Characteristics of the three-dimensional deep electrical structure in the Helan Mountains-Yinchuan Basin and its geodynamic implications

Located in the north segment of the North-South seismic belt where the Alxa block (AB) and the Ordos block (OB) contact, the Helan Mountains-Yinchuan Basin (HLM-YCB) constitutes a typical normal faulting basin-mountain structure on the Chinese mainland. The 1739 M 8.0 Pingluo earthquake occurred in the Yinchuan fault depression basin with such a basin-mountain structure. Data on five magnetotelluric profiles encompassing distinct segments of the HLM-YCB were utilized for three-dimensional (3D) joint inversion in order to collect fine 3D electrical structure information at a crustal and upper mantle scale across the entire region. The electrical structure between the main blocks in the HLM-YCB and adjacent areas is characterized by east-west horizontal blocks OB, YCB, and HLM, which are divided by the Yellow River fault (F5) with the HLM eastern piedmont fault (F2) as electrical boundary zones on the east and west sides. The two main block units, AB and OB, exhibit an obvious layered resistivity structure. Besides, the HLM-YCB is characterized by a typical basin-mountain structure with the mountains as a high-resistivity body and the basin as a low-resistivity body, and in the northern YCB a large-scale low-resistivity structure exists, extending to the upper mantle, probably derived from the upwelling of mantle-derived materials. It is speculated from a combination of recent 3D crustal movement field information and other data that the HLM-YCB is an active tectonic zone formed via regional tensile action. The formation of the HLM-YCB lies in the interaction of the Tibetan Plateau (TP), AB, and OB and abnormal mantle activities beneath the YCB. The HLM-YCB reflects the joint action of upwelling and diffluence caused by the underplating of hot materials from the deep mantle with gravity and the redistribution of regional tectonic stress on the earth’s surface, which may be the main dynamic reason for the 1739 M 8.0 Pingluo earthquake.