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Thermal travertines are an archive of CO2 sources and sinks in hydrothermal systems. Two major regional factors control travertine precipitation: water availability and CO2 supply. Thus, travertines form a valuable archive of hydrodynamic variability and sources of main contributions to dissolved inorganic carbon (DIC). It is relevant to determine the main DIC sources of thermal waters (i.e., organic-matter degradation, recycled from older carbonates, emission of deep-seated magmatic CO2), as they are key inputs to calculate the lithosphere–atmosphere CO2 budget. The Antuco travertine from the Central Andes represents one of such archives, with a 500 ky record of accretionary periods (dated as 425–320, 260, and 155 ky BP) related to high hydrothermal activity of hot springs. It consists of two travertine units: (1) a lower massive unit displaying large calcite pseudomorphs after aragonite that precipitated in proximal ponds with abundant water, and (2) an upper stratified unit showing more distal facies bearing siliciclastics and manganese and iron oxides. The replacement of aragonite by calcite in the lower unit was related to the decrease of water salinity in the thermal system through time. In the Antuco travertine, DIC {delta}13C values of travertine parental waters of around –9{per thousand} suggest that CO2 was related to igneous processes and volcanic activity, and released along deep-seated faults. Relative water/rock ratios derived from {delta}18O values and fluid-inclusion microthermometric data from travertine carbonates are consistent with an interpretation of greater water availability in the hydrothermal system during the late Pleistocene than at present. The different petrographic features and isotopic signatures are interpreted to reflect increased water availability during more humid periods in the Altiplano, which triggered precipitation of travertine bodies. Travertine growth took place during interglacial-humid climate periods between Marine Isotope Stages (MIS) 3 and 9, which correspond to highstand events in large lakes of the Andean Altiplano. Results of this study illustrate that volcanic activity, furnishing rather constant CO2 and heat fluxes, were the key controls of thermalism in the Altiplane region during the Quaternary, whilst climatic changes (humid vs. arid periods in the late Pleistocene) controlled mineralogy, facies, and architecture of the travertines. The combined use of {delta}13C and 87Sr/86Sr signatures in carbonate precipitates has been proven to be of major relevance to evaluate the CO2 sources along fault zones in this study, This work was supported by the Spanish Ministry of Education and Science (MEC) through the projects AYIN93-000547; ANDESTER (BTE2001-3225); BTE2001-5257-E, LAVOLTER (CGL2004-00683), and GEOBILA (CGL2007-60932).

Carbon and Sr-isotope profiles in Upper Cambrian platformal carbonate Formations in the Precordillera, western Argentina (Zonda, La Flecha and La Silla Formations), were constructed for three representative sections: (a) Quebrada de la Flecha, Eastern Precordillera, (b) Cerro La Silla, Central Precordillera and (c) Quebrada de La Angostura, northern part of the Central Precordillera. At Quebrada de La Angostura, upper part of the La Flecha Formation, δ13Ccarb varies continuously up-section from - 2.0 to + 5.6‰ (PDB) and records the SPICE anomaly (+ 5‰) reported for the first time in South America. The peak of this excursion is characterized by intercalated 2 m thick beds of black shale with marl and limestone that record the onset of a sea-level change. The Steptoean Zonda Formation dolomites at the Quebrada de la Flecha exhibit a total δ13C range from - 2.7 to + 0.6‰ with discrete positive anomaly about 200 m from the transition to the overlying Sunwaptan La Flecha Formation. Pronounced C-isotope anomaly (- 5.6‰) is observed in the La Flecha Formation at about 300 m below the transition to the La Silla Formation. At the Cerro La Silla section, the Zonda Formation exhibit δ13C values of ∼ - 1‰, increasing slightly at the transition to the La Flecha Formation (- 1 to 0‰). The transition of the La Flecha to the La Silla Formations is characterized by alternation of black shales and dolomitic limestone with a discrete positive C-isotope excursion, probably corresponding to the SPICE. At the Quebrada de La Flecha, 87Sr/86Sr for the Zonda Formation varies from 0.70924 to 0.70955 and for the La Flecha Formation from 0.70908 to 0.70942. At Cerro La Silla this ratio varies from 0.70914 to 0.70923 for the La Flecha Formation, and from 0.70898 to 0.70980 for the La Silla Formation. At the Quebrada de La Angostura, ratios for the La Flecha carbonates range from 0.70918 to 0.70993. The overall variation of 87Sr/86Sr is consistent with globally reported Upper Cambrian seawater values at ca. 500 Ma. The unambiguous record of SPICE in the La Flecha Formation at the Quebrada de La Angostura supports a Steptoean age for its deposition and allows precise local, regional, and global stratigraphic correlation. The pronounced negative C-isotope excursion recorded in the La Flecha Formation carbonates at the Quebrada de La Flecha is likely equivalent to that registered in Sunwaptan carbonates of North America and Australia, and might be tied to a global event, as a valuable tool in stratigraphic correlation (SNICE, acronym for Sunwaptan negative isotope carbon excursion). Fil: Sial, A. N.. Universidade Federal de Pernambuco; Brasil Fil: Peralta, Silvio Heriberto. Universidad Nacional de San Juan. Facultad de Ciencias Exactas, Físicas y Naturales. Instituto de Geología "Dr. Emiliano Aparicio"; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Ferreira, V. P.. Universidade Federal de Pernambuco; Brasil Fil: Toselli, Alejandro Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto Superior de Correlación Geológica. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Departamento de Geología. Cátedra Geología Estructural. Instituto Superior de Correlación Geológica; Argentina Fil: Aceñolaza, Florencio Gilberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto Superior de Correlación Geológica. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Departamento de Geología. Cátedra Geología Estructural. Instituto Superior de Correlación Geológica; Argentina Fil: Parada Reyes, Miguel Angel. Universidad de Chile; Chile Fil: Gaucher, C.. Universidad de la República. Facultad de Ciencias; Uruguay Fil: Alonso, Ricardo Narciso. Universidad Nacional de Salta. Facultad de Ciencias Naturales; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas; Argentina Fil: Pimentel, Marcio. Universidade do Brasília; Brasil

The history of the Puna Plateau and its marginal basins and ranges in the Eastern Cordillera and the northern Sierras Pampeanas structural provinces in northwestern Argentina impressively documents the effects of tectonics and topography on atmospheric circulation patterns, the successive evolution of orographic barriers, as well as their influence on erosion and sedimentation processes. In addition, this region exemplifies that there are several pathways by which tectonic activity may be coupled to the effects of climate and erosion. Apatite fission track and sedimentologic data indicate that distributed, diachronous uplift of ranges within the present Puna Plateau of NW Argentina began as early as Oligocene time, compartmentalizing a foreland region similar to tectonically active sectors along the current eastern plateau margins. However, fission track data from detrital apatite in sedimentary basins and vertical profiles along the eastern plateau margin document that wholesale plateau uplift probably affected this region in mid to late Miocene time, which may have been associated with mantle delamination. This coincided with the establishment of humid conditions along the eastern Puna margin and a sustained arid to hyper-arid climate within the plateau region. A common feature of the Puna Plateau is that its location corresponds to hyper-arid areas of the landscape in which channels fail to incise deeply into basin sediments or surrounding basement ranges. Importantly, the local base-level is hydrologically isolated from the foreland. This isolation occurs where the incising power of regional drainage systems has been greatly reduced due to a combination of diminished precipitation related to regional climate and local orography, and exposure of resistant bedrock. Hydrologic isolation of the plateau from the foreland permits deposition within basins as material is eroded from the surrounding ranges, reducing the relief between basins and adjacent peaks. While a variety of deformation styles and possibly combinations of different processes may have generated the high elevations observed in the Puna Plateau, the observed low-relief morphology requires evacuation of material via regional fluvial systems to be restricted. Therefore, the low-relief character of the orogenic plateau may be a geomorphic, rather than a tectonic phenomenon. At the eastern plateau margins similar basin histories can be observed in fault-bounded intramontane depressions that straddle the eastern Puna border. However, these basins remain only transiently isolated and internally drained due to their proximity to the high precipitation gradients which were established by orographic barriers in the course of Pliocene uplift. These outlying barriers focus precipitation, erosion, promote headward erosion, stream capture, and ultimately basin exhumation. This conspiring set of processes thus prevents these areas to become incorporated into the plateau realm, while the interior of the orogen conserves mass and may influence deformation patterns in the foreland due to high lithostatic stresses. Sustained aridity in the core of the orogen may thus be responsible for the creation, maintenance and potential for future lateral growth of the plateau, thus emphasizing the coupling between tectonics, climate and erosion.

We have measured the boron concentration and isotope composition of regionally expansive borate deposits and geothermal fluids from the Cenozoic geothermal system of the Argentine Puna Plateau in the central Andes. The borate minerals borax, colemanite, hydroboracite, inderite, inyoite, kernite, teruggite, tincalconite, and ulexite span a wide range of δ11B values from −29.5 to −0.3‰, whereas fluids cover a range from −18.3 to 0.7‰. The data from recent coexisting borate minerals and fluids allow for the calculation of the isotope composition of the ancient mineralizing fluids and thus for the constraint of the isotope composition of the source rocks sampled by the fluids. The boron isotope composition of ancient mineralizing fluids appears uniform throughout the section of precipitates at a given locality and similar to values obtained from recent thermal fluids. These findings support models that suggest uniform and stable climatic, magmatic, and tectonic conditions during the past 8 million years in this part of the central Andes. Boron in fluids is derived from different sources, depending on the drainage system and local country rocks. One significant boron source is the Paleozoic basement, which has a whole-rock isotopic composition of δ11B=−8.9±2.2‰ (1 SD); another important boron contribution comes from Neogene-Pleistocene ignimbrites (δ11B=−3.8±2.8‰, 1 SD). Cenozoic andesites and Mesozoic limestones (δ11B≤+8‰) provide a potential third boron source.

[1] Along the central Andes a pattern of vertical axis rotations (CARP) has been paleomagnetically identified. These rotations, which are counterclockwise north of Arica and clockwise to the south, have been detected in Mesozoic and Cenozoic rocks cropping out in the Andean forearc and the Altiplano-Puna. Different models have been proposed to explain the geodynamic evolution of the central Andes, but the origin of these rotations remains controversial. Such models involve oroclinal bending and/or local block rotations. This lack of agreement is partly due to the scarce paleomagnetic database, particularly for the Argentine Puna. Here we report new paleomagnetic data obtained in the northern Argentine Puna from 57 sites drilled from Miocene volcaniclastic rocks and 12 sites from the Cretaceous-Eocene Salta Group. It was determined that (1) the Morro Blanco area did not undergo significant vertical axis rotation since at least the middle Miocene; (2) the Loma Blanca mine area underwent a clockwise vertical axis rotation of ∼10° later than the late Miocene; and (3) the Coranzuli area underwent a clockwise vertical axis rotation of ∼40° later than the Cretaceous-Eocene. The central Andes database was analyzed, and some features of the CARP were determined. These features were explained considering “domino”-style models for the forearc and a local block rotation model in response to penetrative shear for the Altiplano-Puna. Penetrative shear would occur along a high electrical conductivity zone. However, the possibility of the existence of an oroclinal bending of

Carbonates of the Las Tienditas Fm., Salta province, recorded the Vendian-Tommotian transition in the top of this carbonate sequence, while carbonates of La Laja Fm., San Juan, recorded this transition next to the base of the Formation. C, 87Sr/86Sr, Si, Mg/Ca, Mn, Fe and Si chemostratigraphic profiles in the uppermost portion of Las Tienditas are very similar to those for the lowermost portions of La Laja Formation, suggesting synchronous deposition. C values ( 0 º/ooPDB) in carbonate lenses of the Caucete Group, Pie-de-Palo complex, coupled with high 87Sr/86Sr (0.709 to 0.710) and presence of Vendian ichnofauna suggest that sediments of this Group were, perhaps, deposited synchronically with carbonates of the La Laja Fm. The high 87Sr/86Sr for all of the Formations studied resulted from the nature of the continental crust (high Rb/Sr, or very ancient crust, or both) being eroded during the time of deposition of these marine carbonates. The moderate shift of C from +1 º/ooPDB to -2 º/ooPDB in this Vendian-Tommotian transition contrasts with strong shifts observed in most localities in the world. Similar behavior has been observed in Siberia and a shift from +5 º/ooPDB to +2.7 º/ooPDB was recorded in Bhander and Sirbu limestones, Vindhyan basin in India. These observations demonstrate a nonuniform isotopic behavior of the Vendian-Tommotian transition, at least, for continents that once were part of Gondwanaland. — ( May 18, 2001 ) Fil: Sial, Alcides N.. Universidade Federal de Pernambuco; Brasil Fil: Ferreira, Valderez P.. Universidade Federal de Pernambuco; Brasil Fil: Martins Pimentel, Márcio. Universidade do Brasília; Brasil Fil: Toselli, Alejandro Jose. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto Superior de Correlación Geológica. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Departamento de Geología. Cátedra Geología Estructural. Instituto Superior de Correlación Geológica; Argentina Fil: Aceñolaza, Florencio Gilberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán. Instituto Superior de Correlación Geológica. Universidad Nacional de Tucumán. Facultad de Ciencias Naturales e Instituto Miguel Lillo. Departamento de Geología. Cátedra Geología Estructural. Instituto Superior de Correlación Geológica; Argentina Fil: Alonso, Ricardo Narciso. Universidad Nacional de Salta; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta; Argentina Fil: Parada, Miguel A.. Universidad de Chile; Chile

The chronology of multiple landslide deposits and related lake sediments in the eastern Argentine Cordillera suggests that major mass movements cluster in two time periods during the Quaternary: between 35 000 and 25 000 14 C yr BP and after 5000 14 C yr BP. The older cluster may correspond to the Minchin wet period (40 000 and 25 000 14 C yr BP) identified in tropical and subtropical South America, suggesting a causal relation between enhanced landslide activity and climate change. The younger cluster predates the Titicaca wet period that began at about 3900 14 C yr BP which also affected other regions in the Andes and the Amazon Basin. No landslide and associated lake sediments are documented during the Tauca wet period (between 16 000 and 8000 14 C yr BP). However, the two clusters correspond to periods where it assumed that the El Nino/Southern Oscillation (ENSO) and tropical Atlantic sea surface temperature dipole (TAD) were active. The analysis of the present-day precipitation patterns in NW Argentina indicates significant spatial and temporal differences between the intra-Andean part of the study area and the Andean foreland. Whereas the TAD seems to consistently increase rainfall, the intensity of precipitation during the El Nino phase of the ENSO is reduced to only 25% of the mean annual average in the intra-Andean basins, whereas the regions east of the Andes receive more than 125%. Similar results, but with an opposite sign, characterize La Nina events. The comparison of this pattern with paleo-precipitation data as inferred from varved lake sediments suggests that increased interannual climate variability and, therefore, increased fluctuations in rainfall and river discharge in narrow valleys may reduce landsliding thresholds.

The Upper Miocene to Pliocene Sijes Formation consists of a thick sequence of continental sediments and is of major economic importance because it contains the beds of borate minerals that are being exploited in the Puna. Radiometrically dated as between 7 and 4 Ma, it contains only a few fossils, principally diatoms, the remains of birds, and plant fragments, none of which have stratigraphic significance. External molds of small gastropods discovered recently in the Esperanza (upper) Member of the Sijes Formation near the Sol de Manana borate mine in the Salar of Pastos Grandes represent the first identifiable remains of this kind of organism in these deposits. The Esperanza Member, about 780 m thick, is composed of beds of clastic alluvial fan sediments that are overlain by fine grained clastics, then gray tuffs and pumicites that have been dated at 5.9 Ma by the fission-track method. A sequence of rythmically bedded lacustrine muds and evaporites overlies the volcaniclastic beds. The gastropod-bearing sediment, an altered volcanic ash, is near the base of this lacustrine section. A fission-track date from a volcanic ash near the top of the section is 4.0 Ma. A disconformity separates the top of the Esperanza Member from a 2,000 m thick Pliocene conglomerate. The gastropods recovered are a new species of Littoridina, a lacustrine genus that is widely distributed in lakes that range from nearly fresh water to very salty water. Generally, living species of Littoridina are small, fragile, and unornamented; they live on a firm substrate of fine-grained sediments in a permanent body of water that does not become highly turbid. Their presence in lacustrine sediments of the Sijes Formation indicates that the lake, though undoubtedly salty at the time, probably was deep enough that the snails generally were below wave base or lived in an environment, perhaps on rooted plants, that did not become highly turbid. Littoridina probably was transported to this lake on the feet or feathers of birds. The genus is dioecious, but a fertilized female of another genus in the family (Amnicola) is capable of depositing viable eggs throughout the year, as long as the water remains at an appropriate temperature.

Fundamental to understanding the late Cenozoic evolution of the central Andes is knowledge of the age that the Puna-Altiplano plateau became a unique tectonomorphic entity within the orogen. The two attributes that define the Puna-Altiplano are (1) its average elevation of ≥3 km and (2) regions of high elevation that correspond spatially to regions with internal drainage. The chronology of plateau uplift to elevations similar to modern ones is equivocal owing to an inability to define the absolute elevation of the plateau surface through time. However, because regions of the Puna-Altiplano have nonmarine evaporites nad because such evaporites must accumulate within regions of internal drainage, the chronology of these strata elucidates when the plateau became internally drained. We show that, for the southern Puna (between lat 24° and 26°S), internal drainage was established between 14.1 and 24.2 Ma, and probably by ∼15 Ma. These results corroborate and refine other workers' inferences that the Puna-Altiplano plateau became uplifted and paleogeographically isolated from the adjacent fore arc and foreland during middle to late Miocene time.

Large volumes of continental evaporites accumulated within the central Andes during Neogene uplift of the Altiplano-Puna plateau and development of the Andean volcanic arc. Halite and gypsum are dominant minerals, along with local and economically important borates. Playa conditions have existed since ca. 15 Ma; halite and borate deposition has occurred for the past 7 to 8 m.y. Evaporites formed in salar environments (e.g., playa lakes) and are characterized by complex mineral assemblages, occurrence, zonation, and geochemistry. Evaporite deposition was controlled by volcanism, geothermal activity, closed drainage, and climate. These Andean deposits, and their controls, differ from evaporites in other continental and marine environments.

The Loma Blanca borate deposit was formed in the muds of playa-lake environments during the Late Miocene and is the fourth Tertiary commercial borax deposit discovered within the borate districts of the world. It is the only South American deposit known to contain any of the minerals colemanite, inyoite, ulexite, borax, tincalconite and teruggite with a unique and characteristic mineral sequence among the other Argentinian borate deposits. The Loma Blanca deposit is characterized by abundant Ca, Na and B, very low Cl and relatively high As, S and Mg concentrations compared with other borate deposits. Thermal springs and hydrothermal solutions associated with local volcanic activity are thought to be the source of the borates. The early colemanite, inyoite, ulexite, borax and teruggite nodules and crystals appear to have been formed directly from brines penecontemporaneously within the unconsolidated sediments, and they continued to grow as the sediments were compacted. Later generations of borate minerals occur in vughs, veins and as thin layers. Diagenetic alterations include the partial replacement of borax by ulexite and tincalconite; when weathered, borates are often almost completely replaced by calcite.

Clastic sedimentary basins have evolved during the past 40 m.y. in the central Andes (lat. 20°-28°S) in response to shifting patterns of magmatism and deformation. The distribution of these basins and their genetic relations to uplifted areas are analogous to the basins and mountain belts of the North American Rocky Mountains during the Late Cretaceous and early Cenozoic. Petroleum exploration has focused on zones underlying the upper Cenozoic strata along the eastern margin of the Andes mountain belt. Between about 40 and 25 Ma, a nonmarine basin extended across the region that is now the Andes Mountains. Between about 25 and 10 Ma, the western part of the former basin became the site of a volcanic arc; sediment accumulation continued in the east, where marine intercalations demonstrate the low elevation of the basin. After 10 Ma, the volcanic arc remained active and locally widened, and crustal shortening caused regionally important thrust and reverse faulted ranges. During the past 10 m.y., up to 4,000 m of coarse clastic debris accumulated in a foreland basin on the eastern flank of the mountains; meanwhile in the interior of the mountains, over 4,000 m of fine-grained strata and evaporites accumulated in local depocenters.

Relationships between geological evolution, tectonic setting, magmatism and mineralization are pointed out. The geological Puna province is considered as two geological subprovinces: a northern and southern one. being the Olacapato megafault the boundary between them. Two principal pulses for the metalic mineralization are recognized: an Ordovician and a Miocenic one. Three north-southward mineralizated belts for the Ordovician time are described: eastern (Zn, Pb, Ag, baritine), central polymetalic (Cu, Fe, Zn, Pb, baritine and quartz) and western (Au, quartz) which would be the southern end of some Bolivian belts. On the other hand, four transversal (WNW-ESE) mineralizated belts for the Miocenic times, associated with the transversal volcanic ranges are described. From north to south: Coyahuaima (Sn, Ag, Pb, Sb); Quevar (Ag, Pb, Zn); Antofalla (Pb, Ag, Zn); Carachipampa (Cu). At last, relationships between the geological evolution of the Puna province and the non-metalic mineral occurrences are described, pointing out that northern Puna can be defined as "metal-bearing region" and the southern Puna as "nonmetal bearing region".

The Cretaceous-Paleocene (K-T) transition has been recorded in sedimentary carbonate rocks in northwestern Argentina and southern Chile. In the Yacoraite Basin, Argentina, this transition has been preserved in a 2 m thick marly layer, at the base of the Tunal Formation, which overlies lacustrine/marine carbonates of the Yacoraite Formation (Cabra Corral dam). The K-T transition is also preserved at Maimara, where Tertiary sandstones overlie a 50 m thick limestone bed of the Yacoraite Formation. In the Magellan Basin, Chile, glauconitic sandstones with calcitic cement and limestone concretions of the Maastrichtian Punta Rocallosa Formation are overlain by sandstones, claystones, and limestones of the Chorillo Chico Formation. The K-T transition is preserved in the lower portion of the Chorillo Chico Formation. Carbonates of the Yacoraite Formation display bulk-rock δ13C values from +1 to +2‰ PDB, with a negative incursion (−4‰ PDB) at the K-T transition. δ13C values in the Tunal Formation marls vary from −...