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This paper presents new petrological, geochemical, isotopic (Nd) and geochronological data on magmatic rocks from the southern Ouaddaï massif, located at the southern margin of the Saharan metacraton. This area is made of low-grade metasediments intruded by large felsic batholiths. The oldest but not dated magmatic activity is represented by amphibolites intercalated within silicilastic metasediments. They are metagabbros characterized by high Mg#, flat, LREE-depleted patterns, and superchondritic εNd, suggesting derivation of the parental basaltic magma from a depleted, probably asthenospheric mantle source. Subsequently, sediments of southern Ouaddaï were intruded by large composite peraluminous meta-granites. U-Pb zircon dating yield ages of 635 ± 3 Ma and 613 ± 8 Ma on a peraluminous biotite-leucogranite and a muscovite-leucogranite, respectively. These plutonic rocks contain recycled Archean and Paleoproterozoic zircon cores and are interpreted as the result of partial melting of metasediments. The last magmatic event is marked by the emplacement of granodiorite, biotite-granite and pyroxene-monzonite defining a metaluminous, high-K calc-alkaline to shoshonitic suite with a trace-element signature similar to calc-alkaline post-collisional series. A biotite-granite of this suite contains zircon grains that yield an age of 540 ± 5 Ma with inherited cores crystallized around 1050 Ma. These intermediate to felsic rocks are interpreted to derive from the differentiation of basic to intermediate magmas generated in an enriched mantle that has been affected by partial melting at the end of the Pan-African orogeny. The Mesoproterozoic zircon cores might reflect the presence of a previously unrecognized Mesoproterozoic basement near southern Ouaddaï. These new data show that the southern Ouaddaï is dominated by Ediacarian to lowermost Cambrian magmatic suites intrusive in a siliciclastic metasedimentary sequence. Amphibolites probably record Rodinia breakup or the development of a back-arc basin before 635 Ma. Early Ediacarian (635-613 Ma) deformed anatectic leucogranites correlate to contemporaneous migmatites and leucogranites in Cameroon and Sudan emplaced during the main Pan-African tectonic-metamorphic event leading to pervasive melting in the deep crust. The end of the Pan-African orogeny is marked, as in the whole Central Africa Orogenic belt, by a high-K magmatic event corresponding to the post-collisional Pan-African stage.

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<p>High-resolution characterisation of land deformation and its spatio-temporal response to external triggering mechanisms is an important step towards improving geological hazard forecasting and management. The work presented here is part of the ResEau-Tchad project (www.reseau-tchad.org), with a focus on the city of N’Djamena. The extraction of groundwater to sustain this rapidly growing capital city has increased the pressure on water supply and urban sanitation infrastructures which are failing to meet the current water demand. In this study we exploit Synthetic-Aperture Radar (SAR) data acquired by the Sentinel-1 satellite to investigate the temporal variability and spatial extent of land deformation to assist in the development of a sustainable water management program in N’Djamena city. </p><p>The objectives of the work are: 1) to analyse the recent evolution of land deformation using two multi-temporal differential interferometry techniques, SBAS and PS-InSAR; and, 2) to investigate the land deformation mechanism in order to identify the factors triggering surface movements. The PS-InSAR and SBAS techniques are implemented on SAR images obtained in both ascending and descending orbits from April 2015 to May 2021 to generate high resolution deformation measurements representing the total displacement observed at the surface. While the pattern of displacement indicated by the two datasets is similar, the average velocity values obtained with PS-InSAR tend to be noisier than the ones derived using the SBAS technique, particularly when the SBAS time-series shows non-linear deformation trends.</p><p>Characterisation of the subsidence areas by means of statistical analyses are implemented to reveal the surface deformation patterns which are related to different geo-mechanical processes. The integration of the spatio-temporal distribution of PS and SBAS InSAR results with geological, hydrological, and hydrogeological data, along with subsurface lithological modelling shows a relationship between vertical displacements, clay sediments, and surface water accumulation. These areas are located mostly in the surroundings of the urban area. The city centre is observed to be mostly stable, which might be the result of the removal of the surface water through the city drainage system. Investigation of the relationship between vertical displacements and seasonal groundwater fluctuations or effects due to the groundwater withdrawal is limited due to the temporally sparse piezometric dataset; however, the recent deformation rates appear to be correlated with the groundwater level trend at some locations.</p>