Plateau.The major achievements can be summarized,as follows:1)Physical modeling suggests that across-strike pre-existing topographic relief controls the deformation partitioning within the fold and thrust belts,while segmentation of along-strike pre-existing topographic relief is one cause of the nucleation of transfer structures.On basis of the critical taper theory,the pre-existing topography relief can be divided into three types,subcritical one,critical one and supercritical one,which were introduced into our modeling as the initial boundary conditions.The kinematics of thrust wedges in our experiments,including the lateral expanding and vertical uplifting,vary with the pre-existing topographic relief.According to whether the pre-existing topographic relief is subcritical or critical-supercritical,two distinct kinematical evolutions occur,associated with contrasting modes of deformation partitioning on thrusts within the thrust wedges.Such differences in the structural evolution lead to the development of transfer structures,like tear fault and laterl ramp,in the experiments consisting of two segments with different pre-existing topographic relief.The Partcile Image Velocimetry analysis also reveals that the nucleation and activation of transfer structures in our experiments,have adjusted the lateral differential strain induced by along-strike segmentation of pre-existing topography.Furthermore,geometrics and kinematics of the tear fault occurred in our modeling are comparable to those of the Xiaoyudong fault,which ruptured in the Wenchuan earthquake.It indicates that along-strike segmentation of topography has contributed to the lateral differences of the Longmen Shan fold and thrust belt in the structural evolution and fault activities.2)The experimental results also demonstrate that,after nucleation of new thrust in the foreland,the active deformation would retreat,and the thrusts below envelop line of regional topography in the hinterland would be reactivated.Because the lengthened thrust wedge turns into subcritical state,thickening in the hinterland would be required to recover the critical geometry.While both the 2008 Wenchuan earthquake and the 2013 Lushan earthquake occurred in the southern Longmen Shan,which may be also undergoing vertical thickening in the hinterland after the nucleation of the Longquan Shan anticline in the west Sichuan Basin.3)Physical experiments reveal that the mechanical properties of decollements and regional shortening rates play important roles in the evolution of duplex structures in the range front.Faster shortening,or stronger decollement leads to thrust stacking by out-of-sequence thrusting beneath the middle-level decollement.Above the middle-level decollement,the range front monocline and closed box folds develop.Moreover,large amount of shortening is accommondated in the deep structures,so the fold belt on the surface would be narrow.The geometrics of duplex structures in basal frictional-decollement experiment and the strong ductile-decollement experiment are similar,but the frictional experiment develops through in-sequence thrusting.In the case of lower shortening rate,and weaker basal ductile-decollement,the deep thrust sheet is longer,while the superficial deformation would propagate more forward.If the shortening is slow enough,for example 5 mm/a in nature,the extremely weak basal decollement would lead to strong coupling between the deep and shallower structures.A deep ramp structrue connecting the basal and middle-level decollements would locate the surface anticline right above,similar with the spaced Hongya Anticline in the southern Longmen shan fold and thrust belt,which is controlled by underlying Hongya blind fault.Therefore,under the low shortening rate,Cambrian salt and mudstone should act as the extremely weak basal decollement which controls the evolution of the fold belt in the west Sichuan Basin.