Magnetostratigraphy of the Xihe loess-soil sequence and implication for late Neogene deformation of the West Qinling Mountains

Abstract

The Qinling Mountain range forms an important climate barrier between southern and northern China. Its western part, referred to as the West Qinling, constitutes a unit of northeastern Tibetan Plateau. The uplift history of Qinling during the Neogene is still a contentious issue. Magnetostratigraphic results from the 150-m-thick Xihe loess-soil sequence (NL-VI), located on the alluvial highlands surrounding an intermontane basin from the West Qinling, indicate 17 reverse and 17 normal magnetozones. They are correlative with the interval from chron C5n to chron C2Ar in the Geomagnetic Polarity Timescale. The age of the NL-VI section can thus be palaeomagnetically constrained to a period from 10.5 to 4.2 Ma. The middle portion of the section has been affected intermittently by surface erosion processes. As loess is sensitive to any erosion that may have been induced by substratum deformation and tectonic uplift, this sequence indicates that the intermontane basin–ridge pattern of the West Qinling had already been formed by 10.5 Ma, thus providing suitable topographic conditions for aeolian deposition. This timing is consistent with a significant deformation phase between ∼14 and 10 Ma reported for the Tibetan Plateau. Meanwhile, the complete preservation of the NL-VI loess-soil sequence precludes both intense deformation of the substrate and rapid uplift of the West Qinling between 10.5 and 4.2 Ma. Although the erosion-affected portion of the NL-VI section reflects some degree of tectonic instability from 8.3 to 6.9 Ma, the intensity must have been sufficiently weak to prevent the erosion of the aeolian deposits. These further suggest that the late Miocene growth of the Tibetan Plateau has not significantly affected the West Qinling. Thus, the studied NL-VI aeolian sequence provides not only independent evidence for the uplift history of the West Qinling within an accurate chronological framework, but also provides new insights into the tectonic changes on the northeastern Tibetan Plateau.