The Study On The Deep Material State Of Yangbajing Area In Tibet Based On The Three-dimensional Electrical Structure Model Of Lithosphere

The Yangbajing area is located in the central and southern Qinghai-Tibet Plateau,and includes the rare geothermal resources in the world.The Yangbajing geothermal field is a typical geothermal resource,which temperature can reach above 93 °.Two major thermal belts are distributed in the study area,and extensive east-west extensional structures and north-south normal faults are around these two thermal belts.Studying the deep thermal state of this area will help us understand the tectonic dynamics and the reasons for the formation of geothermal resources in this area.In this paper,the data of 90 ° E-92 ° E and 29 ° N-32 ° N of the magnetotelluric observation network of the Deep Exploration Technology and Experimental Research Project(SINOPROBE)deployed on the Qinghai-Tibet Plateau were used to proceed three-dimensional inversion to obtain the reliable 3-D electrical structure.Through the control of water content,carbon dioxide content,pressure and other parameters,the Nernst-Einstein equation and Hashin-Shtrikman(HS)boundary conditions are used to obtain the distribution of the temperature and melting percentage in the lithospheric mantle.The modified Archi's law waw used to calculate the distribution of the melting percentage in the mid-lower crust.The results showed that under the condition of the same water content,the temperature and melting magtitude in the west of the Yadong-Gulu Rift were higher than those in the east of the rift.The temperature and melting percentage near the Bangong-Nujiang suture zone were relatively high,and the temperature and melting percentage of the Tethys-Himalaya block were relatively low.In addition,the temperature and melting magtitude of the lithospheric mantle decreased as the water content increased.Combined with the previous studies,the results of the lithospheric mantle with a water content of 150 ppm were used in the subsequent study of tectonic dynamics.Under the condition of this water content,the lithospheric melting percentage can reach more than 5%,and the maximum temperature of the lithospheric mantle can reach more than 1100 ° C.Meanwhile,a large-scale zone with higg melt fractiosn wa distributed in the middleand lower crust.According to the distribution of the temperature and melting percentage,it was summerised that taking the IYS as the boundary,the TethysHimalaya terrane in the south of IYS showed the feature of the low temperature and low melting fractions,and the Lhasa terrane in the north of IYS showed the feature of high temperature and high melting fractions.It is indicated that the subduction,rollback and break-off of the Indian plate and possible detachment of the lithosphere of the Lhasa terrane may cause the asthenosphere to upwell along the subduction channel and lithosphere fault zones,resulting in the lithospheric mantle and the midlower crust are partially molten.The subduction of the Indian Plate may generate a large amount of tectonic heat,which further caused the partial melting of the midlower crust,and may form magma chambers in the mid-lower crust.These magmaticthermal materials and fluids would also migrate upward along the crustal fault zones,and heated the circular system of the underground water to form the Yadong-Gulu thermal belt.Parts of the hot materials and fluids ascended to some areas along some shallow fault belts,forming the widespread hot springs.