Interpretation of subhorizontal crustal reflections by metamorphic and rheologic effects in the eastern part of the Pannonian Basin

The geologic origin of subhorizontal reflections, often observed in crustal seismic sections, was investigated by establishing metamorphic facies and strength of rocks in depth, and correlating these properties to seismic reflection sections from eastern Hungary. Estimation of the depths of metamorphic mineral stability zones utilized the principles developed by Fyfe et al. and known geothermal data of the area. The strength versus depth profile was derived by relating local seismic P -wave interval velocities to Meissner et al. 's activation energy. The results show that the series of subhorizontal reflections, observed in the Pannonian Basin, are a consequence of combined metamorphic and rheologic changes in depths. The synthesis of the integrated data set suggests that the retrograde alteration of the pre-Tertiary basement above the percolation threshold was made possible by the softening effect of shear zones and their water-conducting capacity. The subhorizontal reflections of highest energy, of the consolidated crust below the percolation threshold, originate in the depths of greenschist, amphibolite and granulite metamorphic mineral facies, which were formed in geothermal and pressure conditions similar to those existing today. These results imply the overprint of earlier (Variscan) metamorphic sequences of the crust by more recent retrograde metamorphic processes.     

Synchrotron IR study of hydrous ringwoodite (γ-Mg2SiO4) up to 30 GPa

High-pressure synchrotron infrared (IR) absorption spectra were collected between 650 and 4,000 cm⁻¹ at ambient temperature for hydrous Mg-ringwoodite (γ-Mg₂SiO₄) up to 30 GPa. The main feature in the OH⁻ stretching region is an extremely broad band centred at 3,150 cm⁻¹. The hydrogen bond is strong for most protons and the most probable site for protonation is the tetrahedral edge. With increasing pressure, this band shifts downward while decreasing its integrated intensity until disappearance at a pressure of 25 GPa. Only one band at 2,450 cm⁻¹ and an absorption plateau persist with a maximum wavenumber of 3,800 cm⁻¹. This behaviour is reversible upon pressure release. We interpret this as a second-order phase transition occurring in hydrated Mg-ringwoodite at high pressure (beyond ∼ 25 GPa). This result is compatible with the observation by Kleppe et al. (Phys Chem Miner 29:473–476, 2002a) who suggested the presence of Si–O–Si linkages and/or partial increase in the coordination of Si. Beyond the phase transition, the protons are delocalized and their environment on the ringwoodite structure is probably quite different from that at low pressure. Data obtained in situ at high pressures and temperatures are needed to better understand the effect of protonation on the structure and to better constrain this phase transition.     

Contested terrains: Cities and hinterlands in post-apartheid boundary delimitations

In South Africa attempts are being made to address the socio-spatial distortions of the apartheid era through a more equitable distribution of resources, and the re-drawing of municipal geographical boundaries. However, boundaries are not neutral geographic lines. Boundary changes are often associated with a redistribution of political power and resources. The aim of this paper is to analyse the effects of the contemporary territorial and administrative restructuring on urban dynamics in South Africa. More specifically, the focus is on how the process of territorial restructuring impacted on metropolitan areas as well as on secondary cities and their hinterlands. Examining and elucidating the manner in which various social, economic and political forces have manifested themselves in the process of boundary delimitation in a major metropolitan centre as well as adjacent rural areas is a central theme of this paper. There were considerable contestations over the delimitation of new local government boundaries. Affluent metro authorities like that in Durban were opposed to the spatial extension of their boundaries because of the costs of the providing services and infrastructure in the deprived margins. Similarly, there was concern that incorporation of rural areas will result in increased municipal service charges being imposed on these communities. Tensions were heightened between urban and rural regions because traditional leaders believed that their territorial jurisdiction and authority were being undermined. In other parts of the country, the merger of traditionally white and black fragments of secondary cities often resulted in many black locations continuing to be marginalized. There appears to be neither the political will nor the economic capacity to upgrade these zones of marginalized urban communities. While the Municipal Demarcation Board was largely successful in eliminating the political geography of apartheid at a macro- scale, this paper suggests that the greater challenge for government and policy makers is to reduce the socio-spatial and economic inequalities which appears to be still very high and perhaps increasing. This revised version was published online in July 2006 with corrections to the Cover Date.     

Simultaneous use of geochemical and geophysical methods to characterise abandoned landfills

. Groundwater sampling and geophysical methods determined a serious contamination problem associated with refilled exploitation sites at the Cal Dimoni area, Llobregat delta, Barcelona, Spain. To characterise this process, hydrogeochemical analyses were performed and showed the following modifications to groundwater chemical composition: increasing pH values, changing redox conditions, significant increases in total organic carbon (TOC) and certain trace elements, and high groundwater conductivity values. Major ion content accumulations were found under the refilled area. In contrast, elements involved in the oxidation–reduction processes, such as iron, manganese and nitrates, clearly diminished. Electromagnetic prospecting methods were also performed and delineated the contamination plume extent. These methods also showed separate sources of contamination, one clearly related to the groundwater–refilled zone leachate interaction, another as a consequence of the manure–accumulation surface site. Geochemical and geophysical methods have shown similar results for locating groundwater contamination sources, and for determining leachate generation mechanisms and flow paths.     

Al hematites prepared by homogeneous precipitation of oxinates: material characterization and determination of the Morin transition

A novel method for synthesis of aluminium hematites, based upon the homogeneous precipitation of Fe and Al oxinates in various proportions, is presented. The precursor precipitates are heated in air at 700?°C. X-ray diffraction, thermal analyses, BET, FTIR, optical reflection analysis, TEM and Mössbauer spectroscopy at room temperature and 80?K of the resulting products indicate that single-phase hematites are formed with structural Al substitution of up to 10 at%. Interestingly, the particle size (>100?nm) is not substantially reduced by the Al content. Although it remains difficult to obtain a homogeneously distributed Al substitution in the final hematite, this processing line offers a unique opportunity to separate the effects of grain size and Al substitution on the Morin transition temperature (T _M) of Al hematite. From the comparison between the present hematites and a series of Al-substituted hematites with lepidocrocite as precursor, it could be shown that the effect on T _M, associated with a change of a factor 10 in grain size, is about 1/3 of the effect caused by a change of 10 in the degree of substitution. Finally, it is suggested that proper thermal treatments under different conditions of the same precursors are likely to produce spinel phases.     

Spatial and temporal variation of the nekton and hyperbenthos from a temperate European estuary with regulated freshwater inflow

The aquatic macrofauna of the Guadalquivir estuary were sampled (1 mm mesh persiana net) at 5 sampling sites located along the entire (except the tidal freshwater region) estuarine gradient of salinity (outer 50 km). A total of 134 fish and macroinvertebrate species was collected but only 62 were considered common or regularly present in the estuary. Univariate measures of the community structure showed statistically significant differences among sampling sites: species richness, abundance, and biomass decreased in the upstream direction, being positively correlated with the salinity. Temporal differences of these three variables were also statistically significant. While a clear seasonal pattern (minimum densities in winter and maximum in spring-summer) was observed for abundance and biomass, no such pattern existed for the number of species. Mysids was the most dominant group throughout the estuary (96% to 99% of abundance; 49% to 85% of biomass), although fish biomass was also important at the outer estuary (36% to 38%). Multivariate analyses indicated highly significant spatial variation in the macrofaunal communities observed along the salinity gradient. These analyses suggest that the underlying structure was a continuum with more or less overlapping distributions of the species dependent on their ability to tolerate different physicochemical conditions. There were also significant temporal (intermonthly + interannual) variation of the estuarine community; the relative multivariate dispersion indicated that monthly variation was more considerable (relative multivariate dispersion >1) at the outer part of the estuary during the wet year (last 20 km) and was higher in the inner stations during the dry year (32 to 50 km from the river mouth). Since a clear negative exponential relationship was observed between the freshwater input (from a dam located 110 km upstream) and water salinity at all sampling stations, it is concluded that the human freshwater management is probably affecting the studied estuarine communities. While the higher seasonal (long-term) stability of the salinity gradient, due to the human control of the freshwater input, may facilitate the recruitment of marine species juveniles during the meteorologically unstable early-spring, the additional (short-term) salinity fluctuations during the warm period may negatively affect species that complete their lifecycle within the estuary.     

Seismic Hazard Estimate at the Iberian Peninsula

—?Seismic hazard at the Iberian Peninsula has been evaluated by using a methodology which combines both zonified and non-zonified probabilistic methods. Seismic sources are used when considering zones where certain calculation parameters may be considered homogeneous, as in zonified methods, while, on the other hand, earthquakes are considered wherever it has taken place, as in non-zonified methods. The methodology which is applied in this paper has been originally used to calculate the seismic hazard maps in the United States. In our case, it has been necessary to adapt the method to the specific features of the seismicity in the Iberian Peninsula and its geographical surroundings, not only with respect to its distribution and characteristics, but also with respect to the properties of the seismic catalog used.¶Geographically, the main feature of the result is the fact that it reflects both historical seismicity and current seismic clusters of the region. Despite the smoothing, maps show marked differences between several seismic zones; these differences becoming more noticeable as exposure time increases. Maximum seismic hazard is found to be in the southwestern region of the Peninsula, especially in the area of the Cape St. Vicent, and around Lisbon. The uncertainty of the results, without considering that due to the attenuation laws, as deduced from the other evaluation parameters, is quite stable, being more sensitive to the parameters b and m _max of the Gutenberg-Richter relation.