Tectonic And Geomorphological Evolution Features In Qilian Mountains During The Cenozoic

Cenozoic uplift of the Qinghai-Tibet Plateau is the great geological events. TheQilian Mountains form the north east boundary of Qinghai-Tibet Plateau. It issignificant that study Cenozoic tectonic deformation characteristics and comparingwith other parts of the plateau. Muli Basin discovered natural gas hydrate in2008.Natural gas hydrate formation has the extremely close relation with the uplift historyof Qilian Mountains, so the question when this area came into the cryosphere isnecessary to be solved.This thesis mainly base on regional geological survey in the Qilian Mountains,combined with the feature of Cenozoic strata, Cenozoic faulting characteristics andtopographical features, and used ESR dating, carbon and oxygen isotopic techniques,we can conclude the tectonic landform evolution features in Qilian Mountains duringthe Cenozoic to find out the relationship between Qilian Mountains and Tibet Plateau.The main conclusions are as follows:1. In the Eocene to early Oligocene, with the collision, which is between IndiaPlate and Eurasian Plate, the Qilian Mountains began to uplift. Based on the ESRdating, the Tertiary sediments include the Eocene to Oligocene HuoshaogouFormation and the Oligocene Baiyanghe Formation. From late Eocene to earlyOligocene, the sedimentary environment changes significantly, and an obviousangular unconformity exists between the upper and lower strata and it can be inferredthat Qilian Mountains area has been a tectonic deformation and uplift, which hascorresponding connection with the early Cenozoic uplift of Qilian Mountains. Usingcarbon and oxygen isotope ancient altimeters, in early middle Cenozoic Era theancient altitude of Qilian Mountains is~2800m, which means the Qilian Mountainshad reached a certain height, and constituted the northern border of the Qinghai-TibetPlateau.2. In the Oligocene and Early Miocene, the geological structure of QilianMountains was stable, and formed the large area of planation surface, carbon andoxygen isotope of Baiyanghe Formation can estimate the ancient altitude of Qilian Mountains, and result is that the ancient altitude of Qilian Mountains is~2658m,which is lower about140m than the Eocene Huoshaogou Formation, reflecting inOligocene Qilian Mountains widespreadly eroded.3. In Miocene epoch and Qilian Mountains uplift strongly, forming basins-mountains tectonic landforms. The north rim fracture of Qaidam Basin, the north rimfracture of Middle Qilian Mountains and the south rim fracture of Middle QilianMountains intensely actived. According to the carbon and oxygen isotopes ofMiocene lacustrine deposition, the ancient elevation was~3302m, higher thanOligocene ancient elevation about644m, which reflects the Miocene is the QilianMountains’ uplift period.4. In the late Pliocene-early Pleistocene Qilian mountains’ tectonic actived again,leading to exist a slight Angle unconformity between the Pliocene and lowerPleistocene, and also exist Angle unconformity between lower Pleistocene andmiddle-upper Pleistocene. The intensity of tectonic activities much more subdued.There is a good corresponding relation between Qilian Mountains’ tectonicgeomorphology evolution and the Qinghai-Tibet Plateau uplift during Cenozoic, in aword, the Qilian Mountains experienced four stages, which are Orogeny-Planation-Strong uplift-Reconstruction. Fossil sand wedges are discovered in the northern shoreof Qinghai Lake. One location is in the east of Gangcha County, another is in the westof Quanji Township. Fossil sand wedges have environmental significances, the annualmean temperature in the northern shore of Qinghai Lake in the third glaciation fromthe last was probably-7.5~-10℃, in penultimate glaciation was about-9.5~-10℃,temperature reduction in glaciation was conspicuous. In early Mid-PleistocenetheQilian Mountains had entered the Cryosphere, gas hydrate may have formed.