Permafrost degradation driven slope instability in the Central Andes of Argentina.

MOREIRAS S.M.; JEANNERET P.

Workshop; Denudation and Environmental Changes in Different Morphoclimatic Zones (DENUCHANGE).; 2018

Landslides in the Central Andes of Argentina is a poorly studied but highly important topic, as these events could affect local streamflow system and water resources. As the study area has an arid to semi-arid climate, the only available source of drinking water for the neighboring settlements is melt-water from winter snow accumulation, high altitude glaciers, rock-glaciers and other periglacial landforms. This means that, in the case of medium to large volume landslide (between 0,1 km2 and 1km2 and over 1km2, respectively) feeding sediments could seriously affect the water quality, or even completely impound drainage system by natural dams. The later also pose a threat of instant outburst floods if the impoundment is breached. All of these cases are not unknown in the studied area and some occurred during historical times, affecting water quality in downstream settlements such as San Juan city and surrounding areas with almost half a million inhabitants by 2010.The basins fed by the Mercedario?s Peak (around 32ºS and 70ºW) have the highest peaks in the region led by Mercedario (6720m asl) and the Ramada range (ranging 6000 m asl), they also have a high local relief with up to 1100m of elevation in only 2km, the area is still undergoing straining tectonic activity due to the Ramada fold-&-thrust belt and has been heavily glaciated during the Last Glacial Maximum and later advances. All possible triggering mechanisms for landslides. After a thorough landslide inventory carried out with remote sensing technics and field work it was noted that almost 30% of the studied 68.300Ha is affected by diverse types of landslide including slowly creeping permafrost affecting all lithologies and taking up to 100km2. Rock falls are the most extended events and accumulate in talus slopes and debris cones without any direct correlation with the lithology, although the main source of blocks are steeply dipping rock slopes (>50°) placed over 3500m asl, coincident with the lower limit of the periglacial environment where cryogenic processes take place. These slopes are normally densely fractured by regional structures and/or freeze-thaw action. Alluvial cones and fans are the second most extensive landforms developed after debris flow events. In this case the source area was also recognized above the 3500m asl limit, indicating a strong periglacial correlation with permafrost-affected slopes, while earth flows are more frequent but less extensive and its source area is directly associated with periglacial landforms such as rock glacier, protalus ramps and solifluxion tongues. Deep gravitational events are extensive but infrequent, with only 4 like-events with an average surface of 1,5km2, the main lithology consist of lateral moraine deposits. On the other side, rock avalanches are more frequent (25 inventoried events) but smaller in size (average of 0,2km2 per event) and the source lithology is variable. The rest are represented by complex events and moraine collapses. Regarding the diverse factors that could trigger and/or affect the slopes that generate the different types of inventoried events, it was noted that rock falls and some rock avalanches are frequent in previously glacial eroded slopes with intense freeze-thaw action within the periglacial environment. As deep gravitational events are mainly composed by moraine deposits it was estimated that the release of the compressing forces exerted by the glacier when they retreat and the melting of the probable ice nucleus within the moraine could be affecting the stability of these landforms and generate a detachment zone that penetrates deep into the moraines which would make them collapse. This effect could also explain other irregular moraine collapses which also has evidence of river impoundments, again, above the periglacial limit. The source area of flow type events such as debris and earth flows were also recognized above the 3500m asl limit, indicating a strong periglacial correlation. These results show that a great percentage of the studied area that is affected by landslides is directly linked to the release of melt water of periglacial landforms. The water saturation of these landforms exert pressure within the pores that not only can cause flow type events but also destabilized entire moraines that developed into deep gravitational events and other complex collapses with evidence of river stream impoundments. As the lower landforms within the basins have suffered from these phenomena, it is estimated that, in a global warming context, the lower limit of the periglacial environment defined by the 0ºC isotherm will climb up to higher altitudes and extreme freeze-thaw action will affect periglacial landforms placed on the upper catchments increasing the frequency of such events.