Descripción: The Andes located in the central Neuquén (38°-39°S), which belong to the southern Central Andes (35°-39°S), have recorded a similar chronology of uplift than the neighbor northern Patagonian Andes (39°-46°S), Both areas have been formed trough successive phases of contraction in the Late Cretaceous, Middle Eocene and Late Miocene respectively. However, the Neuquén Andes have experienced two discrete phases of orogenic relaxation, during the Late Oligocene and Pliocene-Quaternary, which make then distinctive respect to the area located to the south. Field studies have shown new evidences of an episodic behavior of the fold and thrust belt in Neuquen, corresponding to compressive phases followed by stages of crustal collapse, at least since the Middle Cretaceous. The study of two main morphostructural units in the arc and retroarc area at 37°-39°S, the Alto de Copahue Pino Hachado, and its continuation to the north in the Chilean Andes (Laguna de la Laja), and the Loncopué trough respectively exemplify this particular behavior. A structural and a stratigraphie study in those areas have given a new evolutionary framework for the Neuquén Andes. © 2005 Asociación Geológica Argentina.

Descripción: The Cordillera de los Andes is the typical example of a subduction-related orogen. Its present topography is the result of post-Miocene uplift, however, Andean compressional deformation and uplift started in the Late Cretaceous, as increasingly recognized in different sectors of the mountain belt. We present evidences of a Late Cretaceous event of compressional deformation in the southern Central Andes (35oS), reflected in syn-orogenic foreland basin deposits assigned to the Neuquén Group in Argentina and the Brownish-Red Clastic Unit in Chile. Comparison of the facies of these units allows us to recognize a sector proximal to the Late Cretaceous orogenic front, a distal sector with sediment provenance from the forebulge and a western sector where the sediments where deposited within the Late Cretaceous mountain belt. On this basis, we assign the orogenic front to an inverted Jurassic normal fault, the Río del Cobre fault, and reconstruct the structure of the easternmost Late Cretaceous Andes at this latitude. The change in the location of the orogenic front north and south of 35oS allows us to recognize a long-lived change in behavior in Andean evolution in this sector, which correlates with a change in the shape and the deposits of Mesozoic Neuquén basin.

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: Longitudinal dependences of stratospheric gravity wave (GW) fluctuations and lower ionospheric irregularities (sporadic E) at midlatitudes are studied by means of radio occultation data of the Global Positioning System/Meteorology Experiment (GPS/MET) satellite mission. The zonal average of temperature variance of GW fluctuations with vertical scales less than 7 km at northern midlatitudes is observed to be similar to that at southern midlatitudes, but there is a significant interhemispheric difference in the longitudinal dependence of GW fluctuations. The GPS/MET data at northern midlatitudes show a rapid change of the gravity wave distribution from 25 to 35 km height, resulting in a broad maximum of temperature variance located over the Atlantic and Eurasia. We only find in the wave distribution at h = 25 km some weak traces of possible orographic effects. On the other hand, the distribution of GW fluctuations at southern midlatitudes has a strong and sharp maximum over Andes, which is obviously due to orographic wave generation by the interaction of surface wind with the Andean mountain ridge. This observation of the new GPS radio occultation technique is in agreement with previous measurements of spaceborne microwave and infrared limb sounders. The amplitude of the average wave field increases with height over Andes, while the amplitude maximum moves westward, against the prevailing wind. The temperature fluctuations have an apparent, dominant vertical wavelength of around 6 km. In situ measurements by a balloon-borne rawinsonde at Ushuaia, Argentina (54.7°S, 68.1°W) are compared to a simultaneous GPS/MET temperature profile. The balloon observations of temperature and horizontal wind are interpreted by a large amplitude mountain wave propagating to the upper stratosphere. Wave characteristics and atmospheric background conditions are investigated in detail for this mountain wave observation. Finally, the GPS/MET experiment indicates enhanced sporadic E in the lower ionosphere over Southern Andes. We assume that these plasma irregularities are generated by enhanced, upward wave flux due to the possible orographic ettect of Andes. Copyright 2002 by the American Geophysical Union.

Descripción: The stratigraphy of Cretaceous marine mudstones in the Fuegian Andes, roughly equivalent to Charles Darwin's clay-slate formation, remains a still unsolved problem. Previous records of Albian, Turonian-Coniacian, and Santonian-Campanian bivalves are combined with new findings of the Late Albian inoceramid Inoceramus anglicus Woods, and the Maastrichtian ammonites Diplomoceras sp., Anagaudryceras sp., Maorites densicostatus (Kilian and Reboul), Maorites sp., and Pachydiscus (Neodesmoceras) sp. to further constrain the Cretaceous stratigraphy of the eastern Fuegian Andes. In addition, new records of distinctive trace fossils and ichnofabric are meaningful for stratigraphic division and delineation of paleoenvironmental settings in these Cretaceous mudstones. The Lower Cretaceous ichnoassemblage of Chondrites targioni (Brongniart) and Zoophycos isp. is consistent with the inferred slope-volcaniclastic apron settings of the Yahgan Formation; Nereites missouriensis (Weller) reflects distal basin plain depositional settings for the Beauvoir Formation. In the Upper Cretaceous, the "Estratos de Buen Suceso" record the earliest extensively bioturbated horizons, reflecting prolonged well-oxygenated bottom conditions. In the Bahía Thetis Formation, organic-rich, channel margin or distal basin slaty mudstones record the last occurrence of inoceramid bivalves in the Austral Basin; the generalized absence of trace fossils is consistent with dysoxic bottom conditions. The thoroughly bioturbated Policarpo Formation, records a marked change in paleoceanographic conditions. The strong contrast in the intensity of bioturbation between the Upper Campanian-Maastrichtian Bahía Thetis Formation, almost devoid of trace fossils, and the highly bioturbated Maastrichtian-Danian Policarpo Formation reflects a change from dysoxic-anoxic to well ventilated conditions, probably associated with a cooling trend of bottom waters in the austral deep oceans.

Descripción: In the Central Andes of southern Bolivia, northern Chile and north-western Argentina, a pattern of clockwise vertical axis rotations has been palaeomagnetically determined in Mesozoic and Cenozoic rocks cropping out in the Andean fore-arc and back-arc. The origin of these rotations is controversial. The scarcity of palaeomagnetic data available for the southern Central Andes, in particular for the Argentine Puna, is one of the problems that prevents the determination of the processes leading to the rotation pattern. With the aim of obtaining new palaeomagnetic data and trying to define the local or regional character of the rotations, 73 oriented samples were collected in the zones of Juncal Grande (25° 50′S - 67° 40′W) and Chorrillos (24° 12′S - 66°35'W), southern Argentine Puna. Remanent magnetizations, which indicate the existence of non-rotated and clockwise rotated localities, were isolated. It is suggested that the observed vertical axis rotations are of local character, controlled by the kinematics of the local structures.

Descripción: Landforms and soils in the Traful River Valley, Neuquén Province. The Andes Patagónicos Region is characterised by the presence of strong west-east bioclimatic, geomorphological and lithostratigraphical gradients, that controlled the genesis and geographical distribution of soils. In this contribution soil-landscape relationships in a sector of a transitional zone in the northern Andes Patagónicos are studied. The Traful River valley is located in Neuquén Province aproximately at 40°40′S and 71°15′W. Diverse geomorphological units were distinguished which were formed through glacial and fluvioglacial action, and partially modified in recent times by fluvial and eolian processes and by mass wasting. Holocen ashes and lapilli were deposited in different volcanic events and constitute the soils'main parent material. We recognize soils belong to three orders: entisols, mollisols and andisols. A strong gradient of variations in soils properties was observed. The Andisolization process decrease with distance from the Cordillera zone. © 1999 Asociación Geológica Argentina.

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: Longitudinal dependences of stratospheric gravity wave (GW) fluctuations and lower ionospheric irregularities (sporadic E) at midlatitudes are studied by means of radio occultation data of the Global Positioning System/Meteorology Experiment (GPS/MET) satellite mission. The zonal average of temperature variance of GW fluctuations with vertical scales less than 7 km at northern midlatitudes is observed to be similar to that at southern midlatitudes, but there is a significant interhemispheric difference in the longitudinal dependence of GW fluctuations. The GPS/MET data at northern midlatitudes show a rapid change of the gravity wave distribution from 25 to 35 km height, resulting in a broad maximum of temperature variance located over the Atlantic and Eurasia. We only find in the wave distribution at h = 25 km some weak traces of possible orographic effects. On the other hand, the distribution of GW fluctuations at southern midlatitudes has a strong and sharp maximum over Andes, which is obviously due to orographic wave generation by the interaction of surface wind with the Andean mountain ridge. This observation of the new GPS radio occultation technique is in agreement with previous measurements of spaceborne microwave and infrared limb sounders. The amplitude of the average wave field increases with height over Andes, while the amplitude maximum moves westward, against the prevailing wind. The temperature fluctuations have an apparent, dominant vertical wavelength of around 6 km. In situ measurements by a balloon-borne rawinsonde at Ushuaia, Argentina (54.7°S, 68.1°W) are compared to a simultaneous GPS/MET temperature profile. The balloon observations of temperature and horizontal wind are interpreted by a large amplitude mountain wave propagating to the upper stratosphere. Wave characteristics and atmospheric background conditions are investigated in detail for this mountain wave observation. Finally, the GPS/MET experiment indicates enhanced sporadic E in the lower ionosphere over Southern Andes. We assume that these plasma irregularities are generated by enhanced, upward wave flux due to the possible orographic effect of Andes. Copyright 2002 by the American Geophysical Union.

Descripción: The analyses of the main parameters controlling the present Chile-type and Marianas-type tectonic settings developed along the eastern Pacific region show four different tectonic regimes: (1) a nearly neutral regime in the Oregon subduction zone; (2) major extensional regimes as the Nicaragua subduction zone developed in continental crust; (3) a Marianas setting in the Sandwich subduction zone with ocean floored back-arc basin with a unique west-dipping subduction zone and (4) the classic and dominant Chile-type under compression. The magmatic, structural and sedimentary behaviours of these four settings are discussed to understand the past tectonic regimes in the Mesozoic Andes based on their present geological and tectonic characteristics. The evaluation of the different parameters that governed the past and present tectonic regimes indicates that absolute motion of the upper plate relative to the hotspot frame and the consequent trench roll-back velocity are the first order parameters that control the deformation. Locally, the influences of the trench fill, linked to the dominant climate in the forearc, and the age of the subducted oceanic crust, have secondary roles. Ridge collisions of seismic and seismic oceanic ridges as well as fracture zone collisions have also a local outcome, and may produce an increase in coupling that reinforces compressional deformation. Local strain variations in the past and present Andes are not related with changes in the relative convergence rate, which is less important than the absolute motion relative to the Pacific hotspot frame, or changes in the thermal state of the upper plate. Changes in the slab dip, mainly those linked to steepening subduction zones, produce significant variations in the thermal state, that are important to generate extreme deformation in the foreland. © 2009 John Wiley & Sons, Ltd.

Descripción: A significant wave activity (WA) in the upper troposphere and lower stratosphere, mainly during winter, was detected at midlatitudes in the southern hemisphere (30-40S) above the Andes Range, from an analysis of Global Positioning System Radio Occultation (GPS RO) temperature profiles retrieved by CHAMP (CHAllenging Mini-satellite Payload) and SAC-C (Satélite de Aplicaciones Científicas-C) Low Earth Orbit (LEO) satellites, between May 2001 and February 2006. The possible main gravity wave sources in this region are: i) orographic forcing, ii) geostrophic adjustment and iii) deep convection. The available vertical resolution of GPS RO soundings does not rule out any of these alternatives. Based on satellite imaginary, the WA enhancements cannot be attributed to deep convection events. Inertia-gravity waves (IGWs) could be generated after a geostrophic adjustment process, following a perturbation of the zonal jet situated above the Andes Mountains by mountain waves (MWs). The monthly WA intensity follows the zonal wind velocity strength according to its seasonal variability at jet altitudes. As the GPS-LEO lines of sight are roughly meridionally aligned and the morphology of the Andes at middle latitudes is predominantly north-south, it was possible to detect MWs as well as IGWs from GPS RO temperature profiles. This characteristic does not apply for other mountain range alignments. From the analysis of a numerical simulation at the time and location of a single RO event with very strong WA, two main modes of oscillation with horizontal wavelength around 40 and 200 km were identified. The first one is attributed to a MW and the second one to an IGW. Copyright 2006 by the American Geophysical Union.

Descripción: The high-altitude Andean lakes (HAAL) in the Argentinean Puna-high Andes region represent an almost unexplored ecosystem exposed to extreme conditions (high UV irradiation, hypersalinity, drastic temperature changes, desiccation, and high pH). Here we present the first genome sequence, a Sphingomonas sp., isolated from this extreme environment. © 2011, American Society for Microbiology.

Descripción: The geological observations made by Darwin in 1835 during his crossing of the Andes from Santiago to Mendoza via the Piuquenes Pass and Cordón del Portillo are compared with the present geological knowledge of the Cordillera Principal and Cordillera Frontal at 33°-34°S. The analysis of the complex stratigraphy of the Cordillera Principal, the imbricated structure of the Aconcagua fold and thrust belt, as well as the stratigraphy and structure of the inter mountain foreland Tunuyán Basin, allows to assess the pioneer observations of Darwin. He recognized the old metamorphic basement and the granitoids and volcanic sequences of late Paleozoic to Triassic age of the Cordillera Frontal, established the Cretaceous age of the marine successions cropping out along the eastern Cordillera Principal and studied the conglomeratic deposits associated with the uplift of the Cordillera in the Alto Tunuyán Basin. Based on the study of clast provenance of the synorogenic deposits of the Alto Tunuyán Basin, Darwin recognized that the Cordillera Frontal was uplifted later than the Cordillera Principal. The present knowledge of this sector of the Andean Cordillera confirms his pioneer observations and show that Darwin was one of the first scientists ever in realizing that in an orogenic system the sequence of uplift and deformation proceeds from hinterland towards foreland, according to a process that is exceptionally well-illustrated along the Piuquenes-Cordón del Portillo transect.

Descripción: The southern part of the Ñirihuau basin at the valley of the Cushamen River region, is deformed and uplifted forming a prominent Precordillera Patagónica fold and thrust belt, east of the Patagonian Andes. The Ñirihuau Basin infill is represented by Oligocene to Miocene sedimentary to volcaniclastic rocks gathered in eleven litho types belonging to four members. These foreland sequences are exposed by a combination of thin-skinned deformation whose shortening is transferred to the basement in the west beneath the Cordón del Maitén. The entire Ñirihuau section as well as the overlying Collón Cura Formation is characterized by the occurrence of progressive unconformities, fact that implies synorogenic sedimentation at the time of the Cordón del Maitén range uplift. This belt is formed by an east-verging basement wedge associated with a series of backthrusts. The reactivation of the deformation associated with growth strata, indicates that the basin has evolved in a wedge-top of a foreland system in different pulses of deformation.

Descripción: We first study the seasonal and geographical behavior of gravity wave activity in the lower stratosphere over the southernmost Andes mountains and their prolongation in the Antarctic Peninsula by global positioning system (GPS) radio occultation (RO) temperature profiles, obtained between years 2002 and 2005 by the CHAllenging Minisatellite Payload (CHAMP) mission. The observed features complement observations in the same zone by other satellite passive remote sensing instruments, which are able to detect different height regions and other spectral intervals of the wave spectrum. Comparisons with previous GPS RO studies in smaller areas than the one covered in our analysis are also established. Significant seasonal variation of wave activity is observed in our work, in agreement with results from other instruments. The locations of significant cases indicate that topography is an important source. Some strong wave activity is also found over open ocean. Critical level filtering is shown to have an attenuation effect, implying that a large fraction of the observed activity can be considered to be an outcome of mountain waves. The studied region has a significant advantage as compared to other regions of our planet: it generates wavefronts nearly aligned with the North-South direction (almost parallel to the mountains), whereby this geometry favors the wave detection by the nearly meridional line of sight characterizing most of the GPS RO observations used. A distribution of the observed gravity waves in terms of amplitudes and wavelengths is also presented.

Descripción: A significant wave activity in the upper troposphere and lower stratosphere at midlatitudes (30-40S) above the Andes Range was recently detected from Global Positioning System Radio Occultation (GPS RO) temperature profiles, retrieved from SAC-C (Satélite de Aplicaciones Cientficas-C and CHAMP (CHAllenging Minisatellite Payload) satellites. Previously, large amplitude, long vertical wavelength structures have been reported in this region, as detected from other limb-sounding devices and have been identified as mountain waves (MWs). The capability of GPS RO observations to detect typical MWs with horizontal wavelengths shorter than 150 km, as well as the proper association of the observed wave activity to mountain forcing is put in doubt. Other three possible sources are discussed. In particular, the generation of inertio-gravity waves by geostrophic adjustment near to a permanent jet situated above the mountains, may constitute another important mechanism in this region. These waves may possess longer horizontal and perhaps shorter vertical wavelengths than those typically expected in MWs and could be more easily detected from limb-sounding profiles. The "jet" mechanism will be discussed in a second paper. Copyright 2005 by the American Geophysical Union.

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: We examined diurnal and nocturnal nectar secretion across sexual stages in protandrous Alstroemeria aurea, a bumble bee-pollinated herb with long-lived flowers native to the southern Andes. We found the following patterns: (1) most nectar was produced diurnally and (2) three times more sugar was secreted during the male than female phase, not only because the male phase lasted longer but also because the rate of nectar production was higher. This 3:1 ratio in nectar production matched the ratio of the minimum number of bumble bee visits required on average to saturate male (pollen removal) vs. female (seed set) functions. Standing crop of nectar, on the other hand, did not differ greatly between male- and female-stage flowers left open to visitors, because the high-production male-phase flowers were visited more frequently than female-phase flowers. In an experiment concurrent with the repeated nectar sampling of individual flowers over their life-span, we removed pollen from anthers or deposited pollen on stigmas by hand. Neither treatment, designed to mimic effects of visits by Alstroemeria's native bumble bee pollinator, affected nectar production. The absence of plasticity in nectar secretion in relation to pollination events may reflect a low cost of nectar production, or may result from developmental constraints related to the evolution of the synchronous protandry that characterizes A. aurea.

Descripción: This study analyses heating degree-days (HDDs) in Argentina during the period 1900-2008. Gridded temperature data provided by the University of Delaware were analysed to calculate monthly and annual cumulative HDDs. Mean, maximum and minimum values as well as the average duration of the heating season are used to characterize the mean features of the region. Spatial variations are driven by latitude and altitude. The analysis of the temporal distribution of HDDs reveals that the centre of the mean heating season varies from mid-June to mid-July. The length of the cold season grows with increasing latitude and westward with increasing altitude. In the high Andes, the heating season extends all year round. S-mode principal component analysis is used to identify sub-groups of grid points with similar temporal variability. Negative trends in annual cumulative HDDs are detected in most of the country. Linear and nonlinear trends as well as temporal statistics are examined for inter- and intra-annual variability of HDDs to discuss its potential incidence on residential use of natural gas. Seasonal increases in natural gas consumption in the country, including the effect of regional price benefits, could be now better explained not only by population growth but also by the spatial and temporal characterization of the HDD season. © 2012 Royal Meteorological Society.