While I'm working on a project about landslides on the Tibet Plateau, I encountered many large and valley-filling deposits that seem to be some kind of mass movement but cannot find their source areas.
The phenomenon is usually observed at 4000 - 5000 m elevation and many areas have permafrost. I attached two screenshots to show one of them which dammed the river (the only one that dammed river in the region). Many other cases can be seen around this area in the google earth kmz file.
I was told it might be slow sliding caused by underlain ice melt or remaining glacier tills. Can someone who has working experience in high-elevation areas identify what kind of phenomenon is it?
Thanks in advance
Chenxiao Tang
I am not familiar with your area of interest, but have seen similar landforms and signs of erosion. I see signs of highly erodible soils, and you mentioned glacial till. I see signs of rilling and gullying as one might expect in steep terrain with erosive soils, and areas with sparse vegetation. If there are native tree species suitable for the barren or sparsely vegetated areas, that might help. I also worked in coastal Oregon, and we had areas with debris avalanche, slumping, earth flows, rotational failures, and we had some quality soil scientists and geologists to help identify and map unstable areas. I believe I see signs of these type instability signs or failures, but it is also helpful to have aerial photo pairs and stereoscope to see the landform detail. Having aerial photos during past years can also help define timing of failure and failure boundaries as well as site visits can be helpful. You mentioned permafrost, ice melting, and so these plus soil soils are loosened in freeze thaw cycles. Permafrost could act as a restrictive layer (a slip plane) that at times in steep terrain, soils might be hydraulically overloaded from water above in melt or rainfall, rain on snow, and with little or no cover and deep rooting, potential for slippage, flowing, rotation or avalanche of soil and debris failure. The failures that extend rapidly from steep slopes to streams are often debris avalanches. When in Oregon, I noticed that working with geologists and soil scientists, as well as identifying and evaluating failures on site and as well reviewing aerial photo pairs helped my ability to identify issues or causes involved. I noted that roads and deep fill slopes, clear cut harvest were a couple of the activities that could contribute to landslides, but certain soil types, steep and shallow bedrock substrate contributed. But combined with the natural soil or landform instability, it was often the hydraulic overload of soil moisture which occurred in improper road surface drainage, loss of ET and root support in overcutting trees that may show up until the new forest stand becomes well established. Snow melt as well as rain on snow events can overload susceptible unstable soils to failure. The highly erodible nature of glacial till and various other soil types often display the rilling and gullying where water is concentrated. A few of my Researchgate entrees might be helpful or interesting. There are some excellent reports discussing types of landslides that you may want to search, and I may still have one in my digital files.
William F. Hansen Thank you very much for your kind answer. I would much appreciate it if you can send me some relevant documents. The curious stuff, in this case, is there is no matching scarp for the deposition body nor any visible cracks, but a strong sign for a possible barrier lake. We suspect it is an old avalanche that the scarp has already been weathered and eroded, or something that exists before the formation of the river. Unfortunately, we do not have much data in the area (that's why we are using google earth) and the goal of our project is to map landslides over the entire Tibet Plateau. We are planning to conduct a field survey in 2021 and see what this feature really is, then I'll get back to this question.
If you can afford LiDAR, it can add much ground detail and save much field work and possibly surveys. I have not worked in glacial till, but know it is very erosive and under conditions of freezing/thawing, rapid melt, both failures and erosion can be rapid, episodic. Just for information, gullies erode due to some lower base level adjustment or instability, and erode up the slope. They can be quite deep also in highly erodible soils, and generally deliver sediment in episodic phases in response to storms or in your instance, rapid melt or rain on snow events. With regard to a barrier lake, melting of an ice plug or barrier overflow could contribute to mass failure. With some delay, I will see what I have on landslides.
This type of very old landslides (undated) / rock slides / glacier avalanches are also present in Hunza valley along Hunza river in Central Karakoram north Pakistan.
Regards
Ijaz
Perhaps this link to USGS reference will help with classification question.
http://iplhq.org/icl/wp-content/uploads/2012/12/Landslide-Classification-from-the-Landslide-Handbook20081.pdf
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