GRAVITY CONSTRAINTS ON THE CRUSTAL STRUCTURE OF NORTHERN TUNISIA

Bouguer gravity data were analyzed to determine the general crustal and upper mantle structure in northern Tunisia. Residual gravity anomalies were determined by removing the gravitational effect of crustal thickness variations imaged by regional seismic experiments. Residual gravity anomalies contain short wavelength anomalies superimposed on a long wavelength component that decreases in amplitude northward towards the Tunisian coastline. An edge enhancement analysis (e.g., enhanced analytic signals) of the short wavelength anomalies suggests a previously unknown east-west trending gravity anomaly south of 370N with source depths between 3 and 7 km. Modeling of the regional, residual and Bouguer gravity anomalies indicate there are two possible solutions for the residual gravity decreasing in northern Tunisia: 1) thickening of Cenozoic and Mesozoic sediments north of a strike-slip fault or 2) a crustal and upper mantle low density zone interpreted to be crustal material of the remnant subducted African plate. The latter result is favored based on seismic tomographic images of the Mediterranean region which implies subducting material exists under the African coast, geologic interpretations suggesting the Tell Atlas may be a thrust wedge accreted by underplating of the African continental crust and seismic refraction models indicating a thinning of sediments in northern Tunisia. The east-west trending gravity anomalies south of 37°N corresponds to an important structural feature that may be related to either to strike slip/transform fault or subduction beneath the African plate.