| Over the past 50 million years,the collision and continued convergence of the Indian Plate and the Eurasian Plate have led to intense deformation of the crust and rapid uplift of the Tibetan Plateau.The continental collision has not only created significant northsouth shortening and uplift,but also been accompanied by eastward extrusion of material from the plateau.Material transport is obstructed by the ancient and stable Yangtze craton in the eastern part of the plateau,resulting in a redirection towards the northeast and southeast.In the Sichuan-Yunnan region located at the southeastern margin of the Plateau,the combined effects of material intrusion,rigid blocking by the Yangtze block,and eastward subduction and rollback of the Myanmar microplate provide a weak window for material escape from the Tibetan Plateau.Therefore,the Sichuan-Yunnan region,as the expanding front of the Tibetan Plateau,has become a hotspot for studying the migration of plateau materials and their dynamic mechanisms.This study integrates three-component continuous waveform data recorded by 300 temporary stations from western Sichuan array and 350 temporary stations from the Himalayan Phase I project of China Seismic Array.Firstly,this study uses ambient noise surface wave tomography and teleseismic surface wave two-plane wave tomography techniques to obtain phase velocities at 5-140 s periods.Then,this study combines the surface wave phase velocities with the cross-convolution of teleseismic body wave waveforms to perform the MCMC joint inversion and constructs a 3D S-wave absolute velocity model of the crust and uppermost mantle(0-250 km)in the Sichuan-Yunnan region.Based on this model,the study constructs a 3D S-wave velocity perturbation model of the crust,upper mantle,and even mantle transition zone(0-800 km)in the study area by jointly inverting the surface wave phase velocities and the traveltime residuals of teleseismic S waves,and then explores the dynamic mechanism of the material escape mode of the Tibetan Plateau and the induction of intra-plate earthquakes and volcanoes.The main achievements and findings obtained in this study are as follows.(1)Based on seismic networks with broad coverage and dense station distribution,this study conducts the joint inversion of ambient noise and earthquake surface wave phase velocities,the cross-convolution of teleseismic body wave waveforms,and teleseismic S wave traveltimes.By fully utilizing the sensitivity characteristics of different signals to velocity structure at different depths and interfaces,this study outlines the complex 3D shear wave velocity structure of the lithosphere and asthenosphere in the Sichuan-Yunnan area,as well as the distribution of crustal thickness,lithospheric thickness,and sedimentary layer thickness in major basins of the study area and provides reliable evidence for an accurate understanding of the region’s dynamic mechanism.(2)The new model reveals three independent low-velocity regions in the middle and lower crust.The crustal channel flow from the Tibetan Plateau is blocked by a highvelocity block extending from the Sichuan Basin to the Emeishan Large Igneous Province in the crust,extending to the southwest,passing through the Zhongdian Fault driven by a positive topographic gradient from north to south and terminating before the Tengchong volcano.The independent low-velocity zone in the middle and lower crust below the Tengchong volcano indicates a magma reservoir from mantle upwelling.The lowvelocity zone near the Xiaojiang fault is confined by the Ca B,the ICB and the highvelocity block in the inner zone of the Emeishan large igneous province,which shows an overall NE-SW strike and is subparallel to the plateau margin.Its formation may be mainly related to crustal thickening and partial melting under southeastward extrusion of the plateau.In the southern low-velocity zone,the crust is thinner,and the low-velocity feature is shallower and mainly distributed along the fault zone.Combined with the significant low-velocity anomaly at the top of the upper mantle and the thinner lithosphere,the low-velocity anomaly may derive from the upwelling of hot mantle material.Alternatively,mantle upwelling may only provide the required heat source for crustal weakening.In addition,shear heating from strike-slip faults may also have influenced the formation of the low-velocity zone.(3)The local low-velocity anomalies at the upper mantle beneath the Songpan-Ganzi and the western Sichuan blocks may reveal mantle upwelling associated with lithospheric delamination,which is related to the compression and thickening of the lithosphere.(4)The high-velocity Yangtze craton lithosphere gradually thickens from about 150 km at the Longmenshan fault to 250 km within the craton.A low-velocity zone in the upper mantle extends beneath the Sichuan Basin,crossing the Longmenshan fault.This study indicates that the flow of mantle material from the Tibetan Plateau is activating the cratonic lithosphere in the northwest of the Yangtze craton.(5)The new model reveals a regenerated lithosphere strengthened by mantle plume within the Emeishan Large Igneous Province,where a low-velocity mantle channel exists at depths of 200 to 350 km in the western part of the lithosphere,toward the south and southeast from the Tibetan Plateau.The low-velocity channel converges to the south with an upwelling low-velocity flow from the mantle transition zone beneath the Tengchong volcano,which may also be closely related to the eruption of the Tengchong volcano. |
