GRAVITY CONTRIBUTION TO THE GEOLOGICAL STUDY OF GRABENS IN THE NORTHWEST REGION OF TUNISIA INCLUDING THE KEF BASIN REGION

The northern Atlas region of Tunisia is characterized by northeast-trending compressional structures that has affected extensionally formed Late Permian to Mid Cretaceous Tunisian Trough. Late Cenozoic extensional forces have formed a series of northwest-trending grabens whose origins are partially controlled by geologic structures formed since the Late Permian. In order to aid in determining how these grabens were formed, a detailed gravity analysis was conducted in the western region of the Tunisian Trough. The gravity analysis included the construction of a residual gravity field using wavelength filtering and the residual gravity field was further analyzed using derivatives, strike-pass filtering and Euler deconvolution. The residual gravity anomaly field consists of a series short wavelength maxima that are mainly related to anticlinal structures and minima related to sediment filled grabens. To determine structure of the basins, three-dimensional (3D) gravity models were constructed. The grabens were found to be approximately 5-10 km in thickness and bounded by listric normal faults. In particular, the Kef Basin region was found to be two separate basins, the Kef Basin and the newly found Ksour Basin, southeast of the Kef Basin. The two basins are separated by a Triassic evaporate cored anticline and both basins are bounded by Triassic evaporate cored regions. The Kef Basin has a maximum thickness of 7 km while the wider Ksour Basin is 9 to 10 km thick. Plus the Ksour Basin has a significant northwest-trending component related to thrust motions along the Teboursouk Fault unlike the other grabens in the region. The presence of Triassic evaporates supports the interpretation that evaporitic movement was at least partially responsible for the extension forces that help create these grabens.