The Study Of Rayleigh Surface Wave Tomography In And Around The Northeastern Margin Of The Tibetan Plateau

The northeastern margin of the Tibetan Plateau is the front part of Qinghai-Tibet Plateau to inland extension, this region has undergone strong orogenic movement and formed several large faults since middle and late Miocene. There is a huge boundary zones between the northeast Qinghai-Tibet Plateau and the Ordos Block, as one of the most active earthquake zones in China. It is of great significance to study the structure of crust and upper mantle in this region to reveal the deep boundaries between different tectonic units, the causes of continental dynamics and continental interior earthquakes. It’s also ideal place to test the models of plateau’s uplift and evolution.The project ChinArray Phase II has deployed 670 broadband seismic stations in and around the northeastern margin of the Tibetan Plateau. It is the highest number and density of earthquake observation array in this region. In this paper, the Rayleigh wave apparent phase velocity from 10 to 120s is studied by surface waves tomography based on eikonal equation, which is obtained using the observation data from the broadband density array. We have fitted the azimuth anisotropy of the phase velocity based on the basic hypothesis of weak anisotropic medium (20-60s). Then we applied Helmholtz equation to correct the phase velocity, which result in structural phase velocity (20-120s).Finally we discussed the crust-mantle structure characteristics and its tectonic implication, the relationship between phase velocity and seismicity, and the implication of azimuth anisotropy results, which is obtained using high resolution phase velocity and azimuth anisotropy distribution results combined with the regional tectonic background and the research achivements of predecessors have made. In this paper, the main results and conclutions are as follows:(1)From periods of 16 s to 60 s, distinct low-velocity anomalies exist in the west of Qilian Fold System, in middle-Qinling Fold System and in Songpan-Ganzi Fold System. It suggests low mechanical strength of the crust in these places, which results in the crust deforming easier under strong tectonic stress.The difference characteristics of the phase velocity within the plateau is closely related to the distribution of topography, the moho depth, and the tectonism.(2)The crustal structure is given priority to high velocity in midlle part of Qinling fold system which lies between Ordos block and Sichuan basin, indicating that there is no obvious layer structure in the middle and lower crust. So even there are crustal channels within the Tibetan plateau, it is hard to flow between the Sichuan basin and the Ordos block, or the channel flow in this area is not obvious.(3)The Ordos block is featured by high-velocity anomaly(16-60s), but its western border is lower than interior block, and shows obvious horizontal uneven characteristics, this phenomenon reflects that the edge of the western region has been affected by tectonic action,which results in certain degree of tectonic deformation. The velocity within north zones of Alashan block is between Ordos block and Tibetan Plateau(12-40s), but closer to the stable Ordos block, we confirm that the middle and lower crust is characteristic of high velocity and small lateral change, it is relatively stable block. The velocity of Alashan block is between Tibetan Plateau and its northern block, which reflects that the Alashan block may be undergoing early damage.(4) The northwestern margin of Ordos block, Linhe fault basin and western margin as Yinchuan fault basin, shows local low-velocity characteristics within short period (12-20s). Unlike Yinchuan fault basin, the low velocity continues to 120s, speculating that there may exist thennal material channel from the asthenosphere. The hot material cause partial melting in lithosphere, which results in significant low velocity in this region.(5) When the sensitive depth reaches to lithosphere and asthenosphere (60-120s), the significant low velocity in margin of northeastern almost is connected into a whole, and extend to western margin of Ordos block, which may reflect the whole mantle lithosphere in the northeast of Tibetan Plateau are influenced by the strong tectonism of plateau uplift, while the hot material cause lithosphere thinning or partial melting. The high velocity in Ordos block reflects its retained high velocity lithosphere from the ancient craton. At the same time, the south of the block is higher than the north, which may reflects the thickness of the lithosphere exists obvious difference. The middle part of Qinling fold system lies between both sides of high velocity, which may indicating thin lithosphere, that suggests low velocity asthenosphere material may flow from Qinling orogenic belt to the east.(6) Most of earthquakes distribute near the block borders and transition regions between different blocks, and these seismically active areas correspond with low velocity or high and low velocity transitions(16s).(7)The fast wave direction has the characteristics of clockwise, which is similar with the extentional orientation. This paper argues that the crust of Qilian fold systems gradually shorten and thicken due to the NE direction’s strong tectonic compression from the Tibetan Plateau, at the same time it is resisted by Alashan block and Ordos block, which results in the orientation of the crystal lattice being parallel to the flow direction of extensions, showing the characteristics of anisotropic direction roughly parallel to the fold.