High Resolution Magnetostratigraphy In The North Of Qaidam Basin And The Sedimentary Response To Tectonic Since Late Cenozoic

As the highest and biggest physiognomy union in the world, Tibetan Plateau has become the focal region that geologists attended to and researched for a long time. But at present, there are many big divarications in core questions such as the time, format, extent of plates collision and the plateau deformation or uplift mechanism in the process of collision. An important reason is the absent of key dating informations in the core positions of Tibetan Plateau. If we want to test the prevenient viewpoints or construct new viewpoints and resolve the core scientific questions about the uplift time, format and process of Tibetan Plateau, we should do high-resolution researches in key positions those record the tectonic action events continually and fully.The research of coupling between basin deposition and mountain tectonic uplift in orogenic belt has approved that the sediments in basin could reflect the formation and evolutional history of orogenic belt. The distribution of basin-mountain in the inner and margin of Tibetan Plateau formed in the process of Tibetan Plateau extrusive deformation and uplift. Thus, according to the mountain-basin coupling hypothesis, the probable approachable way to reconstruct the uplift history of the Tibetan Plateau should base on the high-resolution dating, analysis of sedimentary basin, tectonic evolution and environmental information of the basin in the interior and adjacent Tibetan Plateau.Qaidam basin that bounded by Kunlun Shan, Altun Shan and Qilian Shan is the biggest intermountain basin in the north of Tibetan Plateau. It is directly controlled by three main faults and the sub-faults, the phenomenon of mountain-basin coupling is very obvious. The huge thickness of Cenozoic sediments shed from the surrounding mountains fully preserve the detailed records of Tibetan Plateau uplift and basin evolution since Cenozoic, so it is the optimal region to test and perfect the existing mode of Tibetan Plateau uplift. Because the lack of absolute age of Cenozoic strata in Qaidam basin, we couldn't establish the credible strata chronology sequence. The further researches about refined sedimentary-tectonic evolvement of Qaidam Basin and the tectonic uplift of Tibetan Plateau has been tied. In order to resolve above scientific problems, the researches of absolute chronology sequence to Cenozoic strata in Qaidam Basin are very imperative and important.We choose the Huaitoutala section in the north of Qaidam Basin as studying object. Through high-resolution magnetostratigraphy studies, combining with the macroscopical age control of fossil mammals, we could establish the detailed chronology sequence of Cenozoic strata. Under the control of strata chronology, the sedimentary evolution of the North Qaidam Basin and the uplift process of Nan Qilian Shan could be revealed through the studies of detailed basin analysis including Cenozoic strata characters, sedimentary characters, space changing rule, paleocurrent direction, sedimentary facies combinations, depositional thickness and provenance analysis. It provides the important information for discussing the formation of basin-mountain physiognomy pattern in the north of Tibetan Plateau.Through our researches in this region, we mainly obtained the following conclusions:1. Under the control of fossil mammals, the magnetostratigraphy research reveals the forming age of Huaitoutala Section is from 15.7Ma to 1.8Ma. In addition, the widely used stratigraphic units the Xia Youshashan, Shang Youshashan, the Shizigou and Qigequan Formations were formed at >15.3Ma, 15.3-8.1Ma, 8.1-2.5Ma and <2.5Ma, respectively. Through the analysis of sedimentary rate, we believe that there are six high sedimentary rate periods in Huaitoutala Section.2. Through the general analyses including the lithofacies, lithology, sedimentary facies combinations, paleocurrent direction and the gravel components, combining with magnetostratigraphy chronology, Huaitoutala region underwent five sedimentary evolutional periods since 15.7Ma. They are respectively as 1) the period of Qaidam Basin (>15.3Ma); 2) earlier division period of north margin of Qaidam Basin (12.6-15.3Ma); 3) the further division period of north margin of Qaidam Basin and the development period of Delingha depression (8.1-12.6Ma); 4) gradual shrinkage period of north margin of Qaidam Basin (2.5-8.1 Ma) and 5) the formation period of intermountain basin (<2.5Ma). According as the changes including sedimentary rate, the average declination of paleomagnetism samples in every groups, the response relations between tectonic action and sedimentary characters such as strata characters, sedimentary grain-size, sedimentary facies, sedimentary structure, grave component and paleocurrent et al., there are seven intense tectonic uplift events happened in this region since 15.7Ma. They respectively happened at 15.3Ma, 14.6-12.4Ma, 8.1Ma, 6.6Ma, 3.6Ma, 2.5Ma and 1.8Ma. In those events, the tectonic actions happened at 15.3Ma, 8.1Ma and 2.5Ma are especially important. The tectonic action happened at 15.3Ma made Nan Qilian Shan fault extended to south and Emonike Shan uplifted, it induced the north margin of Qaidam Basin divided from Qaidam Basin; the tectonic action happened at 8.1 Ma made Nan Qilian Shan and Emonike Shan uplifted again and Nan Qilian fault extended to Qaidam Basin, it made Delingha Depression in the north margin of Qaidam Basin shrunk; the tectonic action happened at 2.5Ma was very intense, it made the Olonbuluk Shan, Nan Qilian Shan and Emonike Shan uplift and changed the basin as intermountain basin. After 1.8Ma the basin uplift again and the deposition was end in the region of Huaitoutala. It made the previous strata deformated and denudated. Contrasted with other tectonic events happened at the wide region in the north of Tibetan, those events have the characters of coherent, synchronous and stepwise. All these provide a new clue to the farther reasonably interpret and construct the history of the Tibetan Plateau formation and evolution model.All above results not only are important to the researches of the uplift of Qilian Shan, the tectonic uplift and process of NE Tibetan Plateau, but also have very important value to understand the destruction, alteration and rebuilding of gas and petrol resouce in the north of Qaidam Basin.