The structural style of the Southern Atlassic foreland in Northern Chotts Range in Tunisia: field data from Bir Oum Ali Structure

Abstract

We used field data collected from the Bir Oum Ali structure (BOAS) and take into consideration recent published geophysical works to evaluate the deformation style and the structural evolution of the Southern Atlassic foreland in Northern Chotts Range in Tunisia. Various structural interpretations have been proposed for the genesis of the Northern Chotts Range. The BOAS was interpreted previously as (1) curved, right-stepping “en echelon” geometry folds as the result of NW-trending lateral strike-slip faulting (2), an anticline structure developed and the result of Ramp-related folding deformed the sedimentary cover (thin-skinned model) over the rigid basement during a single tectonic event (3), response of salt tectonic that began in Early Jurassic and allows the migration to nucleated folding during tertiary compressional events. New field data of the BOAS show a deformation style, in which shortening is differently accommodated in the eastern, southern, and northern areas. Data highlight a tight folding and steeply to overturned strata related to the N100–110° E-trending Bir Oum Ali–Hachichina fault systems (BOAHFS). This later exhibits fault kinematics, with striation showing a multiphase history. The second-order fault systems associated to the main trend of the BOAHFS are usually apparent strike-slip faults. Unfolding of the structure shows an inherited normal faulting. In addition, the normal faulting generates syntectonic conglomerates associated to the first normal faulting movement. The E-trending fold-related fault reactivation of the E-trending Northern Chotts Range might result from the reactivation of inherited Mesozoic faulting. A new structural data interpretation give information of tectonic inversion starting in Late Cretaceous (Campanian–Maastrichtian transition?). The Late Cretaceous to present-day history is dominated by two major events, i.e., Eocene Atlassic contractional event and Middle–Late Miocene to present-day Alpine event. The folding style, the partial reverse–reactivation of the pre-existing E-trending BOAHFS major faults, and the abundant thick tertiary siliciclastic growth strata sequences together with the recent published geophysical data provide a coherent model, in which the thick-skinned tectonic style (development with basement inversion at the depth) is synchronous of thin-skinned tectonic (shallow decollement in the sedimentary cover).