Uppermantle Thermal Structure Beneath The Southeast Tibetan Plateau Constrained From MT

The temperature inside the earth is a very important physical parameter in the study of geophysics,rock physics and geodynamics.The temperature in the upper mantle mainly determines their density and strength(rheology),thus the dynamic evolution of the upper thermal boundary layer and mechanical boundary layer in the convecting mantle and the dynamic process of plate subduction.It is also the possible reason for lithospheric thinning and deep tectonic evolution of the earth.Therefore,it is of great significance to study the uppermantle thermal structure.However,it is a challenge to determine the temperature structure in the upper mantle.So far,the temperature is mainly inferred from seismic wave velocity and resistivity structure,combined with mineral physics and geothermal data and methods.Compared with seismic wave velocity,resistivity is more sensitive to uppermantle temperature and fluid/melt,so it plays an important role in the study of uppermantle temperature structure.The southeastern margin of the Qinghai-Tibet Plateau is located in the southwest China.This region,with strong crustal deformation and frequent strong earthquake occurrence,is the key position of the weak material migration in the Qinghai-Tibet Plateau.Studying the temperature structure in the upper mantle is not only helpful to understand the expansion mechanism of the Qinghai-Tibet Plateau,but also to understand the interaction between blocks.Therefore,combined with rock physics methods,the uppermantle temperature structure is inferred from a resistivity profile across the southeastern margin of the Qinghai-Tibet Plateau obtained by magnetotelluric sounding(MT)in this thesis.This profile is about 750 km long,which starts from the LanpingSimao block,crosses the Sichuan-Yunnan diamond block,and ends in the South China block.The method used in this thesis is the same as those of previous investigators,but compared with previous work,the research ideas are novel and the results are more reasonable in this thesis.Main points are as follows.(1)A more reasonable uppermantle mineral composition model is adopted.When previous investigators studied uppermantle temperature structure,the experimental data used to build the mineral composition model was collected from both the continent and subduction zone,which was unreasonable.Therefore,we use the experimental results of rock samples completely from the continent to build the mineral composition model in this thesis;(2)A depth-dependent water content of the whole rock is adopted.Previous investigators used a uniform water content of the whole rock to determine the temperature structure in the upper mantle.Our results indicate that the resistivity structure cannot be completely explained by using any uniform whole rock water content.In order to address this issue and obtain a reasonable temperature structure,it is necessary to use a depth-dependent whole rock water content;(3)Craton and noncraton regions are separated.The distribution ratio of mineral water differs significantly in craton and non-craton regions.A same distribution ratio was adopted in previous work for these two regions,which was unreasonable.The results in this thesis show that the uppermantle mineral composition models without water or with water but without partial melting cannot explain the whole resistivity structure beneath the southeastern margin of the Tibetan Plateau,while the model with both water and partial melting can address it;The temperature structure calculated from a uniform whole-rock water content model is reasonable only in a certain depth range,that is,the reasonable uppermantle temperature structure in the depth range covered by the resistivity profile cannot be obtained by using a uniform wholerock water content model.A depth-dependent whole-rock water content model must be used for this goal.The water content is about 4.69 wt%at 40 km depth,then decreases sharply to about 0.2 wt%at 80 km depth,and at last decreases slowly to about 0.13 wt%at 150 km depth.Using the whole rock water content model,we obtain the uppermantle temperature beneath the southeastern margin of the Qinghai-Tibet Plateau under the constraints of the resistivity structure and the depth-averaged global temperature structure obtained by previous investigators.Temperature is between 400℃ and 1300℃,showing a general trend of increasing from shallow to deep parts.In the area shallower than 70 km,the uppermantle temperature beneath the South China block(craton area)is about 400-800℃,which is significantly lower than 800-1000℃ beneath the Sichuan-Yunnan and LanpingSimao blocks(non-craton area).Compared with the area deeper than 70 km,the lateral temperature variation is relatively strong.In the area deeper than 70 km,the uppermantle temperature difference between the South China block and the Sichuan-Yunnan and Lanping-Simao blocks is much smaller than that in the region shallower than 70 km,and the lateral variation of temperature is very small,showing obvious stratification characteristics.The melting degree of uppermantle minerals beneath the southeastern margin of the Tibetan Plateau is between 0 and 1.4%,and shows an overall trend of increasing first and then decreasing with increasing depth.The uppermantle minerals almost do not melt beneath the South China block in the area shallower than 65 km;then,with the deepening depth,the melting becomes stronger and stronger.The average melting degree reaches the highest,about 0.51%,at about 70 km depth.The local strongest mineral melting,about 0.91%,occurs beneath the eastern part of the South China block;after that,the degree of mineral melting gradually decreases.The uppermantle minerals almost do not melt shallower than 45 km beneath the Chuandian and Lanping-Simao blocks;then with the increase of depth to about 70 km,the degree of mineral melting gradually increases.The maximum mineral melting percentage is about 0.8%,significantly higher than that beneath the South China block.Local strong melting areas exist near the Moho surface beneath Panzhihua and Longpan-Qiaohou fault zones,Heqing-Eryuan fault zone and Chenghai fault zone,and the maximum percentages are about 1.4%and 1.1%,respectively;afterwards,the degree of melting of the mineral decreases gradually.The results in this thesis are helpful in understanding the uppermantle thermal structure beneath the southeastern margin of the Tibetan Plateau,but the water content of the xenoliths from the upper mantle is much lower than the whole rock water content obtained in this thesis,probably indicating that the model needs to be further improved,which will be our next work.