Aptian−Albian extension in Tethyan rifted continental passive margin of Tunisia inferred from sequential restorations of balanced cross-sections and expansion index

This article presents an overview of regional extensional deformation across the north-facing southern Tethyan continental passive margin of Tunisia during the Aptian and Albian syn-rift stages. Because of the Late Eocene, Late Miocene and Pleistocene orogenic overprinting, the earlier extensional history of the Tunisian Atlassic province had to be reconstructed from balanced and sequentially restored transects across four key fault systems active at the time. These were used to (i) estimate the amount and timing of extension and (ii) calculate the expansion index (EI) for each fault system. The analysis demonstrates the region experienced a very significant amount of mid-Cretaceous extension, representing the last of several Permian-Mesozoic rift events. Two closely linked styles of deformation were recognised: thick-skinned basement involved faulting and shallow listric faulting detached on decollement surfaces within late Triassic evaporites. Both styles of faulting triggered salt diapirism.

Sequential pre-orogenic restorations of a transect across the Tebaga Kebili and Orbata faults in the southern part of this province suggest both experienced a large amount of extensional displacement during the Aptian and Albian (Aptian-Albian extension = 2.6 km), recorded by footwall erosion, growth strata, rollover structures and changes in sedimentary facies. A similar restoration across the Labaiedh (M'rhila-Cherichira) fault system further north, identified both thick and thin-skinned extensional movement associated with diapiric growth and salt extrusion reaching a maximum during the Aptian (Aptian-Albian extension = 0.55 km). A pre-Tertiary restoration of the Teboursouk fault system in the northern part of the province, demonstrated thick-skinned extension peaking during the Albian (Aptian-Albian extension = 0.4 km). This accommodated a thick sequence of basinal shales to the north, deformed by active diapirism with sea-floor salt extrusion, triggered by deep basement faulting and enhanced by the plastic incompetent character of the thick shale dominant overburden.

Regional extension across the Tunisian Atlassic province during the Aptian-Albian was contemporaneous with a significant change in intra-plate stress across northern Africa, the opening of the South Atlantic and structural reorganization along the East African continental margin. This synchroneity across such a wide region may reflect some still poorly understood deep crustal processes, ultimately responsible for reactivating older rift faults and regional tilting across the province. The associated gravity salt gliding and diapirism bear comparison with the post-rift deformational style of the South Atlantic passive margin. However intra-evaporite decollement surfaces were more local, limited by active thick-skinned faulting with diapirism typically soft linked to deeper fault movement.