Research Of Lithospheric Discontinuity Structure Beneath NE Tibetan Plateau From Receiver Function

The lateral growth and deep processes of the Qinghai-Tibet Plateau uplifting are the keys to understand the mechanism of continental collision deformation and evolution.Based on geophysical and geological studies,many of the tectonic processes models of the Tibetan Plateau growth have been conducted in recent years,such as underthrusting model,lower crustal flow model,distributed shortening model,intracontinental subduction model,delamination model and extrusion model.In the south Tibetan,subduction or underthrusting of India toward Asia has played an important role in the uplifting.However,the deformation mode of northeastern Tibetan Plateau(NETP)remains controversial for lack of sufficient proofs.In recent years,the results from surface wave inversion,receiver function inversion or joint inversion,ambient seismic noise and travel time tomography method showed that the softer NETP has a slower seismic velocity while Alxa block and Ordos block of North China Craton(NCC)are opposite,which indicates that the NETP is resisted by the harder NCC.However the dynamic process of the NETP uplifting remainscontroversial because the previous research scale is limited or the data distribution is insufficient.Since the Quaternary,the Ordos terrane has been counter-clockwise due to the dragging of the southeastern mantle and the uplifting of NETP.The Alxa block moves slowly eastward under the joint regulation of large-scale faults around it,and converges with the Helan Mountains and the Ordos block.The boundary zone between the Alxa block and the Ordos block is the Yinchuan-Hetao basin,and its origin is thought to be the mantle convection,which is caused by the westward subduction of the Pacific plate and the collision of the Indian-Eurasian plate.The deep structure of the area is crucial for understanding the deformation between blocks and the dynamic model of the NETP.The seismic observational data is relatively insufficient,so the acquisition of deep seismological evidence in the region is particularly important.In this paper,we used the teleseismic broad-band waveforms,recorded from the China Array seismic experiment during 2013-2015 and QL temporary stations which we deployed across the Haiyuan Fault and Kunlun Fault during 2016-2017.We apply 3-D Common Conversion Point(CCP)stacking technique with the receiver functions to image the lithospheric and upper mantle velocity discontinuities structure in the northeastern Tibetan plateau,which covers the Alxa block,the Ordos block,the Qilian Orogenic,the Kunlun-West Qinling block,and northern part of the Songpan-Ganzi Terranes.We try to provide an explanation for the dynamic process of growth and uplifting in the NETP.Furthermore,we want to detect the crustal structure and obtain the deformation mode between the Alax and Ordos blocks.Our results show that:1)Our observations reveal the thickest lithosphere in the North Qilian(NQL)and it has been inserted by lithosphere of Central Qilian(CQL).Meanwhile,the main features in the crust include offset of Moho beneath NQLF,shallower crust thickness below between the NQLF and LSSF and a continuous positive interface over the Moho in the north of the LSSF.2)Interpretation of these features about lithospheric interface structure in the area,combined with previous research conclusions,implys that lithosphere had been passive underthrust and localized crust had been shortened and thickened in the NETP.At the same time,the middle crust in the interior of south edge of Alxa block has been affected by the above deformation.We argue that this is the main dynamic process of the uplift of the NETP.3)The Moho and Conrad discontinuity in Ordos are 38-42 km and 18-22 km,respectively.The crustal thickness of Alxa block is 38-45 km,and the Moho lies at a depth of ?52 km beneath the Hetao basin.The deepest Moho depth is ?55 km,located around Helan Mountain.4)Combining our results with the previous research from different aspects,we further confirmed that the Alashan block and the Ordos block belong to different terrestrial tectonic units.At the same time,we conjecture the existence of crustal thickening in the east of Helan Mountain and the western edge of Ordos.In recent years,the method of S-wave receiver function(SRF)has become one of the most important technical means in the research of Lithosphere Asthenosphere Boundary(LAB),and the changing variation of LAB was a crucial seismic evidence of regional geodynamics.In anisotropic media,the Ps converted phase arrival time of receiver function(RF)has cosine variation with the backazimuth.The delay SRF converted phase arrival time of different backazimuth is more obvious because SRF' ray path and direct wave incident angle are larger than PRF'.The anisotropic medium(the seismic wave with different directions of propagation exist the speed variation)can affect the arrival times and amplitudes of the converted phases,so it may have a strong impact on the receiver function migration technique.The receiver function migrationmeans RF phase arrival time is transformed to the depth of subsurface discontinuity where seismic wave refraction occurred,and there will be deviation in the imaging of interface depth for seismic wave propagating through the strongly anisotropic media.According to many geophysical studies of the NE Tibetan Plateau conducted in recent years,there are a large number of seismic anisotropy observations in this region.In the strongly anisotropic region of the NE Tibetan Plateau,the delay times of crust and upper mantle are as large as 0.4 s and 1.6 s,which means that a deviation of more than ten kilometers will be brought in when the SRF migration technique is used to image LAB.Another aim of this study is to correct the Sp converted phase arrival time of SRF in anisotropic medium to improve the accuracy of RF migration imaging.We first revealed the discontinuities of crust and upper mantle by using the synthetic receiver functions in the different models of anisotropic medium.Then we developed a technique for correcting the variation of the anisotropic S-wave receiver function converted phase arrival time to avoid the influence of anisotropy(HTI)on SRF migration imaging.Our results show that:5)We developed a useful technique for correcting travel time variation of SRFs based on HTI model,successfully aligning converted phase of Moho and LABs with different back-azimuth receiver function in anisotropic media with single and multiple layers(the same or different fast axis directions)velocity model.6)The results show that the energy of summed conversion phases has been enhanced and the depth of converted interface after travel time correction becomes easier to identify.Our method would be of great importance in improving the quality in the case of 3-D RF imaging.