Late Cenozoic Environmental Change In The Linxia Basin,NE Tibetan Plateau

Late Cenozoic Tibetan Plateau uplift, Asian monsoon development and Inner Asian aridification are important geological and environmental phenomenons, however, it seems too complicated to be understood well although many researchers have focused on them during the past few decades. Recently, many scientists have continued studying them by using advanced scientific technology. They have springed a series of new opinions on the formation and development of Asian monsoon, Inner Asian aridification and uplift of the Tibetan Plateau. So far,the Tibetan Palteau are still a hot area for testing the relationship between the tectonic and climate.The Linxia Basin, located at the NE Tibetan Plateau, lies at the convergence place influenced by the East Asian summer monsoon, East Asian winter monsoon, Westerlies and the climate of the Tibetan Plateau itself. The Cenozoic deposition in the basin is thick and almost continuous bearing many layers of mammal fossils (rich numbers and species), which provides ideal materials to detect the paleoenvironment, paleoclimate and their relationships with the tectonics of the Tibetan Plateau. In this thesis, the Heilinding section is selected to study the environmental magnetic measurement (isothermal remanent magnetization, anhysteretic remanent magnetization,magnetization curves and hysteresis loops) in combination with other proxies (diffuse reflectance spectroscopy, grain size, colours and calcium carbonate).Through a comprehensive analysis we can get the following conclusions:(1) The magnetic susceptibility of the Heilinding section shows a clear shift at -8.5 Ma from relatively low values (mean 22.8×10-8 m3kg-1) with low fluctuations to remarkably increased values (mean 34.0×10-8m3kg-1) with high fluctuations before and after 8.5 Ma. Environmental magnetic analysis shows that the main magnetic minerals of sediments are magnetite, maghemite and hematite,containing a small amount of goethite,the increase of magnetic susceptibility values 1 were caused by the increase of ferrimagnetic minerals content.(2) Through comprehensive study of the environmental magnetic parameters, combined withthe detailed description of lithology and sedimentary facies analysis,we get that the pedogenesiswas not the main cause of increasing ferrimagnetic minerals content, and there is no directrelationship between climate change and magnetic susceptibility. The conclusion is also supported by the results of the other proxies. The ferrimagnetic minerals content increase may be due to thechange of source area.(3) The denudation and transport rate of source detritus in Heilinding section was acceleratedby the Tibet Plateau uplift. Because of weakened weathering the magnetite deposited before fullyoxidized and only a small portion is oxidized to hematite, which caused the magnetic susceptibility increase after 8.5 Ma. Through comprehensive environmental magnetic analysis with Maogou section, we get that the change of magnetic susceptibility in Linxia Basin was driven mainly by tectonic uplift, and climate change had some extent impact on the environment magnetic parameters, but not the controlling factor.(4) By summing up the late Cenozoic tectonic uplift phases and climate records of the NE Tibetan Plateau we conclude that the intense tectonic movements occurring at ?8 Ma have driven the large-scale climate dries in the NE Tibetan Plateau. The wet environment at some places at -8 Ma may be caused by the local topography also driven by the Tibetan Plateau uplift. In future, we will continue to study the chronology of the sediments, the tectonic proofs and climatic proxies to provide further reliable evidences for the climate change, the uplift of the Tibetan Plateau, and the tectonic-climate interactions at this region.