[H-GM22_30AM2] 地形

[HGM22-08] 10Be,26Alによって示される北東チベットにおける第四期後期の変動地形発達過程

*白濱 吉起¹、宮入 陽介²、何 宏林³、傅 碧宏⁴、狩野 謙一⁵、越後 智雄⁶、横山 祐典²、池田 安隆¹ (1.東京大学大学院地球惑星科学専攻、2.東京大学大気海洋研究所、3.中国地震局地質研究所、4.中国科学院地質・地球物理研究所、5.静岡大学理学部、6.財団法人地域地盤環境研究所)

キーワード：チベット高原, 柴達木盆地, 変動地形, 表面照射年代

Tibetan Plateau has been growing up due to collision between the Indian and Eurasian plates and expanding its area laterally by invading marginal forelands and making them involved into deformation. The mechanism of expansion at the northeastern margin of the plateau is still a subject of much debate due to the scarcity of tectonic researches. In the Kumkol Basin at the northeastern margin of the plateau we made detailed geomorphological mapping using satellite images, and revealed that there is a huge anticlinorium that consists of many thrusts and folds covered with significantly deformed fluvial or fluvio-glacial fans or terraces along the Pitileke River. The development and deformation rate of the anticlinorium would give an important clue to understanding the growth mechanism of the plateau. In order to estimate the deformation rate, we dated depositional surfaces by surface exposure dating by using cosmogenic radionuclides (CRNs), such as ¹⁰Be or ²⁶Al. Field investigations were conducted in 2011 and 2013, and mainly pebbles of vein quartz were collected at 22 points on the surface of fans and terraces. Depth-profile samples were collected also from 3-m deep sections at two points; each depth profile of ¹⁰Be and ²⁶Al concentrations were analyzed for the exposure age, erosion rate, and inheritance by the Monte Calro simulation. In addition, grain-by-grain CRN concentrations were measured for surface samples from two points to estimate the origin of sediments. Following three inferences were obtained: (1) the sediments of the lower part of terraces and the present river contain reworked sediments; (2) depositional processes and erosion processes are strengthened in the glacial and interglacial period, respectively; (3) the uppermost two steps of terraces were formed before the MIS6 and in the transition period from MIS6 to MIS5, respectively.