Miocene Wudaoliang Formation Magnetostratigraphy And Paleoenvironmental Evolution In The Tuotuohe Area Of The Qinghai-Tibetan Plateau

The Qinghai-Tibetan Plateau has brought significant climatic and environmental changes to Asia during its long uplift history leading to the third pole of the earth. Thus, various researches have been done to investigate the uplift reason, process and the related environmental effects to the earth.Now, most of the researches focused on the Qinghai-Tibetan Plateau uplift history process recover through the basin-mountain coupling process approach based on the strata records, tectonic and environmental evolution analysis. However, if no precisely chronological framework provided to demarcate these achievements, it will be difficult to contrast in a large scale and improve the whole recognition of the Qinghai-Tibetan Plateau, which uplifts periodically with different characteristics. And without precisely chronological framework also give difficulties for environmental evolution investigation of the Qinghai-Tibetan Plateau and those adjoining areas of the plateau.Samples from the interest sections are demagnetized by AF demagnetization or thermal demagnetization and the original data regulated by Fisher and PCAo means according different prupose and the results show that magnetite is the dominant magnetic mineral in YP section of Yaxicuo formation lay under WP section of miocene Wudaoliang formation which dominant magnetic mineral is hematite. And nine normal polarity intercalate eight reversed polarity zones correlated with chrons from 6An.2n to 5ADn of the GPTS are identified after local magnetic polarity stratigraphy(LMPS) of Miocene Wudaoliang Formation in the Tuotuohe area of the Qinghai-Tibetan Plateau are constructed through experiment. The bottom boundary age of the WP section Wudaoliang formation is about 20.7Ma, and the top boundary age of it is about 14.2Ma, reprsent the deposit of Earlay Miocene. There are 3Ma depositional break distinguished in WP section between Wudaoliang and Yaxicuo formation corresponding with about 180m deposit deletion.on the basis of magnetostratigraphy results and regarding sedimentary lithofacies as the symbol of climate change, plus climate-lithological combination research of the Miocene Wudaoliang formation located in Tuotuohe area of the plateau hinterland, it was shown that during 16.7Ma～16Ma a quickly uplift event happened in the study area, but until 18Ma when the Second uplift stage ended, the ASL of this area is still under 2Km with a humidly climate. Compared with Linxia basin located in NE edge of the Plateau, the results show that this interest section deposited at 7Ma recorded three different cold climate change events in 17.4Ma, 16.3Ma and 15.8Ma. These climate changing events are also coincided with the evidence of sporopollen and fossil plant founded in Linxia basin. According to the strata records in the study area and former research results, lakes distributed throughout the Plateau broke up at about 14Ma, may pointed that a new whole uplift stage started in north Tibet. This uplift beginning time correspond with a global climate-temperature decrease event happened in the end of the early-Miocene.Secondly, it was found that during early Miocene the uplift rates vary to a certain degree in difference area of the Qinghai-Tibetan Plateau. Uplift rate of western area is greater than Eastern area, and also the adjoining area is greater than hinterland contrasting with the rates of the adjoining major mountain system uplift-denuding, based on different deposition rate at different sedimentary stage and the sedimentary rates comparison between them.This study has given new evidences for reorganization of the difference uplift characteristics and the environmental evolution processes of the Qinghai-Tibetan Plateau. And for the first time, the Miocene clironologic sedimentary sequence of the hinterland in the Qinghai-Tibetan Plateau was set up based on the paleomagnetic dating technique. This work will provide chronologic framework to the toctonic uplift history recorded in deposit and depositional environmental evolution contrast between internal basins and external basins.