Magnetotelluric Evidence for Distributed Lithospheric Modification Beneath the Yinchuan‐Jilantai Rift System and Its Implications for Late Cenozoic Rifting in Western North China. Issue 3 (5th March 2022)

• Record Type: Journal Article
• Authors: Li, Xin; Chen, Yun; Tian, Xiaobo; Liu, Shaozhuo
• Is Part Of: Journal of geophysical research. Volume 127:Issue 3(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-03-05
• Subjects: Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2021JB022585
• Languages: English
• ISSNs: 2169-9313

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. Plain Language Summary: Throughout the Cenozoic, the western North China Craton has undergone widespread extensional deformation, forming large‐scale rift systems around the stable Ordos Block (OB). Although these rifts are active today, the tectonic driving forces and related deep processes remain poorly constrained. We use magnetotelluric (MT) data, which are highly sensitive to deep fluid distribution, to investigate the rift‐related fluid dynamics beneath the Yinchuan‐Jilantai rift system (YJRS) along the northwestern margin of the OB. In deep crust, we find several isolated, subvertical high‐conductivity features that spatially correlate with the rift‐parallel normal faults, likely representing pathways for deeply sourced fluids. Looking deeper, we identify a ∼400‐km‐wide zone of enhanced electrical conductivity in the uppermost mantle, which is interpreted as evidence for lithospheric modification through low‐degree melting of the volatile‐enriched mantle. By comparing our results with previous MT observations, we find that the lithosphere of the YJRS has undergone a larger extent of modification than that of the Shanxi rift system along the eastern margin of the OB. Such a difference matches with the surface features of the two rift systems and can be attributed to the inherited lithospheric heterogeneity and the eastward decreasing far‐field impacts from the India‐Eurasia collision. Key Points: Rift‐related, fluid‐rich fault zones are imaged as subvertical, crustal‐scale conductors Enhanced conductivity in the uppermost mantle is attributed to volatile‐enriched melts Inherited lithospheric rheology and the India‐Eurasia collision induced far‐field forces control the rifting structures