2023-07-20 06:00:05
Although appearing immutable to us, high mountain peaks are, on the scale of geological time, ephemeral: their shape and their altitude are constantly changing in response to the opposing actions of tectonic uplift and erosion. 830 years ago, Nepal witnessed the disappearance of its fifteenth peak, which peaked at 8,000 meters above sea level (Annapurnas IV).
Dizzying summit made up of vertical layers of Annapurna limestone (central Nepal) with in the foreground the gray and compact mass of sedimentary breccias produced by the giant collapse of Annapurna IV 830 years ago.
© L. Bollinger
This sudden disappearance demonstrates that a new collapse is possible at any time. Such events are rare but can have a considerable impact on the valleys through the export of enormous quantities of sediment by the rivers and this, for hundreds of years.
Decryption of this phenomenon with Jérôme Lavé, CNRS researcher at the Center for Petrographic and Geochemical Research (CRPG).
Why is Nepal an interesting study area?
(JL) The Himalayas are the most emblematic mountain range on a global scale, formed by the India-Eurasia collision. It is an interesting object of study because it is a so-called active zone with an important tectonic (The tectonic (from the Greek “Ï„?κττων” or “tekt?n”) activity but also monsoon phenomena each summer which result in a strong erosive process. antagonists: tectonic uplift and erosion Our study is part of the continuity (In mathematics, continuity is a topological property of a function….) of 25 years of work in the Himalayas dedicated mainly to these questions of the construction and evolution of the topography of mountain ranges.
The tectonic aspect translates into a question: “what thickens and builds mountain ranges?”. The thickening of the earth’s crust in these regions is a direct consequence of the convergence (The term convergence is used in many fields:) between India and Tibet: it leads to both the uplift of the Tibetan plateau and the construction of the Himalayan range. At the level of the Himalayas, large overlapping faults will accommodate this convergence and induce the construction and uplift of the chain.
Regarding the erosion of mountain ranges, apart from glaciated areas, landslides are commonly considered to be the main erosive process. The rivers incise themselves into the topography and thus increase the height and the steepness of the slopes which dominate them. This increase in height continues until the slope becomes mechanically unstable and collapses (on average the so-called “mechanical stability” angle of the slopes is around 35-40°). There are different types of slope instabilities ranging from rockfall to very large landslides involving the entire slope.
In the Himalayas, recent studies indicate that at high altitudes, along permanently frozen slopes and ridges, landslides or rockfalls are rarer than at lower altitudes, and erosion rates are much lower. This induces a contrast of erosion between valleys and icy mountain peaks, which could in theory lead to an endless increase in the relief and altitude of the peaks. Nevertheless, reality tells us that this increase is limited: we have to imagine an effective erosion mechanism at these high altitudes. It is this process that we are exploring in our study conducted in the Annapurna massif (central Nepal).
Map of the Himalayas.
© J.Lavé
How was the study carried out?
(JL) In the Himalayas, much past research has focused on the average rates of erosion of the range and the evolution of erosion over several million years. Despite these numerous studies, however, we do not really know how and at what speed the very high peaks of the chain are eroding. We had the opportunity somewhat by chance to provide an answer to this question. As part of a study focusing on the evolution of Himalayan erosion during the transition between the glacial and interglacial period, we carried out boreholes in the Ganges plain in order to watch the evolution of the isotopic and geochemical signature of the sediments from the chain.
One of the boreholes had an abnormally high limestone concentration in its upper part compared to the rest of the series (see image). However, we know that this type of lithological signature is only present in the western part of the Narayani basin (central Himalayas in Nepal), in the Dhaulagiri and Annapurnas massifs. We therefore thought of strong erosive activity in these massifs, possibly linked to a giant landslide. After having explored these different massifs on satellite images, we have identified between Annapurna III and Annapurna IV (Sabche glacial cirque) unusual geomorphological formations and objects. The area had not been studied in detail by geologists before, because of the difficulties of access. They had only been able to observe these formations from afar, from the Pokhara valley, and attributed them to glacial deposits.
After mapping the area in satellite images, we carried out reconnaissance in a microlight, before carrying out a helicopter flight (A helicopter is a rotary-wing aircraft whose rotor(s)…) to carry out direct observation (Observation is the action of attentively monitoring phenomena, without the will to…) and sampling (Sampling is the selection of a part in a whole. This is an important concept…) of these formations. These turned out to be breccias related to massive landslide deposition. Subsequently, we carried out two additional sampling missions in the Seti valley, which descends from the Sabche circus, as well as in the Pokhara basin further downstream. Back from the mission, the samples were sent and analyzed in different laboratories, in order to carry out geochemical, isotopic and cosmogenic analyzes to date the samples and identify their lithological nature.
What does the study contribute to the understanding of tectonic and erosive forcings on the mountain range?
(JL) On a geological time scale, a mountain range is constantly changing in response to thickening (tectonic forcing) and erosion. During its evolution, it will initially grow. But the higher the topography, the steeper the slopes and the more effective the erosion. The mountain system will gradually tend towards a state of equilibrium. When erosion and tectonic forcing are equal, the average altitude of the topography will be more or less constant and we then speak of a stationary state.
Nevertheless, beyond a certain altitude (> 6,000 m), the rate of erosion of the cliffs by falling rocks or small landslides seems to decrease sharply because the rocks there are in some way sealed by the presence of interstitial ice. If the erosion is very low, nothing more comes to counterbalance the tectonic uplift. To maintain a balance with the tectonic forcing, from the example of the collapse of Annapurna IV described in the study, we consider the preponderant role of gigantic gravitational instabilities. These giant landslides would be rare enough not to be observed on the scale of decades or hundreds of years, but large enough to effectively bare topography over the long term. In the case of Annapurna IV, it can be estimated that a giant landslide such as that which took place 830 years ago will recur approximately every 300,000 years.
This study made it possible to identify and date the deposits of a giant rock collapse that occurred around 830 years ago in the Annapurna massif, involving a colossal volume of rocks of around 23 km3. By estimating the average stability of the slopes of the region, we were able to reconstruct the probable geometry of the high summit having collapsed, and which culminated at more than 8000 m in altitude. This collapse, as well as others in the region, suggests that the high Himalayan peaks erode very episodically during rock megaslides: the slow increase (2-3 mm/year) of their altitude by tectonic forces is thus brutally annihilated by the reduction of several hundred meters of this altitude. These giant collapses also generate enormous quantities of sediments that are easily transported by the rivers, and which can have dramatic consequences on the populations living downstream.
Image giving an idea of ​​the total volume of debris (~27 km3) produced by the Annapurna IV mega landslide.
© Copernicus / Lavé
What are the causes of the collapse of Annapurna IV? What risks does such a catastrophic event entail?
(JL) We could not establish a direct cause for the collapse of Annapurna IV. It could have been triggered (A triggered (or triggered roll) is an aerobatic figure.) by an earthquake (Tremors are abnormal involuntary, rhythmic and oscillatory movements, weak…) of the earth, but its age does not correspond to the large earthquakes documented at this period in Nepal – for lack of exhaustive paleo-seismic archives for this period -, we cannot nevertheless rule out this mechanism. Independently, the collapse could be related to global warming (Global warming, also called global warming, or…) during the period of the medieval climatic optimum (Medieval climatic optimum sometimes called global warming of…) with the melting of glaciers and permafrost at high altitudes which would have destabilized the slopes of the mountains. If this mechanism was at work, then of course the question arises of the impact of current global warming on the stability of the current Himalayan peaks. We can observe in the Alps, in particular in the Mont Blanc massif, an increase in rock falls and rock slides for two decades linked to the melting of permafrost at altitude. Without witnessing any megaslides, it is the southern summit of the Fluchthorn (3300m – Austrian Tyrol) which, last June, found itself amputated from its summit by about twenty meters by a large rockslide. In the Himalayan range, as elsewhere, recent studies document a retreat and a reduction in the mass of glaciers. At the same time, the permafrost is probably undergoing severe degradation. Currently, however, it is difficult to predict the exact consequences. Will the degradation of the permafrost and the loss of cohesion of the rocky substrate at altitude, associated with the melting of glaciers at the foot of the cliff, generate large landslides, or, conversely, promote erosion and the fall of small blocks which will delay the occurrence of these megaslides? Or will the two phenomena increase concomitantly? In any case, climate change will have a significant impact on the erosion of the Himalayan and mountain peaks more generally.
To assess the damage of a potential future megaslide, three parameters must be taken into account: the average slope, the difference in level (or height of fall) and the quantity of water incorporated. During landslides with a large proportion of water (half a volume of water or more), a fairly fluid flow is obtained which will descend the valley for tens of kilometers (The meter (symbol m, from the Greek metron, measure) is the basic unit of length of the System…). This is called torrential lava, like the one that occurred in 1970 in Peru following a collapse of ice and rock in the western flank of Huascaran (highest peak in Peru) and which totally submerged the city of Yungay killing around 20,000 people.
On the contrary, for the example of the megaslide of the study, there were 23 km3 of rock and 0.5-1 km3 of ice available, that is to say a marginal quantity of water. The gravity flow gave rise to a “dry” rocky granular avalanche. Most of the gravity flow mass remained in the Sabche area, due to the closed topography of the cirque. Of this volume of fragmented rocks, only 3km3 descended the valley to the basin of Pokhara (now the second largest city in Nepal), and about 80% of the debris remained at altitude in the cirque, 830 years ago.
These finely crushed debris and sediments, and stored at altitude, were then quickly evacuated with each monsoon by the Seti River towards the valley further downstream leading to an accelerated accumulation in the Pokhara basin (1m/year of sediments). If such a landslide were to occur today, it would therefore cause major damage. After a dry granular avalanche, potentially followed by one or more debris flows by incorporation of water into the sediments further downstream, there would be rapid sedimentation over the following century which would require all infrastructure in the valley to be buried and wiped off the map. Nevertheless, a short-term disaster can take on the face of a longer-term benefit: the pile of sediment in the Pokhara Valley has shaped a large flat surface of almost 100 km2. This allowed, in a mountainous area, to have a large area to develop agriculture and then build a large city.
Words transcribed by Julie Amblard.
View of the Seti Valley, the northern part of the Pokhara Basin and the vast alluvial terraces built by the rapid sedimentation of debris along the Seti Valley, and in the background the Annapurna Barrier (from left to right: Machapuchare, Annapurna III, Annapurna IV and Annapurna II), and the void left between the peaks of Annapurna III and Annapurna IV by the collapse of the paleo- Annapurna IV.
© J.Lavé
Reference
Washed et al., Medieval demise of a Himalayan giant summit induced by mega-landslidePhysical Sciences, 2023.
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