Magnetostratigraphic ages of the Cenozoic Weihe and Shanxi Grabens in North China and their tectonic implications

Abstract

It has long been debated whether the formation of the Cenozoic Weihe and Shanxi grabens in North China was induced by the Indo–Asia collision or Pacific Plate subduction. Deciding between these two models requires precise knowledge of the timing of the graben formation. In this study, we used high-resolution magnetostratigraphy to suggest that the age of a ~ 1644-m thick syn-rift section conjoining the two grabens spanned from ~54.3 to ~3 Ma. The section is marked by three unconformities dated at ~26, ~12, and ~ 7 Ma. The precise ages, combined with field observations, indicate the following: the Weihe Graben underwent four rifting stages that started at ~54.3 Ma, whereas the Shanxi Graben opened at ~7 Ma. The synchronous timing between the basin development and uplift history of the Qinling Range strongly suggests that the Cenozoic extension across the Weihe and Shanxi grabens was coupled with left-slip faulting bounded at the northern edge of the Qinling. As the Qinling left-slip fault indicates eastward extrusion during the northward penetration of India, our work supports the early hypothesis that the opening of the north-trending rifts in North China was associated with the Cenozoic Indo–Asia collision. As the basin history across the Weihe and Shanxi grabens can also be correlated with the Bohai Bay evolution along the eastern margin of East Asia, the Pacific Plate may have also played a role in forming the two grabens. We conclude that the Cenozoic extension across North China has been controlled by both the Indo–Asia collision and back-arc tectonics induced by westward subduction of the Pacific Plate.