Magnetic Stratigraphy Of The Paleocene In The Jiuxi Basin On The Northeastern Margin Of The Tibetan Plateau And Its Significance

As a natural laboratory for the study of continent-continent collision patterns,the Tibet Plateau is popular among geologists for a range of scientific questions about the timing and mechanisms of its uplift expansion.A large number of sedimentary basins have developed around the Qilian Mountains since the Cenozoic,and the formation and evolution of these basins record the uplift history of the plateau and the activity of the circumferential tectonic zone.In this paper,we use sedimentological and magnetostratigraphic methods to systematically investigate the stratigraphic chronology,sediment-filled evolutionary history and uplift time frame of the Qilian Mountains and AFT in the Cenozoic Huoshaogou and Baiyanghe formations in the Hongliuxia section of the Jiuxi Basin,guided by basin-mountain coupling.The main research results obtained are as follows:(1)The sedimentary facies in the Hongliu Xia section can be divided into foothill alluvial fan facies,fan-delta facies,braided river facies and lake facies(divided into shallow coastal lake subphases and semi-deep lake-deep lake subfacies).The spatial distribution of the sedimentary facies indicates that the sediments of the Huo shaogou Formation originated from the northern part of the basin in the Beishan,Heishan and Kuan Tan Mountains.The sediment source of the Bai yanghe Formation varies from north to south of the basin.(2)Rock magnetism and thermal demagnetization experiments show that the primary(characteristic)remanent magnetic carriers of the Huoshaogou and Baiyanghe Formations are magnetite,magnetohematite and hematite,and the secondary remanent magnetic carriers are needle iron ore.Analysis of the primary remanent magnetization based on the types of magnetic carriers indicates that the Hongliu Xia section of the Huoshaogou Formation records eight positive polarities and seven negative polarities,with a magnetic stratigraphic age of 40-35 Ma;the Baiyanghe Formation records 14 positive polarities and 14 negative polarities,with a magnetic stratigraphic age of 28.5-21.5 Ma.(3)The sedimentation rate shows that the section records three tectonic uplift events,36-40 Ma,26.5-24 Ma and 22.5-20 Ma.The tectonic rotation reveals that the basin was counterclockwise rotated by the Qilian Mountains retrogradation before 24 Ma,and clockwise rotated by the ATF left-slip and the escape of plateau material to the east after 24 Ma.It also suggests that the ATF has been active from the Late Oligocene.