Download Presentazione di PowerPoint - GOCE Italy

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: [email protected]
Benue Trough
Cameroon
Volcanic Line
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.
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
a
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
1) Presence of non-uniform stretching
with depth, that implies a larger subcrustal extension than the basin’s
width
2)
c
Fig.3 a) b) c) modeling of the
gravity fields in terms of
lithosphere structure over the
three sections
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):
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.
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.
CONCLUSIONS
1)Gravity observations and application of
the McKenzie model constrain
lithosphere model of the Benue Trough
1)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
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