Ambient Noise Tomography In Northeastern Margin Of The Tibetan Plateau And Its Adjacent Areas Based On Dense Array

Since the Indo-Eurasian collision has resulted in significant uplift and intense intracontinental deformation of Tibetan Plateau.The Northeastern margin of Tibetan Plateau is a tectonic transition zone between Tibetan Plateau and North China Craton.Since late Cenozoic,the tectonic deformation in this area has been intense,and it is a key area for studying the uplift,lateral growth and the interaction between the plateau and the surrounding blocks.The study of the crust and upper mantle structure based on various tools to obtain different subsurface structure parameters can provide an significant insight for understanding the continental dynamics and evolution process.Ambient noise tomography based on seismic wave interferometry technique is an effective tool for subsurface structure detection in recent years and has been widely used in the study of crust and upper mantle structure at different scales.Based on the development of ambient noise interferometry technique,this paper studies the crust and upper mantle structure of Northeastern margin of Tibetan Plateau and its adjacent areas by using the continuous waveform records of the dense array deployed in the northern of North-South seismic belt.The main achievements of this paper are as follows:1.The ambient noise cross-correlation functions of 9 components(ZZ,ZR,ZT,RZ,RR.RT,TZ,TR,TT)between about 230,000 station pairs were obtained by using the threecomponent continuous waveform data recorded by 666 broadband stations during 2015.2.The Moho reflected wave(PmP)with a period of 2-10s was extracted from the ZR component of noise cross-correlation functions using array stacking method,and the phase weighted stacking method for common reflection point is applied to obtain the depth of the Moho.The results show that,the thickest crust is located in the western of Qilian Orogen ranging from 55 to 65km,meanwhile,the crust thickness of the northwestern of Ordos Block and Western Qinling Orogen is even thinner,ranging from 30 to 40km,consistent with the previous result from the receiver functions.In addition,sharp PmP phase are observed under Liupanshan fault,Haiyuan fault and Tianjingshan fault,suggesting the steep Moho interface structure and local crustal thickening.The results show superimposed Moho structure in the northern margin of Qilian Orogen,which indicates shortening and thickening of the crust.3.We have developed a frequency-wavenumber domain(F-K)filtering method for noise cross-correlation functions to remove or suppress interference signals such as surface waves or negative slowness signals.Then,we have extracted the P410S and P660S reflected and converted phases of the upper mantle discontinuity from noise cross-correlation functions with the period band of 5-10s,using the phase weighted stacking method for common conversion point.The results indicate that the average depths of the 410-km and 660-km in the study area are 415km and 665km,and the average thickness of the mantle transition zone is about 250km,which is consistent with the global average.The depression of the 660-km discontinuity is observed beneath the Northeastern Songpan-Ganzi block and Western Qinling Orogen,with the thickening of the mantle transition zone,suggesting that it may be related to the stagnant Indian subduction slab.4.We obtain the Rayleigh and Love wave phase velocities of 8-40s period in Northeastern Tibetan Plateau using Eikonal tomography based on waveform crosscorrelation.Based on the surface wave phase velocity results,we have constructed a high resolution 3-D azimuthal anisotropic shear wave velocity model.Our model indicates that,in the shallow crust,the alignment of fast direction of Northeastern Tibetan Plateau is parallel to the direction of large faults,and the low velocity layer is generally distributed in the middle and lower crust with strong anisotropy.The azimuthal anisotropy and fast direction in the lower crust in Western Qilian Orogen are basically consistent with the upper crust,showing near E-W and WNW-ESE directions,parallel to the boundary of Northeastern Tibet Plateau,with stronger anisotropy in the lower crust.It is suggested that the low-velocity layer and stronger azimuthal anisotropy of the lower crust in the Qilian Orogen are predominantly due to the shortening and thickening of the ductile crust.It has strong anisotropic strength in the northeastern of Songpan-Ganzi block,with the same fast wave directions in the upper and lower crust,showing the NW-SE direction,indicating coupled deformation between the crust and upper mantle.So,it is indicated that the low-velocity layer and strong anisotropy in the middle and lower crust may be related to the upwelling of thermal material in the asthenosphere.The strong anisotropy of the lower crust may be related to local melting caused by the upwelling of thermal materials from the mantle in Hetao Basin.The anisotropy is weaker in Haiyuan-Liupanshan fault,which may be the northeastern boundary of the Tibet Plateau,suggesting that the counterclockwise rotation of Ordos block may help to weaken and absorb the deformation.5.We obtain the radial anisotropy of the crust in Northeastern Tibetan Plateau,using Transdimensional Monte Carlo nonlinear inversion method within Bayesian framework with joint inversion of Rayleigh and Love wave phase velocity maps.Meanwhile,On the basis of previous studies,we develop the code of nonlinear Monte Carlo surface wave anisotropy inversion.The results show that there are significant radial anisotropy differences in the study area.The upper crust is mainly characterized by negative radial anisotropy(Vsh<Vsv)in Northeastern Tibet Plateau,which may be related to the development of vertical fractures or fault near the surface,but it exhibits positive radial anisotropy(Vsh>Vsv)beneath Ordos and Alxa blocks;In the lower crust,our results show positive radial anisotropy(Vsh>Vsv)in most areas,however,it displays slightly negative radial anisotropy(Vsh<Vsv)in the mid-to lower crust of the western Qilian Orogen.It is suggested that it may exist shorten laterally and thicken vertically in the western Qilian Orogen due to the extrusion between the Alxa and Tibet block,resulting in slightly negative radial anisotropy.In addition,the lower crust exhibits negative radial anisotropy beneath the southwestern of the Ordos block,indicating that the crust material in Northeastern Tibetan Plateau was pushed into the Ordos block,which is probably the convergence and extrusion between Tibetan and Ordos block,resulting in the uplift of the Liupanshan and the vertical thickening of the lower crust.