PIVETTA T., BRAITENBERG C.

1. THE LITHOSPHERE STRUCTURE BENEATH THE BENUE TROUGH FROM MODELING GRAVITY FIELDS OF GOCE AND EGM08 PIVETTA T., BRAITENBERG C. Dipartimento di Geoscienze, Università di Trieste, Italy, email: berg@units.it INTRODUCTION AND OBJECTIVES The Benue Trough is a Cretaceous basin, located in Central-Nothern Africa, that is important for its high hydrocarbon potential (Fig. 1). Its origin is related to the opening of the Atlantic Ocean (Binks and Fairhead, 1992), in particular it has been associated to a sinistral tectonic movement on a strike slip fault. From gravimetric and geologic observations, Fairhead and Okereke (1987;1990) stated that the Benue Trough could be a good example for the application of a stretching model of the lithosphere as the one proposed by McKenzie (1978). Our work improves the knowledge of this basin by modeling the new data from GOCE (Migliaccio et al, 2010) and the EGM08 gravity and gradient fields (Pavlis et al., 2008) constrained with the application of the McKenzie model and the data from literature. Cameroon Volcanic Line Benue Trough Atlantic Ocean a b Fig.1 a) The area of study of the Benue trough. Black lines represent the sections of previous works (Tokam et al., 2010; Fairhead and Okereke, 1987), while the red lines are the sections modeled in our study. b) Bouguer map derived from GOCE at 4000m b a SYNTHETIC SIGNALS FROM McKENZIE MODEL Before studying the Benue Trough in detail, we analyzed the McKenzie model by the production of density models of the lithosphere and calculating over them the gravity signals gz and Tzz (Braitenberg et al., 2010) at the height of GOCE orbit. In these models the stretching factor (beta) and the initial crustal thickness (yc) have been varied. Results: 1) Recurrent and recognizable pattern of gz and Tzz 2) Non-linear relationship between the depth of basin and the amplitude of the gravity minimum (dependent on β and on the initial crustal thickness) 3) Gradient is a good tool to get informations on the width of basin Fig.2 a) gravity signals gz and Tzz calculated at 250km height with initial crustal thickness of 35km. b) the same gravity signals (gz, Tzz) calculated with initial crustal thickness of 20km. b a 1) Presence of non-uniform stretching with depth, that implies a larger sub-crustal extension than the basin’s width 2) Introduction of a body with intermediate density (w.r.t. mantle and crust) that replaces the mantle under the crust: it has been interpreted as underplated basaltic magma. Results: In the final part of this work we evaluated the stretching factors acting on crust and mantle over three sections (Fig.3): as you can observe, the Benue Trough (3 a) and b)) has been subjected to a higher stretching w.r.t. its eastern arm, Yola rift (Fig.3 c)). This characteristic could also explain the different volumes of magma found beneath these two zones: a smaller stretching value implies a reduced volume of magma, while an important stretching is underlined by greater magmatic activity. • CONCLUSIONS • Gravity observations and application of the McKenzie model constrain lithosphere model of the Benue Trough • Introduction of an underplated body under the crust demonstrates that there was important magmatic activity. This fact could be also important for the understanding of the nearby CVL GRAVITY MODELING OF BENUE TROUGH We used seismological results such as Tokam et al. (2010), Fishwick (2010) and previous gravity studies (Fairhead and Okereke, 1987) as constraints for our model, and then we used the approach of McKenzie to calculate the densities of crust, mantle and asthenosphere. We found that we have to introduce some modifications to the McKenzie model in order to get a good fit of the Bouguer anomalies (we calculated them at 4000m to take advantage of the high detail of EGM08 model): c REFERENCES Binks R. M., Fairhead J. D. (1992), Tectonophysics, 141‐151 Braitenberg C., Mariani P., Ebbing J., Sprlak M. (2010), GSL Fairhead J. D., Okereke C.S. (1987), Tectonophysics, 143, 141‐159 Fishwick S. (2010), Lithos, 120, 63‐73 McKenzie D. (1978), Earth Planet. Sci. Lett., 40, 25‐32 Migliaccio F., Reguzzoni M., Sansò F., Tscherning C. C., Veicherts M. (2010), Bergen, June 27 – July 2, Bergen, Norway, 2010 Pavlis, N. K., Holmes, S. A., Kenyon, S. C., Factor, J. K. (2008), Vienna, Austria, April 13 ‐ 18, 2008 Tokam A. P. K., Tabod C. T., Nyblade A. A., Julià J., Wiens D. A., Pasyanos M. E. (2010), Geophys. J. Int., 183 (2), 1061-1076 Fig.3 a) b) c) modeling of the gravity fields in terms of lithosphere structure over the three sections ACKNOWLEDGEMENTS: We thank Albert Eyike of the University of Douala, Leonardo Uieda of the Observatorio Nacional of Rio de Janeiro and the Italian Space Agency (ASI) for supporting the GOCE-Italy project. Partially the work was supported by PRIN contract 2008CR4455_003.