Crustal And Uppermost Mantle Velocity Structure In The Southeastern Tibetan Plateau By Two Plane Wave Tomography

The Tibetan plateau,the highest,largest and youngest plateau on Earth created by the Indian-Eurasian continental collision since 50 Ma,is characterized by complex geological structure and high seismic activities.The southeastern margin of the Tibetan plateau(SE Tibet)which is a strong deformation zone,has been an important region to study the processes of mountain building and continental collision.Several models have been proposed to explain the deformation of the Tibetan plateau,such as rigid block extrusion,mid-lower crustal flow,and the continuous deformation.The SE Tibet is not only an important site for mid-lower crustal plastic flow,but also an important channel for the “escape” of the excess crust.Therefore,studying the three-dimensional shear velocity of the crust and uppermost mantle is of great significance for understanding the tectonic dynamics evolution process of the SE Tibet.In this study,we analyze a large amount of teleseismic data recorded by 350 temporary seismic stations under the project “China Seismic Array-Phase 1” from June 2011 to March 2014.We select 605 teleseismic events with magnitude larger than 5.5 and epicentral distances between 30°-120°.By applying the two-plane-wave tomography method,we first generate Rayleigh wave phase velocity maps from 20 s to 143 s.Then three-dimensional shear wave velocity structures from the surface to 150 km are inverted based on surface wave dispersion data.The results show that(1)the Tengchong volcano is clearly featured with low velocity below 60 s,indicating that the low-velocity zone(LVZ)below the Tengchong volcano may come from the top of the uppermost mantle.(2)We find two LVZs around the main strike-slip faults in SE Tibet.One of the LVZs extending southward to 24°N,mainly distributes along the Xiaojiang fault,and the other LVZs extending southward from the Lijiang fault to the Tengchong fault.(3)In the uppermost mantle,the speed shows obvious partitioning features.North of 26°N it is depicted with high speed,but south of 26°N it is delineated with low velocity.Combining with previous research results of SKS and GPS measurements,we infer that the deformation in the lower crust and uppermantle of this area might be decoupled.