The changing role of hydrogeology in semi-arid southern and eastern Africa

Much of southern and eastern Africa is semi-arid and heavily groundwater dependent. Borehole drilling commenced over a hundred years ago with magnetic and electrical resistivity surveys for borehole siting being introduced from 1936. Formalised training of hydrogeologists led in the 1970s to an almost standard approach to hydrogeological investigation and a period of stability followed, during which some major investigations were carried out. A period of decentralisation and fragmentation has since taken place in many parts of southern and eastern Africa, and groundwater monitoring and management are inadequate in many countries. All but six of the 14 SADC (Southern African Development Community) member states reportedly have an adequate monitoring network in place. However, groundwater demand is increasing and hydrogeologists need to promote the use of appropriate methodologies as an essential part of tackling the severe issues now facing the water sector in the region.     

P-T paths and problems of high-temperature polymetamorphism

Many Precambrian granulite-facies metamorphic complexes contain so-called straight gneisses, which are massive rocks with a clearly pronounced blastomylonitic texture, lineation, and gneissosity. These rocks occur exclusively in high-temperature ductile shear zones, which can develop either during the primary exhumation of rock complexes or during the overprinting by high-temperature dynamometamorphism. The main criterion for distinguishing between these two types of straight gneisses is the configuration of their P-T trajectories, which are recorded in the mineral assemblages in these rocks and their host gneisses. Ductile shear zones developed in Archean granulite gneisses simultaneously with their exhumation, and, hence, their P-T trajectories are segments of decompression and/or isobaric cooling paths. Straight gneisses in Proterozoic polymetamorphic complexes commonly compose high-temperature ductile shear zones overprinted on Archean granulite complexes, and the P-T paths of these rocks are Z-shaped. This means that, at a constant pressure in the middle part of the continental crust, the T _min of the older P-T trajectory corresponded to T _max of the younger trajectory, and often T _max–T _min > 100°C. Such ductile shear zones commonly have a strike-slip morphology and can be easily seen in aerial photographs and discerned during structural geological surveying. These zones can overprint older gneisses without any notable thermal effect on the latter. Relations of this type were identified in the granulite complexes of Limpopo in South Africa, Sharyzhalgai in the southwestern Baikal area, and Lapland in the Kola Peninsula. The results of our research propose a solution for the well-known problem of the significant discrepancies between the isotopic ages in high-temperature-high-pressure complexes and the partial or complete distortion of radiogenic isotopic systems under the effect of a newly inflowing metamorphic fluid. The application of geochronologic techniques to these situations is senseless, and only P-T trajectories provide insight into the actual age relations between the discrete tectono-metamorphic stages. It is thus expedient to conduct not only structural studies of metamorphic complexes but also their detailed petrological examination and the calculation of their P-T paths before geochronologic dating.     

Some aspects of the interrelations between magmatism and ore formation: Evidence from oxygen isotope data on ore-magmatic systems, Armenia

The results of oxygen-isotope study of the Kokhb-Shnokh (J₃) and Vokhci (N₁) intrusive complexes and the related Tekhut and Kadzharan Cu-Mo porphyry deposits of different types, Armenia, were used to discuss the relevant problems of rock and ore genesis and their relationships. It was established that the assimilation of parental mantle basaltic melts by crustal, mainly feldspathic material is a decisive factor in the formation of multiple gabbro-granite complexes. Specifics in the composition and crystallization of the hybrid melt causes the enrichment of the residual melt and hydrothermal fluids in Fe and other elements, in particular, Cu and Mo. The ore-bearing solutions are generated over the solidification time of the melt. They are removed by the hydrothermal system of directionally moving solutions, a process that gives way to a convective-circulation hydrothermal system. A change in the spatial position of the hydrothermal solutions during the formation of the deposit supplies them to different sites, creating a seeming discontinuity in ore deposition, which is observed at many deposits. In fact, the supply of the ore-bearing solutions is continuous process. The ore stage is not a time interval with particular ore-bearing solutions but rather a certain physicochemical state of the solutions attained during their interaction with the environment. This process resulted in the simultaneous precipitation of different mineral assemblages and the asynchronous precipitation of the same assemblages. The formation of deposits of various metals related to a common intrusive complex, as changes in the composition of mineral assemblages, is caused by the influence of various geochemical barriers rather than by compositional variations in the initial hydrothermal solutions.     

Secular variations of the upper crust composition: Implication of geochemical data on the Upper Precambrian shales from the Southern Urals western flank and Uchur-Maya region

In the mid-1980s, it was concluded based on geochemical study that Th, Sc, La concentrations and ratios Th/Sc, La/Sc and Eu/Eu* did not wary significantly in the post-Archean time. It was impossible to judge about compositional variations of upper crust during the Riphean and Vendian, because data of that time characterized a limited number of samples from the post-Archean basins of Australia, New Zealand, and Antarctic. Considered in this work are variations of Eu/Eu*, LREE/HREE, Th/Sc, and La/Sc ratios in Upper Precambrian fine-grained siliciclastic rock of the Southern Urals western flank (Bashkirian meganticlinorium) and Uchur-Maya region (Uchur-Maya plate and Yudoma-Maya belt). As is established, only the Eu anomaly in the studied siliciclastic rocks is practically identical to this parameter of the average post-Archean shale. Three other parameters plot on the Riphean-Vendian variation curves with positive and negative excursions of diverse magnitude, which do not coincide always in time. It is assumed that these excursions likely mark stages of local geodynamic activity, destruction of pre-Riphean cratons, and progressing recycling of sedimentary material during the Riphean.     

Conceptual hydrodynamic model of the Pamukkale hydrothermal field, southwestern Turkey, based on hydrochemical and isotopic data

The results of study on the hydrochemical and isotope characteristics of shallow and deep waters at Pamukkale hydrothermal field Turkey are described in order to obtain a better understanding of the hydrological circulation. The field can be grouped into two groundwater sub-systems; cold water springs of Ca–HCO₃ type (10–12 °C), and CO₂-rich thermal waters of Ca–HCO₃–SO₄ type (25–58 °C). The occurrence of these water types is closely related to the morphology of the region, where intense tectonism formed horst and graben structures. Hence, two hydrogeological systems were defined: a deep geothermal system which is related to extensive and deep circulation of meteoric water in the regional flow system, and a shallow system which is related to local groundwater flow through sedimentary strata. The meteoric water falling at higher elevations percolates to the local groundwater system at a shallow level and flows to the deep geothermal system. During a deep convection cycle from a recharge to discharge area, the cold water attains heat from the asthenospheric intrusions, causing it to ascend. Variations of chemical and isotopic composition of thermal waters result from their mixing with cool groundwater in a shallow aquifer during their ascent to the surface.

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Résumé Les résultats d'une étude portant sur les caractéristiques hydrochimiques et isotopiques de puits phréatiques et profonds situés dans le champ hydrothermal de Pamukkale, sont décrits de telle manière à éclairer le fonctionnement des circulations hydrologiques. Le champ peut étre divisé en deux sous-systèmes d'eaux souterraines, l'un avec des eaux de sources froides (10–12 °C) de type Ca–HCO3, et les eaux thermales (25–58 °C) riches en CO₂ et de type Ca–HCO₃–SO4. L'occurrence de ces types d'eaux est fermement liée à la morphologie de la région, oùne tectonique intense a engendré des structures en horsts et en grabens. Dés lors deux systèmes hydrogéologiques ont été définis : un système profond, qui est lié à la circulation extensive et profonde des eaux météoritiques dans le système régional d'écoulement, et un système phréatique lié aux écoulements locaux des eaux souterraines à travers les strates sédimentaires. Les eaux météoritiques aux altitudes élevées, percolent jusqu'aux systèmes locaux phréatiques, puis coulent jusqu'aux systèmes géothermaux plus profonds. Durant le cycle de convection profond des zones de recharge jusqu'aux zones de décharge, l'eau froide atteint les zones chaudes liées aux intrusions athenosphériques, provoquant la remontée. Les variations de la composition chimique et isotopique des eaux thermales résultent dans leurs mélanges avec des eaux souterraines froides dans les aquifères phréatiques durant leur remontée jusqu'à la surface.

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Resumen Se describen los resultados del estudio de las características isotópicas e hidroquímicas de las aguas someras y profundas para obtener un mejor entendimiento de la circulación hidrológica del campo hidrotermal Pamukkale. El campo puede agruparse en dos sub-sistemas de agua subterránea: manantiales de agua fría del tipo Ca–HCO₃ (10–12°C) y aguas termales ricas en CO₂ del tipo Ca–HCO₃–SO₄(25–58°C). El ambiente de estos tipos de aguas se relaciona estrechamente con la morfología de la región donde el tectonismo intenso ha formado estructuras extensionales tipo graben y horst. Se definieron dos sistemas hidrogeológicos: un sistema geotermal profundo que se relaciona con la circulación profunda y extensa de agua meteórica en el sistema regional de flujo y un sistema somero el cual se relaciona con flujo local de agua subterránea a través de estratos sedimentarios. El agua meteórica que cae en altas elevaciones percola al sistema local de agua subterránea en un nivel somero y fluye hacia el sistema geotermal profundo. Durante un ciclo de convección del área de recarga hacia la zona de descarga, el agua fría se calienta a partir de los intrusivos astenosféricos lo que ocasiona que asciendan. Como resultado de la mezcla de las aguas recalentadas, con agua subterránea fría en un acuífero somero durante el ascenso hacia la superficie, se derivan variaciones en la composición química e isotópica de las aguas termales.

     

The Spačice eclogite: constraints on the P–T–t history of the Gföhl granulite terrane, Moldanubian Zone, Bohemian Massif

Summary In the Kutná Hora Complex, the Běstvina Formation, which is similar to Gf?hl granulite, contains eclogite that has escaped widespread retrograde recrystallization. The eclogite assemblage, garnet + omphacite + quartz + rutile ± plagioclase, yields an estimate for peak metamorphic conditions of 18–20 kbar and 835–935 °C, which is comparable to that determined from felsic granulite, 14–20 kbar and 900–1000 °C. Garnet in eclogite exhibits both prograde and retrograde compositional zoning, from which constraints on thermal history of the Gf?hl terrane can be derived by diffusion modelling. At 900 °C, a garnet grain of 800–1000 μm radius would homogenize in 7.5–11.7 million years, but the existence of compositional gradients on a length scale of 100–200 μm suggests that the duration of peak metamorphism may have been limited to ∼500,000 years. Diffusion modelling of retrograde zoning in garnet yields a cooling rate of 150–100 °C/m.y. for a radius of 800–1000 μm and initial temperature of 900 °C. The relatively brief duration of high-pressure/high-temperature metamorphism and rapid cooling and exhumation of the Gf?hl terrane may be a consequence of lithospheric delamination during Early Carboniferous collision of Bohemia (Teplá-Barrandia) and Moldanubia (Franke, 2000).     

New Generation of Probabilistic Seismic Hazard Assessment for the Area Cologne/Aachen Considering the Uncertainties of the Input Data

A new earthquake catalogue for central, northern and northwestern Europe with unified M_w magnitudes, in part derived from chi-square maximum likelihood regressions, forms the basis for seismic hazard calculations for the Lower Rhine Embayment. Uncertainties in the various input parameters are introduced, a detailed seismic zonation is performed and a recently developed technique for maximum expected magnitude estimation is adopted and quantified. Applying the logic tree algorithm, resulting hazard values with error estimates are obtained as fractile curves (median, 16% and 84% fractiles and mean) plotted for pga (peak ground acceleration; median values for Cologne 0.7 and 1.2 m/s² for probabilities of exceedence of 10% and 2%, respectively, in 50 years), 0.4 s (0.8 and 1.5 m/s²) and 1.0 s (0.3 and 0.5 m/s²) pseudoacclerations, and intensity (I₀ = 6.5 and 7.2). For the ground motion parameters, rock foundation is assumed. For the area near Cologne and Aachen, maps show the median and 84% fractile hazard for 2% probability of exceedence in 50 years based on pga (maximum median value about 1.5 m/s²), and 0.4 s (>2 m/s²) and 1.0 s (about 0.8 m/s²) pseudoaccelerations, all for rock. The pga 84% fractile map also has a maximum value above 2 m/s² and shows similarities with the median map for 0.4 s. In all maps, the maximum values fall within the area 6.2–6.3° E and 50.8–50.9° N, i.e., east of Aachen.