The Reconstructions Of Megafloods In The Middle Reach Of The Yarlung Tsangpo River,Tibet

Glacial moraines and/or landslide-dammed lakes blocking the Yarlung Tsangpo River may have caused massive outburst floods from failure of these dams during the Quaternary,which may play a vital role in the surface processes and regional tectonic deformation.Our work focused on a large of amount of evidence of megaflood deposits in the middle reach of the Yarlung Tsangpo River.By analyzing sedimentary sequence and internal characteristics,the hydrodynamic characteristics and evolution of the flood,such as the water depth and paleoflow directions,are interpreted.Optically stimulated luminescence(OSL)dating used to estimate the chronological limits of the studied hydrological events,and the shallow-water equations were used to generate water-surface profiles using step-back-water calculations for various discharge values.In addition,we carried out a comparative analysis to assess the potential origin of megaflood deposits.The preliminary conclusions are as follows:(1)We identified a large of amount of sedimentary evidence of cataclysmic flood(s)along 30 km in the middle reach of the Yarlung Tsangpo River,which can be categorized as eddy bars or point bars.The sedimentological variations in different parts of 7 giant bars can be explained by cataclysmic megaflood(s).Considering the significant differences of sediments among grain size,bed thickness,bed contacts,bed geometry and internal sedimentary structures in 1~#giant bar,the hypothesis of two separate floods is also possible.At 2~#giant bar,the lacustrine sediments overlain by cross-stratified megaflood deposits can be regarded as the maximum age of the megaflood events,which may be residual products of the giant glacial-dammed lake at the entrance of the Yarlung Tsangpo gorge.(2)As a result of the low luminescence sensitivity and prominent contamination of the quartz signal by feldspar,some samples were considered unsuitable for acquiring reliable OSL ages.14 ages were used to estimate chronological limits of megaflood events excluding overestimated or perturbable dates.The result shown that at least two separate flood events(ⅠandⅡ)occurred in this basin.The megaflood eventⅠhas occurred during 12.14±1.06 ka～11.05±0.90 ka,dated by megaflood deposits collected from 1~#,2~#and 4~#giant bars.The megaflood eventⅡhas occurred during 12.14±1.06ka～11.05±0.90 ka yield by megaflood deposits collected from 3~#giant bar.In addition,the basal paleo-floodplain deposits are observed to underlie the NV 3~#giant bar yield one age of 10.82±0.89 ka,which may be hiatuses/erosional contacts to support the hypothesis that multiple flood events occurred.(3)The water-surface profiles for discharges between 0.3×10~6m~3/s～1.3×10~6m~3/s at increments of 0.1×10~6m~3/s estimated to contrast with the top elevation of giant bars.The result suggested that the peak discharge with 0.3×10~6m~3/s estimated by 1~#and 3~#giant bars,which indicated a low-magnitude event occurred in this basin(megafloodⅡ).The other giant bars yield a wide range of peak discharge values from 0.5×10~6to1.0×10~6m~3/s,which can be ascribed to the limitations of flow modeling,the complexity of the flow and the complexity of the local geometry.Therefore,we propose a reasonable estimate of peak discharge of approximately 0.8×10~6m~3/s resulting from a single cataclysmic outburst flood event(megafloodⅠ).(4)After systematic research on sedimentary characteristics,chronological limits and peak discharges,the result shown that there are two separate flood events(ⅠandⅡ)occurred in the Yarlung Tsangpo River.The megafloodⅠ,occurred during 12.14±1.06ka～11.05±0.90 ka,with a peak discharge of approximately 0.8×10~6m~3/s would have completely inundated most of the bars,and the discharge uncertainties of giant bars 4~#and 5~#may be caused by changes in channel geometry and stream gradient.The peak discharge of the megafloodⅡoccurred during 5.26±0.47 ka～3.20±0.24 ka is approximately 0.3×10~6m~3/s.Through contrastive analysis,we suspect that the Jiedexiu dammed lake was far more likely to have collapsed and caused a catastrophic flood event that relates to our megaflood evidence.The glacial dam was located at the entrance of the Jiacha Gorge,and the glacial dam collapsed and released large volumes of water,forming massive flood deposits.The sediment rapidly deposited and developed a series of giant bars after the flood flowed out of the Gorge.This study identifies new records for the previously studied megafloods,which could further explain the focused erosion in the steep topography of the Yarlung Tsangpo Grand Canyon.