Seismic Anisotropy In Typical Tectonic Zones Of Eastern Margin Of Qinghai-tibet Plateau

Driven by crashing, extrusion and continuous drifting of Indian plate in the south,and synchronously prevented by Russian-Siberia platform in the north, theQinghai-Tibet plateau, on the one hand, absorbs the north-south strong compressionthrough the crust thickening and shortening, so that it forms complicated structure inthe northeast of Qinghai-Tibet plateau. On the other hand, the plateau materialsescape to east and west respectively. However, no matter what the escape way theyare, the Sichuan-Yunnan regions in the south-eastern Tibetan plateau are the domainswhere the tectonic escape and lower crust flow pass through. Investigating the crustand mantle anisotropy and deformation mechanisms, will be helpful to know the deepstructure, the characteristic of deformation field, the stress-strain field and tectonicprocess and so on. Furthermore, it is very important to make acquaintance of themechanism of the plateau uplift and the escaping of the deep materials.Based on teleseismic PKS，SKS and SKKS phases (XKS) recorded by41broadband stations in Gansu province in northeastern margin of Qinghai-Tibet plateau，this study obtains the splitting parameters of fast polarization direction and delay timebetween the fast and slow shear waves at each station using the minimum transverseenergy method，and then，plots the distribution map of upper mantle anisotropyaround this area. Furthermore， combined with GPS velocity field and crustalanisotropy from near-field shear-wave splitting using SAM method，we discuss thecharacteristics and formation mechanism of the crust-mantle anisotropy in thenortheastern margin of Qinghai-Tibet plateau.The main results demonstrate that the fast wave polarization trends WNW-ESEin the west part of Altun fault，siting at a certain degree angle to the strike of regionalstructures. In Qilian-Hexi Corridor tectonic region，the XKS fast direction is NW-SE，consistent with strike direction of the main fault in this area. In Longzhong Basin andadjacent regions， which located in the transition zone between the active Qinghai-Tibet block and comparatively stable Ordos block，with more complextectonic background, the fast direction trends WNW-ESE, but owing to local feature’sinfluence，the results vary obviously with these stations.By comparing the crustal anisotropy of our study with that from othershear-wave splitting analysis of direct S-wave, we find that the fast direction obtainedfrom near-field shear-wave splitting is NE-SW，consistent with GPS velocity relativeto stable Eurasian Continent, and different with XKS fast direction, which suggeststhat the deformation mechanism of them may be different. In Longzhong Basin andadjacent regions, the fast directions in the crust and in the upper mantle are consistentby and large，which means that the crust and the upper mantle possibly have the samedeformation.Based on continuous ambient noise recorded by56broadband stations in Yunnanprovince in southeastern margin of Qinghai-Tibet plateau，this study obtains thedistribution of Rayleigh wave phase velocity and the splitting parameters of fastpolarization direction and delay time between the fast and slow waves in5-40s periodusing ambient noise correlation. Furthermore，combined with anisotropy separatelyfrom shear-wave splitting using S-wave, Pms and XKS, we discuss the characteristicsand formation mechanism of the crust-mantle anisotropy in the southeastern marginof Qinghai-Tibet plateau.The results of phase velocity distribution shows that, at5-12s period, the phasevelocity is related to shallow crust structure. Low velocities exist between theLancangjiang fault and Chenghai-Honghe fault, west of Puduhe fault and north ofMile-Shizong fault. High velocities exist between Yimen fault and Puduhe fault. At16-26s period, phase velocity mainly reveals the speed variation in the middle-lowercrust. The Sichuan-Yunnan rhombic block, which enclosed by Honghe fault, Xijiangfault and Lijing-Jianchuan fault, scatters low velocity in large regions. And highvelocities mainly exist in northeast of Yunnan. At30-40s period, phase velocitymainly reveals the speed variation from the lower crust to the upper mantle.Remarkably, the velocity in Sichuan-Yunnan rhombic block gradually turns to highvelocity anomaly. According to the variation of Sichuan-Yunnan rhombic block, it is possible that the middle-lower crust is channel flow of Qinghai-Tibet Plateau.The results of azimuthal anisotropy distribution show that, at5-12s period, thefast wave polarization is influenced by the strike of the regional structures. At16-26speriod，fast wave polarization direction coincides with upper crust. In nearby Yimenfault and Puduhe fault in middle Yunnan block, the fast wave polarization rotates fromNS to WN, and in the west of Yimen fault, the direction trends NW, which suggestthat plateau materials flow east, and Sichuan-Yunnan block is squeezed by theQinghai-Tibet plateau. At30-40s period, in Yunnan-Burma block and Indosinianblock, the fast wave direction trends NS and NNW. And in middle Yunnan block,there is clockwise rotation about direction.According to comparison with S-wave splitting, Pms splitting and XKS shearwave splitting, we find that along with the increase of the period, the fast wavepolarization is similar to the XKS shear wave splitting, and sites at a certain degreeangle to the direction of fast wave in the crust. In short, crust-mantle formationmechanism may be different in the southeast of Qinghai-Tibet plateau.