Precambrian and Early Paleozoic evolution of the Andean basement at Belen (northern Chile) and Cerro Uyarani (western Bolivia Altiplano)     

G. Wrner  J. Lezaun  A. Beck  V. Heber  F. Lucassen  E. Zinngrebe  R. Rssling  H. G. Wilke 《Journal of South American Earth Sciences》2000,13(8):717

Exposures of metamorphic basement in the Central Andes are scarce and reconstructions of the history of the Pacific margin of Gondwanaland must rely on a few isolated outcrops. We studied two areas of exposed basement in northernmost Chile (Belen) and westernmost Bolivia (Cerro Uyarani). The Belen metamorphic complex has been known for some time and consists of fault-bounded amphibolites, gneisses, schists, and minor quartzites overlain by folded Mesozoic to Cenozoic strata. The Cerro Uyarani is the only basement outcrop on the Bolivian Altiplano and has only recently been found and studied by geological reconnaissance. It consists of foliated mafic and felsic granulites, charnockites, and amphibolites. How do these basement occurrences compare and how do they relate to the other Precambrian crustal domains in the Central Andes? To answer these questions, we used geothermobarometers to reconstruct the P–T conditions of metamorphism, as well as geochemical analyses and petrological methods to study these rocks. The two basement blocks were found to have distinct geological histories and are probably separated by a major crustal domain boundary. Isotopic fingerprinting by Pb-isotopes clearly exclude Laurentian crustal components either in the protoliths or as reworked material. This signature is quite distinct from basement rocks farther south in Chile and northwestern Argentina.  相似文献   

Manuscript Reviewers for Volume 54     

《The Professional geographer》2013,65(4):587-588

  相似文献   

Provenance of middle to late Palaeozoic sediments in the northeastern Colombian Andes: implications for Pangea reconstruction     

《International Geology Review》2012,54(15):1914-1939

ABSTRACT

Global-scale Palaeozoic plate tectonic reconstructions have suggested that Laurentia was obliquely approaching against the northwestern margin of Gondwana until the final agglutination of Pangea. In this contribution integrated petrographic analysis, heavy mineral analysis, and tourmaline geochemistry were done, and U–Pb detrital zircon geochronology was obtained, in late Palaeozoic sedimentary and meta-sedimentary units from the Floresta and Santander Massifs in the Eastern Colombian Andes in order to constrain their provenance and related it with the magmatic, sedimentary, and deformational record of the Gondwana–Laurentia convergence until the late Carboniferous to Permian formation of Pangea. Late Devonian to early Carboniferous sandstones from the Floresta Massif changed from sublithoarenites to lithoarenites, tracking the progressive uplift and unroofing of sedimentary and metamorphic rocks, with associated volcanic activity. The U–Pb detrital zircon geochronology from the sedimentary and metasedimentary of Floresta and Santander documents Mesoproterozoic and Palaeoproterozoic sources, and younger Ordovician to Silurian age populations, that can be related to the early to middle Palaeozoic plutonic rocks and the Amazon Craton. The limited Silurian to Early Devonian detrital ages that contrast with the more significant Middle to Late Devonian zircons that document the erosion of contemporaneous magmatic sources formed after a late Silurian to Early Devonian reduction on the magmatic activity along the proto-Andean margin. These rocks were apparently deformed and metamorphosed between the late Carboniferous and the early Permian. It is suggested that the filling and deformation record of these rocks documented the changes in plate convergence obliquity at the western margin of Gondwana associated with the migration of Laurentia until its final position in Pangea. Between the late Carboniferous and the early Permian, peri-Gondwanan continental terranes also collided with the continental margin. Over-imposed Mesozoic tectonics have contributed to the final redistribution of these terranes to their current position.

Abbreviations:LA: laser ablation inductively couple mass spectrometer; CL: cathodoluminiscence  相似文献   

Environmental factors controlling spatial variation in sediment yield in a central Andean mountain area     

Armando Molina  Gerard Govers  Jean Poesen  Hendrik Van Hemelryck  Bert De Bivre  Veerle Vanacker 《Geomorphology》2008,98(3-4):176

A large spatial variability in sediment yield was observed from small streams in the Ecuadorian Andes. The objective of this study was to analyze the environmental factors controlling these variations in sediment yield in the Paute basin, Ecuador. Sediment yield data were calculated based on sediment volumes accumulated behind checkdams for 37 small catchments. Mean annual specific sediment yield (SSY) shows a large spatial variability and ranges between 26 and 15,100 Mg km^− 2 year^− 1. Mean vegetation cover (C, fraction) in the catchment, i.e. the plant cover at or near the surface, exerts a first order control on sediment yield. The fractional vegetation cover alone explains 57% of the observed variance in ln(SSY). The negative exponential relation (SSY = a × e^{−b C}) which was found between vegetation cover and sediment yield at the catchment scale (10³–10⁹ m²), is very similar to the equations derived from splash, interrill and rill erosion experiments at the plot scale (1–10³ m²). This affirms the general character of an exponential decrease of sediment yield with increasing vegetation cover at a wide range of spatial scales, provided the distribution of cover can be considered to be essentially random. Lithology also significantly affects the sediment yield, and explains an additional 23% of the observed variance in ln(SSY). Based on these two catchment parameters, a multiple regression model was built. This empirical regression model already explains more than 75% of the total variance in the mean annual sediment yield. These results highlight the large potential of revegetation programs for controlling sediment yield. They show that a slight increase in the overall fractional vegetation cover of degraded land is likely to have a large effect on sediment production and delivery. Moreover, they point to the importance of detailed surface vegetation data for predicting and modeling sediment production rates.  相似文献   

Glacier mass balance variability in the Cordillera Blanca, Peru and its relationship with climate and the large-scale circulation   总被引：1，自引：0，他引：1  

Mathias Vuille  Georg Kaser  Irmgard Juen 《Global and Planetary Change》2008,62(1-2):14-28

A 41-year-long reconstructed annual mean glacier mass balance record from the Cordillera Blanca, Peru, was investigated for its climate sensitivity toward temperature, humidity and precipitation, and its links with the large-scale atmospheric circulation. On interannual timescales precipitation variability appears to be the main driver for glacier mass balance fluctuations in the Cordillera Blanca. This is corroborated by an analysis of the relationship between mass balance variations and local- to regional-scale precipitation variability. Temperature tends to enhance precipitation in driving the mass balance signal, as dry years are often characterized by warm conditions, while wet years usually coincide with cold anomalies. In some years, however, warm and wet or cold and dry conditions coincide, under which circumstances temperature minimizes or even neutralizes the effects of precipitation. Surface energy balance studies have shown that changes in atmospheric humidity significantly affect the melt rates of tropical glaciers, but the lack of long and high-quality in-situ measurements precludes a detailed quantitative assessment of its role on interannual timescales in the Cordillera Blanca. Sea surface temperature anomalies (SSTA) in the tropical Pacific exert the dominant large-scale forcing on interannual time scales, leading to negative mass balance anomalies during El Niño and above average mass balance during La Niña episodes. In general the teleconnection mechanism linking ENSO with glacier mass balance is similar to what has previously been described for the Bolivian Altiplano region. Changes in the upper-tropospheric zonal flow aloft associated with ENSO conditions determine the amount of snowfall during the wet season and thereby significantly affect the glacier mass balance. Because this teleconnection mechanism is spatially unstable and oscillates latitudinally along the subtropical Andes, it affects the Cordillera Blanca in most, but not all years. The relationship between ENSO and glacier mass balance is therefore characterized by occasional ‘break downs’, more common since the mid-1970's, when El Niño years with above average mass balance and La Niña events with negative mass balance have been observed.  相似文献   

A recharge model for high altitude,arid, Andean aquifers   总被引：1，自引：0，他引：1  

John Houston 《水文研究》2009,23(16):2383-2393

Evidence for groundwater recharge in arid zones is mounting, despite early ideas that recharge was unlikely where evaporation greatly exceeded precipitation. The mechanisms and magnitude of groundwater recharge in the Andes and Atacama Desert are not well known but the subject of current research. Diffuse recharge is expected to be limited to high altitude areas with coarse‐grained soils devoid of vegetation. A recharge model for this environment is developed based on a simple soil moisture budgeting technique and the calculation of actual evaporation based on empirical studies. The model is run with data for the Linzor basins, over 4000 m elevation at 22·2°S on the west slope of the Andes. It is checked against independent estimates based on the chloride mass balance (CMB) method and flood events measured downstream in the Río Salado and found to provide robust and reliable results. The results indicate that irregular and volumetrically limited amounts of diffuse recharge occur at high elevations in half of all years, with a tendency to cluster during La Niña episodes. For the Linzor Basins, mean annual recharge is found to be equivalent to 28 mm a^?1, although no recharge occurs in years with precipitation less than 120 mm, and increases proportionately with annual rainfall amounts above this limit. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

Impacts of land use on the hydrological response of tropical Andean catchments          下载免费PDF全文

Boris F. Ochoa‐Tocachi  Wouter Buytaert  Bert De Bièvre  Rolando Célleri  Patricio Crespo  Marcos Villacís  Carlos A. Llerena  Luis Acosta  Mauricio Villazón  Mario Guallpa  Junior Gil‐Ríos  Paola Fuentes  Dimas Olaya  Paúl Viñas  Gerver Rojas  Sandro Arias 《水文研究》2016,30(22):4074-4089

Changes in land use and land cover are major drivers of hydrological alteration in the tropical Andes. However, quantifying their impacts is fraught with difficulties because of the extreme diversity in meteorological boundary conditions, which contrasts strongly with the lack of knowledge about local hydrological processes. Although local studies have reduced data scarcity in certain regions, the complexity of the tropical Andes poses a big challenge to regional hydrological prediction. This study analyses data generated from a participatory monitoring network of 25 headwater catchments covering three of the major Andean biomes (páramo, jalca and puna) and links their hydrological responses to main types of human interventions (cultivation, afforestation and grazing). A paired catchment setup was implemented to evaluate the impacts of change using a ‘trading space‐for‐time’ approach. Catchments were selected based on regional representativeness and contrasting land use types. Precipitation and discharge have been monitored and analysed at high temporal resolution for a time period between 1 and 5 years. The observed catchment responses clearly reflect the extraordinarily wide spectrum of hydrological processes of the tropical Andes. They range from perennially humid páramos in Ecuador and northern Peru with extremely large specific discharge and baseflows, to highly seasonal, flashy catchments in the drier punas of southern Peru and Bolivia. The impacts of land use are similarly diverse and their magnitudes are a function of catchment properties, original and replacement vegetation and management type. Cultivation and afforestation consistently affect the entire range of discharges, particularly low flows. The impacts of grazing are more variable but have the largest effect on the catchment hydrological regulation. Overall, anthropogenic interventions result in increased streamflow variability and significant reductions in catchment regulation capacity and water yield, irrespective of the hydrological properties of the original biome. Copyright © 2016 The Authors. Hydrological Processes. Published by John Wiley & Sons Ltd.  相似文献   

Geomechanical model for a seismically active geothermal field: Insights from the Tinguiririca volcanic-hydrothermal system     

L. Giambiagi  P. Álvarez  S. Spagnotto  E. Godoy  A. Lossada  J. Mescua  M. Barrionuevo  J. Suriano 《地学前缘(英文版)》2019,10(6):2117-2133

In this work,we develop a multidisciplinary approach to investigate a geothermal system located at the volcanic arc of a subduction-related orogen and highlight the interplay between active tectonism.stress field and fluid migration.By using results of field investigations from the Tinguiririca geothermal field in the High Andes of Chile(35°S),empirical analysis,and numerical models of static stress variations,we proposed a geomechanical model for evaluating the distribution of hydrothermal manifestations in a seismically-active region.The present geomechanical model follows four major steps:(1) development of the 3 D structural model of fault pattern;(2) estimation of the in-situ stress field;(3) calculation of the resolved-shear-to-normal-stress ratio(slip tendency) on each fault with varying geomechanical parameters(coefficient of friction,pore pressure and cohesion) as inputs;and(4) estimation of Coulomb static stress changes as a consequence of failure in a nearby fault.Through combination of all these analyses,we characterize in detail both the active deformation in the geothermal field and its relationship with hot fluid migration.  相似文献   

Differences in 10Be concentrations between river sand,gravel and pebbles along the western side of the central Andes     

《Quaternary Geochronology》2015

Cosmogenic nuclides in river sediment have been used to quantify catchment-mean erosion rates. Nevertheless, variable differences in ¹⁰Be concentrations according to grain size have been reported. We analyzed these differences in eleven catchments on the western side of the Andes, covering contrasting climates and slopes. The data include eight sand (0.5–1 mm) and gravel (1–3 cm) pairs and twelve sand (0.5–1 mm) and pebble (5–10 cm) pairs. The difference observed in three pairs can be explained by a difference in the provenance of the sand and coarser sediment. The other sand–pebble pairs show a lower ¹⁰Be concentration in the pebbles, except for one pair that shows similar concentrations. Two sand–gravel pairs show a lower ¹⁰Be concentration in the gravel and the other five pairs show a higher ¹⁰Be concentration in the gravel. Differences in climate do not reveal a particular influence on the ¹⁰Be concentration between pairs. The analysis supports a model where pebbles and gravel are mainly derived from catchment areas that are eroding at a faster rate. The five gravel samples with high ¹⁰Be concentrations probably contain gravel that were derived from the abrasion of cobbles exhumed at high elevations. In order to validate this model, further work should test if pebbles are preferentially exhumed from high erosion rate areas, and if the difference between pebbles with high ¹⁰Be concentrations and sand decreases when the erosion rate tends to be homogeneous within a catchment.  相似文献   

Petrological inferences on the evolution of magmas erupted in the Andagua Valley,Peru (Central Volcanic Zone)     

E.V. Sørensen  P.M. Holm 《Journal of Volcanology and Geothermal Research》2008

Major and trace element and Sr and Nd isotope data is presented from the Andagua valley scoria cone and lava field (15°32′ S 72°19′ W), Southern Peru in the northernmost part of the Central Volcanic Zone (CVZ). The rocks are all quite evolved in composition (SiO₂ = 55–64 wt.%) and classify as benmoreites, latites and few mugearites and trachytes. Samples are characterized by high Na₂O (4.2–5.2 wt.%), Sr (600–1300 ppm), Ba (800–1600 ppm). The main difference between the benmoreites and latites is in the Na₂O content that reach the highest so far reported from CVZ for these SiO₂ concentrations. The rocks are generally nearly aphyric but latites and trachytes are more porphyritic. Amphibole microphenocrysts generally are only present in latites and trachytes. The difference between benmoreite and latite samples is reflected in lower P₂O₅ and Zr content of the latite samples documenting the existence of two compositional different parental magma types. The investigated volcanic activity spans the Pleistocene to Recent with the historic activity concentrated in the area just south of Andagua. Combined relative stratigraphy, petrography and geochemistry define volcanic units and demonstrate that rocks from Chilcayoc Grande, Chilcayoc Chico 2, Jenchana, Sucna 1 and Chilcayoc Chico 1 represent the most recent volcanic activity. The main trend samples, each form a co-magmatic group resulting in sub-parallel trends in many variations diagrams. It is furthermore shown that these trends point towards calculated mixing lines relating the individual units through a binary mixing process, thus indicating a two stage evolution. In the case of Jenchana, Sucna 1 and Chilcayoc Chico 1, the samples define positive correlation trends in the Sr vs. Rb diagram that can be extrapolated back towards origo indicating nearly perfect incompatibility of Sr and Rb. This together with generally high Sr/Y (50–105) and low Y content (< 16 ppm) suggest lack of plagioclase fractionation and residual garnet in the source and is taken as evidence for relatively high pressure (lower crustal) origin of the mixing event. The amphibole bearing samples form individual co-magmatic groups that cannot be related to each other. This means that the amphibole bearing samples originates from different magmas. The lavas of the Ninamama group are comparable in age to the main trend samples but different in petrography and composition, why the two compositional different magmas must have existed within a small confined area within a limited time span.  相似文献