Late Cenozoic Sedimentary,Tectonic And Geomorphic Evolution In The Northeastern Qilian Shan

The northeastern Qilian Shan lies in the northeastern margin of the Tibetan Plateau,adjacent to the Alxa Block in the north and directly contacting with the Tibetan Plateau in the south.Since the Cenozoic,affected by the northeastward expansion of the Tibetan Plateau,the North Qilian Orogenic Belt has gradually uplifted into mountain and formed a series of Cenozoic basins on the north side,forming a basin range geomorphology.It is of great significance to find out the late Cenozoic uplifting history of the northeastern Qilian Shan for analyzing this unique geomorphology and tectonic growth process and dynamic mechanism of the Tibetan Plateau.The Wuwei Basin located in the northeastern Qilian Shan.The thick late Cenozoic sediments in the basin make it a key area to study the regional late Cenozoic sedimentary,tectonic and geomorphic evolution.Based on large-scale geological mapping,sedimentary characteristics analysis,detrital zircon provenance analysis and regional structure analysis,especially on the high-resolution magnetostratigraphy of Wuwei Basin in the late Cenozoic,this paper establishes the sedimentary,tectonic and geomorphic evolution in the northeastern Qilian Shan,and discusses its dynamic mechanism.The main achievements are as follows:1.According to the sedimentary characteristics of WW-01 and Fengle Basin,the Neogene Gansu Group in the northeastern Qilian Shan is divided into Fengle and Guoyuan Formations.The Fengle Formation is a set of fan delta shore shallow lacustrine facies deposits,with the grain size gradually becoming fine from the bottom to the top,and the overall color is orange red brick red.The Guoyuan Formation is a set of brownish red to yellowish yellow fluvial and lacustrine sediments,with a set of light brick red conglomerate developed at the bottom,and the grain size gradually becomes finer upward.Since the Quaternary,there have been at least four periods of alluvial fans with different sedimentary characteristics in the northeastern Qilian Shan,and stable gravel and gravel beds have been filled in the Wuwei Basin.2.According to the high-precision magnetostratigraphy and Cosmogenic nuclide dating results of WW-01 borehole in Wuwei Basin,combined with regional stratigraphic correlation,the late Cenozoic stratigraphic age of the northeastern Qilian Shan is obtained for the first time.The age of Fengle Formation is ?21–8.25 Ma,and that of Guoyuan Formation is ?8.25–2.25 Ma.The Fengle and Guoyuan Formations are in unconformity contact in the front of north Qilian Shan.3.Based on the detrital zircon U-Pb geochronology of WW-01 borehole in Wuwei Basin,the fluctuation information of the provenance of the basin since Late Cenozoic is analyzed:since ?11.15 Ma,the North Qilian Orogenic Belt and Alxa Block have been competing to provide material sources for Wuwei Basin.The provenance of Wuwei Basin is mainly from North Qilian Orogenic Belt at 10.34–9.51 Ma,8.18 Ma and 3.51–0 Ma,and the provenance of8.69 Ma and 8.14–4.05 Ma is mainly from Alxa Block.4.There are at least four periods of tectonic deformation in the late Cenozoic in the northeastern Qilian Shan:(1)NW-SE extensional deformation occurred in the early Miocene late Miocene(?21–8.25 Ma),which controlled the deposition of Fengle formation;(2)NW-SE shortening deformation occurred in the late Miocene(? 8.25 Ma),Which resulted in the parallel unconformity interface between Fengle and Guoyuan Formations;(3)NE-SW shortening deformation occurred in the late Miocene to the end of Pliocene(?8.25–2.58 Ma),which controlled the deposition of the Guoyuan Formation,and the initial time of this deformation may represent the start time of the northeastward extension of the Tibetan Plateau to the northeastern Qilian Shan.5.Based on the information of sedimentology,magnetostratigraphy,structural deformation characteristics and provenance fluctuation,three main geomorphic evolution stages of late Cenozoic in the northeastern Qilian Shan can be divided:(1)NW-SE trending extensional and faulted basins developed in early to late Miocene,NW-SE trending weak compression and sedimentary basin inversion in late Miocene.(2)NE-SW trending compression and compression basins developed in late Miocene Pliocene,and the northeastward compressional stress of the Tibetan Plateau reached the northeastern Qilian Shan.(3)Since the late Pliocene,the Longshou Shan has been uplifted at ? 3.6 Ma,and the regional basin range geomorphology has initially formed;at ?2.58 Ma,the North Qilian Orogenic Belt was strongly uplifted,the lake in Wuwei Basin was broken,which eventually led to the finalization of the basin range geomorphology.