Topography Of The Upper Mantle Discontinuities In The Northeastern Margin Of Qinghai-Tibet Plateau Studied By Dense Broadband Array Receiver Function Data

The 410 km discontinuity(’410’)and 660 km discontinuity(’660’)are the two most important discontinuities in the upper mantle.They may exhibit different characteristics under different temperature and pressure conditions.At the same time,the depth fluctuations of the two discontinuities can reflect the distribution of high and low velocity anomaly in the upper mantle to a certain extent.It also has important significance for analyzing and inferring upper mantle dynamic structure,mantle heat flow and plate subduction in our research area.In this paper,more than 80,000 original waveform data collected by 150 broadband intensive mobile stations that located on the southeastern part of Gansu province.The receiver functions method and grid stacked method are used to extract the Ps converted waves arrival time of the two most important upper mantle discontinuities(’410’ and ’660’).Referring to the Iasp91 model,the regional 3D model provided by Steve Grand combined with Crust1.0 model,Lei and Zhao’s 3D regional velocity model,the depths of the upper mantle ’410’ and ’660’ discontinuities are estimated by ray tracing method.Particularly,the results of the wave velocity perturbation obtained by tomography method are used to discuss the distribution of high and low velocity anomalies beneath this area,and then,other distribution properties in our research area are further discussed.our main research area is the northeastern margin of the Qinghai-Tibet Plateau and its adjacent areas,which are located at the convergence zone between the Qinghai-Tibet,Alxa,South China and Ordos blocks.For a long time,due to the interaction of several blocks,the deep structure of this region is extremely complex,and the internal structure changes are very dramatic.Therefore,the deep structure of this region is further explored by using the dense broadband array data,which has a certain representativeness in global range.Our results show that:(1)The P410 s and P660 s converted waves generated by the two upper mantle discontinuities are slightly behind the prediction results of the global theoretical model.The P410 s and P660 s arrival times have delays of almost 1 second(s)compared with the global model.Contrasting to the theoretical results the MTZ thickness of our research results are also thickened.(2)The synchronous lag of P410 s and P660 s is consistent with the low velocity anomalies in the upper mantle formed by the eastward materials migration beneath the northeastern margin of Qinghai-Tibet Plateau.(3)Combining with the previous tomography results,the thickening MTZ beneath this area is mainly caused by the depression of the ’660’ discontinuity,which may consist with the existence of the Big Mantle Wedge model proposed by previous tomography results.However,due to the limitation of our data,we can’t conduct a further study,meanwhile,we also need more data to analyze the specific details of the Big Mantle Wedge model with our precisely research methods.In the future,we expect to do more further study on the distribution properties of the upper mantle discontinuities beneath this area through a large amount of data.