Structural and geodynamic study in central Tunisia using field and geophysical data: new structural interpretation of the N–S axis and associated Atlassic structures

Abstract

A structural and geodynamic evolution of central Tunisia is constructed from field studies and geophysical data within the Tunisian Atlassic domain. Bouguer gravity and horizontal gradient magnitude (HGM) maps indicate that computed anomalies are related to subsurface structures and are in accordance with the general structural trends of central Tunisia. Near Cherichira fault, the strike is ENE–WSW, whereas it is N–S along the N–S axis, a major discontinuity between the Pelagian shelf to the east and Central Atlas Mountains to the west. E–W-oriented HGM profiles on the north side of the Cherichira fault show moderate HGM variations, which correspond to the NE–SW fault system of the Ousselet–Bou Dabbous. This trend is also expressed by the second-order vertical derivative map of this area. However, on the south side of the fault, a dominant peak is indicated rather than the anomalies characteristic of the north side of the fault. This anomaly is associated with the N–S axis discontinuity. Seismic reflection data are also used to elucidate the structural configuration and style north and south of the Cherichira fault. The seismic lines demonstrate that faulting and associate halokinesis in Ousselet–Bou Dabbous area are not comparable to the paleogeographic fault system of the N–S axis southward. To the south of Cherichira fault, the seismic lines show massive Triassic evaporates piercing along the major paleogeographic faults associated with thickness variations of Mesozoic and Cenozoic stratigraphic units and a reverse slip of western structural elements. It seems that the N–S axis corresponds to a steeply dipping major zone of weakness in the Tunisian Atlassic Mountains and indicates evidence of several stages of activity, including the rise of Triassic evaporates rise up which outcrop at several localities such as Rheouis and Mezzouna. The loading and upward rise of Triassic evaporites along this weak zone are a key factor in the thinning of the Mesozoic–Cenozoic sedimentary cover along this paleogeographic discontinuity. These new data indicate the northeast extrapolation of fold and grabens of the Central Atlas of Tunisia to the northeast of Kairouan, whereas the northern extent of the N–S axis is limited to south of the Cherichira fault. The Cherichira fault is NE–SW oriented and parallels the Ballouta–Zaghouan fault system, which reflects the regional strike of the Tellian orogeny and the associated nappes to the north. The geometry and kinematics of these Tellian features are linked to the tectonic evolution of the northern African margin, including Mesozoic extension separating the African and European plates to the inversion of these structures since the Late Cretaceous. The N–S axis is correlated in its direction and tectonic framework to the strike-slip fault system of the African craton, which is present to the south in the Algerian Saharan domain of Hoggar.