The Structure Of The Crust Beneath Eastern Tibetan Plateau

The Tibetan plateau,known as the "roof of the word",is the product of the collision between the Indian and Eurasian plates during the Late Cenozoic.The formation,uplift and expansion mechanisms of the Tibetan plateau have been paid attention by so many researchers.Since 1920s,a variety of models have been proposed to explain crustal shorting and thickening,lateral extrusion,and "disappearance" of lithospheric material,such as underthrusting model,extrusion model,continuous deformation model,multi-factor driving model,lower crustal floe model and so on.Compared with the flat central Tibet,the eastern Tibetan plateau is still in the primary stage of tectonic deformation and is an ideal place to study the lateral growth of the plateau.In this paper,the high-quality seismic data recorded by the ChinArray is used to explore the shear-wave velocity(Vs)and radial anisotropy(ξ)of the crust in the southeastern(SE)margin and northeastern(NE)margin of the Tibetan plateau and its surrounding regions,thus to probe their implications for plateau growth mechanism.We jointly invert the surface-wave dispersion,Rayleigh-wave ZH ratio,and P-wave receiver functions(RFs)with a stepwise linearized joint inversion method to obtain a highresolution 3D Vs structure in the SE and NE Tibetan plateau and its surrounding regions.Surface-wave dispersion can constrain the smooth variation of absolute Vs structures,whereas RFs are sensitive to discontinuities in the crust and upper mantle.The ZH ratio provides localized structural constraints beneath a station,similar to RF data.The ZH ratio is very sensitive to shallow crustal structures,of which the depth extent of sensitivity kernels is approximately half that of Rayleigh-wave dispersion at the same period.Therefore,the ZH ratio can provide complementary information for crustal structure inversion.In this paper,joint inversion of three data sets can provide much better constraints on the crustal Vs structure beneath the study area.Additionally,crustal ξhas also been studied in the NE margin of the Tibetan plateau and its surrounding regions,because studying ξ can not only provide the most direct evidence of the flow and deformation in the earth’s interior,but also can be combined with the analysis of velocity structure to better understand the crustal deformation model.The Vs structure in the SE margin of the Tibetan plateau shows that there are two main NS-trending low-velocity zones(LVZs)observed in the mid-to-lower crust.One of the LVZs is bounded by the Anninghe-Lijiang-Ninglang-Chenghai fault as the southern boundary,starts from Chuanxibei sub-block and continue to extend southwest to Yunnan-MyanmarThailand block.The other one follows the trace of Xiaojiang fault and Puduhe fault until about 24°N.These two LVZs are separated by the central Dianzhong sub-block(the inner zone of Emeishan Large Igneous Province(ELIP)in the middle crust.The relations between LVZs and major faults suggest the major faults play an important role in the tectonic evolution of the SE margin of the Tibetan plateau.Analysis with Vs<3.4 km/s,high electrical conductivity,strong attenuation,high Vp/Vs ratio,high surface heat flow,and positive ξ,suggests that these two LVZs are probably attributed to partial melting.We further suggest that these two LVZs are mid-to-lower channel flow.From 10 km below the inner zone of the ELIP to the Moho,the high Vs anomalies generally appear to be caused by volcanism and basic and ultrabasic rocks intruding into the crust during the deformation of the Permian ELIP.The crustal Vs and ξ structures beneath the NE margin of the Tibetan plateau and surrounding regions show that the negative ξ exhibited in the upper crust of the study region caused by the shear motion of large strike-slip faults and preferred vertical alignment for cracks.Although the mid-to-lower crust of the study region generally exhibits low Vs and positive ξ,the tectonic deformation mechanism is not completely consistent.The LVZ and positive ξ caused by partial melting beneath the western Qinling orogen and the Songpan-Ganze block many be due to the combined effect of shear heating and asthenosoheric upwelling.The LVZ and positive ξ in the northern Qilian orogen are more likely to be related to the ductile crustal thickening.Furthermore,we suggest that there is no mid-to-lower crustal channel flow in the NE margin of the Tibetan plateau.Almost equivalent Vs anomalies and the distribution of ξ,as well as the consistent fast directions of SKS phase are observed in the Alxa block and Ordos basin,we propose that the Alxa block and the Ordos basin as a whole are a part of the North China Craton.By analyzing the Vs and ξ models obtained by joint inversion of surface-wave dispersion,Rayleigh-wave ZH ratio,and RFs and DRadiSurfTomo methods in the NE margin of the Tibetan plateau and surrounding regions,we speculate the crustal structure beneath the Alxa block may be modified by the far-field stress that originated from the collision between the Indian and Eurasian plates,and the deformation is not only confined to the boundary areas,but also has a complex tectonic deformation inside the block.The crustal Vs structure obtained by joint inversion shows that relatively insignificant low Vs anomalies are observed at the depth interval of 20-35 km beneath the Yinchuan-Hetao graben.Since the velocity of the LVZ(3.43.5 km/s)is not lower than the standard value of partial melting,the formation of the LVZ is more likely to be related to the underplating of mafic magmas,rather than possibly caused by partial melting.Based on the results in the SE margin and NE margin of the Tibetan plateau as well as its surrounding regions,we suggest that the eastward expansion of the Tibetan plateau material only deflects in the southeast direction being blocked by the rigid Sichuan basin,instead of being divided into the northeast and southeast branches as predicted by Clark and Royden(2000).The crustal flow in the NE margin of the Tibetan plateau can flow smoothly with the help of major faults,but in the NE margin,the crustal flow cannot be formed under the blocking of the rigid Alxa block.