The gravity imaging as a mean to characterize the structural mechanism in the Medjerda Basin (Northern Tunisia)

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The middle Medjerda valley belongs to the Tellian zone of Tunisia; it behaves as post-orogenic basin where surface structural indices are almost completely absent. It imports to highlight that Alpine and Atlasic tectonic prints are well expressed in the surrounding depression. This paper presents an integrated study considering a set of surface geology and gravity data. The aim is to improve knowledge of subsurface structuring of the Medjerda valley in order to characterize its structural pattern aiming to integrate the area in the regional framework of the geodynamic evolution of Northern Tunisia. Subsurface geological modeling was based on gravity data interpretation including the complete Bouguer anomaly, Upward Continuation, residual and derivative fields. Euler Deconvolution was essential for subsurface geometric features characterization. In spite of its sub-horizontal and smoothed topography and the low density of the homogenous quaternary series covering the area, gravity responses are well contrasted and variable reflecting the heterogeneities of the geological substratum. The enhanced horizontal gravity gradient confirms structural systems that are already evidenced by surface geology. It proves, also, the presence of new masked fault systems. It consists on: (i) NW directed Grabens located at Mellegue and Sidi ez Zaouam areas, (ii) Triassic diapirs associated to NE thrust faults of El Merdja, Bousalem, Ben Bechir and Mjez el Assa. These structures are generally sealed by the quaternary series. The presence of distensive and compressive structures within a compressive regime is explained by the solicitation of preexisting fault systems in the area. During the NW Upper Miocene shortening, the geodynamic evolution of Northern Tunisia was dominated by strike slip thrust movement through the reactivation of deep major inherited faults: e.g. (i) the dextral EW oriented fault system of Ghardimaou- Thibar, (ii) the sinister NS accident of Dehmani- Bousalem- Ras Rajel. These accidents shaped the Medjerda basin and contribute to its complex geological architecture.