The luminosity oscillation of X-ray bursters and recurrent novae

Thermo-mechanical oscillations of a radiating spherically-symmetric shell containing a fermion gas (i.e., oscillation where only mechanical, thermodynamical, and eventually radiation phenomena are taken into account) are studied. With a reasonable choice of the three relevant parameters all other observational data are similar to the ones of X-ray bursters and recurrent novae.Therefore, thermo-mechanical oscillations could play an important role in the oscillation spectrum of these astronomical objects.     

Rainfall-induced shallow landslides: a model for the triggering mechanism of some case studies in Northern Italy

The article relates the main findings of a recent investigation aimed at modeling the triggering of shallow landslides. A simplified model for assessing the safety factor of potentially unstable slopes, directly related with rainfall trends, was developed. Based on the geometric characteristics of the slope, the geotechnical properties, and strength parameters of the soil, the model makes it possible to define a safety factor of a slope as a function of time. The model is based on the limit equilibrium method and takes into account the seepage of underground water. The safety factor is, in turn, related to the seasonal rainfall. The model was applied on a local scale to some historical cases which had occurred recently in Northern Italy. The paper shows how the results of the application of the model on a local scale achieve a good agreement between the instability condition and the real date of each considered event.     

Plio-Quaternary volcanotectonic activity in the northern sector of the Main Ethiopian Rift: relationships with oblique rifting

Deformation and magmatism within the 90 km wide northern Ethiopian Rift system is concentrated along a narrow zone - the Wonji Fault Belt. Two key areas (the Nazret-Dera and Asela-Ziway areas), located along the eastern margin of the north-northeast to northeast trending Main Ethiopian Rift, have been investigated in order to reconstruct the recent tectonomagmatic evolution of the northern branch of the Main Ethiopian Rift. In these areas, Early Pleistocene volcanic products (Wonji Group) overlie Pliocene volcanic rocks (Eastern Margin Unit). Detailed stratigraphical reconstructions have revealed the presence of several tectonomagmatic units which can be correlated between the two study areas. The stratigraphical and petrological study of these units outlined (1) the bimodal composition (basalts-pantellerites) of the oldest and youngest units and the unimodal character (pantellerites) of the products erupted during the intervening period; (2) the mainly fissural origin of the ignimbrites and oldest basalts; and (3) a mafic/felsic volumetric ratio of 1:5.The geological data suggest that, around the Pliocene-Quaternary boundary, a change in the stress field occurred in this Main Ethiopian Rift sector, passing from a direction of extension roughly orthogonal to the rift shoulders, to oblique rifting related to an east-west trending extension. In this framework the change in the style of volcanism observed in the Nazret-Dera and Asela-Ziway areas can be related to the change of the stress field. A new geodynamic model is presented for the Late Pliocene to Recent evolution of this sector of the Main Ethiopian Rift. According to this model, a large volume of rhyolitic products was erupted during an oblique rifting phase, following a previous period of pure extension. The change in the tectonic regime favoured partial melting of the underplated basalts as a decrease in the pressure and an elevation of isotherms occurred.     

Predictive Power Evaluation of a Physically Based Model for Shallow Landslides in the Area of Oltrepò Pavese,Northern Italy

The use of real-time landslide early warning systems is attracting the attention of the scientific community, since it allows to assess “where” and “when” a shallow rainfall-induced landslide might occur by coupling rainfall amounts, hydrological models and slope-stability analysis. The paper deals with the main results of a back analysis, which refers to the application of a physically based stability model [Shallow Landslides Instability Prediction (SLIP)] on regional scale. The analysis concerns the occurrence of some recent rainfall-induced shallow landslides in the municipal territory of Broni, in the area of Oltrepò Pavese (Northern Italy). The study area is a hilly region 2.4 km² wide, where more than 40 % of the territory has slopes steeper than 15° and altitudes are between 90 and 250 m a.s.l. As regards the geologic setting, clayey-silty shallow colluvial deposits, with a maximum thickness of about 3 m, overlap a bedrock made of clayey shales, calcareous flysch and marls. The SLIP model is based on the limit equilibrium method applied to an infinite slope and on the Mohr–Coulomb strength criterion for the soil. By assuming that the main hydro-geotechnical process that leads to failure is the saturation of parts of the soil, the model allows to take into account the condition of partial saturation of the soil. The safety factor (F _S ) of a slope is also function of previous rainfalls. After the implementation of the model at territory scale, the input data have been introduced through a geographic information systems platform. In the current paper we mainly intend to evaluate the performance of SLIP at catchment scale, by comparison to (1) observed landslide events and (2) another well-established physically based model (TRIGRS). Further, we want to assess the suitability of the model as early warning tool. The results produced by the model are analyzed both in terms of safety factor maps, corresponding to some particular rainfall events, and in terms of the time-varying percentage of unstable areas over a 2-year span period. The paper shows the comparison between observed landslide localizations and model predictions. A quantitative comparison between the SLIP model and TRIGRS is presented, only for the most important event that occurred during the analyzed period. Overall, the results of the stability analyses based on observed rainfalls show the capability of the SLIP model to predict, in real-time and on a wide area, the occurrence of the analyzed phenomena.     

Carbonate-hosted siliceous crust type mineralization of Carnic Alps (Italy-Austria)

A carbonate-hosted stratabound siliceous crust type (SCT) mineralization (base metal sulphides, barite, fluorite) occurs over large areas of Carnic Alps and Karawanken in the Eastern Alps. It concerns a pervasively silicified lithological unit, up to some tens of metres thick, which caps the unconformity landscapes developed on epicontinental Devonian–Dinantian carbonates. The SCT mineralization is directly overlaid by different transgressive siliciclastic sediments, which range from Lower Carboniferous to Lower Permian. The presence of fragments of the SCT mineralization in the transgressive siliciclastic sediment bounds its whole lithological evolution within a short stratigraphic interval of Lower Carboniferous age. Selected features of the regional and lithostratigraphic setting are discussed. The chemical characterisation is based on the statistical evaluation of compositional data of 581 selected samples. Three significant groups of elements have been distinguished: (1) the hydrological and metasomatically active elements (Si, Ba, F), which show a strong negative correlation amongst themselves and characterise the silica-saturated aqueous solutions; (2) the terrigenous elements (Al₂O₃, K₂O, Fe_tot, TiO₂, B, Be, Ce, La, Nb, V, Y, Zr), which suggest a continental margin environment for silica deposition; (3) the sulphide metals (Cu, Pb, Zn, Ni, Sb, As, Hg, Cd), which define the metalliferous signature of the SCT mineralization.Some consistent, but still debatable genetic aspects of the SCT mineralization are as follow: (1) silica may be supplied by illitization of clay-rich basinal sediments during their diagenesis. δ¹⁸O of microcrystalline quartz ranges from +18.5‰ and +24.6‰ and is very similar to δ¹⁸O of authigenic quartz deriving from diagenetic processes of illitization of clay-rich basinal sequences. (2) The diagenetic evolution of these sediments may trigger off the movement of silica-rich marine pore waters. δ³⁴S of barite ranges from +15.5‰ to +19.3‰ with an average of +17.7‰ and are in good agreement with δ³⁴S of sulphate in ocean waters of Upper Devonian–Lower Carboniferous age. (3) A convective hydrological system, connected with sinsedimentary transtensive tectonics, active in the Carnic Alps since the Frasnian, may be the transport mechanism of aqueous solutions. (4) A weak drop in pH in the dominant carbonate environment represents the conditions for silica precipitation.SCT mineralization, showing persistent, independent and distinct characters, occurs over large areas also in other sites of the Alpine belt and outside Italy and Austria. Therefore, it points to important markers for some sedimentary sequences as well as to a worldwide significant cyclic metallogenic event. It represents a new ore deposit-type within the carbonate-hosted mineralization.     

Integration schemes with substepping algorithms for creep analysis in geomaterials

This paper describes a robust and efficient methodology for predicting displacements, deformations, and stresses in geomaterials that are susceptible to creep. The methodology is based on two integration schemes, which consider substepping algorithms. The first scheme is used for integrating space-time relations in a global sense, whereas the second scheme is used for integrating stress-stain relations in a local sense. Different from previous studies, both integration schemes are easy to implement and general in the sense that they can be applied to any type of creep law. Through an in-house finite element simulator, several numerical tests are performed. They include triaxial and wellbore closure analyses considering soft soils and salt rocks. The results show that the combination of both schemes leads to stable and accurate solutions with reduced computational time.     

Spatial assessment of groundwater use potential for irrigation in Teesta Barrage Project in Bangladesh

In Bangladesh, development of the groundwater resource for irrigation is a vital component of the government’s agricultural strategy to attain food self-sufficiency. Amidst reports of falling groundwater levels in many parts of Bangladesh, the potential of groundwater use has been investigated in the Teesta Barrage Project (TBP) in which large-scale groundwater development for dry-season irrigation has taken place in the recent past. Several techniques and tools have been applied such as the combination of analysis of groundwater hydrographs and mathematical modelling to derive key hydrogeological variables, calculation of net irrigation requirement and the use of geographical information systems. The results show that the economically attractive high-yielding variety (HYV) Boro (dry season) rice cultivation during the groundwater irrigation season may not be sustained in large parts of the project area if the current trends in abstraction are continued. However, due to spatial variation in abstraction, nine thanas (sub-districts)—out of a total of 21 in the project area—may still be able to expand groundwater-irrigated cropland and a groundwater-use potential of 40 mm/year may be created if deep-set shallow tubewells are used by the farmers to abstract groundwater. A structured approach, based on zoning of potential areas, is recommended for groundwater development and use.

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Resumen En Bangladesh, el desarrollo de las aguas subterráneas para riego es un componente vital del la estrategia agrícola del Gobierno para obtener la auto-suficiencia alimentaria. Un informe intermedio evidencia un descenso de los niveles de aguas subterráneas en muchas partes de Bangladesh, el uso potencial de aguas subterránea ha sido investigado en el Proyecto Tessta Barrage (TBP) en el cual ha tenido lugar el desarrollo de las aguas subterráneas a gran escala para el riego en la estación seca en un pasado reciente. Se ha aplicado algunas técnicas y herramientas, como la combinación del análisis de datos gráficos de aguas subterráneas y modelización matemática para obtener variables hidrogeológicas clave, el cálculo de la necesidad neta de riego y el uso de Sistemas de Información Geográfica. El resultado muestra que el cultivo, económicamente atractivo, de arroz HYV Boro (estación seca) durante la estación de riego con aguas subterráneas puede no ser sostenible en gran parte del área del proyecto si continúan las tendencias actuales en las extracciones. Sin embargo, debido a la variación espacial en la extracción, nueve thanas (subdistritos)—de un total de 21 en el área del Proyecto—pueden todavía sufrir un aumento de la tierra cultivable regada con aguas subterráneas y que se puede asumir un uso potencial de agua subterránea de 40 mm/estación si los granjeros utilizan sondeos someros instalados en fosas para la extracción. Se recomienda una aproximación estructurada para el desarrollo y uso de las aguas subterráneas, basada en la zonificación de áreas potenciales.

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Résumé Au Bangladesh, l’exploitation des ressources en eau souterraines pour l’irrigation est une composante vitale de la stratégie agricole gouvernementale pour atteindre l’auto-suffisance alimentaire. Parmi les baisses de niveaux piézométriques rapportées sur de nombreuses régions du Bangladesh, le potentiel d’utilisation des eaux souterraines a été étudié sur le Projet de Barrage de Teesta (TBP), où, dans un passé récent, les eaux souterraines ont été massivement exploitées pour l’irrigation en saison sèche. De nombreux outils et techniques ont été utilisés, comme l’analyse combinée des chroniques piézométriques et des modèles mathématiques pour dégager les variables hydrogéologiques clés, le calcul des besoins nets pour l’irrigation, et l’utilisation des systèmes d’information géographique. Les résultats démontrent que la culture en saison d’irrigation de la variété de riz HYV Boro (saison sèche), économiquement attrayante, n’est potentiellement pas viable sur de nombreux secteurs de la zone étudiée si les prélèvements perdurent selon la tendance actuelle. Cependant, du fait de la variation spatiale des prélèvements, neuf thanas (sous-districts)—sur un total de 21 sur le secteur d’étude—pourraient encore augmenter les surfaces irriguées, et une disponibilité en eau de 40 mm/saison pourrait être générée si les agriculteurs utilisaient des puits peu profonds pour exploiter les ressources souterraines. Une approche structurée, basée sur le zonage des secteurs potentiels, est recommandée pour l’exploitation et l’utilisation des eaux souterraines.

     

Isotope geochemistry of Paleoproterozoic metacarbonates from Itatuba,Borborema Province,Northeastern Brazil: Evidence of marble melting within a collisional suture

Strongly-deformed marbles may be easily confused with linear and elongated carbonatite intrusions. Both rocks may present similar texture and foliation to the host rock, or even cross cutting field relationships, which could be interpreted either as igneous or high-grade metamorphosed marble. Diagnostic criteria are even more complex when there is evidence of melting of the metasedimentary carbonate rock, such as has been described in the Himalayas and in the Eastern Ghats, India.In the Alto Moxotó Terrane, a high-grade gneissic domain of the Borborema Province, Northeastern Brazil, there are metacarbonates associated with banded gneisses and different metaplutonic rocks. Field evidence indicates the absence of other metasedimentary rocks associated with these marbles, thus suggesting that these carbonates were separated from other siliciclastic metasedimentary rocks. The presence of marble also suggests that it may represent the initial stage of a crustal carbon recycling into the mantle. These marbles present many field similarities to carbonatites (e.g., fluid-flow structure) and, together with metagranites and metamafic intrusions, may represent a major collisional tectonic suture.A detailed study of the carbon, oxygen and strontium isotopic composition of these marbles is presented. This study aims to identify the origin of the different isotopic components. It is argued that these rocks were subjected to temperature and pressure conditions that were sufficiently high to have melted them. The isotopic data presented here support this interpretation and indicate the mixing of two components: (i) one characterized by radiogenic Sr isotopes and mantle-like carbon isotopes, which is associated with the gneissic and mafic rocks, and (ii) another characterized by low ⁸⁷Sr/⁸⁶Sr ratios and highly positive δ¹³C values. Available geochemical data for the upper Paleoproterozoic indicate that the ⁸⁷Sr/⁸⁶Sr ratio of ocean water, varying between 0.7050 (2.25 ± 0.25 Ga) and 0.7047 (1.91 Ga), falls within the lower range of the samples from Itatuba and thus reinforces the interpretation that these marbles are sedimentary-derived and were partially contaminated by interaction with the host gneissic and mafic rocks.