A Study On Detrital Apatite Fission Tracks In Sediments Of The Jiudong Basin Since 6Ma

The Tibetan Plateau,which is the highest and largest plateau around the world,is still uplifting actively.Its uplift has had a far-reaching effect to the geological and geomorphological pattern of Asia as well as global climate change.The peripheral mountain system of the Tibetan plateau is the most sensitive part in response to the Cenozoic tectonic process of the plateau.Therefore,the research on the uplift process and mechanism of the mountain around the plateau has become a critical issue which concerned by the geosciences in the past decade.The Qilian mountain,located in the northeast margin of Tibetan plateau,which is considered as the last mountain to be involved in the plateau.Since the late Cenozoic,the tectonic deformation of Qilian mountains has been active.It is the leading edge of the plateau's northeastward growth,and an ideal region to study the mechanism of plateau uplift and extension.There are abundant studies on fluvial geomorphology,sedimentology,magnetic stratigraphy and low temperature thermochronology in Qilian mountains,which provide important basic data for further study on the history of tectonic uplift in this region.At present,there is a big controversy about the time of the strong uplift of Qilian mountain and the northward extension of the Qilian orogenic belt.Jiudong basin,which is located in the north of Qilian mountains.The Cenozoic strata are contiouous and complete.These sediments contain plenty of informations related to regional tectonic uplift deformation.Thus Jiudong basin is an ideal area to study the history of regional uplift and extension by detrital fission tracks.In this study,we selected the detrital apatite in MH borehole of jiudong basin as the research object.By the apatite fission track(AFT)method,we obtained the AFT age distribution and lag time's change features of detrital sediments in the Jiudong basin.Combined with previous research results,discussed the uplift and expansion of the cenozoic qilian mountains,obtained the following preliminary conclusions:(1)The AFT age of clastic apatites from MH is mostly within the range of 80-160 Ma,accounting for about 90% of the total particles,which confirms the rapid cooling and denudation event during the late Jurassic to Cretaceous period in the qilian mountains.(2)AFT component ages-depositional age curve shows that the lag time of P1 and P1 were getting s Maller during 5.9-3.4 Ma.It shows that the uplift and exhu Mation rate in the northern Qilian mountains gradually accelerate and the tectonic activity gradually increases.It indicates that the topographic relief of the Qilian mountains was increasing during the period.(3)There are two Marked breaks in the curve,respectively at 3.4-2.4 Ma.The component ages of P1 and P2 get older and the lag time of them become longer conformably,and there were AFT ages older than 200 Ma appear in MH after 3 Ma.We speculate that at 3 Ma,the north Qilian orogenic belt expanded northward and the Yumu mountain was involved in the orogenic belt.