Southern Puna Plateau Research Articles

Plateau Formation Controlled by Lithospheric Foundering Under a Weak Crust

AbstractLithospheric foundering is hypothesized to contribute to the formation of Earth's largest continental plateaus, but the predicted effects remain poorly constrained, especially considering variations in crustal strength. Here we propose that lithospheric foundering involving a hot, weak crust can explain aspects of topography and crustal deformation in mountain belts. We explore this hypothesis using numerical models of orogenesis and foundering in the Southern Puna Plateau in the Central Andes. Contrary to previous models of foundering involving a strong crust, which are characterized by subsidence and shortening, a weak crust results in surface uplift and upper‐crustal horizontal extension, which is accommodated by horizontal shortening in peripheral regions. Our model explains data such as the timing and location of exhumation and basin sedimentation as a response to foundering. The model also shows that foundering is capable of converting a high‐relief, broken‐foreland region into a high‐elevation, low‐relief plateau in a convergent tectonic setting.

Spatial variability of the modern radiocarbon reservoir effect in the high-altitude lake Laguna del Peinado (southern Puna Plateau, Argentina)

Abstract. The high-altitude lakes of the Altiplano–Puna Plateau in the Central Andes commonly have large radiocarbon reservoir effects. This, combined with the general scarcity of terrestrial organic matter, makes obtaining a reliable and accurate chronological model based on radiocarbon ages a challenge. As a result, age–depth models based on radiocarbon dating are often constructed by correcting for the modern reservoir effect, but commonly without consideration of spatial and possible temporal variations of reservoir ages within the lake and across the basin. In order to get a better constraint on the spatial variability of the radiocarbon reservoir effects, we analyse 14C ages of modern terrestrial and aquatic plants from the El Peinado basin in the southern Puna Plateau, which hosts Laguna del Peinado fed by hydrothermal springs. The oldest 14C ages of modern samples (> 18 000 and > 26 000 BP) were found in hot springs discharging into the lake, likely resulting from the input of 14C-depleted carbon from old groundwater and 14C-free magmatic CO2. In the littoral and central part of Laguna del Peinado, 14C ages of modern samples were several thousand years younger (> 13 000 and > 12 000 BP) compared to the inflowing waters as a result of CO2 exchange with the atmosphere. Altogether, our findings reveal a spatial variability of up to 14 000 14C years of the modern reservoir effect between the hot springs and the northern part of the Peinado lake basin. Temporal changes of reservoir effects in sediment records are more difficult to quantify, but 14C ages from a short core from Laguna del Peinado may suggest temporal reservoir age variations of a few thousand years. This study has implications for accurate 14C-based chronologies for palaeoclimate studies in the Altiplano–Puna Plateau and similar settings. Our results highlight the need to consider spatial and likely also temporal variations in the reservoir effects when constructing age–depth models.

Reviewing the geodynamic impact of aseismic ridges subduction on the tectonic-magmatic evolution of the Southern Puna plateau

The Southern Puna (∼24° S to 27° S) is an outstanding segment of the Central Andes where the subduction of aseismic ridges, multi-scale lithospheric foundering, and the transition from normal Andean subduction to the Chilean-Pampean flat slab occur. The exponential increase in the volcanological and geophysical knowledge of the Southern Puna provides a unique opportunity to reappraise its geodynamic evolution. This paper presents an updated review of the spatial-temporal distribution of the volcanism along with a reassessment of the main geophysical and morphostructural features of this region. Contrasting our observations with new kinematic reconstructions of the Taltal and Copiapó ridges, we infer that their subduction under the Southern Puna plateau has played a fundamental role in its Neogene-Quaternary geodynamic evolution (e.g shallow subduction, lithospheric delamination). Specifically, we propose a new alternative hypothesis where the Transversal Volcanic Chains (TVCs), characterizing the Southern Puna, are related to slab tearing. Slab tearing is a valid explanation for the occurrence of local anomalous peaks of melt productivity in a relatively cold retreating mantle wedge during slab shallowing. In our model, slab tearing under the Southern Puna was influenced by the flexural deformation related to slab dip changes as a result of the anomalous slab buoyancy during the subduction of Copiapó and Taltal ridges.

Comparing the biotite and amphibole single-mineral thermobarometric models: A case study from the Cordillera de San Buenaventura volcanic rocks, Puna plateau of Central Andes (Argentina)

We applied a new thermobarometric model based solely on biotite composition to the trachyandesitic to rhyolitic volcanic rocks of the Cordillera de San Buenaventura, a long-lived magma plumbing system (spanning from the Late Miocene to the Holocene) in the Southern Puna plateau (Central Volcanic Zone of the Andean Cordillera). Within these rocks, biotite is a widespread mineral phase found in association with amphibole, as either glomerocrysts, mutual inclusions, or free phenocrysts. This allowed us to compare the biotite-only model with other thermobarometric estimates that rely on amphibole, mineral-melt, and mineral-mineral composition. Our goal was to determine the conditions of formation for biotite and its petrogenetic significance in the evolution of the Cordillera de San Buenaventura magma system. The results indicate that biotite crystallization occurred mainly at middle-to upper-crustal depths (2.1–5.1 kbar) with temperatures below 1000 °C (763–919 °C). The biotite-only T-P estimates for individual lithologies are consistent with those obtained using the amphibole-only model, and this is particularly true for the thermometer. Furthermore, our findings align with the existing petrogenetic models, which suggest extensive fractional crystallization, and with the analyses of mineral textures, that indicates events of magma replenishment. Therefore, our results support the validity of the biotite-only model and its ability to elucidate magmatic processes, including magma differentiation via fractional crystallization, magma ascent and cooling, and magma replenishment and/or mixing events.

Hominins and carnivores in the Early Pleistocene site of Barranco León (Orce, Baza Basin, SE Spain): Comment on Courtenay et al. (2023)

This work investigates questions regarding obsidian circulation among the late populations (since ca. 1100 BP) of Antofagasta de Sierra (Catamarca Province, Argentinean Southern Puna Plateau). By means of geochemical provenance studies conducted on obsidian artefacts, the sources of obsidian that these groups accessed have been identified. The samples considered in this study were collected at three radiocarbon-dated sites in the micro-region: La Alumbrera, Bajo del Coypar II and Campo Cortaderas.The results obtained are compared with those available for other contemporary sites and Formative contexts (ca. 3000–1100 BP) within the area of study. A preliminary discussion is presented on the basis of the environmental, political and social tendencies that characterized the times after ca. 1100 BP at the micro-region and south-central Andes.

The rise and fate of a long-lived deep crustal ‘hot zone’ beneath the neogene-quaternary Cordillera de San Buenaventura in the Southern Puna Plateau (NW Argentina)

ABSTRACT Arc magmatism plays a key-role in the growth and differentiation of continental crust. In particular, the prolonged magmatic flux events control the genesis of deep crustal hot zones where magma accumulation and fractionation favour the continental arc crust evolution and geochemical stratification. In this view long-lived magmatic systems located in arc domains, with their erupted products spanning from basalts to rhyolites are formidable archive for the understanding the evolution of transcrustal magmatic plumbing systems and the construction of a stratified continental crust. This study focuses on the long-lived (ca. 9 Ma) Miocene-Holocene Cordillera de San Buenaventura volcanic system in the Southern Puna Plateau (NW Argentina), a unique natural laboratory where exploring the building up of MASH-zones (or deep crustal ‘hot zones’) and related sustained volcanism. Synthesis of new and published data, together with new thermobarometry and fractional crystallization modelling, indicates a cyclic scenario with mantle melts undergoing fractional crystallization dominated incipient and waning stages alternate to major mafic magma recharge events during the building up phase of the MASH-zone. This magmatic scenario is also discussed in the light of the coeval geodynamic framework dominated by the subduction of the Nazca plate and the eastward migration of the frontal arc magmatism.

A westerly wind dominated Puna Plateau during deposition of upper Pleistocene loessic sediments in the subtropical Andes, South America

The Tafí del Valle depression (~27° S) in the eastern Andes of Argentina provides a record of late Pleistocene dust deposition in the subtropics of South America. We present large-n U-Pb geochronology data for detrital zircons from upper Pleistocene loess-paleosol deposits. When compared to regional data, the age spectra from the Tafí del Valle samples are most like the southern Puna Plateau, supporting derivation largely from the west and northwest. This runs counter to hypotheses suggesting these loessic sediments were derived from the low elevation plains to the east or extra-Andean Patagonia. Mapping of linear wind erosion features on the Puna Plateau yield a mean orientation of 125.7° (1 s.d. = 12.4°). These new data and existing records are consistent with a westerly-northwesterly dominated (upper- and lower-level) wind system over the southern Puna Plateau (to at least ~27° S) during periods of high dust accumulation in Tafí del Valle.

Eocene to Quaternary Deformation of the Southern Puna Plateau: Thermochronology, Geochronology, and Structural Geology of an Andean Hinterland Basin (NW Argentina)

AbstractCordilleran orogens are complex tectonic systems, strongly influenced by lithospheric deformation and exhumation. However, in the type region of the Central Andes, the timing and mechanisms of deformation and mountain building remain poorly constrained, especially for the high‐elevation Puna Plateau of NW Argentina. We use geologic mapping, structural cross‐sections, low‐temperature thermochronology, and U‐Pb dating of ash and detrital zircons to elucidate the history and style of orogenic deformation in the Antofalla region of the Puna Plateau. Cooling ages of major reverse fault hanging walls and dated growth strata indicate that crustal shortening activated major reverse faults during the Eocene and continued out‐of‐sequence through the Early Miocene. This protracted period of shortening, which resulted in the filling of the Antofalla Basin, was followed by a transient episode of upper‐crustal E‐W extension on a major normal fault system during the Late Miocene. Pliocene to Quaternary time saw E‐W shortening return to the region. Our structural cross‐section permits only a low magnitude of shortening (∼15–25%), which stands in contrast to the 60‐km thick crust. Triassic to Cretaceous cooling ages from footwall rocks and minor structures indicate that the entire region experienced little exhumation or sedimentation during the Mesozoic, perhaps retaining a crust thickened during multiple episodes of Paleozoic orogenesis. The crust may also reflect lower‐crustal thickening due to the growth of a lithospheric drip during the Late Miocene.

Late Cenozoic Extensional Formation of the Antofalla Depression, Southern Puna Plateau, Argentina: An Effect of Lithospheric Foundering?

AbstractDeformation within orogenic plateaus functions to establish a dynamic equilibrium between tectonic boundary stresses and plateau gravitational potential energy. Temporal changes in deformation kinematics record perturbations to boundary stresses or internal plateau processes, such as lithospheric foundering. We integrate new mapping, field observations, and geochronologic ages with published data to document complex late Cenozoic upper crustal deformation in the region of the Antofalla depression, a ∼125 km long, sublinear basin within the southern Puna orogenic plateau, Argentina. The juxtaposition of stratal ages across the depression requires >900 m vertical offset on a surface‐breaking fault. Regional geologic structure, basin geomorphology, and our observation of a breccia paralleling the depression margin suggest formation of the depression by normal faulting. We interpret published stratigraphic logs to suggest that the depression formed between ca. 16 and 11 Ma following Andean shortening. Folded Late Miocene to Quaternary strata on the eastern depression margin indicate that extension ended and shortening resumed before present, revealing toggling between extensional and contractional kinematic regimes. The kinematic evolution of the Antofalla depression contrasts with the rest of the southern Puna plateau, which underwent shortening until latest Miocene to Quaternary time, followed by extension and strike‐slip deformation. Taken together, the spatial and temporal variations in late Cenozoic deformation of the southern Puna plateau are inconsistent with mechanisms that would affect the entire orogen, such as slowing convergence, but are compatible with lithospheric foundering.

Magmatic evolution and architecture of an arc-related, rhyolitic caldera complex: The late Pleistocene to Holocene Cerro Blanco volcanic complex, southern Puna, Argentina

Abstract Through the lens of bulk-rock and matrix glass geochemistry, we investigated the magmatic evolution and pre-eruptive architecture of the siliceous magma complex beneath the Cerro Blanco volcanic complex, a Crater Lake–type caldera complex in the southern Puna Plateau of the Central Andes of Argentina. The Cerro Blanco volcanic complex has been the site of two caldera-forming eruptions with volcanic explosivity index (VEI) 6+ that emplaced the ca. 54 ka Campo Piedra Pomez ignimbrite and the ca. 4.2 ka Cerro Blanco ignimbrite. As such, it is the most productive recent explosive volcano in the Central Andes. The most recent eruptions (younger than 4.2 ka) are dominantly postcaldera effusions of crystal-rich domes and associated small explosive pulses. Previous work has demonstrated that andesitic recharge of and mixing with rhyolitic magma occurred at the base of the magma complex, at ~10 km depth. New isotopic data (Sr, Nd, Pb, and O) confirm that the Cerro Blanco volcanic complex rhyolite suite is part of a regional southern Puna, arc-related ignimbrite group. The suite defines a tight group of consanguineous siliceous magmas that serves as a model for the evolution of arc-related, caldera-forming silicic magma systems in the region and elsewhere. These data indicate that the rhyolites originated through limited assimilation of and mixing with upper-crustal lithologies by regional basaltic andesite parent materials, followed by extensive fractional crystallization. Least squares models of major elements in tandem with Rayleigh fractionation models for trace elements reveal that the internal variations among the rhyolites through time can be derived by extensive fractionation of a quartz–two feldspar (granitic minimum) assemblage with limited assimilation. The rare earth element character of local volumes of melt in some samples of the Campo Piedra Pomez ignimbrite basal fallout requires significant fractionation of amphibole. The distinctive major- and trace-element characteristics of bulk rock and matrix of the Campo Piedra Pomez and Cerro Blanco tephras provide useful geochemical fingerprints to facilitate regional tephrochronology. Available data indicate that rhyolites from other neighborhood centers, such as Cueros de Purulla, share bulk chemical characteristics with the Campo Piedra Pomez ignimbrite rhyolites, but they appear to be isotopically distinct. Pre-eruptive storage and final equilibration of the rhyolitic melts were estimated from matrix glass compositions projected onto the haplogranitic system (quartz-albite-orthoclase-H2O) and using rhyolite-MELTS models. These revealed equilibration pressures between 360 and 60 MPa (~10–2 km depth) with lowest pressures in the Holocene eruptions. Model temperatures for the suite ranged from 695 to 790 °C. Integrated together, our results reveal that the Cerro Blanco volcanic complex is a steady-state (low-magmatic-flux), arc-related complex, standing in contrast to the flare-up (high-magmatic-flux) supervolcanoes that dominate the Neogene volcanic stratigraphy. The silicic magmas of the Cerro Blanco volcanic complex were derived more directly from mafic and intermediate precursors through extensive fractional crystallization, albeit with some mixing and assimilation of local basement. Geochemical models and pressure-temperature estimates indicate that significant volumes of remnant cumulates of felsic and intermediate composition should dominate the polybaric magma complex beneath the Cerro Blanco volcanic complex, which gradually shallowed through time. Evolution to the most silicic compositions and final equilibration of some of the postcaldera domes occurred during ascent and decompression at depths less than 2 km. Our work connotes an incrementally accumulated (over at least 54 k.y.), upper-crustal pluton beneath the Cerro Blanco volcanic complex between 2 and 10 km depth. The composition of this pluton is predicted to be dominantly granitic, with deeper parts being granodioritic to tonalitic. The progressive solidification and eventual contraction of the magma complex may account for the decades of deflation that has characterized Cerro Blanco. The presently active geothermal anomaly and hydrothermal springs indicate the Cerro Blanco volcanic complex remains potentially active.

The Architecture of the Southern Puna Magmatic System: Integrating Seismic and Petrologic Observations With Geochemical Modeling

AbstractImprovements in geochemical, petrologic, and geophysical methods and data have led to a paradigm‐shift in understanding magmatic system architecture away from isolated magmatic chambers toward transcrustal “mushy” magmatic systems. Geochemical and petrologic studies indicate that magma storage and differentiation occurs over a range of depths throughout the crust, while geophysical techniques delineate in situ spatial distributions of melt at similar scales. However, seismic properties are nonunique with respect to composition and melt percentage, and must be integrated with geochemical observations to better understand the architecture and chemical evolution of magmatic systems. The southern Puna Plateau represents a locality where extensive geochemical and seismic data can be combined to investigate the role of depth‐dependent processes in generating compositionally diverse volcanism. By combining geochemical modeling with seismic images of shear‐wave velocity and attenuation, we find that magma storage and differentiation occurs predominantly at two depth ranges: (a) near the base of the crust (>40 km), where shear‐wave velocities are anomalously slow compared to what is expected (<4.0 km/s) and geochemical signatures indicate fractional crystallization and crustal assimilation in the garnet stability field, and (b) in the mid‐crust (∼20 km) beneath the Cerro Galan Caldera, where shear‐wave velocities necessitate the presence of melt (∼2.7 km/s) and europium (Eu) anomalies indicate low‐pressure differentiation. These geochemical and seismic results provide evidence for multi‐level melt evolution and are consistent with petrological features of exposed arcs, whose transcrustal architecture is characterized by discrete and compositionally evolved intrusive bodies in the mid‐crust and a compositionally heterogeneous lower crust.

Full Waveform Inversion Beneath the Central Andes: Insight Into the Dehydration of the Nazca Slab and Delamination of the Back‐Arc Lithosphere

AbstractWe present a new seismic tomography model for the crust and upper mantle beneath the Central Andes based on multi‐scale full seismic waveform inversion, proceeding from long periods (40–80 s) over several steps down to 12–60 s. The spatial resolution and trade‐offs among parameters are estimated through the multi‐parameter point‐spread functions. P‐ and S‐wave velocity structures with spatial resolution of 30–40 km for the upper mantle and 20–25 km for the crust could be resolved in the central study region. In our study, the subducting Nazca slab is clearly imaged in the upper mantle, with dip‐angle variations from the north to the south. Bands of low velocities in the crust and mantle wedge indicate intense crustal partial melting and hydration of the mantle wedge beneath the frontal volcanic arc, respectively, and they are linked to the vigorous dehydration from the subducting Nazca plate and intermediate depth seismicity within the slab. These low‐velocity bands are interrupted at 19.8°–21°S, both in the crust and uppermost mantle, hinting at the lower extent of crustal partial melting and hydration of the mantle wedge. The variation of lithospheric high‐velocity anomalies below the back‐arc from north to south allows insight into the evolutionary foundering stages of the Central Andean margin. A high‐velocity layer beneath the southern Altiplano suggests underthrusting of the leading edge of the Brazilian Shield. In contrast, a steeply westward dipping high‐velocity block and low‐velocity lithospheric uppermost mantle beneath the southern Puna plateau hint at the ongoing lithospheric delamination.

Geology, geochemistry and geochronology of Lindero porphyry gold deposit in the Southern Puna plateau, Argentina

The Lindero gold deposit is located in the Southern Puna plateau, northwest Argentina. The deposit is centered in a cluster of six subvolcanic intrusions emplaced at the margin of the Arizaro Basin. Three alteration types were recognized: (i) Ca–Na silicate (clinopyroxene + magnetite + K-feldspar + quartz + calcite ± plagioclase), (ii) K-silicate (K-feldspar + quartz + magnetite ± biotite ± anhydrite) and (iii) chlorite-calcite alteration. The highest ore grades are linked to the K-silicate alteration. The proven plus probable reserves of Lindero are 84,226 t with average grades of 0.63 g/t Au and 0.11% Cu. A previous study assigned Lindero to the iron oxide copper-gold (IOCG) deposit type but many features of Lindero suggest that it is a porphyry gold deposit, including (i) the temporal and spatial link between alteration and the intrusive bodies, (ii) the alteration distribution pattern, particularly the small volume of rock affected by Ca–Na silicate alteration, (iii) the Au-rich and Cu-poor mineralization style.The magmatic complex at Lindero comprises an early-mineral unit (FPD), four inter-mineral units (CPD1, PBFD, CPD2 and DDP) and one post-mineral unit (PMI). In-situ U/Pb SIMS dating of the oldest (FPD), an intermediate (PBFD) and the youngest (DDP) intrusive units, confirms a middle Miocene age. The weighted mean ages of the oldest and youngest units are indistinguishable, with 15.36 ± 0.13 Ma (n = 21) and 15.47 ± 0.11 Ma (n = 16), respectively. Individual ages from each unit range by ~1 m.y. and the overall spread of zircon ages is 15.92 ± 0.23 to 14.44 ± 0.33 Ma. We suggest that emplacement of the subvolcanic stocks took place within this span time, likely at the lower end of this range (15.0–14.4 Ma). Two 40Ar/39Ar ages of hydrothermal biotites from the K-silicate alteration (14.99 ± 0.16 Ma and 14.93 ± 0.12 Ma) indicate that hydrothermal alteration began practically simultaneously with the emplacement of the porphyry units.All of the intrusive units are similar compositionally. They show a fine- to medium-grained porphyric texture (1–4 mm) comprising plagioclase, amphibole, clinopyroxene and scarce quartz phenocrysts (40–55 vol % of phenocrysts) in a K-feldspar ± quartz microcrystalline (0.02–0.07 mm) groundmass, except in the post-mineral unit which has a cryptocrystalline groundmass. Whole-rock analyses reveal a narrow range of dioritic composition (58.6–61.9 wt % SiO2) and high-K calc-alkaline character for all units. Trace element features (low Ba/Nb ratios, high Nb) of the Lindero magmas indicate a back-arc affinity, similar to those from the Southern Puna and distinct from the Central Volcanic Zone (CVZ) frontal arc. The Sr and Nd isotope ratios (87Sr/86Sr = 0.706042 to 0.706607; 143Nd/144Nd = 0.512501 to 0.512582) from Lindero intrusives are also similar to Southern Puna back-arc volcanic rocks. The Pb isotope ratios of Lindero (206Pb/204Pb = 18.79 to 18.83; 207Pb/204Pb = 15.60 to 15.63; 208Pb/204Pb = 38.66 to 38.74) overlap with both back-arc and arc magmas in the CVZ. The narrow age range, spatial association and uniform chemical and isotopic composition of Lindero porphyry units suggest that were derived from a common magma source, which underwent fractionation and/or crustal assimilation before emplacement as suggest by the low concentrations of Mg, Cr, Ni and Sr. The Lindero porphyry units show chemical and isotopic similarities with those from porphyry gold deposits in the Maricunga belt, Chile, and with the porphyry copper deposits of Argentina located in a back-arc setting; however, they differ from porphyry copper deposits in the frontal-arc setting of Chile, notably by the lack of an adakite-like signature (high Sr/Y ratio).

Lithospheric Delamination Beneath the Southern Puna Plateau Resolved by Local Earthquake Tomography

AbstractWe present a local earthquake tomography to illuminate the crustal and uppermost mantle structure beneath the southern Puna plateau and to test the delamination hypothesis. Vp and Vp/Vs ratios were obtained using travel time variations recorded by 75 temporary seismic stations between 2007 and 2009. In the upper crust, prominent low Vp anomalies are found beneath the main volcanic centers, indicating the presence of magma and melt beneath the southern Puna plateau. Beneath the Moho at around 90‐km depth, a strong high Vp anomaly is detected just west of the giant backarc Cerro Galan ignimbrite caldera. This high Vp anomaly is only resolved if earthquakes with an azimuthal gap up to 300° are included in the inversion. However, we show through data subset and synthetic tests that the anomaly is robust due to our specific station‐event geometry and interpret it as a delaminated block of lower crust and uppermost mantle lithosphere under the southern Puna plateau. The low velocities in the crust are interpreted as a product of the delamination event that triggered the rise of fluids and melts into the crust and induced the high topography in this part of the plateau. The tomography also reveals the existence of low‐velocity anomalies that link arc magmatism at the Ojos del Salado volcanic center with slab seismicity clusters at depths of about 100 and 150 km and support fluid transport in the mantle wedge due to dehydration reaction within the subducted slab.

Carbon dioxide diffuse degassing as a tool for computing the thermal energy release at Cerro Blanco Geothermal System, Southern Puna (NW Argentina)

This work presents the first carbon dioxide diffuse degassing survey carried out in the Cerro Blanco Geothermal System (CBGS; Southern Puna plateau, NW Argentina) with the aim to estimate the thermal energy release. The survey was divided into (i) a prospecting stage of the degassing sites within the CBGS and (ii) mapping of the selected diffuse degassing sites. The purpose of the prospecting stage was to elaborate two transects, crosscutting the nested caldera of Cerro Blanco Volcanic Complex (CBVC) and Los Hornitos thermal site, both belonging to CBGS. More than 60 soil diffuse CO2 flux and soil temperature measurements were carried out in the 16-km and 1-km long transects. Significant soil diffuse emissions were only found within the Cerro Blanco caldera, where a detailed mapping of CO2 flux was produced. In this site, named CBa, a single diffuse degassing structure releasing 22.44 kg d−1 of deep-sourced CO2 into the atmosphere was identified. Any other geologic feature of the CBVC and Los Hornitos hydrothermal site presented very low CO2 flux values. According to statistical and geochemical analyses, soil diffuse CO2 degassing is fed by a deep-seated (hydrothermal) and a soil respiration source, which mix each other at different degree. The thermal energy release associated with the diffuse degassing process at CBa is estimated to be ~2.4 kJ/s. This low magnitude thermal energy release is probably a consequence of an efficient cap-rock that likely buffer the surficial expressions of the geothermal resource potentially occurring at depth.

Multi-banded pumice in the Campo de la Piedra Pómez rhyolitic ignimbrite (Southern Puna plateau): Pre-eruptive physical and chemical interactions between mafic and rhyolitic melts

The rhyolitic Campo de la Piedra Pómez ignimbrite crops out in the Southern Puna of NW Argentina and it is related to the youngest caldera-complex (Cerro Blanco caldera complex) of the Central Andes (73 - 4 kyr). The presence of rhyolitic pumice and mafic enclaves with different compositional and textural features, which variability can be observed within a single juvenile clast (multiple-banded pumice), characterized these deposits. The enclaves are associated with hybrid (trachydacitic) pumice and sporadic remnants of rhyolitic material included in the trachydacite. To unravel the possible role of the mafic recharge as eruption trigger, the occurrence of mixing events and the mechanisms of enclave formation, we studied the enclaves and silicic pumice material (petrography, whole rock analyses, mineral and glass chemistry) to decipher the magmatic interaction between the host rhyolitic melt and the enclave-forming magmas. Results allowed recognizing two main mafic recharge events. During the first episode, the mixing of the rhyolite with the injecting magma generated sporadic dacitic products. Mixing was favored by the relatively high temperature of both the injecting magma and the rhyolitic melt, as revealed by clinopyroxene-liquid, plagioclase-liquid and two-pyroxene geothermometers (≥875 °C). The second mafic recharge event involved magma that remained confined at the bottom of the reservoir and crystallized with differential cooling rates. At the interface with the silicic host, the magma generated sub-millimetric mineral assemblage in which amphibole has normally zoned rims. Differently, within the body of the mafic intrusion, crystallization proceeded with a lower undercooling degree, generating a coarser crystalline assemblage in which amphibole crystals do not display zoning. The convergence of different thermobarometric models (applied to the rhyolite, trachydacite, and enclaves) suggests that these magmas interacted at a crustal depth of ca. 2.7 Kbar, here interpreted as the base of the Campo de la Piedra Pómez rhyolitic reservoir (~10 Km b.s.l.). A time lapse occurred between the last mafic recharge and the eruptive events, where the felsic magma cooled down to ~800 °C and the amphibole re-equilibration took place.

Eruptive style and flow dynamics of the pyroclastic density currents related to the Holocene Cerro Blanco eruption (Southern Puna plateau, Argentina)

The Pleistocene-Holocene Cerro Blanco Volcanic Complex (CBVC), one of the youngest caldera complexes in the Southern Central Andes, is the source of possibly one of largest Holocene eruptions on Earth, the 4.2 ka, Cerro Blanco eruption. This caldera forming eruption is the younger of two major explosive events from the CBVC. Previous work has estimated the range from VEI 6 to 7, yet to date there is no detailed study of the stratigraphy and volcanology of the proximal deposits and dynamics of the Cerro Blanco eruption. Here we present the first detailed analysis of the eruptive products of the Holocene Cerro Blanco eruption that reveal the eruptive sequence highlighting the flow dynamics of the related pyroclastic density currents (PDCs). The PDCs were mainly inertia-dominated, however, channelization of parental PDCs into deep valleys resulted in the flow transformation to forced convection-dominated flows. In addition, topographic constriction in valleys enhanced the sedimentation rate producing regressive bed forms and ultimately the avulsion of the main path of the PDCs resulting in flooding of secondary valleys. A model is presented whereby simultaneous convective and collapsing eruptive column dynamics were established and sustained throughout the eruption. Towards its end, instabilities of the column occurred in response to the climax of a protracted incremental caldera collapse. This eruptive sequence is similar to those observed in well-documented small collapse calderas. An important unresolved issue for the CB eruption is it volume. The currently estimated volume of 83 km3 (DRE) by Fernando-Turiel et al. (2019) is inconsistent with the size of the Cerro Blanco caldera and to date the over thickening of the distal ash by local rework is poor assessed. Further work is needed to fully evaluate this mismatch and accurately estimate the volume of this important Holocene eruption.

Geomorphology and spatial distribution of monogenetic volcanoes in the southern Puna Plateau (NW Argentina)

Monogenetic volcanoes are the most common volcanic landforms on Earth. Several works have shown that the study of geomorphology and spatial distribution of such small volcanoes reveals important aspects related to the volcanic field, from eruption dynamics to tectonic processes. The Altiplano-Puna Plateau (15° to 27° S, Central Andes) comprises the largest non-collisional plateau on Earth. Characterized by intensive deformation, uplift, and volcanism, this region is marked by the presence of hundreds of mafic monogenetic volcanoes. Despite its discrete occurrence, the monogenetic volcanoes show a close relationship with large scale, lithospheric foundering events in the region. In this contribution we applied a Geographic Information System (GIS) approach in order to build the first complete catalog of monogenetic landforms present in the southern Puna Plateau (24° 30′ to 27° 00′ S, NW Argentina), exploring their evolution and relationship with geotectonic events. Combining spatial analysis with a geomorphological study of each volcano, it was possible to identify the presence of 285 monogenetic volcanoes in the region, which could be grouped in seven clusters. Southern Puna shows a predominance of cinder cones and lava domes, suggesting a small influence of water in the eruption dynamics. Despite its high clustering degree, southern Puna monogenetic vents present a low density (8.3 × 10−3 vents/km2) when compared to other monogenetic fields around the world. In addition, Puna Plateau cones show a close relationship with monogenetic cones associated with an extensional environment, considering their back-arc context. Based on relative and available absolute ages, it was possible to map the volcanism evolution through time. The results indicate that the first monogenetic volcanoes appeared in the central region of the Plateau (Sierra de Calalaste), mainly associated with large-scale NNE-SSW reactivated faults. The activity then expanded (Antofalla and Antofagasta regions) and, finally reached the borders of the Plateau (Pasto Ventura and Arizaro regions), which show volcanoes associated with NW-SE normal faults.

Obsidian circulation in south-central Andes after ca. 1100 BP: A contribution based on geochemical studies in Argentinean Southern Puna Plateau

This work investigates questions regarding obsidian circulation among the late populations (since ca. 1100 BP) of Antofagasta de Sierra (Catamarca Province, Argentinean Southern Puna Plateau). By means of geochemical provenance studies conducted on obsidian artefacts, the sources of obsidian that these groups accessed have been identified. The samples considered in this study were collected at three radiocarbon-dated sites in the micro-region: La Alumbrera, Bajo del Coypar II and Campo Cortaderas.The results obtained are compared with those available for other contemporary sites and Formative contexts (ca. 3000–1100 BP) within the area of study. A preliminary discussion is presented on the basis of the environmental, political and social tendencies that characterized the times after ca. 1100 BP at the micro-region and south-central Andes.

First data on volume and type of deflated sediment from Southern Puna Plateau and its role as source of the Chaco-Pampean loess

Abstract This paper presents the first attempt at qualitatively and quantitatively evaluating the sedimentary contribution of the Puna plateau to the loess and soils of the Argentinean Pampas. The origin of the Pampean sediment has been traditionally ascribed to northern Patagonia, but new evidence supports a dual supply: western Central Andes/North Patagonia and Puna/Altiplano Plateau. Three different lines of evidence show here that Puna dust supply is active and produces significant sedimentary deposition up to places like Buenos Aires city. The first evidence is meteorological and we show here one event related to an abnormal rainfall that deposited large quantities of a reddish-brownish silt eroded out from the Puna, calling the attention of the environmental agency of Buenos Aires city. The second evidence is the sampling of active wind transport collecting extremely coarse material carried out in the saltation layer, suggesting the fine grained material escapes the area as fugitive dust. This also demonstrates that observed gravel-bearing bedforms of the Puna are active, and that the transport system seems to be activated by polar fronts only. Results show that the saltation curtain is abnormally thick and coarse, suggesting that the kinetic energy peaks ca. 7–10 times higher than for normal saltation, while the transported grain size coincides with the lagged sediment in this basin composing different bedforms, varying between sandy gravel and gravelly sand. The third evidence is the quantification of the exported material by defining a reference surface as the initial time of evolution, and the present day surface. The initial surface is an alluvial surface of an estimated age of 11–12 ka. BP. Volume balance was achieved by a detailed topographic, stratigraphic and geophysical survey, reconstructing old alluvial and present surfaces. Two gridding methods were used, and more conservative results of the volume balance, indicate a total erosion of ca. 3.386 hm 3 of sediment in 72 Ha, spliced in 0.963 hm 3 of sediment that was left within the place forming gravel dunes, while the remaining volume of ca. 2.423 hm 3 is sediment truly exported from the surveyed area. Results imply a continuous blanket of ca. 4 m thick of sediment exported out of this basin in the Holocene. These findings prove that the Puna contribution to the Chaco-Pampean plain is significant, although not unique. Thus, true proportions of sediment contribution to soils and loess from these multiple sources should be better evaluated in future studies.