Magnetostratigraphy Of Early Oligocene-Middle Miocene Deposits In The Xunhua Basin On The Tibet Plateau,China,and Their Paleoclimate Significance

The formation of the Qinghai-Tibet Plateau is the most remarkable global geological tectonic event since the Cenozoic.The uplift of the plateau is closely related to the formation of the Asian monsoon and the aridification process of the Asian interior.The northeastern part of the Qinghai-Tibet Plateau has a special geographical position,where the westerly circulation,the East Asian monsoon,and the Indian monsoon converge and jointly influence the evolution of the climate and environment in the region.With the continuous uplift and expansion of the Tibetan Plateau to the northeast,an inter-basin and inter-mountain geomorphological pattern has been formed.A series of inter-mountain basins in the northeast of the Tibetan Plateau have deposited huge Cenozoic sediments,which have recorded the tectonic and climatic environmental evolution information of the northeast of the Tibetan Plateau,providing a good research vehicle for studying the uplift of the Tibetan Plateau,the aridification of the Asian interior and the formation and evolution of the Asian winter and summer monsoons.The Xunhua Basin in the northeastern part of the Tibetan Plateau mainly deposits a set of aeolian sediments from the early Oligocene to the Quaternary,and the strata are interspersed with transient river and lake deposits,which are good materials for studying plateau uplift and climate change.In this paper,a continuous sedimentary stratum mainly composed of aeolian red clay embedded with fluvial and lacustrine phases at about 238 m in the Hongzhuang section of the Xunhua Basin was sampled systematically at 20 cm intervals,and a high-resolution stratigraphic age scale was obtained by magnetostratigraphy,which laid the foundation for paleoenvironmental studies in the region.The environmental evolution history of the Xunhua Basin is also revealed through magnetic susceptibility and other related environmental magnetic indicators.On this basis,the coupling relationship between the uplift of the Tibetan Plateau,global temperature change,and the evolution of the East Asian monsoon was further explored.The main conclusions are as follows.(1)The study of the Hongzhuang section of the Xunhua Basin on the northeastern margin of the Tibetan Plateau reveals that the section records continuous magnetic polarity changes from C5ACn～C10r,with an interval spanning 30 Ma～14.3 Ma from the early Oligocene to the middle Miocene,indicating that the Xunhua Basin first began to receive deposition at an age of ～30 Ma during the Cenozoic.(2)Based on the establishment of a high-precision paleomagnetic dating framework,the changes in stratigraphic deposition rates in the Hongzhuang section were analyzed.A comprehensive analysis of the magnetic susceptibility,frequency magnetic susceptibility,and global deep-sea oxygen isotopes in the Hongzhuang section concluded that the changes in magnetic susceptibility were influenced by global temperature,probably due to the enhancement of weathering denudation caused by the increase in temperature,which led to more near-source materials entering the basin.Meanwhile,the anomalous increase in magnetic susceptibility at ～27 Ma and ～15 Ma shows a significant correlation with the deposition rate,indicating that the input of coarse-grained magnetic material due to plateau uplift is one of the main reasons for the increase in magnetic susceptibility.The fitted relationship between the magnetic susceptibility and the frequency magnetic susceptibility shows that the increase in superparamagnetic particles at ～27 Ma and ～15 Ma is also one of the main reasons for the increase in the magnetic susceptibility.(3)By comparing the magnetic susceptibility,frequency magnetic susceptibility,global deep-sea oxygen isotopes and sedimentation rate of the Hongzhuang section in the Xunhua Basin,we suggest that the northeastern margin of the Qinghai-Tibet Plateau experienced two obvious uplift events at ～27 Ma and ～15 Ma,which directly strengthened the East Asian summer wind,indicating that the East Asian monsoon formed as early as ～27 Ma or earlier.This suggests that the East Asian monsoon was formed at ～27 Ma or earlier and affected the northeastern part of the Tibetan Plateau.Meanwhile,the increase in global temperature was also one of the factors influencing the strengthening of the East Asian monsoon at ～15 Ma.