A Multi-profile And Multi-phase Tomographic Study Of The Crustal And Upper-mantle Structure Of South Tibet

In order to map the P-wave velocity structure beneath central-eastern Gangdese,we performed teleseismic P-wave travel-time tomographic inversion to the data recorded by 197 portable broadband seismic stations.Our resultant images reveal complicated spatial pattern of the velocity structure in the study region.Firstly,a strong low-velocity anomaly is seen lying beneath Tethyan Himalaya north of the South Tibet Detachment(STD).In places far from the north-south rift systems,this anomaly is clearly blocked by the Yarlung Zangbo Suture(YZS)on its northern side,quite different from those stated in published papers.We also observe some low-velocity anomalies beneath the Yadong-Gulu(YGR)and the Comei-Sangri(CSR)rift systems,but both of which are less bold and possess smaller depth extension than the conspicuous one beneath Tethyan Himalaya.Instead,a strong high velocity anomaly is seen in the crust between the YGR and the CSR in the southern Lhasa Terrane.These observations favor the channel flow,an essential role in the crustal deformation and evolution of the Tibetan plateau,only widely distributed beneath Tethyan Himalaya,but confined to finite space centered on the two rift systems beneath Gangdese on the north of the YZS in the study region.Secondly,the high-velocity anomaly in the upper mantle,interpreted as a manifest of the Indian lithospheric mantle,is seen extending more than 100 km north of the YZS in the western part of the study region,but stagnates under High Himalaya,?100 km south of the YZS in our eastern study region.This is consistent with some previous results which show a systematic west-to-east change of the subduction angle of Indian lithospheric mantle in the study region(eg.Li Chang et al.,2008).Finally,our tomographic result confirms the observation that deep-seated low-velocity anomalies root into the base of upper mantle,situated to the southeast of Gangdese(eg.Ren Yong et al.,2008),but we attribute this phenomenon to the eastward subduction and westward retreat of the Burmese arc during clockwise rotation of the Sunda block(ca.Russo,2012).In order to deepen our comprehension about the crustal and upper mantle structure beneath southeastern Tibet revealed by teleseismic tomograms,we also performed measurement of shear-wave splitting parameters.Our new results confirm the occurence of west-to-east variation of crustal and upper mantle structure at-91.5° E in southeastern Tibet.We have not observed apparent shear-wave splitting west of?91.5°E(also see Huang et al.,2000;Chen WP et al.,2010),where India Plate is suggested to be underplating or thrusting under(see Shi et al.,2016),with its northernmost extent upto the vinicity of the Jiali Fault.Complicated anisotropic pattern is observed east of-91.5°E in central-eastern Gangdese,above the athenospheric wedge resolved by receiver fuction profiles(Shi et al.,2015),north of India mantle lithosphere subducting at steep angle(Shi et al.,2016).Conspicuous shear-wave splitting is lacking above the area where an asthenospheric upwelling has been proposed,perhaps due to near vertical flow in the asthenospheric mantle(cf.Fu et al.,2008),but is present in some other regions,where mantle flow may have occurred in sub-horizontal directions.We speculate that the complex flow directions in asthenospheric mantle are responsible for the complex pattern of anisotropy revealed by the measurement of shear-wave splitting and the complicated crustal and upper mantle structure revealed by teleseismic tomograms.