Descripción: A new and important rock avalanche not previously recognized is described, which is located 25 km west of Andacollo town. It shows two scars (northern and southern ones) where the southernmost one is interpreted as the most important, developed in the western flank of a reduced basaltic plain, where at its northern part of Cerro Los Cardos is located. The movement is complex showing several breakaway zones developed into the landslide deposit, considered as secondary scars, all of them, main and secondary ones, have a NW trending. The movement mechanics seems to have started as a planar landslide considering the local almost vertical bedding of the Lileo Formation. However, the concave nature of the breakaway zones allows to infer a rotational movement after the initial stage of sliding. The movement corresponding to the southern breakaway zone was projected as a flux to the south of the Lileo valley, presumably in successive pulses as indicated by the multiple secondary scars. The movement corresponding to the northern break away zone moved to the west and after-wards seems to be controlled by the slope of the valley, moving through to the Palao valley. The western slope of the Palao valley was covered by avalanche deposits corresponding to a run-up. Los Rojos lake is located in a depression presumably generated by the landslide and consequent fracturing of a rock block detached from a Los Cardos's lavic structural plain. Several factors, particularly the structure, favoured the development of the landslide. We propose a seismic trigger for the Los Cardos's avalanche, having into account the regional seismotectonic characteristics (neotectonic front) and the local structure (lineaments, faults). Other landslides, some of them with volume similar to an avalanche deposit, have been included in a regional map. Moreover, modifications to the regional stratigraphy, particularly referred to the volcanic units, are proposed based on morphological criteria. Other geoforms related to other geomorphological processes, fluvial, volcanic and glacial are identified. © 2005 Asociación Geológica Argentina.

Descripción: The geomorphologic analysis of the study area led to distinguish some formations not considered in previous geological research, as well as to explain the characteristics of the present relief. Thirteen principal geomorphic units produced by different geomorphic processes are recognized. Their variety and incidence through the time are expressed in the glacial morphogenesis concentrated in the western part of the Cordillera del Viento, in the volcanic landscape that predominates in the northern and eastern sectors (calderas, volcanoes, lava flows), in the structural control that defines the morphology of the fluvial process in the central zone and a firstly recognized important landslide to the north of Tricao Malal. As a result of the methodology employed two geological units of different age in the western part of the Tromen-Waile volcanic field were distinguished, which were initially interpreted as a single formation. A limited but consistent structural analysis of the region is also included.

Descripción: Large landslides of different types not previously recorded are located in the left bank of the Río Curri Leuvú valley, north of the Tricao Malal village. There are situated among the Cerro Palao and the Palao pond and Riscos Negros site to the north. The largest landslides (Malal Cura and Arroyo Mogolillo landslides) correspond to the multiple rotational progressive type. They are developed on Mesozoic sedimentary rocks, covered by Cenozoic basalt flows. The debris and earth flows are developed in areas covered by the volcanic rocks of the Charilehue Formation (Middle Miocene) and continue north of study reaching the latitudes of the Grande and Medio streams and also the Colimamil stream valley southeast of Cerro Domuyo. The impact of these phenomena in the fluvial system was important and produced ephemeral and permanent blocking as seen in the La Turbia stream with coeval ponding. It is hypothesized that the landslide occurrence is related to excess or long-lasting periods of rain in postglacial times. It is also indicated the presence of rock glaciers in the Cerro Palao.

Descripción: In the region of the Main Andes of San Juan, exists a high concentration of landslide deposits that can originate natural dams. The outburst of these dams generated by slide is a common process in this morphology. The analyses of satellite imagery and air photos of previous years allow reconstructing the characteristics and the origin of the slides that formed those natural dams. In this region of the Main Andes, the rapid uplift and erosion of the mountain chain has created abrupt slopes and an internal structure in the materials that determined an increase of the susceptibility to the slope collapse without triggers as the seismic activity or precipitations. The objectives of the present work are (1) to analyze the causes that formed the natural dam and produced the landslide dam and development of the Los Erizos lagoon in the middle course of the Santa Cruz river; (2) the outburst of the dam and their consequent flood; and (3) the morphologic characteristic of the study region. The temporary comparison between air photos with satellite imageries indicates that the natural dam was generated by slip of the western hillside of the Santa Cruz mountain range, located below the Cerro Estrella. However, the constant water flow to the lagoon, has increased its water level until reaching the maximum benchmark of the dam, produced the beginning of the lake drainage and weakening of the dam causing the outburst and its consequent flood. Based on satellite imagery, a temporary analysis of the lagoon is presented, indicating that monitoring is an important tool to alert in the whole San Juan river basin for possible natural dikes, preventing hazards to the populations down water. The alluvium of November 12 of 2005, discharged 32.100.000 m3 in 67 minutes and traveled 254 km in less than 12 hours.

Descripción: The previous analysis of the prehistoric slide phenomena in the main Andes of Neuquén Province in Argentina (36o-38oS), triggered the study of those located south of this la-titude. Therefore a new set of mass wasting deposits has been identified, whose trigger factors and corresponding tectonic framework, where they are located, differ from the northern slides. Their study was performed in three specific areas: Aluminé, Chapelco and Arroyo Limay Chico. Earth flows predominate in relation to slumps. The only exception is the Chapelco rock avalanche. Their occurrence seems to be related to the presence of Limay Chico member of the Caleufú Formation, whose lithology (expanding clays), particular rheology before water saturation, and particular disposition underlying basaltic lava and conglomeradic plains were conditioning factors. The trigger factor of the Present and prehistoric earth flows has been intense orographic precipitations. Water saturation reduced friction and cohesion in materials, incrementing water pore pressure and producing slope instabilities. Rotational slides are related to the slope basal erosion produced by lateral river migration and additionally to precipitations. Slope orientation has been important in determining location of main slides. Lateral slope release through the flanks of glacial valleys due to debutressing because of the glacial retire in the area, has produced instability and rock fracturation that led to slide phenomena. The Chapelco rock avalanche is particularly related to copious precipitations during postglacial times. Thus sliding main trigger factors south of 38oS differ from those pro-posed for the northern area where earthquakes dominated. Their specific age is still unknown, although their geomorphologic analysis allow to constraint a postglacial age for all these occurrences. However, temporal relationships are lacking for the specific case of the arroyo Limay Chico, beyond the glaciated area.

Descripción: Studies performed during the last five years have shown the existence of 74 prehistoric rock-avalanches in the northern Neuquén Andes (36°-38°S). These are mainly related to volcanic grounds of Pliocene to Pleistocene age corresponding to the Hualcupén Formation. In this work we describe their main characteristics and general homogeneous morphology. They are also characterized by a single tectonic control associated with the occurrence of their break-away and mainly postglacial ages, fact revealed by their systematic position into previous glacial valleys and limited radiometric ages. The obstruction produced by these features altered the shape of the valleys giving to local lakes. Eighteen percent of the cases are characterized by landslide dams preserved nowadays while only six percent by ephemeral ones. Two of the dams were actually catastrophically collapsed: the Cerro Pelán and Navarrete rock-avalanches: The former in Barrancas river valley with the consequent formation of the Carrilauquen lake and the later with the generation of Navarrete lake. The Carrilauquen's dam broke during historical times (29/12/1914) forming an outburst-flood that passed from the Barrancas valley to the Colorado river valley destroying their local economy and reaching after 1,500 km the Atlantic coast. The highest computed volumes in the entire region for the deposits related to rock-avalanches correspond to Varva Co Campos avalanche (3 km3). Cosmogenic isotopes have revealed its age and the age of the neighbor deposit of Varvar Co Tapia rock-avalanche, resulting in 30 ka and 60 ka respectively. Further cosmogenic isotope data revealed that the Cerro PeMn avalanche is only 2 ka. A wrong assumption, presently revaluated, was related to the assignment of these deposits to glacial accumulations because of their chaotic structure, their hummocky topography and the induced dams. Finally, several topographic, geomorphological and seismotectonic criteria are exposed to prove a seismic trigger for these avalanches. Graphics and one table are used to display the distribution of their deposits and its dose association with the orogenic front at these latitudes and the crustal seismicity of the region. © 2006 Asociación Geológica Argentina.