Lateral ramp-related folding evidences in the Tellian domain of Tunisia: Tectonic implications

During the Alpine compression, the basal decollement propagation throughout the North-African margin of Tunisia resulted in the development of frontal ramps (parallel to the Tellian thrust-belt). This study demonstrates that lateral ramps have also formed synchronously with the frontal ramp folds. According to the location of the ramp faults, two fold types whose axes lie parallel to the transport direction are distinguished: (i) lateral ramp folds pertaining to the allochthonous series originally resting above a lateral ramp; and (ii) drape folds which formed above lateral ramps, following a cover substitution. By mean of geometric and tectonosedimentary criteria displayed by two chosen field examples, we show that many of the Northern-Tunisian lateral folds can be considered as lateral ramp folds and that the lateral ramps took place near inherited synsedimentary normal faults, striking sensibly orthogonal to the thrust-belt strike. We suggest that the highly non-cylindrical feature of the Tellian fold-belt from Tunisia, accounts for the likely abundant laterally growing tectonic edifice of this type.