| Thanks to the collision of the India plate and Eurasian plate, Tibetan Plateau, a zone which the area exceeds 2.5 million square kilometers, the average altitude surpasses 5000 m has formed. It has a distinct influence on climate not only in our country but also in the world because of the vast area and the high altitude. The collision of the India plate and the Eurasian plate leads to the formation of the Tibetan Plateau and most importantly, this process is still continuing. Thus, the Tibetan Plateau is key natural laboratory to study the collisional orogenic belt, continental deformation and its geodynamics. So far, it is still in controversy about the processes and mechanism of the Tibetan Plateau uplift. Sedimentary basin is an important place to study the uplift. The basins surrounding the Tibetan Plateau has been researched with a high level. However, the research on the basins locating at its central part has fallen behind. Tuotuohe basin is a part of the Hoh Xil basin and locates in the South. To the north of the Tuotuohe basin is Jinshajiang suture and to the south is Tanggula thrust fault. A suit of strata deposited in the Tuotuohe basin, their age spans from late Cretaceous to Miocene. They are Fenghuoshan Group, Tuotuohe Formation, Yaxicuo Formation, Wudaoliang Formation. This suit of strata may have recorded the tectonic response of the collision between India plate and Eurasian plate and the strata have a great useful to understand the process of plateau uplift. At the same time, the strata can help to give a clue to the geochronology problem of the Fenghuoshan group. This thesis has studied the main magnetic minerals, paleoclimate change through magnetostratigraphy, rock magnetism, sedimentary facies, anisotropy of magnetic susceptibility and biostratigraphy. The conclusions are as following:1. The X-T, IRM curves and the CLG model indicate that the main magnetic minerals are hematite and magnetite. In the process of thermal demagnetization, magnetic intensity of some samples decline to 15% level of the primary intensity. This suggests that there is also some goethite.2. The Tuotuohe basin profile can be divided into Tuotuohe Formation, Yaxicuo Formation, from the lower to upper. Magnetostratigraphy and biostratigraphy indicate that the geochronology of the Tuotuohe Formation is 59.8-52.6Ma, Ya xicuo Formation is 51.8-24.1Ma. If Fenghuoshan Group was the same strata as Tuotuohe Formation, then the geochronology of the group is older than 52.6Ma at least. However, if Tuotuohe Formation stayed at the up of the Fenghuoshan Group with an unconformity, the geochronology of it would be older than 59.8Ma.3. Sedimentary facies indicates that Tuotuohe basin has five sedimentary facies changes. They are: The first obvious change starts at 52.6Ma. The climate was arid before 52.6Ma and then got wet after it. Through the research of sporopollen, fossils, the regional contrast and the global temperature change, the subtropical high pressure belt and the decreasing trend of the global temperature lead to the arid. Early Eocene Climate Optimum contributes to the wet climate. Taking consideration of the rainfall variability and the distribution range of wet climate, monsoon may have initiated. We can not exclude the impact of Tibetan Plateau because Tuotuohe basin is at central part of it. The second obvious change starts at 43.2Ma. Gypsum appears at the stratigraphy since that time. The third obvious change starts at 41 Ma. There is thick-layer gypsum. These two times of acidity are the result of Tibetan Plateau uplift, retreat of Paratethys and the global cooling. Because of the retreat of Paratethys is under the control of global climate change and the anisotropy of magnetic susceptibility indicates there is a tectonic event at 41 Ma, thus the uplift of Tibetan Plateau may have much more influence on the Tuotuohe climate than the global cooling. Magnetic indices also support this conclusion. The fourth obvious change starts at 34.5Ma. There is also thick-layer gypsum appeared. This aridity can still be thought as the result of Tibetan Plateau uplift, retreat of Paratethys and the global cooling. At this period, the global factor has diminished while impact of the Tibetan Plateau has expanded. The reasons are the following: first, the retreat of Paratethys is under the control of global climate change; second, marine δ18O indicates that global temperature changes dramatically while the magnetic indices of Tuotuohe basin decreases gently. These two factors allude that Tibetan Plateau impact may have outweighed the global climate change. The fifth obvious change starts at 26.9Ma. There is extremely thick-layer gypsum appeared. Magnetic indices allude that Tibetan Plateau has absolute influence on the zone climate.4. Anisotropy of magnetic susceptibility reveals Tuotuohe basin has experienced strong tectonic strain. There are four strain period from early to the late stages. They are: Segment T1: geochronology is 59.8-52.6Ma. Strain orientation is southwest, which reveal the collision between India plate and Eurasian plate has began before this period. Segment T3: geochronology is 46.8-44 Ma. Strain orientation is southwest. This time of strain is strong and can be compared with other regions. There is gypsum appeared at the stratum. This means that there is high elevation geomorphology at the south of Tuotuohe basin, which can obstacle moisture penetrating into central of the Tibetan Plateau. Segment T5: geochronology is 41.1-34.3Ma. Average declination drifted obviously at 41.1Ma. Strain orientation is southwest. It is noticeable there is southeast orientation appeared. This means the southeast collision of the twoplates has started. Average altitude elevated and the obstacle effection is much more prominent. Segment T7: geochronology is 26.2-24.1Ma. Strain orientation is southwest and southeast. Magnetic indices display that Tibetan Plateau tectonic is much more intensively.5. Relationship among magnetic susceptibility, frequency susceptibility, frequency susceptibility(%) is out phase. This reflects SP particles nearly have no contribution to magnetic susceptibility. In addition, magnetic indices decrease overall at 51.8-41 Ma and 41-32 Ma. This trend is in phase with the declining global temperature and gradual aridity of Tuotuohe basin. Indicating that global climate has impact on the regional climate at 51.8-41 Ma and 41-32 Ma. However, values of the relevant indices(41-32Ma) exceed overall the values of 51.8-41Ma、32-24.1Ma. Based on analysis, there is a tectonic event at 41Ma(results of anisotropy of magnetic also support this conclusion). This tectonic has a great influence on the aridity of Tuotuohe basin. Of course, it is still superimposed on the global climate change. At 32-24.1Ma, magnetic indices decrease wholly, global temperature changes dramatically. The two are out of phase, which alludes Tibetan Plateau has much more influence on Tuotuohe basin than global climate change. |
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