Crustal And Uppermost Mantle Velocity Structure Beneath The Central-eastern Tibetan Plateau From P-wave Tomography

Being the main region of the Tibetan Plateau,the central-eastern Tibetan Plateau consists of the Qaidam block,the Baryan Har block,the Qiangtang block and the Lhasa block.It is an important area for studying the formation and evolution of the Tibetan Plateau and verifying the existence of low-velocity zones and channel flows in the middle and lower crust because of having a great number of faults,frequent earthquakes and large quantities of media migration from the middle to the east.Studying the velocity structure and physical properties of the crust and upper mantle in the centraleastern Tibetan Plateau can help us understand the distribution of velocity anomalies and heat flow in the crust and upper mantle,as well as verify the distribution of channel flow,which plays an important role in understanding the mechanism of continental collision and growth process.Based on joint inversion with a large dataset of P-wave(Pg and Pn)first arrival times picked automatic from event waveforms,we obtained regional 3-D P velocity structure beneath the central-eastern Tibetan Plateau,including crustal P velocity,uppermost mantle P(Pn)velocity,and Moho depth.The results show that the crustal and uppermost mantle structure is strongly heterogeneous,which generally correlates with geologic blocks.The result shows that crustal P velocity ranges from 5.2 to 7.2 km/s,the Pn velocity ranges from 7.7 to 8.4 km/s,and the crustal thickness ranges from 48.0 to 68.6 km.Prominent low velocity zones exist in the middle and lower crust.We found great difference in the distribution of high and low velocity anomalies between upper crust and mid-lower crust.High velocity anomalies primarily exist in the upper crust in the Qiangtang block and the Baryan Har block,while become lower as depth increases.The Qaidam Basin shows low velocity anomaly in the upper crust while high in the lower crust.High Pn velocity exists in the Qaidam Basin and the Lhasa block,with a maximum velocity of about 8.4 km/s,while the Baryan Har block and the Qiangtang block mainly show low Pn velocity anomalies,which reach as low as 7.7 km/s.Crustal thickness is mainly characterized to be thick in the south and thin in the north.The crust is thicker in the eastern Qiangtang block and the Lhasa block,while thiner in the Qaidam Basin,the eastern Baryan Har block and the western part of the Qiangtang block,which shows the characteristics of lithospheric deformation due to the northward subduction of the Indian plate.