The Cerro Morado Andesites: Volcanic history and eruptive styles of a mafic volcanic field from northern Puna, Argentina     

A.P. Cabrera  P.J. Caffe   《Journal of South American Earth Sciences》2009,28(2):113-131

The Upper Miocene Cerro Morado Andesites constitutes a mafic volcanic field (100 km²) composed of andesite to basaltic andesite rocks that crop out 75 km to the east from the current arc, in the northern Puna of Argentina. The volcanic field comprises lavas and scoria cones resulting from three different eruptive phases developed without long interruptions between each other. Lavas and pyroclastic rocks are thought to be sourced from the same vents, located where orogen-parallel north-south faults crosscut transverse structures.The first eruptive phase involved the effusion of extensive andesitic flows, and minor Hawaiian-style fountaining which formed subordinate clastogenic lavas. The second phase represents the eruption of slightly less evolved andesite lavas and pyroclastic deposits, only distributed to the north and central sectors of the volcanic field. The third phase represents the discharge of basaltic andesite magmas which occurred as both pyroclastic eruptions and lava effusion from scattered vents distributed throughout the entire volcanic field. The interpreted facies model for scoria cones fits well with products of typical Strombolian-type activity, with minor fountaining episodes to the final stages of eruptions.Petrographic and chemical features suggest that the andesitic units (SiO₂ > 57%) evolved by crystal fractionation. In contrast, characteristics of basaltic andesite rocks are inconsistent with residence in upper-crustal chambers, suggesting that batches of magmas with different origins or evolutive histories arrived at the surface and erupted coevally.Based on the eruptive styles and lack of volcanic quiescence gaps between eruptions, the Cerro Morado Andesites can be classified as a mafic volcanic field constructed from the concurrent activity of several small, probably short-lived, monogenetic centers.  相似文献   

Contribution of non-pollen palynomorphs to the paleolimnological study of a high-altitude Andean lake (Laguna Verde Alta,Venezuela)     

Valentí Rull  José A. López-Sáez  Teresa Vegas-Vilarrúbia 《Journal of Paleolimnology》2008,40(1):399-411

This paper evaluates the potential usefulness of non-pollen palynomorphs or NPPs (microfossils other than pollen and spores present in palynological preparations) contained in lake sediments in the paleolimnological reconstruction of high altitude environments (>4,000 m) from the Venezuelan Andes. A synthetic, quantitative approach is employed, instead of the classical analytical and mostly qualitative approach commonly used so far for NPPs. The main sources of variation are the Pediastrum–Botryococcus alternation and the relationship between these two algae and animal remains such as Acari legs, postabdomina of Cladocera, mandibles of other invertebrates, and an unknown type called LVA-1. Other significant microfossils are remains of Rivularia-type and turbellarian oocytes, including Gyratrix. The sequence initiates around 15,000 calibrated years before present (cal BP) with the deglaciation of the lake catchment, high water levels and still cold climates. A phase of lower lake levels was recorded between about 12,000 and 6,000 cal BP. Temperatures increased by around 9,000 and 7,000 cal BP, and then decreased until 6,000 cal BP. Since that time, both lake levels and temperature increased again and stabilized at about 4,000 cal BP, when they reach modern-like values. These results show a good agreement with previous studies based on pollen, diatom and oxygen isotope analyses, and provide additional paleoecological information, as for example the possibility of a previously unrecorded Younger Dryas signal. The inclusion of quantitative NPP analysis in routine paleolimnological studies using synthetic methods is thus recommended. Possible future improvements are suggested, mainly those related with the development and use of NPP modern analogs.  相似文献   

Crustal structure and tectonic setting of the south central Andes from gravimetric analysis     

Cecilia Weidmann  Silvana Spagnotto  Orlando Álvarez  Marcos Sánchez  Federico Lince Klinger  Mario Giménez  Patricia Martínez 《Geofísica Internacional》2013,52(3):197-208

Based on terrestrial gravity data, in this paper we prepared a map of Bouguer anomalies, which was filtered to separate shallow and deep gravity sources. Based on a density model and gravimetric inversion techniques, the discontinuous crust-mantle boundary and the top of crystalline basement were modeled. Subsequently, the equivalent elastic thickness (Te) was evaluated, considering information from the crust-mantle discontinuity and topographic load, finding high Te values in the eastern Andean foothills and west of the Velasco range. These results are consistent with the positive isostatic and residual Bouguer anomaly values, which suggest the presence of high-density rocks in the mid-to upper crust. In addition, petrographic and geochemical analysis conducted in surface outcrops suggest a mantle origin.  相似文献   

Sedimentary Crust and Metallogeny of Granitoid Affinity: Implications from the Geotectonic Histories of the Circum‐Japan Sea Region,Central Andes and Southeastern Australia     

Kohei Sato 《Resource Geology》2012,62(4):329-351

Metallogeny of granitoid affinity was reviewed from the aspect of geotectonic history of the continental crust, particularly of the genesis of sedimentary crust involved in magmatism. The redox state of granitoids and related mineralization shows a remarkable contrast between the east and west sides of the Pacific Rim, but if examined closely, the reduced‐type and oxidized‐type granitoid provinces are juxtaposed in three regions: the circum‐Japan Sea region, the central Andes, and the Lachlan Fold Belt in southeastern Australia. Comparative study of these regions revealed that the reduced‐type magmatism associated with Sn mineralization generated in thick sedimentary crust which formed in three geotectonic environments: (i) accretionary terrane along a subduction zone (e.g. Jurassic East Asia), (ii) continental rift (e.g. Early Paleozoic Andes), and (iii) mega‐fan (e.g. Early Paleozoic southeastern Australia). A collisional orogen can provide large amounts of clastic sediment to these environments. The age gap between the magmatism and sedimentation varies depending on the tectonic evolution of individual regions. Thin sedimentary crust may not play an essential role for the reduced‐type magmatism. The oxidized‐type magmatism associated with porphyry Cu and other mineralization generated in the crust which was initially carbon‐free igneous crust or modified from sedimentary crust by magmatism. Subduction‐related basaltic magmas are relatively oxidized, and may enhance fO₂ conditions of granitoid activity. Repeated magmatism in a monotonous convergent margin may be favorable for porphyry Cu mineralization as exemplified in the eastern Pacific Rim.  相似文献   

Variability in erosion rates related to the state of landscape transience in the semi‐arid Chilean Andes     

Germán Aguilar  Rodrigo Riquelme  Joseph Martinod  José Darrozes  Eric Maire 《地球表面变化过程与地形》2011,36(13):1736-1748

We quantify erosion rates in the higher sectors of the Huasco Valley, in the Main Cordillera of the semi‐arid Andes of Chile, using elevation differences between three successive geomorphic markers (pediments and paleo‐valleys) and the present day valley. Available Ar‐Ar ages of Neogene pediments are used to estimate mean erosion rates for the three periods (16 to 13 My, 13 to 8 My, and following 8 My). The landscape of the Huasco Valley is in a transient state, as indicated by well‐preserved pediment surfaces in interfluves, valleys deeply incised by fluvial and glacial erosion and scarped head‐valleys that represent the current knickzones. Higher erosion rates (45–75 m/My) are calculated for the more recent period (< 8 My) during which deep incision developed compared to previous periods (6–31 m/My). Quantitative data indicate that glaciers had a much higher erosional capability than fluvial activity in the higher sectors of the Main Cordillera. Comparison with erosion rates calculated in other drainage basins of the Chilean Andes suggests that the variability of erosion rates depends on the landscape's transient erosive state. The landscape's geomorphologic response to the uplift of the Main Cordillera results in the retreat of a knickzone, for which retreat velocity depends on precipitation rate pattern and glacial erosion intensity. Copyright © 2011 John Wiley & Sons, Ltd.  相似文献   

Vegetation and topographic controls on sediment deposition and storage on gully beds in a degraded mountain area   总被引：1，自引：0，他引：1  

Armando Molina  Gerard Govers  Felipe Cisneros  Veerle Vanacker 《地球表面变化过程与地形》2009,34(6):755-767

Active gully systems developed on highly weathered or loose parent material are an important source of runoff and sediment production in degraded areas. However, a decrease of land pressure may lead to a return of a partial vegetation cover, whereby gully beds are preferred recolonization spots. Although the current knowledge on the role of vegetation on reducing sediment production on slopes is well developed, few studies exist on the significance of restoring sediment transport pathways on the total sediment budget of degraded mountainous catchments. This study in the Ecuadorian Andes evaluates the potential of vegetation to stabilize active gully systems by trapping and retaining eroded sediment in the gully bed, and analyses the significance of vegetation restoration in the gully bed in reducing sediment export from degraded catchments. Field measurements on 138 gully segments located in 13 ephemeral steep gullies with different ground vegetation cover indicate that gully bed vegetation is the most important factor in promoting short‐term (1–15 years) sediment deposition and gully stabilization. In well‐vegetated gully systems ( ≥ 30% of ground vegetation cover), 0.035 m³ m^–1 of sediment is deposited yearly in the gully bed. Almost 50 per cent of the observed variance in sediment deposition volumes can be explained by the mean ground vegetation cover of the gully bed. The presence of vegetation in gully beds gives rise to the formation of vegetated buffer zones, which enhance short‐term sediment trapping even in active gully systems in mountainous environments. Vegetation buffer zones are shown to modify the connectivity of sediment fluxes, as they reduce the transport efficiency of gully systems. First calculations on data on sediment deposition patterns in our study area show that gully bed deposition in response to gully bed revegetation can represent more than 25 per cent of the volume of sediment generated within the catchment. Our findings indicate that relatively small changes in landscape connectivity have the potential to create strong (positive) feedback loops between erosion and vegetation dynamics. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

Evolution of the late Pleistocene Mojanda–Fuya Fuya volcanic complex (Ecuador), by progressive adakitic involvement in mantle magma sources     

Claude Robin  Jean-Philippe Eissen  Pablo Samaniego  Hervé Martin  Minard Hall  Joseph Cotten 《Bulletin of Volcanology》2009,71(3):233-258

The Mojanda–Fuya Fuya Volcanic Complex consists of two nearby volcanoes, Mojanda and Fuya Fuya. The older one, Mojanda volcano (0.6 to 0.2 Ma), was first constructed by andesites and high-silica andesites forming a large stratovolcano (Lower Mojanda). This edifice was capped by a basaltic andesite and andesitic cone (Upper Mojanda), which collapsed later to form a 3-km-wide summit caldera, after large phreatomagmatic eruptions. The Lower Fuya Fuya edifice was constructed by the extrusion of viscous Si-rich andesitic lavas and dacitic domes, and the emission of a thick sequence of pyroclastic-flow and fallout deposits which include two voluminous rhyolitic layers. An intermediate construction phase at Fuya Fuya is represented by a mainly effusive cone, andesitic in composition (San Bartolo edifice), the construction of which was interrupted by a major sector collapse in the Late Pleistocene. Finally, a complex of thick siliceous lavas and domes was emplaced within the avalanche amphitheatre, forming the Upper Fuya Fuya volcanic centre. This paper shows that the general evolution from an effusive to an explosive eruptive style is related to a progressive adakitic contribution to the magma source. Although all the rocks of the complex are included in the medium-K field of continental arcs, the Fuya Fuya suite (61–75 wt.% SiO₂) shows depletion in Y and HREE and high Sr/Y and La/Yb values, compared to the less silicic Mojanda suite (55–66.5 wt.% SiO₂). The Mojanda calc-alkaline suite was generated by partial melting of an adakite-metasomatised mantle source that left a residue with 2% garnet, followed by fractional crystallization of dominant plagioclase + pyroxene + olivine at shallow, intra-crustal depths. For Fuya Fuya, geochemical and mineralogical data suggest either (1) partial melting of a similar metasomatised mantle with more garnet in the residue (4%), followed by fractional crystallization involving plagioclase, amphibole and pyroxene, or (2) mixing of mafic mantle-derived magma from the Mojanda suite and slab melts, followed by the same fractional crystallization process.  相似文献   

Phosphorus fractions and sorption dynamics in the sediments of two Ca-SO4 water reservoirs in the central Argentine Andes     

Jos&#; Gabriel Le&#;n  Fernando Luis Pedrozo  Pedro F&#;lix Temporetti 《国际泥沙研究》2017,32(3)

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Streamflow elasticity,in a context of climate change,in arid Andean watersheds of north-central Chile     

Cristián Barrera  Jorge Núñez Cobo  Maxime Souvignet  Jorge Oyarzún 《水文科学杂志》2020,65(10):1707-1719

ABSTRACT

Based on a future temperature increase of 0.5°C and precipitation decrease of 25%, the climate elasticity of streamflow to precipitation and temperature changes in 12 Andean watersheds of the Coquimbo Region, north-central Chile, was assessed. Also, the possible relationships between this elasticity and specific physiographic characteristics of the watersheds (area, average elevation, slope distribution, terrain roughness, slope orientation, vegetation cover) were studied. Climate elasticity of streamflow ranged between 0 and 2.8. Watersheds presenting higher elevations, with a fairly well-balanced distribution of slope exposure tend to exhibit lower elasticity, which could be explained by the contribution of snowfall to the hydrological regime, more significant in those watersheds. Results should be considered when downscaling climate model projections at the basin scale in mountain settings. Finally, uncertainties in the approach, given by factors such as streamflow seasonality, data availability and representativeness and watershed characteristics, and therefore the scope of the results, are discussed.  相似文献   

Late Pleistocene–Holocene environmental and climatic history of a freshwater paramo ecosystem in the northern Andes     

Luisa Patiño  Maria Isabel Velez  Marion Weber  César A. Velásquez-r  Santiago David  Manuela Rueda  Ivonne Castañeda  Diana Arboleda 《第四纪科学杂志》2020,35(8):1046-1056

This study presents the first multi-proxy palaeoenvironmental and palaeoclimatic history for northern South America based on the palaeolimnological reconstruction of a pond located in a dry paramo at 3570 masl. During the Last Glacial Maximum (LGM), the study area was under glacial conditions, then during global events Heinrich Stadial 1 (HS1), Bølling–Allerød (BA), and the Younger Dryas (YD), the pond expanded, accumulation rates and proxies for erosion reached the highest values, indicative of humid conditions, with maxima in humidity during the BA and YD. Dry conditions and pond desiccation occurred in the Greenlandian–Northgrippian and by 6010 cal a bp the area was transformed into the mire of today. Comparisons with records from other sites in South America indicate that changes in humidity are most likely controlled by the Intertropical Convergence Zone, mainly during the glacial and postglacial, and by changes in the Pacific Ocean, more pronounced after the YD.  相似文献