青藏高原东北缘合作—大井剖面地壳电性结构研究

摘要

Magnetotelluric (MT) data has been collected along a profile from Hezuo to Dajing across the northeastern margin of the Tibetan plateau. The electrical resistivity model calculated for this profile indicates that the electrical structure of crust is characterized by distinct layering in vertical direction and block division in horizontal direction. Layers of high conductivity were found to be wide spread in the middle and lower crust. The major faults in this region, such as the north edge fault of the West Qinling, the south edge fault of the North Qilian, the north edge fault of the North Qilian (the Haiyuan fault), and the Longshou Shan fault, all present as electrical gradient zones or low-resistivity zones. The horizontal division of the electrical structures in the crust is consistent with the tectonic blocks division at the surface, and the electrical properties of these blocks are quite different. Although the north edge fault of the West Qinling is generally considered as a large-scale plate boundary, we did not find any clear traces of overthrusting or underthrusting. Thus it is probably being dominated by sinistral strike slip motions. Judging from our model, the boundary between the northward overthrusting North Qilian block and the southward underthrusting Alashan block is the Longshou Shan fault rather than the Haiyuan fault. The high-conductivity layers in the crust under the West Qinling orogen are comparable with those within the Tibetan plateau, which are likely due to the existence of partial melts and saline fluids, while the high-conductivity layers under the Central Qilian are only associated with saline fluids. The high-conductivity layers under the transition zone between the North Qilian and Hexi Corridor are presumably the traces of tectonic motions of plates underthrusting and overthrusting, and generated by fluids in the fractured zone.%青藏高原东北缘合作—大井剖面的大地电磁探测结果表明,该区域的电性结构呈明显的纵向分层、横向分块的特点,中下地壳普遍存在高导层.青藏高原东北缘西秦岭北缘断裂带、北祁连南缘断裂带、北祁连北缘断裂带(海原断裂带)及龙首山南缘断裂带等区域性断裂带在电性结构模型中均表现为电性梯度带或低阻异常带.电性结构的横向分区与构造上的地块划分有明显的一致性,各个地块的电性结构存在明显差异.西秦岭北缘断裂带作是一个大型的板块边界,但板块结合带附近没有明显逆冲或俯冲痕迹,可能主要以左旋走滑为主.北祁连地块向北仰冲与阿拉善地块向南俯冲边界可能不是海原断裂带,而是龙首山南缘断裂带.西秦岭造山带内的壳内高导层与青藏高原内部存在的高导层具有可对比性,可能是由于部分熔融与含盐水流体共同作用的结果.中祁连地块内的高导层可能是含盐水流体引起的.而北祁连与河西走廊过渡带内的高导层则可能是板块俯冲或仰冲的构造运动痕迹,也可能是由含盐水流体引起的.