High-resolution 3-D Shear-Wave Velocity Model Of The Tibetan Plateau And Its Tectonic Implication

The Tibetan Plateau,known as "the roof of the world",is the highest plateau on the Earth,with its average topography of more than 4500 meters.During the Mesozoic,the Neo-Tethyan Ocean subducted northward under the Eurasian plate,and was completely consumed at about 65 Ma.The Indian plate collided with the Eurasian plate subsequently.The collision caused the lithosphere strongly folded and formed orogenic belts,resulting the high topography and thickened crust of the Tibetan Plateau.The plateau is composed of several terranes from south to north,including Himalaya,Lhasa terrane,Qiangtang terrane and Songpan-Ganzi terrane.As the world's largest and youngest continental collision zone,the Tibet Plateau has undergone complicated deformation,leaving many geological evidences,for example,several large fault systems that developed on the plateau surface.In addition,during the Neo-Tethyan Ocean subduction and the India-Eurasia continental collision,the Tibetan lithosphere underwent several stages of magmatism and metamorphism.As an important part of the Tethyan metallogenic domain,plenty of ore deposits have developed inside the plateau,in which case Tibet has become the strategic mineral storing of China.Therefore,tomographic research on the Tibetan structure has both academic significances and economic benefits.With more permanent stations and temporary stations deployed,researchers now have more detailed and insightful observations of the Tibetan Plateau.We collected continuous waveform data of permanent stations inside or around the plateau and the data from SANDWICH temporary array,recorded from 2014 to 2015.From ambient noise cross-correlation functions between stations we extracted empirical Green's functions and surface wave phase velocity dispersion at the period range from 5 s to 50 s.In addition,we included Rayleigh wave phase velocity dispersion used in previous research.We finally obtained more than 10000 dispersion curves,covering most of central,southern and eastern Tibet.We inverted for a high-resolution 3-D isotropic shear-wave velocity model of the plateau with lateral resolution of 150 km,using the direct inversion method of surface wave.Our velocity model shows that the crustal structure of the Tibetan Plateau is complicated.The low-velocity features are widespread across the middle-lower crust heterogeneously,and have certain relationship with tectonic units.We observed four prominent low-velocity zones(LVZs)and relatively narrow low-velocity bands connecting the LVZs.Our model shows significant crustal low-velocity structures with lateral variations along the Himalaya and the Lhasa terrane.In the eastern segment of the Lhasa terrane,the LVZ is spatially correlated with Miocene porphyry Cu deposits which are probably related to strong tearing of the subducting Indian lithosphere,while pre-existing weak zones contribute to the LVZ in the western segment.Meanwhile,the LVZ along the Bangong-Nujiang suture could be considered as a channel for eastward extrusion of ductile material,i.e.crustal flow on geological timescales.This "flow" in the middle-lower crust probably contributed to the formation of the V-shaped conjugate strike-slip fault system in central Tibet.The development of conjugate strike-slip faults,however,ceased near 90°E,since the eastward”flow" was blocked by an intra-crustal high-velocity block,which contains Amdo area.This rigid zone hence influences the pattern of material transport inside the Tibetan crust and the deformation of the plateau.We then use Rayleigh wave dispersion and isotropic shear-wave velocity model to invert for crustal azimuthal anisotropy.Generally,the azimuthal anisotropy patterns of the Tibetan Plateau are complicated,with obvious differences between north to south,and between layers at various depths.In northern Tibet,from the Eastern Kunlun Mountain to the eastern Songpan-ganzi terrane,the azimuthal anisotropy of shallow crust is strong,whose fast polarization axes are correlated with geological units,such as faults.This azimuthal anisotropy grows weaker in depth.In central and southern Tibet,there are no clear patterns of azimuthal anisotropy in the shallow crust,except observation of near east-west fast polarization axes in the Lhasa terrane.However,in the deeper structure,we observed strong azimuthal anisotropy in southern and eastern Tibet,whose directions are pointing toward the outside of the plateau and have lateral variation of their azimuths.This indicates that the growth of the plateau may be attributed to a combination of several tectonic models.In this paper,high-resolution 3-D shear-wave velocity model(including isotropic part and azimuthal anisotropy part)of the Tibetan Plateau is obtained using ambient noise tomography methods,which provides a seismic reference for studying the Tibetan lithosphere and its deformation.Based on numerous geophysical and geological observations,we discuss the complicated deformation of plateau and its relationship with the metallogenic process.