基于电性结构模型的青藏高原东缘上地幔热结构研究

研究青藏高原东缘地区的深部物质结构对于理解青藏高原的隆升及扩张机制具有重要的科学意义.本文将青藏高原东缘实测大地电磁测深剖面反演所得的岩石圈电性结构模型与高温高压岩石物理实验测得的上地幔矿物和熔融体导电性定量关系相结合,通过Hashin-Shtrikman(HS)边界条件建立上地幔电导率与温度、熔融百分比等参数的定量关系,在此基础上计算得到了青藏高原东缘上地幔热结构及熔融百分比分布模型.研究结果表明在青藏高原东缘地区通过大地电磁测深方法所探测到的上地幔低阻体可以解释为由高温作用所产生的局部熔融区域.其中,松潘—甘孜地块上地幔高导体对应的温度介于1300～1500℃之间,熔融百分比可高达10%,支持前人将松潘—甘孜地块内部的低阻体解释为局部熔融的观点.龙门山断裂带以东、四川盆地西缘的上地幔高导体温度介于1200～1400℃之间,熔融百分比介于1%～5%左右,表明扬子克拉通的西缘可能正在经历一定程度的活化作用.龙门山断裂带下方的上地幔高阻体温度介于1100℃附近,基本没有发生局部熔融,具有较冷的刚性块体特征,与该区域频发的地震活动相吻合.四川盆地东部的扬子上地幔温度介于800～900℃之间,没有发生局部熔融,符合古老稳定的克拉通块体的基本特征.

Upper mantle thermal structure beneath the eastern margin of the Tibetan Plateau inferred from electrical structure model

The eastern margin of the Tibetan Plateau plays a significant role in controlling the uplift and expansion of the plateau. Previous Magnetotelluric (MT) studies have been conducted to better understand the lithospheric structure and physical properties of this region. In this study, to interpret the previously published MT models more quantitatively, we establish a quantitative relationship between the electrical conductivity and temperature, as well as melt fraction of the upper mantle, by combining the two-dimensional MT inversion model with laboratory derived formulae for electrical conductivity of major upper mantle minerals and melts using the mixing law of Hashin-Shtrikman (HS) bounds. The resultant upper mantle thermal structure as well as distribution model of upper mantle melt fraction suggest that the upper mantle conductors observed from previous MT studies can be interpreted as regions with partial melting under high temperature. The large scale upper mantle conductor beneath Songpan-Garzê terrane corresponds to temperature ranges between 1300 and 1500℃, with melt fraction as high as 10%, which is consistent with the qualitative interpretations of partial melting from previous MT studies. A smaller conductive region in the upper mantle beneath the western Sichuan Basin represents a region with the temperature of 1200~1400℃ and 1%~5% melt fraction, which indicate upwelling of the asthenosphere and modification of the western margin of the Yangtze Craton. The upper mantle resistor beneath the Longmen Shan fault zone corresponds to a relatively cold region with temperature around 1100℃, and very limited amount of melt fraction (0~0.1%), which is consistent with its rigid behavior as a seismogenic zone. The coldest region locates beneath the eastern Sichuan Basin with upper mantle temperature of 800~900℃ and no partial melting, which represents the ancient, stable, cratonic Yangtze block.