Magnetostratigraphic Chronology Of The Neogene Strata Along West Hongliuxia Section In The Jiuxi Basin And The Tectonic Implications

Sedimentary deposits in the interior and outside of the Tibetan Plateau archive the uplift history attributed to the Cenozoic India-Asia collision.The Jiuxi Basin,located in the northeastern of Tibetan Plateau and junction region between Altyn Tagh Fault and North Qilian Fault,is the most ideal region to research this northerly response to the collision effect,the process of northward growth of Tibetan Plateau and the activity history of Altyn Tagh Fault.In this study,new Neogene section,the west Hongliuxia section,is researched by sedimentary facies and magnetostratigraphy.The magnetostratigraphic data combined with basin evolution and previous studies.This study discussed the tectonic uplift of North Qilian Shan and activity of Altyn Tagh Fault during the middle Miocene to early Pliocene.1)The Shulehe Formation and the Yumen Conglomerate Formation can be subdivided into four facies,we identified the facies in the Shulehe Formation as shallow lacustrine,delta fan,fluvial facies,the Yumen Conglomerate Formation as alluvial fan system.2)The demagnetization and rock-magnetism experiments show that characteristic remanent magnetization(ChRM)is mostly carried by a combination of magnetite and maghemite,and hematite is second one.The goethite is secondary remanent magnetization carrier.3)We identified 24 normal magnetic chrons(marked as N1-N24)and 24 reverse magnetic chrons(marked as R1-R24)in west Hongliuxia section,with each magnetozone defined by two or more specimens.Magnetostratigraphic correlation suggest that the Shulehe Formation spans from 17.8 Ma to 4.5 Ma.4)Paleocurrent direction indicate the river of the west Hongliuxia flowed southwardly before 6.7 Ma,and the Altyn Tagh Fault controlled the tectonic evolution of Jiuxi Basin.The North Qilian Fault controlled initially tectonic evolution of the basin after 6.7 Ma.In addition,paleomagnetic declination results from the section indicate 6 degree counterclockwise rotation of the Jiuxi Basin since 6.7 Ma.5)Rather than integral growth of the western part of North Qilian Shan.The Laojunmiao region,located in the central part,is earliest uplift gradually in the middle Miocene(~13 Ma).The tectonic force propagated along the North Qilian Fault from east to west Hongliuxia until late Miocene(6.7 Ma),and paleocurrent flowed northwardly.The whole North Qilian Shan occurred rapid uplift since the early Pliocene(~5 Ma).