3-D Velocity Structure Of The Crust And Upper Mantle In Tibet And Its Geodynamic Effect

The crust and upper mantle structure under Tibet is the direct resultof the India plate subducting and colliding with the Eurasian plate. Thestructure heterogeneity has recorded the process of the two plate collision.It supplies the basal information for clarifying the uplift mechanism andthe geodynamic process. Obviously, exploring the crust and upper mantlevelocity structure of Tibet and revealing the process and the effect ofcollision are crucial for solving the puzzle of the Tibet uplift and thecontinent-continent collision. It is a supplement to the Plate Tectonicstheory.3-D seismic velocity structure of the crust and upper mantle under Tibetwas determined by using the Tomo3D tomography program developed by Prof.Dapeng Zhao. The present author collected all the available arrival time datarecorded by the temporary and permanent digital seismic stations in Tibet.In the tomographic inversion she used 139,021 P-wave arrival times from 9649te]eseismic events recorded by 305 seismic stations. Most of the data werepicked up manually by the author, which took the author three months ofextraordinary hard-working. This is the first tomography study using sucha large amount of data. The data cover from southern Ganges plain to northernTarim basin and Qaidam basin. This study has revealed the subducted India]ithospheric mantle and a prominent low-velocity zone beneath the QiangtangTerrain. The major results of this study on the Tibetan 3-D velocity structureare summarized as follows:1) The Tibetan crust velocity structure is generally consistent with thesurface tectonic features which are oriented nearly east-west. But themain trend of the velocity anomalies in the upper mantle is generallyoriented in the north-south direction.2) Low-velocity anomalies in the crust are clearly visible under the Himalaya Mountain. At 75 km depth, low-velocity zones are separated intoa few blocks from the east to the west, which are considered to be a resultof east-west extension.3) In the deeper areas, the location of theNNE strike low-velocity zoneis consistent with the N-S strike negative aeromagnetic data. Consideringthe depth to the Curie temperature, the negative aeromagnetic anomaly isinductive aeromagnetic, which may be controlled by the low velocity zone.4) The biggest rift in Tibet, Yadong-Gulu rift, exists between ahigh-velocity zone and a low-velocity zone in the crust. However, in the]ithospheric mantle part this rift shows high velocity. Under thelithospheric mantle it shows tow velocity again. This indicates that thedeep structure and process control the development of the rift. Both of thefeatures suggest that there exists a north-south striking significantextending structure.5) The subducting India plate shows high velocity anomaly. Beforecollision, the Indian plate is continuous. After subducting beneath theTibetan plateau, the Indian slab subducting angle and its front locationsare different under different areas. The geometry of the subducting Indianlithosperic mantle is not flat. The eastern and western parts of thesubducted slab are shallow, while the middle part is deep. Its general shapeis like a scoop.In the western region, theTarimbasin lithosphericmantleis subductingunder the West Kunlun Mountain witha steep angle, and it is colliding withthe Indian lithospheric mantle face to face.In the middle region, the Indian lithospheric mantle is underthrustingwith an angle of about 22°beneath the center of Qiangtang terrain at about34°N, and it has reached to 400 km depth.In the eastern region, the Indian mantle has nearly horizontallyunderthrusted under the Tibet from Ganges plain to 33°N. Then, the Indianmantle broke off down to the asthenosphere. 6) Following the collision, the Indian crust was melted under SouthernTibet. The tomography image shows that the Indian crust was held up by theEurasia plate and has not plunged beneath the Tibetan plateau too far.7) There is a huge low-velocity body which looks like a mantle plumebeneath the Qiangtang terrain. Such a prominent low-velocity body isimpossible to be the partial melting products, and is speculated to be eitherthe subducted delamination of the Indian lithospheric mantle according toits location and extending depth with Chaidam block holding back the thermaldisturbance in the area, and so resulting in higher temperature which begetsfalling velocity, or be the mantle upwelling materials along the surface ofIndian lithospheric mantle. The volcanic activity and low-velocity anomalyin northern Tibet may be the direct results of this hot mantle upwelling.8) The southward subduction of the Eurasian plate is not found. Thereare few seismic stations in the north of the profile, so this result needsto be validated by future studies.