青藏高原东缘川滇构造区深部电性结构特征

本文对位于青藏高原东缘川滇构造区的贡山一绥江大地电磁测深(MT)剖面数据进行反演,获得沿剖面的深部电性结构,为研究喜马拉雅东构造结、川滇菱形地块与华南地块的构造变形特征、壳幔耦合关系、地块间接触关系以及相互作用等问题,提供电性结构的依据.研究发现:(1)电性结构揭示澜沧江断裂带和小金河断裂带为深大断裂带,控制着研究区的深部结构特征和形变机制;(2)澜沧江断裂带和金沙江断裂带之间的高阻体,可能是扬子古地块的残留部分;小金河断裂带和安宁河断裂带之间的高阻体,则是峨眉山大火山省喷发形成的冕宁一越西杂岩带;(3)在滇西地块、川滇地块和大凉山地块均存在低阻层,它们的介质属性有所不同,滇西地块下的低阻层"疑似"高热状态的岩浆囊,主要由缅甸弧向东俯冲运动引起的,中上地壳的高热状态使地块的活动性增强;川滇地块内部的壳内低阻层的成因为:理塘断裂带和小金河断裂带之间的地表低阻层由破碎带充水所致,而金沙江断裂带和理塘断裂带之间的中地壳低阻层可能是由局部熔融物质或含盐流体导致的,其为壳内物质运移的通道.从而在地下物质发生大规模走滑运动的过程中起到引导作用;川滇地块东部和大凉山地块西部的壳内低阻层可能与地慢物质的上涌有关;马边断裂带附近的低阻体可能与破碎带变宽和破碎带内的流体有关.

Deep electrical structure beneath the Sichuan-Yunnan area in the eastern margin of the Tibetan plateau

In response to the ongoing convergence between the Indian and Eurasian plates, significant intracontinental deformation has been accommodated by crustal thickening on large-scale strike slip faults. Several models have been proposed to explain the deformation of the eastern Tibetan plateau, including: (1) lateral extrusion of rigid blocks, in which deformation is primarily localized along strike-slip faults that bound the blocks; (2) continuous deformation of lithosphere; and (3) ductile channel flow in the mid-lower crust. The major characteristics are dominated by the large-scale clockwise rotation around the eastern Himalayan syntaxis in the southeast margin of the Tibetan plateau. However, the mechanisms for Tibetan plateau's deformation and expansion remain enigmatic due to the resolution limitations of methods and data. The debates of these models center on whether the deformation is localized to the mantle or to the upper crust. The southeast margin of the Tibetan plateau, which has undergone significant crustal thickening and tectonic extrusion, is particularly important for understanding the kinematics and dynamics of intracontinental deformation.
In order to study the deep electrical structure of the eastern Tibetan plateau, the MT data along L3 were collected during the period from September to November in 2012 supported by State Key Laboratory of Earthquake Dynamics. The observation equipment used in this survey was the MTU-5A system manufactured by Phoenix of Canada. The total length of the MT profile is 530 km. The numbers of observation MT sites and collected MT sites were 43 and 32, respectively. The ranges of frequency are 320 Hz~2000 s and 320 Hz~1000 s, respectively, which permitted to invert the structure of the crust-uppermost mantle in the area. The remote reference MT technique and the robust data processing method are employed. The results of 2D calculation show that the skewness of most sites is less than 0.3, and that of partly longest periods is greater than 0.3. The electric strikes of most sites are in N—S, largely in accordance with the tectonic strike and perpendicular to the profile. The non-linear conjugate gradients (NLCG) method was used in the 2D inversion. The initial models were constructed with 100 Ωm uniform half-space and an incorporated topography. According to the L-curve analysis, τ=30 is an optimal selection. We also compared the model roughness obtained with different polarization data. We selected the TM mode, because the model roughness of the TE mode did not tend to be stable, which also affected the results of the joint TE+TM mode. The error floors were set to 5% for the TM apparent resistivity and TM phase in the NLCG inversions. The root mean square (RMS) misfit of data is 2.86.
The final results of the inversion process are a smooth model with very distinctive features. The conductivities show distinct contrasts, ranging from several thousand Ωm in the resistive parts to as low as around 1 Ωm. There are some agreements between the surface geology and lateral changes in conductivity that support the correlation between electrical features and lithological units. The Lancang River fault and Xiaojin River fault are lithospheric faults, which control the deep geologic structure and deformation mechanism beneath survey zones. A high-resistivity body situated between the Lancang River and Jinsha River faults may be a residual portion of the ancient Yangtze land; another high-resistivity body located between the Xiaojin River and Anning River faults corresponds to the Mianning-Yuexi tectonic complex zone, which was presumably derived from eruption of Emeishan flood-basalt province. A low resistivity zone widely distributed beneath the survey area is then interpreted as a weak crustal zone in the mid-lower crust. Another low resistivity body beneath west of the Yunnan block is inferred to be a magma chamber probably caused by subduction of the Burma arc, where the thermal state of upper crust has enhanced the activity of the block. A shallow low-resistivity layer between the Litang and Xiaojin River faults is probably caused by a fracture zone filled with water. A low-resistivity layer in the crust between the Jinsha River and Litang faults may be a channel of material migration which is caused by the partial melting of the substance or saline fluid. The low resistivity nearby the Mabian fault might be the consequence of fracture zone and fluid therein.