Magnetic Stratigraphy Of The Laolongwan Basin In The Northeastern Margin Of The Tibetan Plateau And Its Tectonic Significance

The Laolongwan Basin is a Cenozoic sedimentary basin developed in the Haiyuan fault zone on the northeastern margin of the Qinghai-Tibet Plateau.Its sedimentary evolution has recorded the structural deformation process of the Haiyuan fault zone since the Cenozoic in detail.In this paper,magnetic stratigraphy,cosmogenic nuclide dating and apatite(U-Th)/He method are used to try to establish the chronological framework of the Cenozoic strata in the Laolongwan Basin.The sedimentary characteristics of sedimentary strata,the distribution of sedimentary facies,and the deformation patterns of the stratum were studied to reveal the Cenozoic activity history of the Haiyuan fault zone on the northeastern margin of the Qinghai-Tibet Plateau.We obtained the following understanding:1)The Laolongwan Basin is now diamond-shaped,with an aspect ratio close to 3:1.The basin is bounded by the central Zihongshan fault.There is a clear difference in formation filling between the south and north sides.The north side mainly develops orange-purple thick mudstone,argillaceous siltstone with coarse sandstone,fine conglomerate and thin gypsum and the sediment size is relatively fine;On the south side,thick orange-brick red detrital quartz medium coarse sandstone with conglomerate and mudstone,coarse medium conglomerate with gravel containing coarse gravel sandstone,and orange-red medium sandstone with thin layer conglomerate are mainly developed and the sediment size is coarser than the north side.The thickness of the sedimentary strata on the south and north sides of the basin is about the same.2)The formations in the northern part of the Laolongwan Basin are similar to the Eocene-Oligocene formations in the Xining-Lanzhou area,indicating that the formation age in the northern part of the basin may also be Paleogene.The detailed paleomagnetic study results of the upper stratum of the Shuiquan section in the southern part of the basin show that the depositional age is 4.6-0.8 Ma.If the deposition rate of the upper and lower sections of the section is similar,it can be inferred that the section began to deposit at ～12 Ma.3)The Laolongwan Basin has various types of sedimentary facies such as braided river facies,lake facies,and alluvial plain.The formations on both the north and south sides of the basin have continuous folds and axial SE,reflecting that the basin has been affected by Northeast-Southwest The compressive stress in the southward direction also developed folds in the axial direction from north to south on the south side of the basin,indicating that the basin was subjected to east-west compressive stress in the late Cenozoic.4)The sedimentary lithology,grain size and sedimentary facies distribution on the southern side of the Laolongwan Basin show that it has the nature of a subsidence basin,indicating that the Haiyuan fault may have been overthrust in the late Miocene,revealing that the extension of the Tibetan Plateau to the northeast direction in the late The Miocene has already reached the area.The north-south distribution of fold-related growth strata in the basin may be closely related to the left-lateral strike-slip of Haiyuan fault.The growth strata may have begun in the Pliocene,indicating that the Haiyuan fault zone began to rotate left-slip in the Pliocene,resulting in the destruction of the Laolongwan Basin and the formation of the present tectonic and geomorphic pattern.