Lithospheric Flexure And Vertical Deformation Mechanism In The Southeastern Margin Of The Tibetan Plateau

The collision and convergence of the Indian and Eurasian plates shaped the complex landform and tectonic activities of the southeastern margin of the Tibetan Plateau.The eastern Himalayan syntaxis and material escape area in the southeastern Tibetan Plateau have been the topical area.The formation mechanism of complex landform,the crustal deformation mechanism and the distribution of the channel flow are important scientific issues in the study area.In this work,we use gravity/GPS joint observation data,gravity model and digital elevation data combined with the lithospheric flexure analysis method to study the problems above,and the main understandings are as follows:?1?Formation mechanism of the landform in the big bend area of the Yarlung Zangbo RiverWe built the first dense gravity network including 107 stations around the big bend area of the Yarlung Zangbo River?YZR?,Tibet,one of the hardest places in the world to reach,and conducted a gravity and hybrid GPS observation campaign in 2016.We compute the Bouguer gravity anomalies?BGAs?and free-air gravity anomalies?FGAs?and increase the resolution of the FGAs by merging the in situ data with EIGEN-6C4 gravity model data.The BGAs around the Tsangpo Gorge are in general negative and gradually decrease from south?-360 mGal?south to north?-480 mGal?.They indicate a uniformly dipping Moho around the Tsangpo Gorge that sinks from south to north at an angle of 12°.The vertical tectonic stress?VTS?of the lithosphere in the area is calculated by using BGAs and elevation data.According to the distribution of the VTS,we find that the lithosphere of the upstream of the Tsangpo Gorge is roughly in an isostatic state,but the lithosphere of the downstream exhibits VTS of⁵0 MPa,which indicates the loss of a large amount of surface material.This result does not support the deduction of the valley bottom before uplift of the Tsangpo Gorge by Wang et al.?2014?.This study thinks that the continuous collision of the Indian and Eurasian plates caused the crustal thickening in the great turning area of the YZR,and the surface of the Earth was uplifted.After that,the erosion of the YZR causes the rapid loss of a large amount of surface material,then forms a great imbalance state in the downstream of the YZR in the southern area of the big bend.?2?Vertical crustal movement in the southeastern margin of the Tibetan Plateau.The plastic flow in the southeastern margin of the Tibetan Plateau is the consequence of the eastern expansion of the Tibetan Plateau.The study area can be divided into three sections by the Xiaojin River fault zone and Red River fault zone,which both contain the normal fault component,consistent with GPS observations.The average vertical rates of the three sections exhibit a step-like distribution,and the southern subsidence rate is about 1–2 mm/yr with respect to the northern area.To explain the vertical movement in the southeastern margin of the Tibetan Plateau,we subtract the vertical rates caused by the horizontal movement from the observed vertical rates and then use a two-layer viscous model to constrain the viscosities of the crust and upper mantle in this area.The fitting results between the vertical observed and model data show that the viscosity of the upper mantle is about 2.8×10²² Pa·s and the viscosity of the crust is less than 1.0×10²⁵ Pa·s.Furthermore,we find that the VTS dominate the driving mechanism for the vertical movement in the southeastern margin of the Tibetan Plateau.At the same time,we speculate that the VTS in the southeastern margin of the Tibetan Plateau is caused by the escape of the material inside the Tibetan Plateau,which makes the lithosphere deviate from the equilibrium state.?3?Distribution of the channel flow in the southeastern margin of the Tibetan Plateau inversed bygravity and topographic dataThe activity and distribution characteristics of the channel flow in the southeastern margin of the Tibetan Plateau are controversial topics in recent years.Previous studies use the methods of magnetotelluric,seismic tomography and numerical simulation to give different range of activities for the channel flow.In this study,the Fan wavelet method is used to calculate the correlation between gravity and topographic data,admittance and coherence.The admittance,coherence model and load deconvolution method are used to inverse the activity and distribution of the channel flow in the area.Inversion experiments are carried out on the different combinations of the admittances and coherences of FGAs and BGAs using the synthetic data.The results show that the joint inversion of the coherences of FGAs and BGAs can recover the optimal distribution of F₂,which denotes the internal load ratio of the crust and indicates the distribution of the channel flow.Further testing of the joint inversion method shows that as the wavelet center wavenumber|6)₀|decreases,the inversed results of the F₂ will gradually deviate from the set value.When|6)₀|takes 5.336 and3.773,the inversion results can restore the set value well.Using the above method combined with digital elevation and gravity model data to inverse the distribution of F₂ in the southeastern margin of the Tibetan Plateau,which can be used to estimate the distribution of the channel flow.The results show that the channel flow rotating clockwise around the eastern Himalayan syntaxis flows from the interior to the southeastern margin of the Tibetan Plateau.The channel flow in the Sichuan-Yunnan rhombic block is diverted to the southwest and southeast by the obstruction of the Emeishan large igneous province.Meanwhile,this study finds that the western channel flow given by the previous study using the magnetotelluric data and seismic tomography results is discontinuous,and we speculate that the result is caused by the extrusion of the Eurasian plate by the Indian and Myanmar plate.In summary,this study suggests that the current landforms,vertical crustal movement and the channel flow distribution in the southeastern margin of the Tibetan Plateau are the results of the mutual squeeze movement between the plates.The tectonic activities in the area are controlled by the continuous collision of the plates,and it is the continuation of the collision activities of the plates.