The structure of the Precambrian and Lower Paleozoic basement of the Central Andes between 22° and 32°S. Lat.

Three major diastrophic cycles, defined by their structural style and their spatial distribution are recognized in the Andean Basement of this region. The oldest structures are related to the Panamerican Orogeny (500 to 700 m. a.) which produced in the Central Craton multiply deformed complexes of schists, gneisses and granites, that are covered discordantly by unmetamorphosed Cambrian and Ordovician beds. West of the Central Craton Ordovician sedimentary beds are folded with a simple structural style and intruded by granites. Both the sedimentry beds and the granites are covered discordantly by undeformed Devonian sequences. The folding of the Ordovician is attributed to the ocloyic phase of the Caledonian movements. West of the ocloyic belt is another foldbelt consisting of strongly folded Devonian beds attributed to the chanic phase (hercynian). The chanic belt is intruded by carboniferous and permian granites and covered discordantly by Carboniferous and Permian sequences.The features observed in the eastern slope of the Andes suggest that the Paleozoic foldbelts are intracratonic. Whether there are accreted terrains in the Pacific Coastal Cordillera is matter of controversy.

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Zusammenfassung Im Andenbasement dieser Region lassen sich drei diastrophische Hauptzyklen, die durch ihren strukturellen Baustil und ihre geographische Verbreitung definiert sind, unterscheiden. Die ältesten Strukturen sind verknüpft mit den Bewegungen der Panamerikanischen Orogenese (500–700 m. a.), welche im Zentralkraton einen polydeformierten Komplex von Schiefern, Gneissen und Graniten erzeugte. Diesem lagern diskordant kambrische und ordovizische Sequenzen ohne Metamorphose und präandiner Deformation auf. Nach Westen zu gibt es einen Gürtel stark deformierter ordovizischer Sedimente, die einen einfacheren Baustil als das präkambrische Basement aufweisen, und diskordant von undeformierten devonischen Abfolgen bedeckt sind. Die Faltung des Ordoviziums wird der Ocloyischen Phase der Kaledonischen Bewegung zugerechnet.Weiter westwärts tritt ein jüngerer Faltengürtel aus devonischen Sedimenten auf, die während der Chanischen Phase der Herzynischen Bewegung intensiv deformiert wurden, und von karbonischen und permischen Sequenzen diskordant überlagert sind.Der Ocloyische Gürtel ist von postordovizischen Graniten die von devonischen Schichten überlagert sind, und der Chanische Gürtel von karbonischen und permischen Graniten intrudiert. Im präkambrischen Basement des Zentralkratons treten einerseits präkambrische Granite auf, denen diskordant kambrische Schichten auflagern. Andererseits findet man Granite mit paläozoischen Isotopenaltern, letztere aber in Gebieten, wo wegen des Fehlens von Deckschichten eine stratigraphische Kontrolle unmöglich ist.Die gemachten Beobachtungen scheinen darauf hinzudeuten, daß die paläozoischen Faltengürtel intrakratonisch sind. Ob es allochtone »terrains« in der pazifischen Küstenkordillere gibt, ist Gegenstand der Kontroverse.

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Resumen En el basamento de los Andes de esta región se distinguen tres ciclos diastróficos mayores definidos por sus estilos estructurales y su distributión espacial. Las estructuras mas antiguas responden a movimientos que culminaron con la Orogénesis Panamericana (500–700 m. a.) que produjo en el Cratógeno Central un complejo de esquistos, gneises y granitos polideformados, al que se le superponen discordantemente secuencias cámbricas y ordovicicas sin metamorfismo ni déformatión preandina. Hacia el oeste hay un cinturón de sedimentos ordovícicos intensamente deformados con un estilo de plegamiento mas simple que el del basamento precámbrico y cubierto discordantemente por secuencias marinas devónicas sin deformatión. Los movimientos responsables de este plegamiento se atribuyen a la fase oclóyica de los movimientos caledónicos. Más hacia el oeste se manifiesta una franja de deformatión mas jóven atribuída a la Fase chánica de los movimientos hercínicos, en ella se encuentran sedimentitas devónicas intensamente plegadas cubiertas discordantemente por secuencias carbónicas y pérmicas.El cinturón oclóyico esta acompanãdo por granitos postordovícicos cubiertos por Devónico. Asimismo, el cinturón chánico (hercínico) esta intruido por granitos probablemente carbónicos. Con respecta a los granitos intruidos en el zócalo antiguo, hay algunos indudablemente precámbricos por estar cubiertos discordantemente por secuencias cámbricas sin metamorfismo, además hay granitos de edad dudosa que han dado valores isotópicos paleozoicos pero en lugares donde el control estratigráfico no es posible por ausencia de la cobertura paleozoica. De acuerdo a los hechos observados en la ladera oriental de los Andes surge la idea de que los cinturones de plegamiento paleozoicos son intracratónicos, aun es materia de controversia si en la Cordillera de la Costa junto al Pacifico hay terrenos alóctonos acrecionados.

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Seasonal occurrence and growth of benthic algae in the Patos Lagoon estuary, Brazil

The seasonal occurrence of 94 benthic algal species, including Cyanophyta, was observed on a monthly basis over one year in the Patos Lagoon estuary (32°S, 52°W), Brazil, and showed three patterns of algal periodicity. An aseasonal group of 46 species was present throughout the year and two groups of 24 species each occurred either during summer and fall or winter and spring. The seasonal growth of 55 green, brown and red algae was correlated with monthly variations of salinity, water temperature, daylength and light radiation energy, which were measured daily in the estuary. Summer/fall peak growth was observed in 15 algae most of which correlated significantly with high salinities alone or in combination with other factors during this period. Growth peak of 11 species in winter/spring was principally a function of low water temperatures alone or together with a reduced light regime.Both periodicity of benthic algae and their seasonal growth, as a more sensitive measure of their floristic affinity, demonstrated the presence of cool temperate and tropical elements in this flora typical for warm temperate biogeographic regions. Correlations between seasonal variation of algal growth and selected environmental parameters aided in the evaluation of causal factors for algal seasonality.     

Thermal insulation of the intertidal zone by the ice foot

Few studies have looked at the ecological significance of the ice foot in intertidal habitats. During the 2007 winter, we quantified the hourly variation of temperature at the intertidal zone and at the upper, dry coast on the southern Gulf of St. Lawrence (Nova Scotia, Canada) using submersible data loggers. While air temperature dropped to − 20 °C at the peak of the winter, intertidal temperature was never below − 7 °C during the winter. In fact, for almost two months when the ice foot was stable, temperature ranged only between − 2.4 °C and − 1.1 °C at the intertidal zone. The intertidal values are higher than published values of lethal temperature for cold-water intertidal invertebrates and seaweeds. Thus, the ice foot may prevent these organisms from experiencing lethal levels of thermal stress, contributing to their long-term persistence in these environmentally stressful habitats.     

Alluvial fan–lacustrine association in the fault tip end of a half-graben, northern Triassic Cuyo basin, western Argentina

The Triassic deposits of Cerro Puntudo in the San Juan province of western Argentina constitute the northernmost exposures of the northern portion of the nonmarine Cuyo rift basin, also known as the Las Peñas-Tamberías half-graben. The local column, with an exposed thickness of approximately 400 m, consists of abundant basal and topmost coarse alluvial fan conglomerates and breccias (facies associations I and II) and a relatively thin (approximately 50 m) intervening sequence of marginal, shallow lacustrine deposits characterized by stromatolitic (domal) limestones, tuffaceous mudstones, and fine-grained sandstones (facies association III). Subaerial exposure in the lacustrine deposits is evidenced by desiccation cracks and brecciation. A very thin (0–6 m), laterally, discontinuous succession of lacustrine deposits with similar characteristics is interbedded with the basal conglomerates. Laterally, this lacustrine interval was eroded by overlying conglomerates. The basal conglomerates commonly show crude normal grading, faint cross-bedding, and b-axis clast imbrication. The predominance of coarse deposits and paleocurrents from NW to SE, indicative of an axial flow pattern, suggest that these exposures correspond to the northern end of the Cuyo basin, which is characterized by a shallow, alluvial fan-encased, carbonate-rich lake margin. This lacustrine interval can be correlated with the thicker lacustrine section exposed to the south at Quebrada del Tigre and Ciénaga Larga along the border fault margin of a nonmarine half-graben. The correlation with these sections suggests that the Cerro Puntudo lacustrine deposits are the shallowest equivalent of more profundal, organic-rich lacustrine sediments exposed in the deepest segment of the border fault margin. These thickness and facies variations are the result of differential subsidence along the border fault margin between the low accommodation, fault tip end represented by the Cerro Puntudo section and the high accommodation, central segment located to the south.     

Early Middle Miocene broken foreland development in the southern Central Andes: evidence for extension prior to regional shortening

Most authors suggest that the main contraction phases in the southern Central Andes started in the Late Miocene. Along the flat‐slab segment, deformation has progressively involved basement and a broken foreland has developed. Recent work suggests that construction of the Andes by late Neogene shortening may have been controlled by lithospheric thinning and crustal structure generated during mid‐Tertiary times in the Southern and Central Andes. Exposures at the eastern border of the Famatina Ranges in western Argentina in the flat‐slab segment document basement involved extension of approximately this age. The Del Abra Formation, the lower unit of a major Andean synorogenic cycle (Angulos Group), reveals a distinct and previously unrecognized early Middle Miocene tectonic event. This is suggested by a 505‐m‐thick thinning‐ and fining‐upward megasequence. Dominantly conglomeratic facies record a continuous progression from fault‐scarp‐related high‐gradient colluvium to relatively distal terminal‐fan facies. The fining–thinning upward megasequence characterizes progressive scarp backstepping and decreasing relief after active extension. Interpretation of the stratigraphic fill and the associated structure (high‐angle hinterland‐dipping fault) favours tectonic inversion of an originally normal fault. This allows reappraisal and new understanding of the early‐stage architecture of the Central Andean foreland. Early Middle Miocene extension may have had an important bearing on the later evolution of the broken foreland.     

Low-P metamorphism following a Barrovian-type evolution. Complex tectonic controls for a common transition, as deduced in the Mondoñedo thrust sheet (NW Iberian Massif)

The Mondoñedo thrust sheet has been studied to investigate the complex dynamic relationships that may be involved in the development of low- and medium-P metamorphic domains. This unit underwent an initial medium-P event during the initial stages of Variscan convergence, related to crustal thickening. Subsequently, the thrust sheet evolved to a low-P baric type of metamorphism, related to syn-convergence thinning and exhumation. Its footwall, cropping out in two tectonic windows, registered a different evolution, with a low-P history that evolved from low- to high-T under a high geothermal gradient. Several different P–T paths of the Mondoñedo thrust sheet and its relative autochthon are traced and interpreted according to the structural evolution of the area. Following the initial crustal thickening, two main syn-convergence extensional shear zones developed. One of them occurs in the hangingwall, whereas the other affects the footwall unit. Both extensional shear zones were contemporaneous with ductile thrusting in the inner parts of the thrust sheet, and their activity is viewed as a consequence of the need for gravitational re-equilibration within the orogenic wedge.The most commonly accepted models of tectonothermal evolution in regions of thickened continental crust assume that low-P metamorphism is essentially a late phenomenon, and is linked to late-orogenic tectonic activity. In the Mondoñedo thrust sheet, our conclusions indicate that low-P metamorphism may also develop during convergence, and that this may occur in at least two cases. One is tectonic denudation of an allochthonous unit during its emplacement, and the other, thinning and extension at the footwall unit of an advancing thrust sheet. As a consequence, the low-P evolution may show different characteristics in different units of an orogenic nappe pile.     

Elemental and Single Particle Aerosol Characterisation at a Background Station in Kazakhstan

Aerosol sampling in Kazakhstan was performed at a remote astronomical observatory in the Tien Shan mountain region. The background character of the site was considered by comparing the elemental concentrations, obtained by Energy Dispersive X-Ray Fluorescence (EDXRF), with those reported for other remote stations. On the basis of the variability of the elemental concentrations and the source of origin, the elements could be classified into two main groups. These findings, complemented with enrichment factor calculations revealed the first group of elements to be originating mainly from local sources, and the second group to be related to long-range transport of anthropogenic aerosol. Automated individual particle analysis by electron probe X-ray microanalysis (EPXMA) and subsequent cluster analysis resulted in the identification of 12 distinct aerosol particle types. Relative particle type abundances were converted to absolute abundances by estimating the particle number concentrations for the detected particles, exploring the seasonal particle variation. Based on the absolute abundances, a strong seasonal pattern was found for Si-Al-Fe-, Si-, Ca-S-Si-, Ca-Si-, Fe-Si- and Ti-Si-rich particles. These particle types can mainly be attributed to the local soil source. For most of the remaining particle types, a completely different seasonal trend was found, which can be related to anthropogenic emissions. Their abundance in winter can be related to long-range transport of anthropogenic aerosols. As a result, these data can be representative for the Tien Shan site and may be useful to climate modellers and environmentalists.