GNGTS 2017 - 36° Convegno Nazionale

558 GNGTS 2017 S essione 3.1 connected to the density contrast between the crust and mantle; 4- to confirm the stability of the residuals, we derived six cross-sections through the anomalies Bouguer map. In each profile, the complete Bouguer anomaly and the free-air anomaly field show similar trends and the stability of the residuals is confirmed by the perfect overlapping of the residual trends with different filters; 5- we made the inversion of the negative values of the residuals for the depth of the basement discretizing the Chad basin with a series of rectangular prisms of size of ~ 10 km. In the inversion, we used different density contrast between crust and sediments. We chose a range of values of density contrast (200-400 kg/m 3 ) on the base of the density of the samples measured with the hydrostatic balance. The density contrast is an important constrain since the depth of the basement depends on it. Indeed, a reduction of this contrast, leads to a proportional increase in the basement depth. Main results. Topography-gravity regression . For low topography (< 1000 m) there is an inverse proportion between the Bouguer gravity anomalies and topography (Fig. 1). This is due to isostatic compensation. For higher topography, the correlation is loose. This indicates the increase of crustal density of the Tibesti volcanic Massif area, the only value of the topography >1000 m. Residual Maps . They have been obtained from the regression analysis using satellite and terrain dataset (Fig. 2). These maps show the gravity signal induced by the crustal density variations occurring between the tectonic features, after the removal of the effect of topography and crustal root. The Residual Map (Fig. 2), shows: 1) a large, generally weak, negative anomaly (< -20 mGal), affecting most of the basin, related to the sedimentary infill. A higher negative anomaly (-30 mGal) with a “U” shape, extending north to the Chad lake is probably related with the Termit rift basin, already identified by seismic reflection profiles (Genik, 1993). 2) We can recognize inside the basin some small positive anomalies (20-30 mGal), probably due to some local basalt dykes. 3) We can observe a negative gravity anomaly (~ -50 mGal) around Sarth, trending NE- SW, corresponding to the Central African Rift (CAR) (Genik, 1992). 4) Along the northwestern edge of the basin there is a pattern of positive anomalies (~ 40 mGal) trending NW-SE, likely corresponding to the volcanic intrusions along the edges of the rifts. 5) An interesting local positive anomaly lineament (~ 50 mGal) discussed by Braitenberg et al. (2011) and Li et al. (2013) and Liégeois et al. (2013) along the southeastern edges of the basin and approximately 1000 m extended. Depth of the basement under the Lake Chad . It has been obtained from the inversion of the residuals (Fig. 3). The depth of the basement under Lake Chad shows a sharp variation from north to south. In the northern part of Lake Chad, the basement is between 4 and 6 km deep, and reaches the depth of ~12 km in the Termit Rifting Basin. This value is consistent with that estimated in the previous study of Genik (1993). In contrast, in the southern part of the Lake Chad, the basement shallows to 2-3 km. The depth of the basement under the city of Bol ranges between 2 and 4 km. The largest depth of the basement (~ 15 km) is observed 100 km west of the Chad Lake, cannot be connected with a specific tectonic feature, because of the poor Fig. 2 - Gravity Residual Map from satellite data (Eigen- 6c4) after the regression analysis.