Seismic Evidence on Different Rifting Mechanisms in Southern and Northern Segments of the Fenhe-Weihe Rift Zone

Abstract

The Fenhe-Weihe Rift (FWR) system is one of the most active rift zones in east China, and plays an important role in the reactivation of the North China Craton. In this work, a high-resolution 3-D lithospheric S wave velocity model of the FWR is built by joint inversion of receiver functions and ambient noise. Strong correlation between topography and tectonic features is observed at shallow depths. At great depths, strong heterogeneities are observed along the FWR: (1) high-velocity anomalies emerge beneath the Weihe and Taiyuan basins in the lower crust; (2) two high-velocity anomalies are imaged in the uppermost mantle beneath the south to central FWR; and (3) two strong large-scale, low-velocity zones are observed beneath the north Trans-North China Oregon: one is at ~60 km and the other one is much larger with a deeper source. Based on the petrologic properties, geochemical analyses, and seismic images obtained in this work, we propose that the rifting mechanisms between the south and north FWR are different. In the south, the rifting could be mainly caused by the passive effects from the uplift of the Tibetan Plateau, while in the north the rifting is due to the combined effects of the asthenosphere upwelling and the counterclockwise rotation of the Ordos block. The uppermost mantle high-velocity anomalies underneath the south to central part of the FWR could be the remnants of strong ancient cratonic root, and act as mechanical barriers to block the rifting processes between the south and north FWR.