STRUCTURES AND TECTONICS OF TUNISIA

In the Tunisian Atlas nothing is known about the nature of the acoustic basement, and in the Saharan cratonic part, only a few deep exploration wells have reached the Precambrian below a thick unfolded Palaeozoic sequence. North of the craton, a major set of faults (W-E or WNW-ESE) represents the southern limit of palaeo-Tethys; thick sequences of marine Late Carboniferous and Permian were developed below the Jeffara and Chott Jerid. Triassic evaporites are thick below the Tunisian Atlas, which has enabled detachment of the cover from the basement and folding into "plis de couverture" structures. Large diapiric intrusions are known, especially in the northwestern Sillon Tunisien (Tunisian trough). From east to west there is a progressive variation from the stable platform on the eastern side to the unstable shelf of western Tunisia. Between these a zone of tectonic instability, named the North-South Axis, corresponds to a deep fault system that acted as a positive suture, characterized by tilted blocks, gaps in sedimentation, and reduced or condensed sequences. In the north, the unstable shelf of western Tunisia grades to the deep furrows where the sediments now forming the Tellian and Numidian nappes were deposited. These nappes overthrust the autochthonous foreland during the Miocene orogeny. The main folding took place in the Pliocene and Early Pleistocene. Recent tectonic activity occurs along the main fault zones.