GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 3.1 559 Fig. 3 - Basement map of the Chad lake area fromgravity inversion. 200 kg/m 3 is the density contrast between rock basement and sediments. knowledge of the geology of the area. Unfortunately, we do not have further geological/geophysical constraints to confirm the value found. In the other parts of the study area (e.g., under the city of Mao and Moussoro) the depth of the basement is quite shallow (1-2 km). Conclusions. In this project, we estimated for the first time the depth of the basement under the Chad basin in a more detailed way, focusing our attention on the city of Bol where in the framework of the project ICDP, it is planned to drill a borehole for paleo-environmental and anthropological research. For this purpose, we made a joint analysis and interpretation of a spherical harmonic gravity model (GOCE) and terrestrial gravity data (BGI) with well-logs data and density values of Cameroon-Chadian rock samples taken from the collection of the University of Trieste. We found a large negative anomaly (-30 mGal) under the Chad basin connected to the presence of low density sediments. Furthermore, there are several positive anomalies around the edges of the basin and a pattern of negative anomalies outside of it. Both types of trends are linked to the presence of rifts and extensional structures. We could observe a sharp variation of the depth of the basement moving from the northern (4-6 km) to southern part (2-3 km) of the Chad lake. The deepening of the basement is connected to the Termit rift basin and the values are consistent with previous seismic surveys. The depth of the basement under the city of Bol is between 3 and 4 km, but unfortunately there are no other geological/geophysical constraints to confirm these values. For the drilling purpose, since in the inversion we used a minimum value of the density contrast (200 kg/m 3 ) among the range defined (200-400 kg/m 3 ), it is possible to assume that the maximum expected depth of the basement is about 4 km. Furthermore, we suggest an integrative geophysical survey, such as a seismic reflection campaign to get more detailed information about the structure of the basement (faults, highs and lows) as well as on the variability of its depth and the thickness of the sediment cover. Acknowledgements A visit at IPGS-EOST (Strasbourg) by F.M has been supported by a grant of the International Mobility program of the University of Trieste. References Braitenberg, C., Mariani P., Ebbing J., and Sprlak M., 2011. The enigmatic Chad lineament revisited with global gravity and gravity-gradient fields. Geological Society, London, Special Publications 2011, v. 357, p. 329-341. Braitenberg, C., Mariani, P., & De Min, A. (2013). ��� �������� ���� ��� ������ �������� ����� ������ �� The European Alps and nearby orogenic belts sensed by GOCE. Bollettino di Geofisica Teorica e Applicata, 54, 321-334. Braitenberg, C. (2015). Exploration of tectonic structures with GOCE in Africa and across-continents. International Journal of Applied Earth Observation and Geoinformation, 35, 88-95. Genik, G. J. 1992. Regional framework, structural and petroleum aspects of rift basins in Niger, Chad and Central African Republic (CAR). ln: Recueil des Communications pp39-53. Géologie africaine, Colloque Géologie Libreville, Gabon. Genik, G.J., 1993. Petroleum geology of the Cretaceous-Tertiary rifts basins in Niger, Chad, and Central African Republic. American Association of Petroleum Geologists Bulletin v. 73, no. 8, p. 153–168.