Magnetostratigraphy Of The Lower Cretaceous In The Jiuxi Basin And The Early Cretaceous Uplift Of The Western North Qilian Mountain

Tectonic uplift research of the North Qilian Mountain has become a hotspot of the earth science.However,most tectonic uplift research focused on the influence of the collis ion between India and Asia in Cenozoic.Research of the same field in Pre-Cenozoic,particularly in the northern margin area of the Tibet Plateau,is the key to understand the tectonic background before the collision between Asia and India.Though many researches have acquired achievements,most of the achievements lack of precise chronological constraints.To explore these issues,we choose the Jiuxi basin,located in the convergency area of Altyn Tagh Fault and northern edge thrust fault of Qilian Mountain,as the research area.Lower Cretaceous in Jiuxi basin recorded abundant information related to the tectonic uplift in Qilian Mountain which provided possibilities of our research.We obtained the chronosequence of Lower Cretaceous in Jiuxi Basin and based on which we recovered the tectonic evolution of northern magin of the Tibet Plateau.Our research section was measured in Hongliuxia area.We systematically collected paleomagnetic and rock magnetic samples from Chijinpu Formation,Xiagou Formation and Zhonggou Formation.Demagnetization and rock magnetism test turns out that the characteristic remanents of most samples were carried by maghemite,magnetite or hematite Paleomagnetism measurement demonstrated that Hongliuxia section recorded 15 normal and 15 reverse polarity zones and can be correlated to the M10Nr to up M"-2r" in GPTS1994 and GPTS2012,Thus we confirm the age of Lower Cretaceous of Hongliuxia section as 135.32103.746Ma,and more detailed:135.32-112Ma(Chijinpu Formation),112-109.1 Ma(Xiagou Formation)and 109.1-103.746Ma(Zhonggou Formation).Sedimentary facies analysis suggest that Hongliuxia section could be divided into 4 facies:alluvial fan,fan delta,semi deep-deep lacustrine and fluvial Through provenance analysis and chronosequence,we divided the tectonic evolution of Qilian Mountain area into three stages:the early intensively uplift stage(135.32-126.3Ma),middle stable uplift stage(126.3-113.3Ma)and the late rapid uplift stage(113.3-103.746Ma).Uplift of the first stage occurred in the Northern Qilian Mountain as well as Dunhuang Block west of the basin.To the second and third stage,tectonic uplift both occurred in the Northern Qilian Mountain.