Electrical Constraints on the channel flow underneath the northeastern Tibetan plateau: Results of the Longriba-Minjiang magnetotelluric sounding profile

摘要

The Minjiang Fault belt on the eastern edge of the Tibetan Plateau is the boundary between the Motianling block and the Songpan-Ganzi fold belt. It has recently been a focus of many geological and geophysical studies. In this paper, a NW-SE trending magnetotelluric (MT) sounding profile which crossed the Minjiang Fault and adjacent regions was used to study the deep structure of this region. The dimensionality analysis results indicate that the structure of the research area is 3-D. This is because most of the Swift and Bahr skew values are larger than 0.2 in the long period data. In contrast, impedance tensor decomposition results show a poorly-defined strike angle of N20 degrees E with the 90 degrees ambiguity removed with regional geological information. A similar strike angle was given by phase tensor analysis as well. Thus, both 2-D and 3-D approaches were applied to produce resistivity models. The resistivity model can be divided into three parts which are the Ruoergai basin, the Maerkang block and the Minshan block from west to east These three parts are separated by the Longriba Fault in the west and the Minjiang Fault in the east. The Ruoergai block and Maerkang block are characterized by highly resistive upper crust and less resistive middle to lower crust. The low resistivity layer is located beneath the Minjiang Fault and connects with the shallow low resistivity body in the upper crust of the Minshan block. The shallow conductor beneath the Minshan block may be caused by fluid and partial melt. In contrast, the widespread low resistivity middle to lower crust beneath the Minshan block, is interpreted as partial melt and can be related to the northeastward channel flow of the weakened Tibetan crust. The channel flow is interpreted to travel through the Longriba Fault in the eastern margin of the Tibetan plateau and was blocked by the rigid Motianling block east of the Minshan block. This blockage caused the upwelling of partial melt in the middle and lower crust, leading to crustal deformation in the weak Minshan uplift zone. As a result, the Minshan uplift was formed as a result of an eastward-thrusting nappe, whereas the left-lateral, strike-slip movement of the Minjiang Faults and Huya fracture accelerates the uplift of the Minshan block. The other portion of the channel flow is blocked by the Motianling block and turns to the southeast, resulting in the Maerkang block moving to the south; this phenomenon corresponds to the dextral strike-slip movement of the Longriba Fault and the sinistral strike-slip movement of the Minjiang Fault.