Paleogene Fission-track Stratigraphy Of The Lanzhou Basin And Its Implications

The Lanzhou Basin,a Cenozoic sedimentary basin,is locataed on the junction of the Qilian Shan and Western Qinling orogenic belts,the northeastern margin of Tibetan Plateau.This basin is filled with Cenozoic sediments,which records the processes and details of the far-field effects of India-Asia collision and Tibetan uplift extended to the eastward of the Western Qinling syntaxis.These documents provide an opportunity for understanding the Cenozoic tectonic events and deciphering t the uplift history of the Tibetan Plateau.However,very few studies have focused on the regional tectonic deformation in the Lanzhou Basin.The detrital apatite fission-track(AFT)thermochronology and magnetostratigraphy are the sensitive and precise tools to solve problems mentioned above.Base on the existed magnetostratigraphy study of Yongdeng section in the Lanzhou basin to achieved the preciously depositional ages,The preciously depositional ages based on the existed magnetostratigraphy study of Yongdeng section in the Lanzhou basin.We conduct an integrity detrital apatite FT thermochronology in this section,combining with the paleocurrent analysis,tectonic-sedimentary deformation and the response of the regional geomorphology,two significant Cenozoic tectonic events are yielded,and as yielded.As an ultimate outcome,we attempt to unravel the Cenozoic morpho-tectonics of the Lanzhou basin and its source regions.The details as in the following:(1)The presence of static peak ages suggesting that the source terranes of sediments in the Lanzhou Basin belong to the crustal quenching type.With the existed magnetostratigraphy of the Yongdeng section of the Lanzhou basin,we adjust the top boundary of the Xiliugou Formation is dated to be 45 Ma;The Yehucheng Formation covers the time span of 45-31.5 Ma;The Xianshuihe Formation covers the time span of 31.5?<16.5Ma,;The lower member of the Xianshuihe Formation spans from 31.5 to 22.0 Ma;The middle member is from 22.0 to 19 Ma.(2)AFT analysis in the Lanzhou basin provides valuable information on the paleogene exhumation history of the Northeastern Tibetan Plateau.Five unreset static peak ages was achieved and the two younger peak ages gradually occurred with the deposition upward,.we We interpreted they are related to the tectonic activities of source region.(3)The provenance analysis of AFT and paleocurrent shows that the potential sources of the Lanzhou basin derived mainly from the western Qinling during Paleocene to the Eocene,but changed to Qilian Shan during Oligocene to Miocene.Combined with previous studies,we concluded that the basin sediments preserve two cooling events based on the thermochronological evidences:(A)the early Eocene(56-43Ma)cooling event,which triggered by the Indo-Asia collision,and in the process of the Western Qinling uplift,the sediments deposited in the Lanzhou basin from 52Ma;(B)the 35?27Ma cooling event,under the north-south compression,the lithosphere of the Tibetan Plateau wae was continuously shortening and thickening,which would result in the comparatively distal effect to the NEM Tibetan plateau.;and Lanzhou basin basement uplift about 31 Ma with the change of the depositional environments.