Three-dimensional Density Structure Of The Eastern Qinghai-Tibet Plateau And Its Geodynamic Significance

The collision between the Indian and Eurasia continent and its continued northward advancement have created the highest altitude,largest scale,and the latest Qinghai-Tibet Plateau on the planet.The deformation and uplift of the Qinghai-Tibet Plateau has always been the focus and core of the international continental dynamics theory.So far,many scholars have put forward various dynamic models to explain the uplift and evolution of the Qinghai-Tibet Plateau,including subduction,shortening,eastward escape,crustal injection,lithospheric convective stripping,channel flow and many other models and hypotheses.These models promote the understanding of the Qinghai-Tibet Plateau's uplift mechanism.Different uplift mechanisms may produce completely different deformation patterns around the Qinghai-Tibet Plateau.Therefore,the eastern margin of the Qinghai-Tibet Plateau is an important place for the plateau material to expand eastward and southeastward(extrusion and escape).It can provide an important geophysics basis for the study of the uplift mechanism of the Qinghai-Tibet Plateau.At the same time,the eastern margin of the plateau is also a region where earthquakes occur frequently.The long-term tectonic evolution of the Qinghai-Tibet Plateau has made its deep structure extremely complex,and it is difficult to obtain the fine structure of the deep crust and mantle by a single geophysical method.The characteristics of underground matter reflected by gravity field can be correlated with seismic,geomagnetic and geodetic data through a certain relationship,so gravity method has obvious advantages.In this paper,we use the Bouguer gravity anomaly data and elevation data of the eastern margin of the Qinghai-Tibet Plateau(98°-105°E,27°-33°N)to analyze the equilibrium state of the eastern margin of the Qinghai-Tibet Plateau.The calculation results show that the eastern margin of the Qinghai-Tibet Plateau is in a basically balanced state.In terms of sub-regions,parts of the Sichuan Basin are basically balanced,while the Qinghai-Tibet Plateau crust is in a sub-equilibrium state.Based on the full use and analysis of previous research results,this paper selects nine intersecting gravity lines,and uses Oasis montaj software to invert the two-dimensional density structure of the underground of the eastern margin of the Qinghai-Tibet Plateau,and obtains three-dimensional results.The results show that the Longmenshan fault zone is the region with the most dramatic change of crustal thickness in the study area.It is bounded by the Longmen Mountains.The western plateau has a thick crust.The depth of the Moho surface in the study area is about 62 km,and the depth of the Moho surface in the eastern basin is about 42 km.The sedimentary layers also show different structures on both sides of the Longmenshan fault zone.The sedimentary layers in the western plateau are thinner,most of which the bedrock are exposed;while the sedimentary layers in the eastern basin are obviously thicker,and the buried depth of the basal sueface can reach 6km.The Longmenshan fault zone acts as a gravity gradient zone near the SN direction and the Moho surface mutation zone,and the seismic activity is strong.