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IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS011 Oral Presentation 2124
Interaction between mantle upwellings and lithosphere as derived from
seismic anisotropy in Africa and Pacific
Prof. Jean-Paul Montagner
Geophysics Seismology IASPEI
Jean-Paul Montagner, Michel Cara, Eleonore Stutzmann, Eric Debayle, Genevive
Roult, Jean-Jacques Lvque
The interaction of continental and oceanic lithospheres with mantle upwellings, has been investigated
for East Africa and Pacific by using 3D anisotropic tomographic model derived from broadband seismic
data . The simultaneous use of Rayleigh and Love surface waves enables to retrieve both azimuthal and
radial anisotropies with a lateral resolution of 500km. The joint interpretation of seismic velocity and
anisotropy in the upper 400 km of the mantle enables to map mantle flow in the upper mantle and
provides information on the complex interaction between mantle upwellings and lithosphere. In
agreement with numerical modeling, it is shown that the flow pattern in the asthenosphere is
significantly affected by mantle upwellings. Two kinds of mantle upwellings can be distinguished. The
first one, such as the Afar plume in Africa originates from deeper than 400 km. From a geochemical
point of view, it is characterized by enrichment in primordial 3He and 3He/4He ratios higher than those
along mid-ocean ridges (MOR). The second one, associated with Cenozoic volcanic provinces (Darfur,
Tibesti, Hoggar, Cameroon) or close to the Mid-Atlantic ridge (Ascension, St Helena, Tristan, Canary,
....), with 3He/4He ratios similar to, or lower than MOR, is a consequence of shallower upwelling,
presumably asthenospheric instabilities (secondary convection). An unexpected strong positive radial
anisotropy is observed below Afar and Hawaii similar to the one found below Iceland (Gaherty, 2001),
which is more difficult to explain from a geodynamic and mineralogical point of view. Both azimuthal
and radial anisotropies show a stratification of anisotropy at depth, corresponding to different physical
processes (shape preferred orientation, lattice preferred orientation) and also related to differential
motion between different layers at depth.
Keywords: anisotropy, lithosphere plume interaction, upwelling