The Lithospheric Structure And Geodynamic Characteristics Beneath Sichuan-Yunnan Region

The Sichuan-Yunnan region which located on the southeastern margin of the Tibet Plateau is one of the regions with the strongest crustal deformation and seismic activity on the Tibet Plateau.It plays an important role in explaining the uplift and evolution mechanism of the Tibet Plateau.Therefore,it is of great significance to study the crust-rmantle structure in the southwest of China for further understanding of the tectonic activities and geodynamic processes of the Tibet Plateau.This paper selects the teleseismic data recorded by a total of 117 permanent seismic stations in the Sichuan,Yunnan and Chongqing from January 2009 to December 2017.We use the S receiver function to deal with the teleseismic data,and obtain the depth distribution of the Moho and the lithosphere-asthenosphere boundary(LAB)in the study region.The span of crustal thickness is large,which ranges from 30km to 69 km in the whole study area.The thickest part of the crust is located in the Luguhu(LGH)-Muli(MLI)of the Northwest Sichuan sub-block,with an average crust thickness of about 67 km;the thinnest part of the crust is distributed in Western Yunnan area and Red River fault,the crustal thickness of this area is basical within 36k11m.The Lonagmenshan fault and the Lijiang-Xiaojinhe fault are the gradient zone of'the Moho deptlh change,which gradually thinning fr-om the northwest side to the southeast side of the fault:the Songpan-Ganzi block and the Northwestern Sichuan sub-block which located on the northwestern side of the faults with the crustal thickness of more than 52km and 60-69km,respectively.But in the southeast side of the faults,there are significant difference in the crustal thickness of blocks.The thickness of the crust in the Sichuan Basin is change mildly,ranging from 40 to 48 km.But conversely,the Moho depth of Central Yunnan sub-block has long span and decrease from?60km in the north to?36km in the south.The crust of the South China block and the Indosimian block are relatively thinner,approximately 36?40km and 30?36km,respectively.The results show that the variation of Moho beneath the study area is consistent with the change of topography and The Bouguer yravity anomalyv that is,in the area with high terrain or with smaller negative Bouguer anomaly,the Mono surface is relatively deeper,and in the area with lower terrain or with larger negative Bouguer anomaly tends to correspond to the shallower Moho surface as well.the crustal material flow escaped from the eastern side of the Tibet Plateau and one part entered the Songpan-Ganzi block along the northeast,and due to the obstruction of the Sichuan Basin,the overthrust nappe tectonic was formed in the Longmenshan fault,resulting a significant thickness gradient between the Songpan-Ganzi block and the Sichuan Basin;the other crustal material flowing in the southeast direction enters the Sichuan-Yunnan diamond-shaped block,which leads to the thickening of the crust in this area.It is an obvious difference in thickness of the crust of the two secondary blocks of Northwest Sichuan and Central Yunnan on account of the blocking of the Lijiang-Xiaojinhe fault.There are significant differences in lithospheric thickness and varied characteristics of each block in the study area.The lithospheric thickness of the Songpan-Ganzi block varies between 100 and 170 km,and most of the areas in a range of 100-130 km,besides,the lithosphere gradually becomes thicker from the west to the east in this block.The lithosphere of the Sichuan Basin is characterized by thick in the center with thin margins,in the most part of the basin is more than 140 km,and a shallow LAB belt with only 100-120 km in the outer edge.The thickest lithosphere is about 170km,located in Meigu(MGU)of Sichuan Basin and Zhongdian(ZOD)of the Sichuan-Yunnan diamond-shaped block,moreover,the change of the LAB depth in the two secondary blocks inside this block is complex:there are obvious abrupt change inside the Northwestern Sichuan sub-block,in the southwestern part of the sub-block is above 170km.while the eastern part of the sub-block is distributed with a shallow LAB area of 100-120km:the other sub-block is the Central Yunnan sub-block where has the thickness of the lithosphere and most areas varies from 70-120km.The lithospheric thickness of the South China block is beyond 120km;the Indosinian block is basically beneath 90km,furthermore,in this block,the thinnest part of the entire study area is Tengchong(TNC)-Baoshan(BAS)located in the Tengchong volcanic area,ranging from 68km to 80km.According to the analysis of the research results,the geodynamic characteristics of the lithosphere in the study area can be obtained:the thinning lithosphere indicates the direction and scale of the asthenosphere material migration.The uplift of mantle hot materials(or the upwelling of the asthenosphere)in the Tengchong volcanic area of Yunnan,resulting in LAB becomes shallower and presents a dome shape obviously.We suggest that thinned lithosphere in west of Red River fault is related to hot mantle material associated with the westward rollback and northeastward subduction of the Indian plate;the mantle hot materials escaping from the Tibet Plateas caused by collision and extrusion between the Indian and Eurasian plates,it is resisted by the thick Sichuan Basin and moved along the northeast and southeast.