Descripción: Results of a detailed geologic survey of the Río Borbollón area, located in the Cordillera Principal of the province of Mendoza are presented. The stratigraphic record of the area corresponds to a Jurassic-early Cretaceous sedimentary succession, the products of Neogene igneous activity and Quaternary deposits. The Tres Esquinas and La Manga Formations of Jurassic age were recognized for the first time in the study area. The main structural features of the area are described and interpreted based on the obtained data and previous works. A balanced structural cross section of the Malargüe fold and thrust belt at 34°15'S depicts the structural style and the amount of orogenic shortening in this region of the Andes.

Descripción: Bracketed between 188-152 Ma, the Chon Aike Silicic Province developed in Patagonia covering an area of about 1,7 x 10 6 km 2 with volcanic products. An extensional tectonic regime that was active since at least the Jurassic-Cretaceous boundary accompanied this volcanic megaevent. Kinematic analyses focused in four selected localities (El Dorado - Monserrat, Norte de Cerro Vanguardia and El Fénix, in the Deseado region; and Lago Ghío, in the southern Patagonian Cordillera) allowed to define the main kinematic axes and to recognize two different kinematic events (a main and a secondary ones). Additionally these data certified the transtensional character of the Jurassic deformational event. When comparing directions of kinematic axes of all the selected localities, two kinematic domains: west and east could be discriminated. Whereas the eastern domain (El Dorado - Monserrat, Cerro Vanguardia north) shows ENE and NNW directions of extension for the main and the secondary kinematic events respectively, the western domain (El Fénix and Lago Ghío) indicate NE and NW directions of extension for the two events. The two kinematic events could be related or correspond to different volcanic facies. Considering the diachronism of the Chon Aike province (younger volcanic rocks westwards), the different kinematic fields could be linked to a large scale, non-coaxial deformation regime that controlled the emplacement of the Chon Aike volcanic province.

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 Huecú depocenter is located in the central sector of the Loncopué trough in western Neuquén. This basin is composed of volcanic, volcaniclastic, fluvial and lacustrine deposits, younger than 1.6 Ma encompassing postglacial and even historical sediments and lavas. During most of its evolution, the Huecú depocenter has constituted a small closed depression (pullapart basin) restricted to the Loncopué trough, a major extensional retroarc basin in the Central and Patagonian Andes. This basin was fed from the north since the beginning of its evolution by a series of big amalgamated post-Pliocene stratovolcanoes located in the Mandolegüe Cordillera. The basin was also filled by fissural lava flows originated in the Agrio caldera located to the east grouped in the Escorial Basalt of 1.6 to 0.8 Ma. Polygenetic volcanic products form in the region big stratovolcanoes among which the Trolón and Colorado volcanic centers were the most prominent. These centers of Pleistocene age were highly eroded by the last glacial activity that affected the region. Nearly at the end of the glacial period two fissural volcanic fields covered the Huecú depocenter corresponding to the Rankül-Lom Basalt and preglacial Tres Hermanos basalts. Finally postglacial volcanic lavas covered most of the Huecú depocenter. These were grouped in the Cerro Artillería Basalts and postglacial Tres Hermanos basalt, which have recurrently dammed the main fluvial basins in the area. The fluvial and lacustrine deposits are represented by El Huecú and Mar Pequeño Formations.

Descripción: Factors of soils formation, parent material, and relief in western part of Santa Cruz province (southermost Patagonia) were studied in rio de las Vueltas valley beetween Laguna del Desierto and Lago Viedma (49oS-49o30′S y a 73o W). There is great soil variability related to a strong north-west-south-east bioclimatic and geomorphic gradient. Soils were studied in a NW-SE transect, where three different sectors could be distinguished. The western area, including the Patagonian Andean Cordillera and piedmont is characterised by strong landscape variations due to glacial processes, colluvial, glacial, cineritc and glacifluvial parent materials, a udic-xeric regime and Nothofagus woodlands. The landscape of the eastern area is mainly of glacial-fluvioglacial, fluvial origin, with aeolian landforms, an and regime, grassland-shrub steppe and large flat structural plain landscape. Because of their different landscape-relief features and parent materials, both areas have different soils associations. The western area is characterized by a distrudepts, udivitrands/hapludands and haploxerolls sequence, meanwhile the eastern by an haplargides, torriortents and haplocalcides trends. In the western area, podzolisation features were recognized. In the north-western sector, Bs and E horizons were recognized, and low pH base saturation values, due to climatic conditions, indicate ongoing podzolization. Elsewhere in the south-eastern sector calcium carbonate from the surface, basic pH and total saturation are soils main features. © 2004 Asociación Geológica Argentina.

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: 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.

Descripción: The Atuel depocentre corresponds to a Late Triassic - Early Jurassic NNW-trending subbasin, located in the northern sector of the Neuquén basin. Based on pre-existing stratigraphical data and present structural analysis we propose that the Atuel depocentre is bounded by the presence of two NNW-trending major normal faults, named Alumbre and La Manga. These faults are inferred to have controlled the development of two west-facing half-grabens: the Río Blanco, a completely emerged half-graben, and the western Arroyo Malo, a completely submerged half-graben. The structural model presented here is based on the assumption that both, the basement structural grain and the regional extension direction, exerted a first-order control in the development and evolution of the Atuel depocentre. During the early stage of rifting (pre-Rhaetian - Middle Hettangian) the pre-Triassic Alumbre and La Manga faults reactivated in an oblique mode. During the second episode of rifting, both Alumbre and La Manga faults continued to play, while WNW-trending normal fault developed in order to accommodate the strain inside both half-grabens. The third extensional event began with an abrupt marine rise inside the Arroyo Malo half-graben during late Middle Hettangian, as a result of the last displacement of the Alumbre fault, and finished with an abrupt marine drop associated with the desactivation of the La Manga fault.