Eocene Stratigraphic Sequence And Tectono-Climatic Implications In The Linxia Basin,Northeastern Tibetan Plateau

The eastern segment of the northeastern Tibetan Plateau(NETP)lies in a critical transition zone,in terms of both tectonics and topography,between the Tibetan Plateau to the central Loess Plateau and the Gobi deserts,and in climatic terms between the Asian monsoon dominated region and the westerly-influenced northwest arid region.Several high-elevation ranges and Cenozoic basins have developed in the region,which record abundant information about landform evolution and environmental change of NETP.Therefore,this region is an ideal area to reveal the lithosphere deformation and related kinetic processes caused by the collision of the Indian and Eurasian plates,and to understand the histories of global/regional climate change and geomorphic evolution in responding to the uplift of orogenic belts.The Linxia Basin is an intracontinental foreland basin which was subjected to flexural deformation exerted by the West Qinling(Mts.)and NE Tibetan Plateau to the south.This basin has accumulated continuous sequence of alluvial-fluvio-lacustrine deposits during Cenozoic.In addition,the Linxia Basin has an advantage of the most abundant Cenozoic mammal fossils that attract worldwide attention.Therefore,the Linxia Basin has recorded in detail important information about aridification of Asia,evolution of the East Asian monsoon,origin of the Yellow River,biological evolution,and uplift of the Tibetan Plateau.However,magnetostratigraphic correlation constraints reveal that the Linxia and Xunhua Basins formed no later than 29 Ma,which is younger than the initial formation stages of neighboring basin,resulting in difficulty of sedimentary sequences correlation in the regional basins and in-depth understanding of the Paleogene orogenic uplift process,basin sedimentary evolution and climate change in this region.In this paper,we select a continuous Cenozoic sediment sequence exposed in the Dahejia area in the west of the Linxia Basin to carry out new high-resolution magnetostratigraphic analysis,and establish an accurate chronological framework.The sedimentary facies change,provenance information(including paleocurrent and detrital zircon U-Pb age spectrum)and magnetic fabric were further employed for detail understanding the process and relation between sedimentary evolution of Longzhong Basin and uplift of surrounding orogen.Moreover,we present the hydroclimate variations based on a combined method of rock magnetism and diffuse reflectance spectroscopy,and seek to understand the impact of the Paratethys Sea,uplift of the Tibetan Plateau,and global cooling on the regional climate change.Detailed results are as follows:1.Detailed field investigation results show that the studied DHJ section has a total thickness of approximately ~500 m and is exposed in front of the Jishi Shan and disconformably overlies the Cretaceous sandstones.Based on lithology and sedimentary structures,three sedimentary facies were identified in this section.The lower part of the section(0–77 m)developed in an alluvial fan sedimentary environment,whereas the middle part(77–279 m)in a braided fluvial sedimentary environment.The upper part of the section(279–492 m)is mainly indicated a mudflat and distal floodplain sedimentary environment.Magnetostratigraphic results show that there are eight normal and eight reversed polarity intervals in the DHJ section.This observed paleomagnetic polarity zones is correlated to the geomagnetic polarity time scale(GPTS)indicating ages of 53–40 Ma.In addition,50-m-thick coarse sediments are preserved under the bottom of the DHJ section where samples were not collected.According to the accumulation rate in both correlations,the basal age of the DHJ section is ~54 Ma,which exceeds that of the oldest strata previously known(29 Ma)and has great potential for recording paleoclimatic and paleontological evolution in the fossil-rich Linxia Basin.2.Integrating our provenance and magnetic fabric results reveals that the U-Pb age distributions of detrital grains from pre-51 Ma strata are characterized by many 200 300 Ma ages;alluvial fans and north-directed paleocurrent data indicate a single and relatively proximal source.The northern segment of the West Qinling was the predominant source supplying sediments to the Linxia Basin before ~51 Ma.Subsequently,Detrital zircon U-Pb age spectra since ~51 Ma reveal significant increases in Precambrian peaks(1500–2000 Ma and 2300–2600 Ma).These observations suggest accelerated growth of the southern segment of the West Qinling and/or eastern segment of the East Kunlun Shan between 51 and 47.8 Ma.Meanwhile,the Linxia Basin was exerted a relatively strong SW-NE compressional strain and subjected to a ~17° counterclockwise rotation.Deformation in the NETP between 51 and 47.8 Ma was followed by a period of relative quiescence between 47.8 and 40 Ma,when regional relief decreased by long-term denudation.These growth of orogenic belt and basin deformation indicate that near-synchronicity of crustal shortening and deformation between northern Tibetan Plateau and the India-Eurasia collision zone.3.Based on a combined method of rock magnetism and diffuse reflectance spectroscopy,our results show magnetic enhancement of fluvial sediments at 51.7 Ma resulted from abundant single-domain magnetite inputs from the surrounding mountains through strong erosion and transport in a relatively wet environment.Subsequently,at 47.6 Ma,magnetic weakening of floodplain sediments corresponds to an increase in hematite concentrations produced via low-temperature oxidation in a prolonged dry environment.Comparisons of paleoclimatic proxies,such as organic geochemistry and rock magnetism from neighboring basins,tectonic deformation of the NE Tibetan Plateau and sea level change in the Paratethys Sea suggest that the hydroclimate variation from relatively wet to dry climate in the early-middle Eocene in this area was mainly controlled by global climatic change.4.Based on the synsedimentary characteristics of the Linxia Basin and its surrounding areas,the regional spatio-temporal evolutions of tectonics-climatesedimentary during the early-middle Eocene are reestablished.During the period of~54–48 Ma,the West Qinling and East Kunlun Shan underwent accelerated growth,which were coeval with the SW-NE compression,anti-clockwise tectonic rotation and inception of Cenozoic sedimentation in the Longzhong Basin.The debris was rapidly eroded and transported in relatively humid climate conditions,resulting in alluvial fan sedimentary environment occupied in the Linxia and Guide basins,and distal floodplain and fan-delta environments occupied in the Xining and Lanzhou basins,respectively.During the period of 48–40 Ma,the topographic relief decreased by exhumation within a relatively stable tectonic setting in the Longzhong Basin.The distal floodplains developed in the Linxia and Guide basins and the playa salt lakes developed in the Xining and Lanzhou basins.Meanwhile,under the control of global cooling,the regional hydroclimate varied from relatively wet to dry,resulting in a relatively low sedimentation rate.