Receiver Function Study Of The Crust And Mantle Structure In Northeastern Tibetan Plateau

The depths of Moho and lithosphere-asthenosphere boundary(LAB)in the northeastern Tibetan Plateau were obtained by using P and S-wave receiver function method.The P-wave receiver function method is used to obtain the Moho,but due to the interference of multiple waves in the crust,the P-to-S converted phase from LAB in the P-wave receiver function is ambiguous.While the S-wave receiver function takes advantage of the long period and low frequency of the S-to-P converted phase to explore the discontinuities,so the S-wave receiver function has been widely used in the researches on LAB.In this paper,we use three-component seismic data from 43 portable broadband seismic stations deployed in the northeastern margin of the Tibetan Plateau operated from May 2008 to July 2010.Firstly,the Moho depth and Poisson's ratio below each station are obtained by H-k stacking and CCP stacking.The results show that the average thickness of the crust in the study area ranges from 44.0 km to 60.6 km.The average thickness of the crust beneath the eastern margin of the Songpan-Ganzi block is about 51 km to 54 km,and gradually thickens from east to west.The thickness of the crust beneath the West Qinling orogenic belt is about 54.3km.The average thickness of the crust beneath the eastern margin of the northern Qilian block is 50 km to58.5km,and gradually thickens from east to west.The thickness of the crust beneath the Songpan-Ganzi block and the West Qinling orogenic belt is thinner than that of the eastern margin of the Qilian block in the north.It is speculated that this may be related to the existence of weak material flow in the middle and lower crust of the Tibetan Plateau and the continental collision.According to H-k stacking results,the variation of P/S velocity ratio is 1.56-1.84,and the Poisson's ratio is 0.15-0.28.The lower wave velocity ratio is mainly caused by felsic acid rocks.It is speculated that the crustal thickening may be caused by the superimposition of the upper crust.The higher wave velocity ratio distribution indicates that the phenomenon of melting of the upper crust is unlikely.The results show that the depth of LAB in the northeast of the Tibetan Plateau ranges from100 km to 150 km,about 100 km to 130 km in the eastern margin of the Songpan-Ganzi block,120 to 140km in the western margin of the West Qinling orogenic belt and 125 to 150 km in the eastern margin of the Qilian block.From the above results,it can be concluded that the lithosphere thickness beneath each station in the study area is gradually thickening from south to north,from Songpan-Ganzi block to the eastern margin of Qilian block.Generally speaking,the lithosphere thickness in the west is thicker than that in the East,and that in the north is thicker than that in the south.Combined with the results of this study and previous geophysical exploration,it is speculated that the LAB may be related to the subduction of the Alxa block,and the blocking effect of the Ordos block also affected the lithosphere outward expansion of the Tibetan Plateau.This resulted in the uplift and growth of the northeastern of the plateau.