10:45 AM - 11:15 AM
[SCG47-01] Studies of tectonic geomorphology at the northeastern edge of the Tibetan plateau by the dating method with in situ terrestrial cosmogenic nuclides
★Invited Papers
Keywords:tectonic geomorphology, active fault, in situ terrestrial cosmogenic nuclide, surface exposure dating, Tibetan plateau
Tibetan plateau has been growing up by the collision between Indian and Eurasian plates and expanding its area laterally. The marginal deformation of the plateau related to the lateral growth formed some major strike-slip faults; Altyn Tagh Fault, Kunlun Fault, and Haiyuan Fault, and many active thrusts and folds around those major faults. Understanding the development and slip rate of these faults and folds is important for revealing the growing process of the northeastern plateau. Due to the cold and dry climate of this region, old topographic surfaces are very well preserved because of the low erosion rate. However, until the 1990s, there were no suitable dating methods available, making quantitative analysis difficult. After the development of the surface exposure dating method using in situ terrestrial cosmogenic nuclides (TCN) in the late 1990s, the TCN dating method has been applied broadly to the geomorphological study of this region. The dating method made quantitative analyses to be conductable such as the estimation of average slip rate along the major strike-slip faults and has become an essential tool for the study of tectonics in the Tibetan plateau. This presentation introduces some examples of the applications of TCN in this region, mainly focusing on the author's research in the Kumkol and Gonghe basins, northeastern Tibet.
In the Kumkol basin near the closing area between Altyn Tagh Fault and Kunlun Fault at the northeastern margin of the plateau, there is a huge anticlinorium that consists of many thrusts and folds. The anticlinorium is covered with significantly deformed fluvial or fluvioglacial fans or terraces along the Pitileke river at the eastern part of the anticlinorium. Geomorphological studies and TCN dating for those tectonic landforms were applied to reveal the development and deformation rate of the anticlinorium. The formation age of the oldest terrace, T1 was determined 252 ± 24 ka by depth profiling. As a result, the age and deformation of the T1 terrace give a maximum uplift rate of c. 1.0 mm/yr and a north-south horizontal shortening rate of 2.5-3.2 mm/yr of the anticlinorium, absorbing a part of shortening between Tibetan plateau and Qaidam basin.
Gonghe Basin, one of the inter-mountain basins at the northeastern margin of the plateau, is about 3200 m above sea level and located at the border of Qinghai Nan Shan and Heka Shan to its north and south, respectively. Previous researches reported that after the Yellow River filled the basin with over 500 m thick clastic sediments, it started to cut it down and formed many fluvial terraces. In order to reveal the filling process, pebbles of quartz were collected at nine points per about 50 m depth in the sediments from the fill top surface to the bottom of the valley, and burial dating by TCN was conducted. The ratio of 26Al concentration to 10Be yields a period since the sample completely shielded from cosmic rays. Calculated burial ages showed in remarkable stratigraphic order from top to bottom, suggesting an average deposition rate of about 70 mm/kyr. The fact indicates that the basin had been constantly filled since the late Miocene.
In the Kumkol basin near the closing area between Altyn Tagh Fault and Kunlun Fault at the northeastern margin of the plateau, there is a huge anticlinorium that consists of many thrusts and folds. The anticlinorium is covered with significantly deformed fluvial or fluvioglacial fans or terraces along the Pitileke river at the eastern part of the anticlinorium. Geomorphological studies and TCN dating for those tectonic landforms were applied to reveal the development and deformation rate of the anticlinorium. The formation age of the oldest terrace, T1 was determined 252 ± 24 ka by depth profiling. As a result, the age and deformation of the T1 terrace give a maximum uplift rate of c. 1.0 mm/yr and a north-south horizontal shortening rate of 2.5-3.2 mm/yr of the anticlinorium, absorbing a part of shortening between Tibetan plateau and Qaidam basin.
Gonghe Basin, one of the inter-mountain basins at the northeastern margin of the plateau, is about 3200 m above sea level and located at the border of Qinghai Nan Shan and Heka Shan to its north and south, respectively. Previous researches reported that after the Yellow River filled the basin with over 500 m thick clastic sediments, it started to cut it down and formed many fluvial terraces. In order to reveal the filling process, pebbles of quartz were collected at nine points per about 50 m depth in the sediments from the fill top surface to the bottom of the valley, and burial dating by TCN was conducted. The ratio of 26Al concentration to 10Be yields a period since the sample completely shielded from cosmic rays. Calculated burial ages showed in remarkable stratigraphic order from top to bottom, suggesting an average deposition rate of about 70 mm/kyr. The fact indicates that the basin had been constantly filled since the late Miocene.