Structural styles of the Tellian fold-and-thrust belt of Tunisia based on structural transects: Insights on the subsurface oil and gas pre-salt plays

Abstract

We present several structural transects throughout the internal part of the Tunisian Tell focusing on some representative structures and domains. We used also an interpreted seismic cross-section and a 3-D structural view to highlight the styles within the Tellian units in the Fernana-Adissa allochthon domain. The structural style is mainly represented by active roof-thrust and duplex structures affecting the Mesozoic-Cenozoic sequences belonging respectively to the infra-Kasseb and Kasseb units. The main decollement thrusting layer is located in the Triassic evaporites, while the roof thrust corresponds to the Upper Cretaceous or to the Paleocene shaly levels. The structural transects allow viewing that the Numidian Flysch unit presents different structural configurations and relationships with its subsurface supporting units; (1) it rests on an apparent stratigraphic contact above the underlying Kasseb nappe or even over pre-existing Triassic salt structures while elsewhere (2) it is detached above these infra-Numidian units reworking an originally downlap contact. The area is also characterized by important halokinetic movements controlling the overall architecture of the Tell fold-and-thrust belt that allows the thrusting of the whole Tellian wedge. In addition, originally apical parts of former diapiric structures were remobilized subsequently as sole thrust allowing the displacement of the Numidian Flysch. The Upper Miocene syn-folding sequences sourced by the erosion of the precedent uplifted reliefs are folded and tectonically transported along some active thrusts by the Tell tectonic units suggesting a recent reactivation and rejuvenation of thrust during the Upper Miocene to Quaternary Alpine events. Since the Late Miocene, shortening is still active today, allowing the refolding of older nappes contacts as well as rejuvenation of inherited faults.