New insights on the structural style of the subsurface of the Tell units in north-western Tunisia issued from seismic imaging: Geodynamic implications

Structural interpretations of seismic sections crossing the Tell area in Tunisia, allow us to outline the structural style of the subsurface along the thrust front of the Numidian “nappe” and its relationships with the substratum north of the Middle Mejerda valley. The seismic interpretations show a structural style characterized by large thrust sheets detached over the Triassic salt, whereas the autochthonous cover is affected by inherited faults and tilted blocks. These inherited faults are connected at depth on the major “décollement” level represented by the interface Triassic salt/Mesozoic-Cenozoic cover. This autochthonous Atlas type cover recorded the Eocene Atlas compressional event outlined by the angular unconformity of the Oligo-Miocene on top of folded Cretaceous strata. Seismic interpretation shows also the occurrence of important halokinetic movements deforming both the autochthonous and allochthonous series. Two types of diapirs can be distinguished: the first ones cutting the autochthonous sedimentary cover, the second ones the Tellian sheets specially the Numidian “nappe” which is the uppermost tectonic unit of the Tellian belt. These latter Triassic levels have pierced the allochthonous cover after the “nappes” emplacement i.e. after the Langhian and during the Late Miocene compressional events. Otherwise, Triassic salt is incorporated as sole thrust level beneath the Numidian “nappe”. A regional structural transect crossing the whole Tunisia thrust belt from the Tell to the southeastern foreland and the Zaghoaun thrust-fault allows us to illustrate the overall structural style of the Domain. It outlines the occurrence, beneath the upper-most Tellian structural level (Numidian) of imbricated units issued from duplexing of the sedimentary cover. It appears that the Numidian “nappe” detaches over Middle-Late Eocene shales which represent the upper-most “décollement” level. The Triassic salts locally exposed at the base the Numidian “nappe” are incorporated mechanically during the southward gliding of the “nappe” above the duplex structures.