Magnetotelluric Evidence for Distributed Lithospheric Modification Beneath the Yinchuan‐Jilantai Rift System and Its Implications for Late Cenozoic Rifting in Western North China
Xin Li, Yun Chen, Xiaobo Tian, Shaozhuo Liu
Abstract
Abstract The Yinchuan‐Jilantai rift system (YJRS) is a prominent Cenozoic intracontinental rift zone located along the northwestern margin of the Ordos Block in western North China. Although the absence of volcanism indicates a passive origin, the tectonic driving forces and rift‐related deep processes remain poorly understood. Here we use newly obtained broadband magnetotelluric (MT) data to image the lithospheric electrical structure along a ∼500‐km‐long profile across the YJRS. At middle–lower crustal levels, the resulting model reveals several subvertical, crustal‐scale conductors that spatially correlate with the rift‐parallel, high‐angle normal faults. We attribute these features to a combined effect of saline fluids and partial melt due to recent supply of heat and volatiles into the crust from below. The crustal‐penetrating normal faults that were reactivated during extension serve as permeable pathways for deep fluid migration. In the uppermost mantle, a ∼400‐km‐wide zone of enhanced conductivity is present and requires the presence of partial melt. We interpret this feature as evidence for lithospheric modification through upwelling and decompressional melting of the volatile‐enriched mantle. When compared with the narrow (<100‐km‐wide) mantle conductors imaged beneath the Shanxi rift system along the eastern margin of the Ordos Block, such a feature indicates the YJRS has experienced a larger extent of lithospheric modification. Combined with additional geologic and geophysical observations, we attribute this essential difference to the inherited lithospheric heterogeneity and the eastward decreasing far‐field impacts from the India‐Eurasia collision.