The Neogene structural evolution of the western margin of the Pelagian Platform, central Tunisia

Abstract

The deformation front of the Atlas Mountains in central Tunisia is a structure termed the North—South Axis. To the west of Kairouan, the North—South Axis consists of NE—SW to NNE—SSW-trending folds and thrusts. A main decollement thrust in Triassic evaporites has been mapped, with secondary detachment of out-of-sequence thrusts in Tertiary rocks and backthrusts at the base of the Tertiary and in Upper Cretaceous sediments. The Upper Cretaceous and Tertiary sediments are backfolded in the hangingwalls of the backthrusts, which are in turn folded in the SE-verging anticlines. The backthrusts are roof thrusts to ‘passive roof’ duplexes and imply the extensive development of thin-skinned ‘blind’ SE-verging thrusts. A similar structural style is seen further west in the Tunisian Atlas, but major NW-verging backthrusts are not exposed. Adjacent to the Pelagian Platform, the thrust structures are affected by later strike-slip faults. The thin-skinned thrusting at the deformation front is Middle Miocene in age and was synchronous with Africa—Europe collision. The North—South Axis is therefore interpreted as the frontal structure of a collision-related thrust belt. The thrust belt is underlain by a NW—SE- to E—W-trending Middle Jurassic to Early Cretaceous rift basin with probable ‘mid’ Cretaceous to Oligocene post-rift fill. The NE—SW-trending thrust belt is detached from NW—SE- to E—W-trending syn-rift basement faults, but changes in thrust belt geometry across the syn-rift faults indicate that basement structures have compartmentalized the thrust belt. The later strike-slip faulting resulted from the reactivation of the syn-rift basement faults in the post-collisional framework of the thrust belt.