The Velocity Structure Of Eastern Tibetan Crust And Its Tectonic Implication

Tibetan plateau is characterized by vast territory, complex topography andparticular tectonic features. It is the most active tectonic region of Eurasian continent.Tectonic deformation and tectonic uplift of Tibetan plateau is the hottest topic andresearch front of present international geoscience community. Combining the geologyand geophysics to study the geoscience transect can reflect the physical properties ofthe crust in different depth and different structure. Besides the coupling relationship ofthe different tectonic units and the tectonic evolutionary character of the lithospherecan be found as well.In this paper, the crustal thickness of southwestern Tibet is55km compared to60-65km of northeastern and northern qiangtang and Songpan-Ganzi block. Aconspicuous feature is that there is a very clear east–west difference as to crustalthickness on both sides of the boundary defined by about90°E. In the south part ofBangong-nujiang suture, the crustal thickness is relatively high.The thesis chooses two typical geoscience transects to research The twogeoscience transects coss the five tectonic units of the Qinghai-Tibet Plateau, and arecut by the major faults. The research shows that the Indian plate is migrating towardsthe northern boundary of Eurasian plate while the Bangong-Nujiang suture becomesthe northern edge of Indian subduction. There is a low velocity area in the middlecrust of Qiangtang block. Generally the emerge of low velocity in the middle andlower crust is thought to be related with the partial melting of the lithosphere,however, some researchers hold that the low-velocity layer is caused by thesuperimposing by the upper crust transshipment downward.The crustal motion observed by GPS shows that the slip rates of major slip faultsare as follows:4.5±1mm/yr of Haiyuan left-lateral strike slip fault in the eastern Tibet;10±2mm/yr of east Kunlun left-lateral strike slip fault in the eastern Tibet;9.4±2.3，9±4and5±2mm/yr of Altyn left-lateral strike slip fault from west to east;3.4±5mm/yr of Karakoram dextral slip fault;10±2mm/yr of Xianshuihe left-lateral strike slip fault;2±2mm/yr of Ailaoshan-Honghe left-lateral strike slip fault.The depth slices of P wave speed shows that in the20-35km depth slice, thevelocity of Sichuan basin is higher than the surrounding block, In the southeast part ofthe research area the P-wave velocity is low in the same depth slice which is inferredto be related with the magmatic activity in this region. In40-50km depth slices for thesame depth, the P-wave velocity difference is relatively small. The speed limit ofSichuan Basin and the surrounding blocks is not obvious. P-wave velocity-depthsections showed that from20km to35km the P-wave velocity decreases rapidlywhile from35km to50km, the P wave velocity of shallow increases continuously Thethesis concludes that this feature is related to crustal flow in the mid-lower crust.