Fission Track Thermochronology Study Of Neogene Exhumation Of The Greater Himalayan Slab In Southern Tibet

Exploring on Earth’s surface climate and tectonic system dynamic interaction of surface geological processes has become an emerging and challenging research field in Earth Sciences. Climate-structure response process is mainly reflected In the orogenic climate and tectonic evolution interaction and constraints between climate and tectonic evolution, geomorphology in orogenic belt, tectonism impact on local climate characteristics and even global climate by surface uplift and increased topography, on the contrary, climatic factors (primarily for rainfall and glaciation) induced and maintained tectonic uplift by surface erosion, generated stress concentration and balanced role in the erosion area by surface erosion. The Greater Himalaya is located between the southern Tibet detachment system (STDS) to the north and the Main Central thrust faults (MCT) in the south, and Its exhumation process that controlled by structure factors or climate factors or both of them is still in dispute.In this thesis, we tried to start from Gyirong-Nyalam apatite fission track dating and com-bined with MCT thrusting and the existing data to carried out with geothermal field structure thermal simulation,Aiming to give quantitative constraints on hree-dimensional in Cooling and exhumation process in temporal of the Himalaya orogenic belt. At the same time, by combining of evolution process of the STDS, MCT and the present geomorphology and rainfall conditions, juxtaposing rock exhumation rate, geomorphic features on the spatial distribution characteristics and coupling relationship, we analyzed and evaluated tectonics, and surface processes in macro-scopic Orogen, and deeply discussed the temporal and spatial process of cooling and exhumation of the Himalayan orogeny and the coupling relationship between Tectonism and climate effects.Bedrock apatite fission track data was gained of two sampling profiles, Gyirong and Nyalam, from the Greater Himalaya region of southern Tibet, obtained apatite Fission Track ages in Gyirong transect were0.7±0.3Ma～1.5±0.4Ma, while in Nyalam Fission Track ages shows a clear step-like distribution, and can be subdivided into two stages:1.2±0.6Ma-4.9±0.8Ma and7.3±2.0Ma～11±1.8Ma. The Greater Himalaya profile Ages and its spatial distribution above indicates that the Greater Himalayan experienced two stages of strong rock cooling and exhumation in middle to late Miocene and since late Pliocene. Preparation on existing zircon fission track data (Gyirong profile:13.1±2.2～2.4±0.5Ma, Nyalam profile:16.1±1.0～3.0±0.5Ma) found that difference between the zircon and apatite age presents a significant reduction from north to south, which may indicate that the Nyalam Greater Himalaya rock cooling exhumation migration process of development from north to south. Further analysis showed that the Miocene middle-later(11.0±1.8Ma～7.3±2.0Ma)age was related to the large-scale stretch of STDS in Miocene.Based on the special pattern of spatial distribution of the apatite fission track ages, in this thesis, analyses on the relationship between precipitation and elevation fitting showed an overall characteristic that precipitation is decreases with increased elevation, and locally concentrated. Based on topography conditions at or near steady state, river channel undercutting rates model (Detachment-limited model), the simulation results show that the rainfall (watershed area and flow) and river channel undercutting effects does not have a significant consistency in space, compared the river channel undercutting rates and spatial distribution of apatite fission track ages found do not fully coupled. Above characteristics suggest that undercutting effects is not the dominanted the rock cooling process of study area, but played an important role in shaping unique topography of Himalayan orogenic belt. At the same time, field surveys and40Ar/39Ar age data exclude the possibility of fault which able to cause the age pattern.Based on the special distribution pattern in space of apatite fission track ages and relationship with the main boundary fault (MCT) thrusting and whether it reflects the climate-structure response in the Greater Himalaya, in this thesis, the Nyalam transect was selected as structural model for carrying out the three-dimensional structure thermal simulation. Simulation results show that the simulation Age can fit well with the newly tested apatite fission track ages and existing apatite, zircon fission track age data, MCT thrusting makes the Greater Himalayan isotherm significant upward close to the MCT migration, then control the main Central Thrust (MCT) hanging wall Greater Himalayan rock cooling ages spatial pattern, consequently, the unique step type age structure in the Greater Himalaya needn’t explain by formation the response structure in climate and erosion conditions. The simulation also shows that the MCT continuing activities at thrust rate on average of1.7～2.7mm/a since at least6Ma, this provided new evidence for MCT activities since the late Miocene. MCT tectonic activity since the late Miocene showed that MCT may be with MBT and MFT adjust and shape the Himalayan modern orogeny together for long-term, and not simply expansion before migration from MCT to the main boundary thrust fault (MBT) and the main forward thrust fault (MFT).Comprehensive Climate-structure analysis shows that, obvious E-W fault that in response of climatic mechanism does not exist in the interior Greater Himalaya. Therefore, on a small scale, rainfall-dominated climates effects played an important role in shaping unique topography in the Himalayan orogenic belt, but not the fundamental driving factors of the Greater Himalayan rock cooling and exhumation. The Greater Himalayan rock cooling exhumation is mainly controlled by MCT thrust tectonism. However, in the entire orogenic belt, the assumption that the continuous thrusting of MCT was partially driven by the intense material erosion in the Greater Himalaya can not necessarily be impossible.