Abstract: The Yarlung Tsangbo fault zone, one of the most important geological interfaces in the Yarlung Tsangbo suture zone which is a huge geotectonic boundary with nearly east-west-trending in southern Tibet Plateau, has undergone a long-term tectonic evolution. Studying this fault zone can help us understand the development and evolution history of the suture zone and the tectonic mechanism of subduction-collision about the Tibet Plateau, so it has always been a hot topic in the field of geology. Most of existing data suggest that the current tectonic activity in southern Tibet is given priority to the rift system with nearly north-south-trending, and the Yarlung Tsangbo fault zone with nearly east-west-trending has relatively weaker activity since late Quaternary. There are only some evidences of Holocene activity found in the Lulang town section near eastern Himalayan syntaxis, and there are few reports about the reliable geological evidences of late Quaternary activity of the section on the west of Milin County of the fault zone.
Based on image interpretation, field investigation and chronological method, we found several fault profiles along the Yarlung Tsangbo fault zone near the Angren Lake in this study. These profiles reveal that loose fault gouge has been developed on the fault plane which nearly extends to the surface and offsets the loess sediments and its overlying alluvial-proluvial gravels. The loess is characterized by coarser grains, higher content of fine sand and tiny small gravels. The results of the two OSL dating samples collected in the loess are(94.68±6.51)ka and(103.84±5.14)ka respectively, showing that the loess revealed at the Angren site should be the middle-late Pleistocene sand loess distributed on the high-terraces along the Yarlung Tsangpo River. Consequently, the Angren segment of the Yarlung Tsangpo fault zone is active since the late Quaternary. In addition, synchronous left-lateral offsets of a series of small gullies and beheaded gullies can be seen near the profiles along the fault, which are the supporting evidence for the late Quaternary activity of the fault.
However, the segment with obvious geomorphology remains is relatively short, and no evidence of late Quaternary activity have been found in other sections on the west of Milin County of the Yarlung Tsangpo fault zone. Existing data show that, in the southern Tibet, a series of near NS-trending rift systems are strongly active since the late Quaternary, cutting almost all of the near east-west-trending tectonic belts including the Yarlung Tsangpo fault zone. In addition, majority of the earthquakes occurring in southern Tibet are related to the NS-trending rift systems. Tectonic images show that the Angren segment locates between the Shenzha-Dingjie rift and the Dangreyong Lake-Gu Lake rift. These two adjacent rifts are special in the rift system in southern Tibet:Firstly, the two rifts are located in the conversion position of the trend of the whole rift system; Secondly, the size of the two rifts varies significantly between the north side and the south side of the Yarlung Tsangbo fault zone. Thirdly, the Shenzha-Dingjie rift seems to be of right-lateral bending, while the Dangreyong Lake-Gu Lake rift shows left-lateral bending. These characteristics may lead to the fact that the amount of absorption and accommodation of the rift activities in the north side of the Yarlung Tsangbo fault zone is larger than that in the south side during the migration of the plateau materials, leading to the differential movement of the block between the two sides of the fault zone. Therefore, the Yarlung Tsangbo fault zone possesses the accommodating tectonic activity, of course, the intensity of this accommodating activity is limited and relatively weaker, which may be the reason why it is difficult to find large-scale tectonic remains characterizing the late Quaternary activity along the fault zone. The scale of the rift system in southern Tibet is systematically different between the two sides of the Yarlung Tsangbo fault zone, so it cannot be ruled out that there are also weak activities similar to the Angren segment in other sections of the fault zone.

Key words: Yarlung Tsangpo fault zone, Angren segment, the late Quaternary, weak activity, rift system in the southern Tibet

摘要： 雅鲁藏布江断裂带是青藏高原南部一条规模巨大的近EW向大地构造边界，其经历了长期的构造演化，一直是地学界关注的热点。已有资料大多认为青藏高原南部地区现今的构造活动以近SN向的裂谷系为主，近EW向的雅鲁藏布江断裂带仅在东构造结部位的鲁朗一带发现有全新世活动的迹象，而米林以西段的最新活动主要发生在早、中更新世。近年来，在沿该断裂带日喀则以西段的调查过程中，在昂仁错一带发现多个断层断错晚第四纪地层的剖面，显示其晚第四纪以来仍有断错地表的活动，但活动遗迹延伸较短。综合分析认为，在区域SN向裂谷的构造环境中，被裂谷系分割的近EW向雅鲁藏布江断裂带可能具次级构造调节作用，从而表现出相对较弱或局部的断错地表活动。

关键词: 雅鲁藏布江断裂带, 昂仁段, 晚第四纪, 弱活动, 藏南裂谷系