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The Quaternary Campo de la Piedra Pomez ignimbrite (CPPI) is a superbly exposed, partially indurated, rhyolitic ignimbrite emplaced on the southern Puna of Argentina. It is characterized by a variety of facies that record in unprecedented detail the flow dynamics of the parent pyroclastic density currents (PDCs). Detailed facies analysis and internal architecture defined using a sequential stratigraphy approach reveal that CPPI was formed by sustained PDCs, generated from a low fountain eruptive style (boiling over). The PDCs had overall flow conditions characterized by high particle concentration and limited capability to surmount topographic obstacles. The mobility of the PDCs was largely controlled by high pore pressure and the continuous supply at the source (sustained forced convection-dominated PDCs). The successive forestepping-backstepping stacking patterns identified in the CPPI reflect a marked unsteadiness of its parent PDCs due to a pulsating discharge rate at the source. The lateral facies variations in the CPPI record the non-uniform character of its parent PDCs as they flowed outward from the base of the collapsing fountain. Proximal-medial areas characterized by steady aggradation rates pass through to distal braided thalwegs with highly variable aggradation rates, to frontal edges characterized by the step aggradations of secondary decoupled pumice-rich lobes. Collectively, the facies variations in the CPPI record the complex dynamics of sustained PDCs that include pulsating discharge rate at the source, progressive modification of the original topography, and lateral flow transformations. [ABSTRACT FROM AUTHOR]

Crystalline domains in rhyolitic domes and coulees are commonly characterized by spherulites and lithophysae. Spherulites typically range from microscopic to a few centimeters in diameter. Larger spherulites, termed "megaspherulites," are rare but have been reported in the USA and Mexico. The uncommon nature of such structures supports the need for a study to explore the factors and special conditions that allow them to reach such a large size. In the Southern Puna of Argentina, El Viejo Coulee includes megaspherulites up to 4 m in diameter. We present observations from field work, petrography, scanning electron microscopy, bulk geochemistry, and Fourier transform infrared spectroscopy. The megaspherulites occur in obsidian lenses that differ in phenocryst content and composition from the foliated coherent facies that forms the rest of the coulee. The obsidian lacks vesicles and microlites and is unaltered. The megaspherulites comprise growth cones consisting of micropoikilitic texture where quartz encloses potassium feldspar. The growth cones are separated by interconal areas composed of lithophysae. We propose that the megaspherulites were formed above the glass transition temperature (Tg) and are the product of primary crystallization of rhyolitic melt. The exceptionally large size of the megaspherulites implies high diffusion rates which are favored by temperatures above Tg during crystallization. The large size also suggests scarcity of nucleation sites, which is consistent with the megaspherulites being hosted by unaltered microlite- and vesicle-free glass. The position of the obsidian lens at the base of the coulee may have played a critical role in maintaining the temperature above the Tg long enough to allow the crystallization of the megaspherulites. These conditions also favored crystallization in the most advanced stage where micropoikilitic texture replaced the fans of crystal fibers typical of spherulites. Crystallization of anhydrous quartz and feldspar in the growth cones led to the concentration of volatiles in the melt in the interconal areas, resulting in volatile exsolution and formation of vesicles that became nucleation sites for lithophysae. The study advances our understanding of some of the special processes that are involved in the cooling and solidification of rhyolitic magmas. Fundamentally, we find that the position of the obsidian at the base of the coulee was critically important because this position favored maintenance of the temperature above the Tg which, in turn, favored high diffusion rates. Also, the scarcity of nucleation sites in the obsidian melt allowed only a small number of spherulites to nucleate; those that nucleated therefore grew very large. The meter-scale megaspherulites may have taken ~ 55 years to create. [ABSTRACT FROM AUTHOR]

A comprehensive morphostratigraphic and chronological study of the complete set of Holocene tephras from Tafí and Santa María valleys (northwestern Argentina), including analyses of compositional characteristics, is presented. Five ash tephras are recognized: V0 (El Rincón), V1a (Carreras 1a ash), V1b (Carreras 1b ash), V2a (Carreras 2 ash), and V2b (El Paso 3 ash). Two of them (V1b and V2b) are described for the first time in the study area. The new 14C and accelerator mass spectrometry ages presented, along with the previously published information, allows for the establishment of a chronological framework. The V0 tephra was deposited in the Early Holocene (about 10,000 yr BP), V1a and V1b were deposited in the Middle Holocene (about 4200 and 3500 yr BP, respectively), and V2a and V2b were deposited in the Late Holocene (after about 800 yr BP). The mineralogical, textural, and geochemical characterizations of the five tephras suggest that their tephra provenance was mainly from the back-arc region. However, the determination of the exact source of each tephra requires more accurate high-resolution tephrochronological studies. At least five major eruptions affected the Tafí and Santa María valleys in the last 10,000 yr. [ABSTRACT FROM AUTHOR]

Pyroclastic density currents (PDCs) are one of the most dangerous volcanic phenomena. The correct interpretation and mapping of PDC deposits in the volcano record is important to establish the eruptive style and play a fundamental role in hazard assessment and risk management. The Copahue volcano is an active intermediate volcano of the Southern Volcanic Zone of the Andes (Argentina-Chile) that presents fragmentary evidence of explosive activity during its evolution, with unusual minor PDC deposits. The recorded historic eruptions were mainly phreatomagmatic due to the presence of a crater lake. In this study, four key deposits previously interpreted as PDC products corresponding to different stages of the Copahue volcano evolution (Pleistocene, Holocene, and Historic times) are analyzed and this origin is discussed: (i) A Pleistocene reddish succession located in the northeast flank of the volcano formed by stretched bombs in a coherent lava is interpreted as clastogenic lavas; (ii) a series of proximal bedded volcaniclastics deposits of Pleistocene age are interpreted as redeposition of hyaloclastic fragments from syn-eruptive subglacial meltwater flows associated with subglacial eruptions; (iii) a distal Holocene deposit located ~12 km east of the active crater consist mainly in fine-sized clasts forming aggregates is reinterpreted as a sedimentary (lacustrine) deposit with volcaniclastic input; and, (iv) a historic whitish-grey clastic deposit located on the eastern flank is considered a product of a mixed avalanche generated during the 1992–1995 activity. Consequently, the PDC occurrence during the Copahue volcano evolution is less than previously thought. Large PDCs are unlikely in the future and their influence area would be reduced near the active crater as observed in recent eruptions. Flows triggered by the melting of snow/ice during volcanic activity and sudden drainage of the crater lake appear to be a more likely potential hazard that should be considered during risk assessment. • Reinterpretation of four deposits previously interpreted as PDCs-products. • Clastogenic lavas, subglacial flows, lacustrine deposits, and mixed avalanches. • Overestimation of the PDCs occurrence in the Copahue volcano. • Melting of snow and sudden drainage of the crater lake are potential hazard. [ABSTRACT FROM AUTHOR]

Establishing the factors that control the pathways of magma ascent is an important issue in the study of monogenic mafic volcanism since it provides information about the relationship between the source of the ascending magma and the regional and local tectonic frameworks. We have quantitatively analyzed the spatial distribution of the volcanic centers in the monogenic volcanic fields (MVFs) of the Southern Puna using Nearest-Neighbor Analysis to assess the degree of randomness between individual centers. We also performed alignment analysis which highlights the structural weaknesses that magmas use to ascend. We integrate these novel data with published structural, petrologic, and geophysical data to propose a source-to-surface model which explains the spatial distribution of monogenic mafic centers of the Southern Puna. We find that MVFs display two distinct spatial patterns related to different magmatic production rates. Specifically, in those areas where the magmatic production is long-lasting and relatively high, the magma exerts strain rates suffciently high to reactivate pre-exisiting discontinuities with random orientations with respect to the current local stress conditions. As a result the volcanic centers are arranged with a clustered spatial distribution. We define these as MVFs controlled by magmatism or high flux fields (MVFs-M). On the other hand, low magmatic production rates tend to produce areas where mafic centers display a Poisson distribution, because strain rates are only sufficient to reactivate pre-existing discontinuities that are nearly parallel to the maximum compressive stress. This latter group is defined as MVFs controlled by tectonism or low flux fields (MVFs-T). Available petrological and geophysical data indicate that both groups are fed by a complex lower crustal MASH zone where magma production is associated with lithospheric foundering and subduction-related mantle melting. Spatial distribution of the MVFs in the Southern Puna are also affected by the development of upper crustal magma storage zones. • Bivariate Gauss-kernels Density allow to define the number of MVF in the Southern Puna. • Nearest-Neighbor and Two-Azimuth quantitative methods reveal the spatial distribution of the MVFs. • Two main types of MVFs were identified: dominated by magmatism and dominated by tectonism. • Both groups are feed by a complex source related to lithospheric foundering and subduction. • Spatial distribution is later affected by lithospheric processes. [ABSTRACT FROM AUTHOR]

This paper presents a detailed geological map at the 1:20,000 scale of the Tocomar basin in the Central Puna (north-western Argentina), which extends over an area of about 80 km2 and displays the spatial distribution of the Quaternary deposits and the structures that cover the Ordovician basement and the Tertiary sedimentary and volcanic units. The new dataset includes litho-facies descriptions, stratigraphic and structural data and new 234U/230Th ages for travertine rocks. The new reconstructed stratigraphic framework, along with the structural analysis, has revealed the complex evolution of a small extensional basin including a period of prolonged volcanic activity with different eruptive centres and styles. The geological map improves the knowledge of the geology of the Tocomar basin and the local interplay between orogen-parallel thrusts and orogen-oblique fault systems. This contribution represents a fundamental support for in depth research and also for encouraging geothermal exploration and exploitation in the Puna Plateau region. [ABSTRACT FROM AUTHOR]

Glaciovolcanism produces distinctive features that are useful paleoclimate proxies for the distribution of past ice sheets and glacier extent. The Copahue volcano located in the Andean Southern Volcanic Zone, Argentina-Chile, is an active composite volcano known to have glaciovolcanic features such as lava bodies with glassy margins and anomalous cooling fractures. However, the emplacement conditions of these products and the influence of Pleistocene glaciations on the evolution of the Copahue volcano remains poorly understood. In this contribution, we propose a model for glaciovolcanic evolution of the Copahue volcano based on the analysis, interpretation, and mapping of its products. Ten lithofacies are described on the eastern flank of Copahue volcano exhibiting several examples of glaciovolcanism. The evolution of the Copahue volcano can be divided into two main sequences: the Ancient Sequence (S1) and the Young Sequence (S2), separated by a major erosive phase. The S1 (early-middle Pleistocene-late Pleistocene) consists of an initial subaerial effusive stage followed by a major glaciovolcanic stage, during which a thick ice cap existed and the edifice grew beneath an englacial lake with the eventual formation of a lava-fed delta. The S2 (late Pleistocene-Present) is defined by mainly effusive activity during periods of glacial advance and retreat recorded by an alternation of unconfined subaerial lavas and ice-confined lavas. The evolution of the Copahue volcano therefore indicates two glaciations in the Copahue-Caviahue area during the late Pleistocene, in contrast to a single glaciation. Based on the glacial history in the area we associate the first glaciation with the end of Marine Isotope Stage 3 (57–29 ka) and/or the Last Glacial Maximum period (26.5–19.0 ka), and the second less-extensive glacial period with the Antarctic Cold Reversal (14.5–12.9 ka). • Copahue volcano comprises abundant and diverse glaciovolcanic deposits. • The edifice comprises two eruptive sequences separated by a major phase of erosion. • Glaciovolcanic indicators include pillow lavas, pillow breccias, fractured glassy lavas and overthickened lavas. • Unique syn-eruptive volcaniclastic deposits record meltwater drainage events. • The lithofacies constrain ice conditions and suggest two late Pleistocene glaciations. [ABSTRACT FROM AUTHOR]

• An integrated conceptual model of the Tocomar Geothermal system is presented. • Compositional data of thermal fluids along with structural, Raman spectroscopic, and in-situ permeability data from the Tocomar Geothermal area are reported. • Cation geothermometers indicates reservoir temperatures up to 235 °C. • The structural control on flow pathways and fluids-rock interaction processes are discussed. • The geothermal electric potential is stochastically assessed. The Argentinean Andean region hosts a vast geothermal resource clustered by active magmatic and tectonic activity. One of the most studied geothermal areas is the Tuzgle-Tocomar geothermal system in Central Puna (NW Argentina). However, despite the existence of several studies since the 1970′s highlighting the geothermal potential of the area, only highly schematic and dissimilar conceptual models have been proposed for the Tocomar Geothermal System. This study presents new detailed geological-structural and hydrogeochemical data, together with in-situ permeability analysis of fault zones and Raman spectroscopic characterization of hydrothermal-alteration minerals. The electrical generating capacity has also been evaluated using the volumetric method and a stochastic approach. A new comprehensive conceptual model of the studied area was constructed highlighting the role of the Calama-Olacapato-El Toro (COT) fault-system in the circulation of hydrothermal fluids. The reservoir of the Tocomar geothermal system has a Na+-Clˉ(HCO 3)ˉ composition and an estimated temperature of ∼235 °C. Such a reservoir is hosted in fractured Ordovician rocks and controlled by the COT-like Chorrillos transpressive fault at 1000–1500 m depth b.g.l. The water isotopic data and hydrological features indicate a regional recharge beyond the Tocomar sub-basin boundaries (>5000 m a.s.l.). Additionally, the main hydrothermal reservoir receives inputs of magmatic fluids from the degassing of the intra-crustal rhyolitic magma chamber of the Tocomar volcanic center. The Monte Carlo simulations suggest that the Tocomar geothermal system has a probable power production capacity above 1.23 MWe (P90), 6.18 MWe (P50) and 11.67 MWe (P10) at different confidence levels. All calculations were biased towards minimum values, thus a tighter definition of the resource size and fracture porosity would significantly impact on the predictions. Notwithstanding, the strategic position of the Tocomar geothermal field encourages move forward towards more in-depth exploration phases. [ABSTRACT FROM AUTHOR]