Role of preexisting faults in the geodynamic evolution of Northern Tunisia, insights from gravity data from the Medjerda valley

Abstract

The middle Medjerda valley belongs to the Tellian zone of Tunisia; it behaves as a post-orogenic basin where surface structural indices are almost completely absent. It is noteworthy to recall that Alpine and Atlasic tectonic prints are well expressed on both sides of this basin. This paper presents an integrated study comprising geology and gravity data. Subsurface geological modeling was based on gravity data interpretation including the complete Bouguer anomaly, upward continuation, residual and derivatives. 2.5D gravity modeling was essential for subsurface geometric features characterization. In spite of its sub-horizontal and smooth topography and the low density of the homogenous Quaternary series covering the area, well contrasted and variable gravity responses are observed reflecting the heterogeneities of the geological substratum. The enhanced horizontal gravity gradient confirms structural systems that are already evidenced by surface geology. It supports, also, the presence of new masked fault systems. The main structural features inferred are (i) NW directed Grabens located at Mellegue and S. ez Zaouam depressions, (ii) Triassic diapirs associated to NE thrust faults of El Merdja, Bousalem, Ben Bechir and Mjez el Assa. These structures are generally covered by the Quaternary series. The presence of both distensive and compressive structures within a compressive regime is explained by the solicitation of preexisting fault systems in the area. During the NW directed shortening occurred at the Upper Miocene age, the geodynamic evolution of Northern Tunisia was dominated by strike-slip thrust movements through the reactivation of deep major inherited faults: e.g. (i) the dextral EW oriented fault system of Ghardimaou-Thibar, and (ii) the sinister NS accident of Dehmani–Bousalem–Ras Rajel. These accidents shaped the Medjerda basin and contribute to its complex geological architecture.