Slope instability driven by permafrost degradation in the Central Andes of Argentina

MOREIRAS, S.M.; JEANNERET, P.

Congreso; 20º INQUA Congress; 2019

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 such as San Juan city, is melt-water from winter snow accumulation, high altitude glaciers and periglacial landforms. This means that, in the case of medium to large volume landslide (0,1 km2-1km2 and >1km2, respectively) feeding sediments could seriously affect the water quality, or even completely impound drainage system by natural dams, which also pose a threat of instant outburst floods by breaching. The basins fed by the Mercedario?s Peak (around 32ºS and 70ºW) have the highest peaks in the region led by Mercedario (6720m asl). With a high local relief, straining tectonic activity and high erosion rates during the Last Glacial Maximum, this area is no strange to landsliding. After a thorough mass removal 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 mass removal processes 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 coming from any outcrop with steeply dipping 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. Debris flows accumulate in alluvial cones and fans and are the second most extensive landforms, originating above the 3500m asl limit, while earth flows are more frequent but less extensive and its source area is directly associated with rock glacier. Deep gravitational events are extensive but infrequent, with only 4 like-events with an average surface of 1,5km2, being derived from lateral moraines. Rock avalanches are more frequent but smaller in size (25 events with an average of 0,2km2 per event) and the source lithology is variable. The rest are represented by complex events and moraine collapses. Regarding the triggering and/or slope debilitating factors, it was noted that rock falls and some rock avalanches are frequent in previously glacially eroded slopes with intense freeze-thaw action within the periglacial environment. Deep gravitational events on moraines could be a consequence of ice melting within the moraine generating a detachment zone between the deposit and the rock slopes. This effect could also explain other irregular moraine collapses. The source area of flow type events such as debris and earth flows also indicate a strong periglacial correlation. These results show that large landslides, debris and earth flows are directly linked to the release of melt water of periglacial landforms with evidence of river 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.