Three-Dimensional Electrical Structure of the Southern Dianzhong Secondary Block, Southeastern Tibetan Plateau

The southeastern margin of the Tibetan Plateau serves as a key pathway for the outward extrusion of plateau material. The deep crust–upper mantle structure and associated material transport processes in this region are therefore crucial for understanding the mechanisms of tectonic deformation of the plateau. The southern part of the Dianzhong secondary block is located at the junction of the Red River Fault Zone and the Xiaojiang Fault Zone, where tectonic activity is particularly intense. However, existing magnetotelluric (MT) studies in the Dianzhong block have mainly focused on its central and northern sectors, while the three-dimensional lithospheric electrical structure of the southern part and its implications for deep material transport remain poorly constrained.

In this study, a three-dimensional MT investigation was carried out in the southern Dianzhong secondary block to image the electrical structure of the crust and upper mantle and to explore its tectonic significance. A total of 105 MT sites were deployed across the study area. Impedance tensor decomposition and phase tensor analysis were first applied to assess the dominant dimensionality and structural strike of the subsurface. The results indicate that the middle to deep crust is characterized by strong three-dimensional features, supporting the application of three-dimensional inversion.

Three-dimensional MT inversion based on a nonlinear conjugate gradient algorithm was subsequently performed, yielding a resistivity model down to a depth of approximately 80 km. The reliability of the major low-resistivity anomalies was further evaluated through sensitivity tests. The inversion results reveal a complex electrical structure in the upper and middle crust, with high- and low-resistivity bodies distributed in an interlaced pattern. Shallow low-resistivity anomalies show a clear spatial correlation with major active faults in the region.

At greater depths, a prominent low-resistivity anomaly extends from the lower crust into the upper mantle and exhibits a noticeable change in geometry near the Moho. Beneath the Xiaojiang Fault Zone, a low-resistivity channel that crosses the Moho is identified. In combination with regional geological and tectonic information, this deep low-resistivity structure is interpreted to represent a pathway for the ascent of thermal material or fluids controlled by deep-seated fault systems. These results provide new electrical constraints on deep material transport processes beneath the southeastern margin of the Tibetan Plateau and the Dianzhong region.