Isotopic dating of the mylonitization of the Azuaga Group in the Badajóz-Córdoba belt,SW Spain

On the basis of Rb-Sr muscovite dating an age of about 355 Ma (Late Tournaisian/Early Visean) is assigned to the M₁ metamorphism and associated mylonitization of the ultramylonites of the Las Grullas Formation in the Azuaga Group, Badajóz-Córdoba belt. This age may possibly be related to the accretion process (suturing of the Ossa-Morena Zone to the Central Iberian Zone) thought to be responsible for the metamorphism and mylonitization. Muscovite from the Valencia de Las Torres Formation displays a somewhat lower Rb-Sr age of about 337 Ma. The corresponding K-Ar ages indicate that after the termination of the metamorphism it took some 20 Ma before the rocks had cooled below the closure temperature of muscovite to K-Ar. Whole-rock Rb-Sr data indicate that the Las Grullas ultramylonites were probably derived from Ordovician granitoids.

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Zusammenfassung Aufgrund von Rb-Sr-Datierungen in Muskoviten ist für die M₁Metamorphose und die damit verbundene Mylonitisierung der Ultramylonite der Las Grullas-Formation in der Azuaga-Gruppe, Badajóz-Córdoba-Gürtel, ein Alter von ca. 355 Ma (Spätes Tournais/Frühes Visé) festgestellt worden. Dieses Alter könnte mit dem Akkretionsproze\ (Sutur der Ossa-Morena-Zone zur Zentral-Iberischen-Zone) verbunden sein, der für Metamorphose und Mylonitisierung verantwortlich gemacht wird.Muskovit der Valencia de Las Torres-Formation spiegelt ein etwas geringeres Rb-Sr-Alter von ca. 337 Ma wieder. Die entsprechenden K-Ar-Alter zeigen, da\ es nach dem Ende der Metamorphose einige 20 Ma gedauert hat, bis die Gesteine unter die Schlie\ungstemperatur des Muskovits für K-Ar abgekühlt waren. Rb-Sr-Daten von Gesamtgesteinsproben zeigen, da\ die Las Grullas-Ultramylonite wahrscheinlich aus ordovizischen Granitoiden entstanden sind.

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Resumen Dataciones Rb-Sr en moscovitas de alrededor de 355 Ma (Tournaisiense tardío a Viseense) se asignan al metamorfismo M₁ y se asocian a la milonitización de ultramilonitas de la Formación Las Grullas en el Grupo de Azuaga, Eje Badajóz-Córdoba. Esta edad podría posiblemente datar el proceso de acreción (sutura de la Zona de Ossa-Morena a la Zona Centro-Ibérica) interpretado como responsable del metamorfismo y milonitización. Moscovita de la Formación Valencia de Las Torres indice una edad Rb-Sr un poco más bajo de cerca de 337 Ma. Las correspondientes edades K-Ar indican que después de la terminación del metamorfismo, se tardó cerca de 20 Ma antes que las rocas se enfriaran por debajo de la temperatura de cierre de la moscovita para K-Ar. Datos Rb-Sr de roca total indican que las ultramilonitas de Las Grullas posiblemente se deriven de granitoides ordovicicos.

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Rb/Sr m₁ Las Grullas Azuaga, Badajoz-Cordiba 355 — / /-. . Rb/Sr , Valencia de Las Torres , , 337 . , , / , , , , k/Ar . Rb/Sr , Las Grallas, , .

     

Landslide risk analysis in the area North of Lisbon (Portugal): evaluation of direct and indirect costs resulting from a motorway disruption by slope movements

A risk analysis is developed for a test site located in the area north of Lisbon using a scenario of a temporary interruption of the motorway A9 due to a landslide of rotational or translational type. Landslide susceptibility is assessed at the regional scale independently for rotational and translational slides, comparing the landslide distribution with a set of assumed independent landslide predisposing factors. Susceptibility models are validated and classified through the computation of prediction rate curves based on the temporal partition of the landslide data sets. Landslide hazard maps are based on a scenario of future landslide occurrence for the next 27 years. These maps allow the definition of two critical areas on the motorway A9 with regards to landslide susceptibility. Direct costs associated with both critical spots are assessed taking into account the probable affected area of the motorway, and the reconstruction costs. Indirect costs derived from the traffic interruption are evaluated considering the alternatives paths to the motorway, and include costs resulting from: (1) additional fuel consumption; (2) decrease in tollgate income; and (3) loss of productive time. Results show that indirect costs may be 24 to 43 times higher than direct costs, assuming a temporary interruption of the motorway for 6 months.     

What do people think about the flood risk? An experience with the residents of Talcahuano city,Chile

In the province of Concepción (Chile), floods are considered one of the main natural hazards. One of the most important cities of this area is Talcahuano. During the last years, Talcahuano has been affected by a number of flood episodes, as a consequence of an increase in the frequency of extraordinary atmospheric events, along with a higher exposure to the flood risk caused by an intense urban development. On 27 February 2010, an 8.8° earthquake (Richter scale) occurred in central southern Chile and originated the tsunami which flooded a large percentage of the residential area and military base of the Talcahuano city. This flood event affected a population higher than 180,000 people (including 23 casualties and invaluable economic and environmental losses). The objective of this study is to investigate the social perception and knowledge of Talcahuano residents affected by different types of flood, including tsunami, emphasizing which are their risks, vulnerability, resilience and coping capacity concepts. In addition, the kind of measures that have been proposed to improve their capacity to face floods after having suffered the natural disaster will be determined. This social assessment has been carried out based on a survey to permanent residents. Research results reveal that their endogenous and exogenous characteristics have resulted determinant to explain their perception.     

Biological,physical and chemical properties at the Subtropical Shelf Front Zone in the SW Atlantic Continental Shelf

The physical aspects of the Subtropical Shelf Front (STSF) for the Southwest Atlantic Continental Shelf were previously described. However, only scarce data on the biology of the front is available in the literature. The main goal of this paper is to describe the physical, chemical and biological properties of the STSF found in winter 2003 and summer 2004. A cross-section was established at the historically determined location of the STSF. Nine stations were sampled in winter and seven in summer. Each section included a series of conductivity-temperature-depth (CTD) stations where water samples from selected depths were filtered for nutrient determination. Surface samples were taken for chlorophyll a (Chl-a) determination and plankton net tows carried out above and below the pycnocline. Results revealed that winter was marked by an inner-shelf salinity front and that the STSF was located on the mid-shelf. The low salinity waters in the inner-shelf indicated a strong influence of freshwater, with high silicate (72 μM), suspended matter (45 mg l⁻¹), phosphate (2.70 μM) and low nitrate (1.0 μM) levels. Total dissolved nitrogen was relatively high (22.98 μM), probably due to the elevated levels of organic compound contribution close to the continental margin. Surface Chl-a concentration decreased from coastal well-mixed waters, where values up to 8.0 mg m⁻³ were registered, to offshore waters. Towards the open ocean, high subsurface nutrients values were observed, probably associated to South Atlantic Central Waters (SACW). Zooplankton and ichthyoplankton abundance followed the same trend; three different groups associated to the inner-, mid- and outer-shelf region were identified. During summer, diluted waters extended over the shelf to join the STSF in the upper layer; the concentration of inorganic nutrients decreased in shallow waters; however, high values were observed between 40 and 60 m and in deep offshore waters. Surface Chl-a ranged 0.07–1.5 mg m⁻³; winter levels were higher. Three groups of zoo and ichthyoplankton, separated by the STSF, were also identified. Results of the study performed suggest that the influence of freshwater was stronger during winter and that abundance distribution of Chl-a, copepods and ichthyoplankton was related to the Plata Plume Waters (PPW), rather than to the presence of the STSF. During summer, when the presence of freshwater decreases, plankton interactions seem to take place in the STSF.     

Three‐dimensional numerical modeling of the Bulle effect: the nonlinear distribution of near‐bed sediment at fluvial diversions

The Bulle effect is a phenomenon in which a disproportionately higher amount of near‐bed sediment load at a fluvial diversion moves into the diverted channel, even for cases in which the proportion of water (with respect to the main flow) entering the diversion channel is relatively small. This phenomenon has wide‐ranging implications for both engineered and natural systems: from efficient design of channels to redirect water and sediment for reclaiming sinking deltas, designing navigational channels that do not need frequent dredging, to morphological evolution of river bifurcations. The first ever, and one of the most extensive set of experiments conducted to explore this phenomenon, were conducted by Bulle in 1926 . In the current study the experiments conducted by Bulle have been simulated using an open‐source, free‐surface finite‐element‐based hydrodynamic solver. The main objectives were to explore to what extent the complex phenomenon of the Bulle effect at the scale of a laboratory experiment can be simulated accurately using Reynolds‐averaged Navier–Stokes (RANS)‐based hydrodynamic solver, and to understand the details of the hydrodynamics that Bulle could not analyze through his experiments. The hydrodynamics captured by the simulations were found to match the observations made by Bulle through his experiments, and the distributions of sediment at the diversion predicted by the numerical simulations were found to match the general trend observed in the laboratory experiments. The results from the numerical simulations were also compared with existing one‐dimensional models for sediment distribution at bifurcations, and the three‐dimensional numerical model was found to perform appreciably better. This is expected due to the complex flow features at the diversion, which can only be captured satisfactorily using a three‐dimensional hydrodynamic model. Copyright © 2017 John Wiley & Sons, Ltd.     

A long-term annual water balance analysis of the Araçuaí River Basin,Brazil

Semi-arid and arid areas exhibit great temporal variability in water availability. In some of these regions, a one- or two-day rainfall is followed by intervening dry periods of variable length. In recent decades, many rivers of the semi-arid portion of the Jequitinhonha Basin have been undergoing a progressive discharge reduction, mainly of their base flow, the Araçuaí River is one of them. In order to understand this transformation, a long-term analysis of the annual water balance of the Araçuaí River Basin was performed, the results of which are presented herein. Satellite images, hydrometeorological and river discharge data were analyzed; mean and variance tests were conducted to determine temporal homogeneity. Historical pluviometric data analysis shows no corresponding precipitation reduction and temperature undergoes only a slight increase in the same period. On the other hand, evaporation is extremely high, higher than precipitation during most of the year, leaving almost no water for infiltration (aquifer recharge) and runoff. Furthermore, the Araçuaí headwaters of its tributaries are now occupied by a monoculture, Eucaliptus sp., used for paper production. Because of the decreased fluvial discharges, its lowlands, usually used for agriculture and pasture, are abandoned and partially eroded.     

巴西Ara(c)uaí河流域长期水量平衡分析

Semi-arid and arid areas exhibit great temporal variability in water availability. In some of these regions, a one- or two-day rainfall is followed by intervening dry periods of variable length. In recent decades, many rivers of the semi-arid portion of the Jequitinhonha Basin have been undergoing a progressive discharge reduction, mainly of their base flow, the are presented herein. Satellite images, hydrometeorological and river discharge data were analyzed; mean and variance tests were conducted to determine temporal homogeneity.Historical pluviometric data analysis shows no corresponding precipitation reduction and temperature undergoes only a slight increase in the same period. On the other hand,evaporation is extremely high, higher than precipitation during most of the year, leaving alwaters of its tributaries are now occupied by a monoculture, Eucaliptus sp., used for paper production. Because of the decreased fluvial discharges, its lowlands, usually used for agriculture and pasture, are abandoned and partially eroded.     

Particle acceleration in flares

Particle acceleration is intrinsic to the primary energy release in the impulsive phase of solar flares, and we cannot understand flares without understanding acceleration. New observations in soft and hard X-rays, -rays and coherent radio emissions are presented, suggesting flare fragmentation in time and space. X-ray and radio measurements exhibit at least five different time scales in flares. In addition, some new observations of delayed acceleration signatures are also presented. The theory of acceleration by parallel electric fields is used to model the spectral shape and evolution of hard X-rays. The possibility of the appearance of double layers is further investigated.Report of Team 3, Flares 22 Workshop, Ottawa, May 25–28, 1993.     

High-energy inundation events versus long-term coastal processes – room for misinterpretation

Coastal boulder deposits and chevrons are two features whose origin have triggered controversial discussions. Boulders are often used as indicators of past tsunamis and storms, with the former interpretation in many cases preferred due to the clast size. Chevrons, defined as large parabolic sand bodies, were previously attributed to (mega-)tsunami, potentially caused by oceanic impacts, because of their dimensions, height above sea level and alignment of the central axis. This study documents that chevrons along the Quobba coast in Western Australia are parabolic dunes and not related to tsunami inundation; their age is consistent with an arid period at about 3·9 to 2·3 ka when the sea level was 1 to 2 m higher than today. The internal age distribution proves an inland migration. Weakly developed soil horizons represent phases of intermittent dune stabilization and later reactivation. The calculated velocities required for wind transport and the prevailing wind directions are consistent with on-site meteorological parameters. The boulders at Quobba are most likely to be remnants of in situ platform denudation that produces shell hash, coral clasts and boulders. An unknown portion of the boulders was certainly moved by tropical cyclones. A previously proposed tsunami origin is unsustainable because the observed features can be explained by processes other than tsunamis. Boulders were tilted during gravitative platform collapse, standing water caused dissolution of the boulder bottoms, creating ‘pseudo-rockpools’, consequently not applicable as upside-down criteria, and ages of attached encrusting organisms document their colonization at higher sea level and (sub)recent frequent inundation by wave splash during rough seas.     

Controls on carbon and nitrogen export in an eroding catchment of south‐eastern Queensland,Australia

Effects of gully and channel erosion on the export of sediments are in general well understood, but the effects on carbon (C) and nitrogen (N) export remain an open question. We examined these effects and the role of flow magnitude, total wet season rainfall, catchment size and the C and N content and solubility of most probable sediment sources in a subtropical catchment. We sampled the baseflow and the rising stage of high‐flow events for one wet season and analysed 5 years of water quality data from event sampling stations. Total suspended sediment was the main variable explaining N export followed by rainfall, flow and catchment size. N was exported mainly in particulate organic form and C in dissolved form. The quality of most probable sediment source fractions explains these results and points to fractionation during transport into C and N richer and C and N poorer fractions, with travel distance ultimately determining the average quality of transported sediment for different flow magnitudes. Erosion would have caused a lower C and N concentration in sediments, a lower proportion of mineralized N, a larger proportion of dissolved organic C and a larger C : N ratios of the soluble fraction as compared with unincised grassed valleys. This would alter the rates of nutrient cycling and energy flow within and across ecosystem compartments in streams receiving this export. Copyright © 2014 John Wiley & Sons, Ltd.