Aerodynamic Roughness Height of Gravel-covered Plains in the Puna of Argentina

Plains covered by gravel-dominated desert pavement in the Puna of Argentina have an aerodynamic roughness height (or length) z0 of ∼1 cm, likely representing a skimming flow regime above the closely spaced gravel particles. Aerodynamic roughness height locally may transition from that of skimming flow over the gravels to a z0 that includes the effects of obstacles considerably larger than those of the gravel particles alone. Among large (>60 cm tall) megaripples, z0 is elevated beyond that of the gravels alone to values of 2–4 cm. These results represent an analog for an improved understanding of the aerodynamics of gravel-dominated desert pavement and megaripples documented by multiple rovers on Mars.Fil: Zimbelman, James R.. National Air and Space Museum; Estados UnidosFil: Spagnuolo, Mauro Gabriel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Estudios Andinos "Don Pablo Groeber". Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Estudios Andinos "Don Pablo Groeber"; ArgentinaFil: de Silva, Shanaka. State University of Oregon; Estados Unido

Quickening the pulse: Fractal tempos in continental arc magmatism

The magmatic history of a continental arc can be characterized as punctuated equilibrium, whereby long periods of low-level activity are interrupted periodically by short bursts of high-volume magmatism (“flare-ups”). Geochronological records, most notably from zircon, reveal episodicity in volcanism, pluton formation, and detrital sedimentation in, and associated with, arc segments and volcano-plutonic suites. Distinct tempos can be recognized at all resolvable spatial and temporal scales and are broadly fractal, with each scale reflecting the timescale of processes occurring at different levels in the arc crust. The tempos of continental arc magmatism thus reflect modulation of the mantle-power input as it is progressively filtered through the continental crust

On synchronous supereruptions

The Youngest Toba Tuff (YTT) supereruption from Toba Caldera in Sumatra at ca. 74,000 years BP is the largest volcanic event recorded in the Pleistocene. Intriguingly, recent radioisotopic dating of the near antipodal Los Chocoyos (LCY) supereruption from the Atitlán caldera in Guatemala finds an identical age within uncertainties to that of YTT. This opens the question of whether these synchronous supereruptions may be a coincidence or could be a consequence of each other? Using the known eruptive record from the past 2 Myr, we find that the likelihood of having two near antipodal supereruptions (>1,000 km3 tephra volume) within centuries (<400 years), as suggested by volcanic proxies and annual counting layer chronology in the ice core records, is very small (0.086%), requiring a non-random cause and effect. Considering this analysis, we speculate that one potential physical mechanism that could explain the temporal relationship between these supereruptions is that seismic energy released during YTT eruption focused on the antipodal region, where concentrated stresses ultimately promoted the eruption of the perched LCY magma system (or vice versa). This supereruption “double-whammy” may thus be the more compelling source of the significant environmental impacts often attributed individually to the YTT supereruption. Improving the existing age information of YTT and LCY, and a better understanding of caldera collapse events will enable further testing of the hypothesis that synchronous supereruptions do not result by pure chance

Correlation of ignimbrites using characteristic remanent magnetization and anisotropy of magnetic susceptibility, Central Andes, Bolivia

Large ignimbrite flare-ups provide records of profound crustal modification during batholith formation at depth. The locations of source calderas and volumes and ages of the eruptions must be determined to develop models for the tectonomagmatic processes that occur during these events. Although high-precision isotopic ages of the ignimbrites are critical, less expensive and more rapid techniques, such as paleomagnetism, can extend the temporal information from dated outcrops. Paleomagnetic and rock magnetic data, including characteristic remanent magnetization (ChRM) and anisotropy of magnetic susceptibility (AMS), from the Altiplano-Puna Volcanic Complex of the Central Andes reliably identify calderas and eight associated Mio-Pliocene ignimbrites. ChRM results indicate a larger between-site error for most ignimbrites, in comparison to within-site scatter. Part of this dispersion may be due to tumescence/detumescence associated with the caldera-forming eruptions, but most of the effect is probably due to the recording of paleosecular variation during cooling and vapor-phase crystallization of the thick ignimbrites. AMS data identify the source calderas for four ignimbrites and provide limits on possible post-emplacement rotations of the deposits. AMS data indicate significant topographic control on inferred flow directions, implying that the flows were dense and/or of low mobility

The role of crustal and eruptive processes versus source variations in controlling the oxidation state of iron in Central Andean magmas

The composition of the continental crust is closely tied to subduction zone magmatism. Elevated oxygen fugacity (fO₂) plays a central role in fostering crystallization of oxide minerals and thereby aids in generating the calc-alkaline trend of iron depletion that characterizes the continents. Along continental margins, arc magmas erupt through continental crust and often undergo extensive differentiation that may modify magmatic fO₂. The importance of the subducting slab and mantle wedge relative to the effects of this differentiation on the fO₂ recorded by continental arc magmas remains relatively unconstrained. Here, we focus on the effect of differentiation on magmatic fO₂ using a suite of 14 samples from the Central Volcanic Zone (CVZ) of the Andes where the continental crust is atypically thick (60–80 km). The samples range in composition from ∼55 to 74 wt% SiO₂ and represent the Neogene history of the arc. Samples are basaltic andesite to rhyolite and span a range of radiogenic isotopic compositions (⁸⁷Sr/⁸⁶Sr = ∼0.705–0.712) that represent 30 to 100% crustal assimilation. We use several proxies to estimate the fO₂ recorded by lavas, pumice, and scoria: (1) whole rock Fe³⁺/ΣFe ratios, (2) Fe³⁺+/ΣFe ratios in quartz-hosted melt inclusions, and (3) Fe–Ti oxide oxygen-barometry. Comparison of the fO₂ calculated from bulk Fe³⁺/ΣFe ratios (post-eruptive) with that derived from Fe–Ti oxides or melt inclusion Fe³⁺/ΣFe ratios (pre-eruptive), enables us to quantify the effect of syn- or post-eruptive alteration, and to select rocks for bulk analysis appropriate for the determination of pre-eruptive magmatic fO₂ using a strict criterion developed here. Across our sample suite, and in context with samples from the literature, we do not find evidence for systematic oxidation due to crystal fractionation or crustal contamination. Less evolved samples, ranging from 55 to 61 wt% SiO₂, record a range of >3 orders of magnitude in fO₂, spanning the fO₂ range recorded by all samples in our suite. Among these less evolved magmas, we find that those erupted from volcanic centers located closer to the trench, closer to the Benioff Zone, and with more geochemical evidence of subducted components in the mantle source (elevated La/Nb) result in magmas that record systematically higher fO₂. We conclude that the slab/mantle source can exert greater control on magmatic fO₂ than processes occurring in even the thickest continental crust. Thus, the fO₂ of arc magmas, and hence their calc-alkaline nature, may be inherited from the mantle.Keywords: oxygen fugacity, wet chemistry, subduction, crustal assimilation, Central Andean volcanic zoneKeywords: oxygen fugacity, wet chemistry, subduction, crustal assimilation, Central Andean volcanic zon

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

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 Laketype 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 ntegrated 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 pressuretemperature 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.Fil: de Silva, Shanaka. State University of Oregon; Estados UnidosFil: Roberege, Julie. Instituto Politécnico Nacional;Fil: Bardelli, Lorenzo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Bio y Geociencias del NOA. Universidad Nacional de Salta. Facultad de Ciencias Naturales. Museo de Ciencias Naturales. Instituto de Bio y Geociencias del NOA; ArgentinaFil: Baez, Walter Ariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Bio y Geociencias del NOA. Universidad Nacional de Salta. Facultad de Ciencias Naturales. Museo de Ciencias Naturales. Instituto de Bio y Geociencias del NOA; ArgentinaFil: Ortiz Yañez, Agustín. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Bio y Geociencias del NOA. Universidad Nacional de Salta. Facultad de Ciencias Naturales. Museo de Ciencias Naturales. Instituto de Bio y Geociencias del NOA; ArgentinaFil: Viramonte, Jose German. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Bio y Geociencias del NOA. Universidad Nacional de Salta. Facultad de Ciencias Naturales. Museo de Ciencias Naturales. Instituto de Bio y Geociencias del NOA; ArgentinaFil: Arnosio, José Marcelo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Bio y Geociencias del NOA. Universidad Nacional de Salta. Facultad de Ciencias Naturales. Museo de Ciencias Naturales. Instituto de Bio y Geociencias del NOA; ArgentinaFil: Becchio, Raul Alberto. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Bio y Geociencias del NOA. Universidad Nacional de Salta. Facultad de Ciencias Naturales. Museo de Ciencias Naturales. Instituto de Bio y Geociencias del NOA; Argentin

Small-Scale Disequilibrium in a Magmatic Inclusion and its More Silicic Host

Basaltic andesite inclusions and their host dacite from the Purico-Chascon complex in northern Chile are isotopically distinct. Textural characteristics of the inclusions are typical of those resulting from magma mingling. Serial sectioning across the interface of an inclusion and its host dacite, complemented by microdrill sampling and detailed microprobe work, has enabled us to examine the scale of mixing and chemical (isotopic and trace element) disequilibrium. The results of this work show that (1) the composition of the inclusion is relatively homogeneous; (2) the dacite host is generally higher in ⁸⁷Sr/⁸⁶Sr and lower in ¹⁴³Nd/¹⁴⁴Nd than the enclave, but it is heterogeneous on a small scale, and probably a “hybrid”; (3) the isotopic composition in the marginal zone, apparently on both host and inclusion sides of the weakly chilled interface, actually shows the highest ⁸⁷Sr/⁸⁶Sr and lowest ¹⁴³Nd/¹⁴⁴Nd; (4) large plagioclase crystals in the inclusions and host are xenocrystic, with higher ⁸⁷Sr/⁸⁶Sr than any of the other samples. These observations are reconciled with a model of magma evolution in a crustal magma chamber. In such a scenario the mafic magma is overlain by a cap of rhyolite - a partial melt of the dacitic ignimbrites which now underlie the Purico-Chascon complex. Overturn of such a magma system gives rise to a hybrid dacite containing discrete mafic inclusions

Resurgent Toba—field, chronologic, and model constraints on time scales and mechanisms of resurgence at large calderas

Highlights: New data reveal for the first time a history of the last ∼33.7 ky of uplift of Samosir. Minimum uplift rates were high (4.9 cm/year) for the first 11.2 ky but diminished after that to <1 cm/year for the last 22.5 ky. Numerical modeling suggests that rebound of remnant magma augmented by deep recharge appears to be the most likely driver for uplift. Detumescence makes a negligible contribution to resurgent uplift. The volume of the resurgent dome is isostatically compensated by magma. Average rates of uplift at Toba are much lower than currently restless calderas indicating a distinction between resurgence and “restlessness”. New data reveal details of the post-caldera history at the Earth’s youngest resurgent supervolcano, Toba caldera in Sumatra. Resurgence after the caldera-forming ∼74 ka Youngest Toba Tuff eruption uplifted the caldera floor as a resurgent dome, Samosir Island, capped with 100m of lake sediments. ¹⁴C age data from the uppermost datable sediments reveal that Samosir Island was submerged beneath lake level (∼900 m a.s.l) at 33 ka. Since then, Samosir experienced 700 m of uplift as a tilted block dipping to the west. ¹⁴C ages and elevations of sediment along a transect of Samosir reveal that minimum uplift rates were ∼4.9 cm/year from ∼33.7 to 22.5 ka, but diminished to ∼0.7 cm/year after 22.5 ka. Thermo-mechanical models informed by these rates reveal that detumescence does not produce the uplift nor the uplift rates estimated for Samosir. However, models calculating the effect of volume change of the magma reservoir within a temperature-dependent viscoelastic host rock reveal that a single pulse of ∼475 km³ of magma produces a better fit to the uplift data than a constant flux. The cause of resurgent uplift of the caldera floor is rebound of remnant magma as the system re-established magmastatic and isostatic equilibrium after the caldera collapse. Previous assertions that the caldera floor was apparently at 400 m a.s.l or lower requires that uplift must have initiated between sometime between 33.7 and 74 ka at a minimum average uplift rate of ∼1.1 cm/year. The change in uplift rate from pre-33.7 ka to immediately post-33.7 ka suggests a role for deep recharge augmenting rebound. Average minimum rates of resurgent uplift at Toba are at least an order of magnitude slower than net rates of “restlessness” at currently active calderas. This connotes a distinction between resurgence and “restlessness” controlled by different processes, scales of process, and controlling variables.This is the publisher’s final pdf. The published article is copyrighted by the author(s) and published by the Frontiers Research Foundation. The published article can be found at: http://journal.frontiersin.org/journal/earth-science.Keywords: Magmastatic equilibrium, Resurgence, Lake sediments, Magmatic intrusion, Numerical modeling, Toba caldera, Carbon-14 dating, Remnant magm

Origin of the Medusae Fossae Formation, Mars: Insights from a synoptic approach

The geologic origin of the Medusae Fossae Formation (MFF) has remained a mystery despite three decades of research. To better constrain its formation, an in-depth analysis of observations made in the literature was combined with a new survey of over 700 Mars Orbiter Camera narrow-angle images of the MFF to identify morphologic characteristics and material properties that define this formation as a whole. While previous work has identified clear agreement on some characteristics, our analysis identifies yardangs, collapse features, and layering as pervasive features of the MFF. Whereas collapse features and layering may implicate several different physical and chemical processes, yardangs provide vital information on material properties that inform about mechanical properties of the MFF lithology. Aspect ratios of megayardangs range from 3:1 to 50:1, and slope analyses reveal heights of up to 200 m with cliffs that are almost vertical. Other yardangs show lower aspect ratios and topographic profiles. These characteristics coupled to the presence of serrated margins, suggest that MFF lithology must be of weakly to heavily indurated material that lends itself to jointing. The characteristics and properties of the MFF are inconsistent with those of terrestrial pyroclastic fall deposits or loess, but are in common with large terrestrial ignimbrites, a hypothesis that explains all key observations with a single mechanism. Yardang fields developed in regionally extensive ignimbrite sheets in the central Andes display morphologic characteristics that correlate with degree of induration of the host lithology and suggest an origin by pyroclastic flow for the MFF

Probabilistic Volcanic Hazard Assessment of the 22.5–28°S Segment of the Central Volcanic Zone of the Andes

Evaluation of volcanic hazards typically focusses on single eruptive centres or spatially restricted areas, such as volcanic fields. Expanding hazard assessments across wide regions (e.g., large sections of a continental margin) has rarely been attempted, due to the complexity of integrating temporal and spatial variability in tectonic and magmatic processes. In this study, we investigate new approaches to quantify the hazards of such long-term active and complex settings, using the example of the 22.5–28°S segment of the Central Volcanic Zone of the Andes. This research is based on the estimation of: 1) spatial probability of future volcanic activity (based on kernel density estimation using a new volcanic geospatial database), 2) temporal probability of future volcanic events, and 3) areas susceptible to volcanic flow and fall processes (based on computer modeling). Integrating these results, we produce a set of volcanic hazard maps. We then calculate the relative probabilities of population centres in the area being affected by any volcanic phenomenon. Touristic towns such as La Poma (Argentina), Toconao (Chile), Antofagasta de la Sierra (Argentina), Socaire (Chile), and Talabre (Chile) are exposed to the highest relative volcanic hazard. In addition, through this work we delineate five regions of high spatial probability (i.e., volcanic clusters), three of which correlate well with geophysical evidence of mid-crustal partial melt bodies. Many of the eruptive centres within these volcanic clusters have poorly known eruption histories and are recommended to be targeted for future work. We hope this contribution will be a useful approach to encourage probabilistic volcanic hazard assessments for other arc segments.Fil: Bertin, Daniel. University of Auckland; Nueva ZelandaFil: Lindsay, Jan M.. University of Auckland; Nueva ZelandaFil: Cronin, Shane J.. University of Auckland; Nueva ZelandaFil: de Silva, Shanaka L.. State University of Oregon; Estados UnidosFil: Connor, Charles B.. University of Florida; Estados UnidosFil: Caffe, Pablo Jorge. Universidad Nacional de Jujuy. Instituto de Ecorregiones Andinas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Ecorregiones Andinas; ArgentinaFil: Grosse, Pablo. Fundación Miguel Lillo; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Tucumán; ArgentinaFil: Báez, Walter. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Bio y Geociencias del NOA. Universidad Nacional de Salta. Facultad de Ciencias Naturales. Museo de Ciencias Naturales. Instituto de Bio y Geociencias del NOA; ArgentinaFil: Bustos, Emilce. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Bio y Geociencias del NOA. Universidad Nacional de Salta. Facultad de Ciencias Naturales. Museo de Ciencias Naturales. Instituto de Bio y Geociencias del NOA; ArgentinaFil: Constantinescu, Robert. University of Florida; Estados Unido