Bouguer gravity anomaly and topography data have been used to
estimate the effective elastic thickness, Te, of the African
lithosphere. The highest values (Te > 100 km) correlate with
relatively old Archaean cratonic regions and the lowest ( 0 < Te <
10) with younger Late Palaeozoic fold-belts and Mesozoic/Tertiary
rift systems. No simple relationship exists, however, between Te
and the age of the lithosphere at the time of loading. Cratons (e.g.
West Africa) and fold-belts (e.g. Atlas) of similar thermal age
display both high and low Te values. A good correlation exists
between Te and the present day surface heat flow over Africa :
regions of low Te correlate with high heat flow whereas high Te
regions have low heat flow. Thus, continental Te depends more on
the present day geotherm and, hence composition, than on the
cooling history of the lithosphere. The Te and heat flow data over
the West Africa craton, Kaapvaal-Zimbabwe craton, Damara fold
belt, Niger basin, and the Central African Plateau, for example,
can be explained by a thermal and mechanical model in which Te
is given approximately by the depth to the 400 oC isotherm. We
have used the Te structure of Africa to isolate that part of the
gravity field which cannot be accounted for by flexure and must
be caused by other processes. The resulting anomalies, which have
amplitudes of up to 25 mGal and wavelengths of about 2000 km,
correlate with regional changes in topography. There is a
suggestion of a lineation of the long-wavelength gravity field in
the direction of absolute African plate motion which, if correct,
implies that even beneath the slow moving plates there may be
some alignment of motions in the underlying mantle.
Hartley, R., Watts, A. B. and J. D. Fairhead, Earth Planet. Sci. Letts.,
137, 1-18,1996