No abstract

In the Lake Léré region, southern Chad, Neoproterozoic terrains are distributed in four lithostructural groups that reveal the geotectonic evolution of a part of the Pan-African orogenic domain. The first group includes basaltic volcanic rocks and fine-grained detrital sedimentary rocks of pre-tectonic basins that were emplaced in an extensional regime, close to a volcanic arc. The second and third groups include calc-alkaline gabbroic intrusions emplaced at an upper crustal level and a midcrustal tonalite, respectively, that are interpreted to be the roots of an active margin volcanic arc. These first three groups experienced WNW to ESE compression, and may belong to a fore-arc basic-volcanic arc-back-arc basin system that was accreted eastward to the Palaeoproterozoic Adamaoua-Yadé Block. The fourth group includes post-tectonic granite plutons invading the older groups. This paper documents the accretion processes in the southern margin of the Saharan Metacraton.

International audienceIn the Sahelian belt, Lake Chad is a key water body for 13 million people, who live on its resources. It experiences, however, substantial and frequent surface changes. Located at the centre of one of the largest endorheic basins in the world, its waters remain surprisingly fresh. Its low salinity has been attributed to a low infiltration flow whose value remains poorly constrained. Understanding the lake's hydrological behaviour in response to climate variability requires a better constraint of the factors that control its water and chemical balance. Based on the three-pool concep-tualization of Lake Chad proposed by Bader et al. (2011), this study aims to quantify the total water outflow from the lake, the respective proportions of evaporation (E), transpiration (T), and infiltration (I), and the associated uncertainties. A Bayesian inversion method based on lake-level data was used, leading to total water loss estimates in each pool (E + T + I = ETI). Sodium and stable isotope mass balances were then used to separate total water losses into E, T, and I components. Despite the scarcity of representative data available on the lake, the combination of these two geochemical tracers is relevant to assess the relative contribution of these three outflows involved in the control of the hydrological budget. Mean evapotranspiration rates were estimated at 2070 +/- 100 and 2270 +/- 100 mm yr(-1) for the southern and northern pools, respectively. Infiltration represents between 100 and 300 mm yr(-1) but most of the water is evapotranspirated in the first few kilometres from the shorelines and does not efficiently recharge the Quaternary aquifer. Transpiration is shown to be significant, around 300 mm yr(-1) and reaches 500 mm yr(-1) in the vegetated zone of the archipelagos. Hydrological and chemical simulations reproduce the marked hydrological change between the normal lake state that occurred before 1972 and the small lake state after 1972 when the lake surface shrunk to a one-tenth of its size. According to our model, shrinking phases are efficient periods for salt evacuation from the lake towards the phreatic aquifer

International audienceThe Pala region, in southwestern Chad, belongs to the northern part of the Central African Pan-African Fold Belt. It is made up of greenschist-facies schists and is characterized by bimodal, mainly mafic, magmatism. This schist unit named Goueigoudoum Series is intruded by pre- to post-tectonic plutonic rocks dated between 737 and 570 Ma and dykes of quartz. Gold is mined artisanally from alluvial deposits and primary chalcopyrite-pyrite-bearing quartz veins, brecciated and silicified zones and shear zones. The majority of the mineralized shear zones and some quartz veins generally trend N-S to NNE-SSW or NW-SE and are interpreted as extensional shear fractures related to regional NE-SW-trending sinistral strike-slip shear zones. The geological context of the Pala region clearly indicates hydrothermal fluids formed along active continental margins during collisional orogenesis, and subsequent associated fluid migration typically occurred during strike-slip events. Although the origin of fluids may be varied (magmatic, metamorphic or meteoric fluids, Proterozoic seawater, or sedimentary basin formation waters), the distribution of the mineralizations along the granitoid intrusions suggests that magmatism played a major role in the dynamics of the mineralizing fluids

Abstract. In the Sahelian belt, Lake Chad is a key water body for 13 million people who live on its resources. It experiences, however, substantial and frequent surface changes. Located at the center of one of the largest endorheic basins in the world, its waters remain surprisingly fresh. Its low salinity has been attributed to a low infiltration flow whose value remains poorly constrained. Understanding the lake's hydrological behavior in response to climate variability requires a better constraint of the factors that control its water and chemical balance. Based on the three-pool conceptualization of Lake Chad proposed by J. C. Bader, J. Lemoalle, and M. Leblanc (Bader et al., 2011), this study aims to quantify the total water outflow from the lake, the respective proportions of evaporation (E), transpiration (T) and infiltration (I), and the associated uncertainties. A Bayesian inversion method based on lake-level data was used, leading to total water loss estimates in each pool (ETI). Sodium and stable isotope mass balances were then used to separate total water losses into E, T and I components. Despite the scarcity of representative data available on the lake, the combination of these two geochemical tracers is relevant to assess the relative contribution of these three outflows involved in the control of the hydrological budget. Mean evapotranspiration rates were estimated at 2070 ± 100 and 2270 ± 100 mm yr−1 for the southern and northern pools respectively. Infiltration represents between 100 and 300 mm yr−1 but most of the water is evapotranspirated in the first few kilometers from the shorelines and does not efficiently recharge the Quaternary aquifer. Transpiration is shown to be significant, around 300 mm yr−1 and reaches 500 mm yr−1 in the vegetated zone of the archipelagos. Hydrological and chemical simulations reproduce the marked hydrological change between the normal lake state that occurred before 1972 and the small lake state after 1972 when the lake surface shrunk to a tenth of its size. According to our model, shrinking phases are efficient periods for salt evacuation from the lake towards the phreatic aquifer.