| River blockage events occur frequently in high mountain terrain due to the strongly internal and external dynamics in the southeastern Tibet Plateau.Paleo dammed lake and outburst floods deposits have been widely reported in the Yarlung Tsangpo and Jinsha Rivers,providing an excellent platform for analyzing of paleo-geological hazards.How to obtain the characteristics and evolution,the impact of tectonic activity and climate change,as well as the transient fluvial response to river blockages are the research hotspots in this field.A considerable amount of research has focused on the paleo dammed lakes and outburst floods in southeastern Tibet,but their work is mostly limited to the description of typical sections and the confirmation of specific paleo hazards.There is a lack of systematic research on evolution and causes of river blockage-dam failure events.Based on previous research,we conducted a systematic investigation of paleo dammed lakes and outburst floods in the Jinsha River and Yarlung Tsangpo Rivers and their tributaries Parlong Tsangpo,Yigong Tsangpo and Dongjiu Rivers,finally discovered and confirmed more than ten paleo dammed lakes.We select several typical paleo damming blockage events for analysis in the southeastern Tibet,including the dammed lakes and outburst floods developed in the Hutiaoxia Gorge of the upper-middle reaches of the Jinsha River,the middle-lower reaches of Dongjiu and the middle reaches of the Yarlung Tsangpo River.We have made a preliminary study on the characteristics,mechanism and geomorphological response of the natural dams.The main progress of the research works are as follows.(1)At least two dam blockage-outburst events caused by glacial debris flows at the western foot of the Yulong Mountain near the Hutiaoxia Gorge in the upper-middle reaches of Jinsha River from the Hengduan Mountains area on the eastern Tibet since the Last Glacial period.The first dam blocking occurred in～20-7ka and the formed dammed lake II,with the lowest lake surface～1,840 m asl,the lake area～40.79 km~2,and the storage capacity～0.63 km~3.During the blocking period,two outburst events occurred at～20-18 ka and～7-6 ka,forming numerous flood deposits and landforms in the Daju Basin.The second dam blocking occurred during～1.5-0.4 ka and formed dammed lake I,with the lowest lake surface～1830 m asl,the lake area～28.20 km~2,and the storage capacity～0.28 km~3.The two dam blocking events corresponded to the cold climate of the Last Glacial period and Neoglacial period respectively.The rapid uplift and exhumation of the Yulong-Haba fault has strengthen glacier activity in the late Pleistocene,which led to repeated dam blockages and failures in the Jinsha River.(2)A large paleo-seismic event occurred in the Early Holocene caused by the activity of the Lulang fault on the western boundary of the Namcha Barwa Syntaxis on the southeastern Tibet,which led to the paleo-landslide dam blocking in middle reach of the Dongjiu River.The Delun paleo-landslide blocked the river and formed a dammed lake at least before～7.1 ka,and the end time was no earlier than～3.6 ka.The lowest lake surface is～2,590 m asl,with the lake surface area～2.27 km~2,and the corresponding storage capacity～0.09 km~3.Eight main lithofacies associations are proposed that represent four main paleoenvironments:a distal lacustrine environment,i.e.representing a lake distal environment;a lake margin environment;an ice-contact lacustrine environment;and a fluvial environment.(3)Two glacial debris flows damming blocking events occurred in the Xigaze–Dazhuka Valley across the middle reaches of the Yarlung Tsangpo River in the interior of Tibet Plateau since the Last Glacial period.The blocking location is located at the exit of the Dazhuka Gorge.The first damming blockage occurred in～52-42 ka corresponding to MIS4 stage,forming a dammed lake with the lowest surface～3820 m.The second damming blockage occurred during the period of～23-11 ka in LGM period and formed a dammed lake with the lowest surface～3760m.The dammed lakes may have gradually disappeared through the continuous overflow of the barrier dams with no outburst events occurred,which induced large amount of deposits preserved in the Dazhuka valleys.(4)Numerical geomorphological analysis and field surveys show that dammed lakes are mainly distributed in wide valley areas with low-slope,while the locations of natural dams are concentrated in high-slope terrain belonging to geo-hazard-prone areas.The dam blocking events developing in the main stream of Yarlung Tsangpo River have been blocked repeatedly,and mainly caused by glaciers damming.The corresponding dammed lakes are large scale,subjecting to the differential uplifting activities of NW trending active fault zone.River blockages occurred in the Parlung Tsangpo and its tributaries have mainly caused by glaciers,landslides,and debris flows.The number of corresponding dammed lakes are significantly larger,but the scale is relatively small.Most of them are affected by the strong uplift of the Namcha Barwa Syntaxis.Taken as a whole,most paleodammed lakes in southern Tibet have been lasted for many thousand years,starting from the Last Glacial period to the early Holocene.The number and scale of them developed in LGM are the largest,reflecting the development of dammed lakes was likely affected by climate change.The dams can affect river longitudinal profile in the short term,but their long-term effects are very limited for rivers with strong erosion capabilities.In general,this paper has reconstructed the dam blocking-outburst process of the Yarlung Tsangpo and Jinsha River through the comprehensive research of geomorphology,sedimentology and chronology,enriching the information for giant paleo-hazards in southeastern Tibet.It also provides an entry point for discussing the interaction of river surface processes with tectonics and climate.In the future,we should strengthen the quantitative calculation and chronology,and deepen the investigation and analysis of regional tectonic activities and climate change. |
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