Understanding salt tectonics in Northern Tunisia - Insight from gravity data analysis

Abstract

Introduction Salt tectonics is one of the key fundamental problems of structural analysis in petroliferous basins, and is of broad practice prospecting in petroleum exploration and development stages. Many authors have dealt with salt dome folding and typology, especially after the discovery of ‘diapir plays’ within gas and oil-fields from the Gulf of Mexico and Iran (Talbot and Jaravis 1984; Talbot 1993; Jackson, Vendeville and SchultzEla 1994; Jackson 1995; Vendeville 2002; Krzywiec 2004; Talbot and Aftabi 2004). The study on salt tectonics in extensional basins (such as the Gulf of Mexico) has made great achievements (McBride et al., 1998), and salt tectonics research on compressive regime of foreland fold belt has also made progress, such as in the Persian Gulf Basin (Letouzey et al., 1995). The salt beds can change the occurrence of overlying horizons, forming various structural traps (Rowan, 1995). The salt bed itself is the most effective caprocks (Liang-Jie et al 2004). Focuses were performed on salt rising mechanism and geologic parameters effect such as: tectonic regime, basement faulting, initial salt layers thicknesses, overburden thickness, rate of sediment aggradations and salt dissolution. The beginning of salt movement is related essentially to extension in the subsalt basement (thickskin) or to extension in post-salt sedimentation (thinskin). Densitydriven diapirism is probably not the initiating mechanism (Perthuisot, 1978), as these diapirs are not developed in stable regions even with the presence of a thick initial salt layer. Regional tectonics play more important role in the initiation of salt movements than stratigraphic density inversion. Northern tunisian Atlas is characterized by numerous NE– SW trending Triassic salt outcrops (Fig.1) in contact with younger rocks. Opinions are divided concerning the mechanism of emplacement of Triassic edifices and their dynamics. For some authors, Perthuisot (1978), Hammami (1999), Chikhaoui (2002), Jallouli (2005), Triassic salts series have pierced their cover as mushroom-like structures whereas some others Ghanmi (2001), Vila et al. 2002) have mapped “salt glaciers” bodies on a regional scale. Gravity data analysis based on recently collected gravity data, observed surface geology and assisted by analogous salt rising models help constraining the Triassic salt structure of outcrops in northern tunisian Atlas and allow deriving the mechanism of their presumably polyphasic motion. It permits the identification of a Triassic dome covered by Quaternary series.