Crustal And Upper Mantle Velocity Structure Of The Tien Shan

The Tienshan orogenic belt is one of the most active intracontinental orogenic belts in the Central Asian orogenic belt,and has undergone complex deformation and evolution.The Tienshan orogenic belt area has frequent seismic activities and the orogenic movement is strong.It is an ideal natural testing ground for the study of intracontinental orogenic and seismic activity.Studying the crustal and mantle velocity structure in this area is of great significance for understanding the deep dynamics of the Tianshan orogen.In this paper,the S wave velocity structure of the Tienshan orogenic belt and its surrounding areas is studied by using the fixed seismic stations of Xinjiang Seismic Network and the wide-band mobile seismic stations that we have set up.The results are as follows.(1)Using the observation data of 52 fixed seismic stations in Xinjiang Seismic Network and 11 new mobile seismic stations in the Tienshan area,the ambient noise tomography method was used to obtain Rayleigh surface wave phase velocity distribution image of cycle 10-50 s and Rayleigh surface wave group velocity distribution image of cycle 6-35 s.(2)Using the phase velocity dispersion data of the Tienshan and adjacent areas obtained by ambient noise tomography method,the Bayesian-based Markov chain Monte Carlo(MCMC)method is used to obtain the S wave velocity structure of the crustal and upper mantle in the study area.The results show that the sedimentary cover in junggar basin is deep in the south and shallow in the north,and the thickest part can reach nearly 15 km.In the vicinity of the basin-mountain junction in the northern and southern parts of the Tianshan orogenic belt,there are some clear lowspeed zones in the crust that is inclined toward the orogenic belt.Combining previous research results,we speculated the subduction of tarim basin and junggar basin to tianshan orogenic belt mainly in central tianshan orogenic belt in China,and eastern and western tianshan are only one-way weak subduction.There are obvious lowvelocity anomalies in the lower crust of the tianshan orogenic belt,which reflect that the medium has low mechanical strength and is more prone to deformation and upliftunder the extrusion environment.The low-velocity anomalies in the lower crust may be closely related to the existence of the ancient plate suture belt,the bidirectional subduction of the continental masses on the north and south sides of the tianshan mountains and the extrusion deformation.(3)The joint inversion of surface wave and receiver function reveals the S wave velocity structure of the crust and uppermost mantle and the crustal thickness below the station beneath the Tienshan area in China.The results show that the thickness of the crust below the Tienshan orogenic belt is between 55 km and 63 km,and the thickness of the crust is the largest near the BLT seismic station in Bazhou of Xinjiang,and it is about 63 km.The average crust thickness in the Tarim Basin is about 45 km,and the crustal thickness in the Junggar Basin is 47 km.there are many obvious low-velocity layers in the crust near the basin-bearing zone in the northern Tienshan Mountains and the southern Tienshan Mountains.There are significant differences in the structural characteristics and distribution range of the low-velocity zone in the northern margin and the southern margin.This result supports the dynamics of the subduction of the Tarim Basin and the Junggar Basin to the lower Tienshan Mountain,and the subduction of the southern margin of the Tienshan Mountains is larger than that of the northern margin,and the subduction of the eastern crust is not obvious or in the early subduction stage.There are many low-velocity layers in the inner crust of the Tienshan orogenic belt,and most of them correspond to the strong uplifting areas that are currently occurring.