Deep Electrical Structure and Geological Significance of Alxa Block-Ordos Block in the Northeast Margin of Qinghai-Tibet Plateau

Abstract:

Due to the subduction of the Pacific plate and the wedging of the Indian plate, the area from Alxa Block to Ordos Block in the northeast margin of the Qinghai-Tibet Plateau has been experiencing intense tectonic activities, forming a north-south contact zone with complex geological structure and developed faults. In this study, a long-distance profile of magnetotelluric sounding distributed across Alxa Block, Hetao Basin-Helan Mountain-Yinchuan Basin-Ordos Block was conducted, and the following conclusions were drawn by dimensional analysis, and three-dimensional inversion research of the measured data, combined with other geological and geophysical data: (1) The Ordos Block shows a relatively stable layered resistivity structure. The shallow structure is mainly characterized by low resistance at 3-5km, the overall high resistance at 5-25km, and the low resistance structure at a large range at 25km. Alxa Block is characterized by high resistance, indicating that it still retains a rigid base, and two high-conductivity anomalies are upwelling to a depth of 20km in the middle of the block. The Hetao Basin-Yinchuan Basin is characterized by a typical basin-mountain structure with high resistance of mountain range and low resistance of basin. (2) F6 is Yellow River Fault, which cut into the upper mantle at a large angle. It is the boundary between Yinchuan Basin and Ordos Block. The eastern foot fault of Helan Mountain meets the Yellow River fault at a depth of about 20km. The F3, 4, 5, and 6 faults form a typical "negative flower structure". The low-resistivity melt from the upper mantle migrated upward in the weak zone and intruded into the high-resistivity Helan Mountain along the fault at the eastern foot of Helan Mountain at a depth of 20km. (3) The lithosphere on the western margin of the Ordos Block has undergone destruction and reconstruction, showing that the high-resistivity basement has been greatly reduced, while Alxa Block still retains a 40km thick basement. There are large-scale low-resistance bodies representing the asthenosphere of the northeastern margin of the Qinghai-Tibet Plateau, indicating that the surface area of the plateau's northeast-extending deformation has reached the Helan Shan-Yinchuan Basin. Alxa and Ordos Blocks were stretched and the low-resistivity fluid rose along the fault, and the low-resistivity fluid in Yinchuan basin invaded the middle part of Helan Mountain.

Keywords: Geophysics; Northeast margin of Qinghai-Tibet Plateau; Magnetotelluric profile; Deep electrical structure;
Acknowledgments:This research was supported by Phase I of the Northwest Region Magnetotelluric Subnetwork(2024ZD1000201),the China Geological Survey project – Regional Geophysical Survey of Wuwei to Yinchuan Region(DD20230255).