Gravity contribution to the geological study of grabens in the northwest region of Tunisia including the Kef Basin region

Abstract

The northern Atlas region of Tunisia is characterized by northeast-trending compressional structures that affected the extensionally formed Late Permian to Mid Cretaceous Tunisian Trough. More recently, 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 which was further analyzed using derivatives, Euler deconvolution and three-dimensional modeling. The resulting residual gravity anomaly field consists of a series of short wavelength maxima that are mainly related to anticlinal structures and minima related to sediment-filled grabens. To better define the structure of the basins, three-dimensional (3D) gravity models were constructed, where the grabens were found to be approximately 3–5 km in thickness and mainly bounded by normal faults and underlain by low density Triassic evaporites. This evaporitic layer was not underlying the entire study area but only in the vicinity of the grabens. In particular, the Kef Basin region was found to be two separate grabens, the Kef and the Ksour located southeast of the Kef graben. The Kef graben has a maximum thickness of 3–4 km while the wider Ksour graben is between 4 and 5 km thick. The gravity analysis cannot determine exact origin of the grabens but extension is the most plausible cause of the grabens with strike-slip motion may have played a role in the formation of the Ksour graben if not the other grabens. The location of the grabens was controlled by the lateral extent of the Triassic evaporates which provided the rheological weakness to initiate the grabens.