Tectono-sedimentary events and geodynamic evolution of the Mesozoic and Cenozoic basins of the Alpine Margin, Gulf of Tunis, north-eastern Tunisia offshore E ´ve´nements tectono-se´dimentaires et e´volution ge´odynamique des bassins me´sozoı ¨ques et ce´nozoı ¨ques de la marge alpine, cas du Golfe de Tunis,

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Trente kilomètres au sud-ouest de Tunis, les deux lames de « Trias », noyées dans les argiles clansayésiennes qui longent les deux flancs de l'anticlinal atlasique de Bir Afou, sont encadrées par des contacts sédimentaires, matérialisés par des conglomérats glauconieux à insolubles triasiques. Ces deux lames résultent, pour nous, d'un épanchement rapide en contexte distensif, pendant l'Aptien supérieur, à partir d'une source nourricière qui serait l'actuelle masse « triasique » de Bir Afou. Ce dispositif correspond à un « glacier de sel » de type Ben Gasseur décrit par Vila et al. [J.M. Vila, M. Ben Youssef, M. Chikhaoui, M. Ghanmi, Bull. Soc. géol. France 167 (1996) 235-246], mis en place en extension et plissé au cours de l'Éocène inférieur. Les dépôts de l'Éocène moyen et supérieur, à caractère transgressif, reposent en discordance sur les argiles aptiennes de la voûte de l'anticlinal. La compression au Miocène supérieur, importante phase tectonique régionale, aboutit à l'installation des structures actuelles qui témoignent de tectoniques superposées bien prononcées, depuis l'installation du « glacier de sel » jusqu'à son évolution dans les structures atlasiques. Cet appareil salifère permet d'étendre, loin vers l'est, la notion de domaine des « glaciers de sel » en Tunisie septentrionale.

The chemical composition of groundwater within the multilayer Jeffara aquifer and the detailed analysis of saturation indices enables identification of the origin of mineralization and hydrogeochemical processes occurring in groundwater. It has been demonstrated that groundwater is mainly characterized by a Ca-Mg-SO 4-Cl water type. Geochemical pattern is controlled by the dissolution of evaporites, largely abundant in the study area and incongruent dissolution of dolomite. Gypsum, anhydrite, mirabilite and thenardite have been examined as potential sources of sulphate dissolved in groundwater. Since Jeffara groundwater is recharged by the Continental Intercalaire (CI) geothermal water, water temperature decreases from the CI to the Jeffara aquifer. Solubility of the majority of minerals is modified by this change in temperature and thus a mixing process of thermal and non-thermal waters was examined.

The Bou Arada Trough is an east-west-oriented structure located 80 km SW of Tunis, characterizing the central Tunisian Atlas. This trough is filled by a thick Quaternary sand and clay series and is bordered by complex systems of folds generally trending NE-SW. Contacts between the Bou Arada Trough and the neighbouring folds are accommodated by NE-SW-and NW-SE-oriented faults. In contrast to the other troughs of the Tunisian Atlas, which are related to the Pliocene-Quaternary orogenic period, the geodynamic evolution of the Bou Arada Trough began in the Maastrichtian and has continued until the present day. Structural, tectonosedimentary and seismic data analyses are undertaken in the study area to better understand the evolutionary scenario of this trough. The results obtained show that the Bou Arada Trough is fragmented into three NW-SE-oriented sub-basins and records a continuous history of downthrow. Indeed, during extensional to transtensional regimes, this trough has evolved in response to the two networks of perpendicular fractures whereas during compressive to transpressive periods, the collapse of the Bou Arada Trough has been induced by a pull-apart mechanism using the same network of faults but with a strike-slip movement. The Bou Arada Trough thus preserves a record of the convergence between the European and African plates since the Maastrichtian.