Inherited Structure of a Paleo‐Subduction Zone Beneath the Northern Tibetan Plateau: Insights From 3‐D Anisotropic Magnetotelluric Imaging

Abstract Ancient sutures within the Tibetan Plateau are genetically weak zones that were reactivated as major tectonic boundaries during the Cenozoic Indo‐Eurasia collision, playing a critical role in the plateau's growth and uplift. Geological studies suggest that ancient subduction occurred beneath the continental crust between the Qaidam Basin and the Qilian Shan in the northern Tibetan Plateau. However, the deep structure of this paleo‐subduction zone remains poorly constrained by previous geophysical studies. By three‐dimensional (3‐D) anisotropic inversion of magnetotelluric data, we reveal a steeply northeast‐dipping low‐resistivity anomaly beneath the North Qaidam Fault, extending downward to the mantle lithosphere. This anomaly becomes shallower and less distinct eastward. Anisotropic anomalies are concentrated between the Qaidam Basin and the Qilian Shan. The low‐resistivity anomaly is probably caused by fluids (volume fraction <10%) that formed and were trapped during subduction, representing a lithospheric “scar” that acts as a persistent weak zone. These features suggest the presence of an inherited paleo‐subduction structure with an in situ suture. We propose that seismicity in the study area originates from fluid migration along faults and fractures, and is associated with reactivation of this “scar” under regional compression. Meanwhile, low‐resistivity sediments in the Qaidam Basin deepen toward the North Qaidam Fault in the west profiles, consistent with its foreland basin setting. Reactivation of the “scar” will promote fault thrusting and eastward expansion of the basin, and also contribute to the growth and uplift of the northern Tibetan Plateau.