Geodynamic Modeling Of The Evolution Around The Southeastern Tibetan Plateau

Previous studies have shown that the uplift of Tibetan Plateau started in response to the collision of Indian Plate and Eurasian Plate at-50 Myrs.During this process,the crust of southern Tibetan Plateau has been greatly thickened which leads to significant elevations because of the subducted Indian Plate.However,the reason for the uplift at other regions in Tibetan Plateau is still in debate,especially around the eastern margin.In spite of the shortening of the southern part,west-east extension is going on in the middle of Tibetan Plateau and the material from Tibetan Plateau is leaking through the east margin.At the southeastern margin,which is a major channel of the leakage,there are three major features.Firstly,when the flow is block by Sichuan Basin,it piles up and causes uplift.The elevation gradient is extremely large at the eastern boundary where Longmenshan Fault exists,dropping from?4500 m to?500 m within a distance of?100 km,while it is more gentle at the south and north sides of Sichuan Basin.Secondly,the flow branches into two,one flows to North China,the other flows to South China and is rotating around the East Himalayan Syntaxis.Thirdly,the geological activity is strong at the southeastern margin of Tibetan Plateau is intense.There are widespread fault zones on which megaseisms happen frequently.In order to understand the evolution of the southeastern borderland of Tibetan Plateau,we set up a new analytical model and numerical models for the Tibet-Sichuan boundary to determine the viscosity distribution in the lithosphere and calculate the uplift process,the distribution of stress and strain rate.We are also trying to design a new three dimensional model to study the effect of near surface structures on the evolution process and the crust-mantle coupling pattern.The uplift of Longmenshan region and the difference of elevation gradient around can be explained with a crustal channel flow theory.However,previous analytical crustal flow models have obvious shortcomings,so we build a more realistic crustal flow model,in which the thickness of the lower crust is variable and dependent on the inflow velocity of crustal materials.The results of the analytical model show that if the thickness of the lower crust can vary during the uplift process,the lower crustal viscosity of the Sichuan Basin needs to be 1022 Pas to fit the observed elevation gradient.Such a viscosity is one-order magnitude larger than the previous results.We further more build up numerical models based on this result.Numerical model results show that the viscosity of upper crust is 1024 Pas,of lithospheric mantle is 1023 Pas.Stress and strain rate are the largest at the left boundary of our model,because the flow is blocked by Sichuan Basin.A stress state change is observed in our models,which may cause the formation of different fault types in the Longmenshan Fault area during its uplift history.Additionally for the anisotropy distribution,the difference of fast polarization may be caused by the crust-mantle coupling pattern.