The Characteristics Of Gravity Anomalies And Lithospheric Thermal Structure In Southeast Tibet

Southeast Tibet is located in the southeast margin of the Qinghai-Tibet Plateau and is the front of the collision between the Indian plate and the Eurasian plate.The geological structure is complex,faults are developed and seismic activity is frequent.The study of the spatial distribution characteristics of gravity anomalies and the three-dimensional thermal structure of lithosphere in Southeast Tibet will help to understand the geological structure and geodynamic process in this area.In this paper,World Gravity Map 2012(WGM2012),a high-precision global Gravity field model,is used to systematically analyze the spatial distribution of gravity anomalies in Southeast Tibet.Two-dimensional wavelet multi-scale decomposition is performed to analyze the gravity anomalies,and the details of each order wavelet and the construction information of the approaching field are analyzed.In order to study the fluctuation of crust-mantle boundary,the ParkerOlderburg method of Bouguer gravity anomaly is used to invert the Moho surface.Based on the data of terrestrial heat flow,crustal magnetic anomalies and Moho surface,the three-dimensional lithospheric thermal structure in Southeast Tibet is established based on one-dimensional thermal steady-state equation.Based on the above research work,the following main results are obtained:(1)The wavelet approximation field of Bouguer gravity anomalies shows that songpan-Ganzi block and northwest Sichuan sub-block are large scale negative anomalies,which corresponds to the phenomenon of material flow eastward on the Qinghai-Tibet Plateau.The gravity anomalies shows different characteristics on both sides of the Lijiang-Xiaojinhe fault and the Longmenshan fault zone,which indicates that there is material adjustment and migration in the crust of this region.The most significant feature of the 5th order wavelet detail field is the positive anomaly in the north of central Yunnan block and the negative anomaly in the east and west of the block,the positive anomaly in the Sichuan Basin and the large negative anomaly in the Songpan-Ganzi block in the west.The distribution characteristics of gravity anomalies indicate that the crustal material flowing eastward in the Qinghai-Tibet Plateau is blocked by the rigid Sichuan Basin and the northern yunnan Block.Part of the crustal material flows northeastward along the Longmenshan fault zone,and the other part flows southward along the Xiaojiang fault zone,red River fault zone and Lancang River fault zone.(2)The moho depth in Southeast Tibet is 30～67km,with an average depth of 43 km.The Moho surface gradually becomes shallower from northwest to southeast,with a variation range of nearly 37 km,confirming the crustal thickening caused by the subduction of the Indian plate to the northeast Eurasian plate.Due to the blocking of material flow and block compression,three moho gradient zones are formed in this region,one is the Longmenshan fault zone,the other is the Lijiang-Xiaojinhe fault zone,and the third is the west side of the Lancang River fault zone.The difference of moho depth between the two sides of the three fault zones is nearly 10 km,and the different distribution of gravity anomaly exists on both sides of the fault zone,which fully indicates that the three fault zones play an extremely important role in the crustal deformation and evolution in Southeast Tibet.(3)The thermal structure of the lithosphere in the study area is characterized by the high temperature region in the west Yunnan,west Sichuan secondary block,SongpanGanzi block and part of the west and south of central Yunnan secondary block,while the middle and low temperature regions in the east Yunnan,southeast Yunnan,north Central Yunnan secondary block and Sichuan Basin.According to the relationship between temperature and crustal mobility and the density distribution characteristics of this region,it is indicated that there is material and energy migration in songpan-Ganzi block along the northwest side of the Longmen Shan fault zone,and there is local high density area in the upper crust of the northern part of the secondary block of Central Yunnan,and the density decreases on both sides of this area at the beginning of the middle and lower crust.It is consistent with the material flow distribution of the middle and lower crust reflected by the 5th order wavelet decomposition of Bouguer gravity anomalies.