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,
The multilayered Djeffara aquifer system, south-eastern Tunisia, has been intensively used as a primary source to meet the growing needs of the various sectors (drinking, agricultural and industrial purposes). The analysis of groundwater chemical characteristics provides much important information useful in water resources management. Detailed knowledge of the geochemical evolution of groundwater and assessing the water quality status for special use are the main objective of any water monitoring study. An attempt has been made for the first time in this region to characterize aquifer behavior and appreciate the quality and/or the suitability of groundwater for drinking and irrigation purposes. In order to attend this objective, a total of 54 groundwater samples were collected and analyzed during January 2008 for the major cations (sodium, calcium, magnesium and potassium), anions (chloride, sulfate, bicarbonate), trace elements (boron, strontium and fluoride), and physicochemical parameters (temperature, pH, total dissolved salts and electrical conductivity). The evolution of chemical composition of groundwater from recharge areas to discharge areas is characterized by increasing sodium, chloride and sulfate contents as a result of leaching of evaporite rock. In this study, three distinct chemical trends in groundwater were identified. The major reactions responsible for the chemical evolution of groundwater in the investigated area fall into three categories: (1) calcite precipitation, (2) gypsum and halite dissolution, and (3) ion exchange. Based on the physicochemical analyses, irrigation quality parameters such as sodium absorption ratio (SAR), percentage of sodium, residual sodium carbonate, residual sodium bicarbonate, and permeability index (PI) were calculated. In addition, groundwater quality maps were elabortaed using the geographic information system to delineate spatial variation in physico-chemical characteristics of the groundwater samples. The integration of various dataset indicates that the groundwater of the Djeffara aquifers of the northern Gabes is generally very hard, brackish and high to very high saline and alkaline in nature. The water suitability for drinking and irrigation purposes was evaluated by comparing the values of different water quality parameters with World Health Organization (WHO) guideline values for drinking water. Piper trilinear diagram was constructed to identify groundwater groups where the relative major anionic and cationic concentrations are expressed in percentage of the milliequivalent per liter (meq/l), and it was demonstrated that the majority of the samples belongs to SO 4 -Cl-Ca-Na, Cl-SO 4 -Na-Ca and Na-Cl hydrochemical facies. As a whole, all the analyzed waters from this groundwater have revealed that this water is unsuitable for drinking purposes when comparing to the drinking water standards. Salinity, high electric conductivity, sodium adsorption ratio and sodium percentages indicate that most of the groundwater samples are inappropriate for ir...
Two orthogonal extensional systems produced the extensional collapse of the Tell and Atlas thrust belts in northern Tunisia during the Late Miocene to Pliocene in a context of NW-SE plate convergence between Africa and Eurasia. The older extensional system shows several low-angle normal faults (LANFs) and associated high-angle faults with ENE-directed transport that produced half-grabens and hanging-wall syncline basins during the late Tortonian to Messinian. The direction of extension swinged towards the SE during the Messinian, cutting into, and tilting the previous detachments. Extension was accompanied by the extrusion of 8-Ma rhyodacites and Messinian basalts, together with the development of mineralized fault breccias. Plio-Quaternary NW-SE directed shortening formed inversion arrowhead structures, reverse faults, refolded extensional rollover anticlines and folded the LANFs. ENE-directed extension is concomitant with the opening of the Tyrrhenian basin. We consequently think that both processes are related and that tearing of the Calabrian slab along the northern Tunisia coast drove the ENE-directed extension. Meanwhile, the SE-directed extension that followed was probably related to SE-directed peeling back of the Tunisian continental lithospheric mantle during NW subduction of the Maghrebian margin. This extension propagated eastwards from the late Tortonian until the Pliocene following the SE migrating subduction front and favored by lateral slab tearing along the Tunisian Atlas dextral Subduction Transfer Edge Propagator boundary. This new hypothesis for the tectonic evolution of northern Tunisia shows for the first time the importance of crustal extension in the denudation of the Tunisian Atlas and Tell foreland thrust belts and its relation to deep mantle tectonic mechanisms.
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