Anisotropic Signature of the Afar plume in the Upper Mantle.
Abstract
Plumes remain enigmatic geological objects and it is still unclear how they are formed and whether they act independently from plate tectonics. The role of plumes in mantle dynamics can be investigated by studying their interaction with lithosphere and crust and their perturbations on flow pattern in the mantle. The flow pattern can be derived from seismic anisotropy. An anisotropic surface wave tomography in the Horn of Africa was performed. The choice of the experiment in the Horn of Africa is motivated by the the presence of the Afar hotspot, one of the biggest continental hotspot. In the framework of the mantle degree 2 pattern, the Afar hotspot is the antipode of the Pacific superswell, but its origin at depth and its geodynamic importance are still debated. Data were collected from the permanent IRIS and GEOSCOPE networks and from the PASSCAL experiment in Tanzania and Saudi Arabia. We completed our data base with a French deployment of portable broadband stations surrounding the Afar Hotspot. Path average phase velocities are obtained by using a method based on a least-squares minimization (Beucler et al.,2002). A correction of the data is applied according to the a priori 3SMAC model (Nataf and Ricard, 1996). 3D-models of velocity, radial and azimuthal anisotropies are inverted for. Down to 250km, low velocities are found beneath the Red Sea, the Gulf of Aden, the South East of the Tanzania Craton, the Afar hotspot. High velocities are present in the eastern Arabia and the Tanzania Craton. These results are in agreement with the isotropic model of Debayle et al. (2002). The anisotropy model beneath Afar displays a complex pattern. The azimuthal anisotropy shows that the Afar plume might be interpreted as feeding other hotspots in central Africa. Deeper in the asthenosphere, a wide stem of positive radial anisotropy (VSH > VSV) comes up, where we might expect the reverse sign. The same observation was made below Iceland (Gaherty, 2001) and Hawaii (Montagner, 2002). Different interpretations of this observation can be proposed, in terms of perturbation of the flow pattern around Afar or of the predominant influence of water-rich plume material where other mechanisms of alignment prevail (Jung and Karato, 2001).
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.S11A1128S
- Keywords:
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- 1236 Rheology of the lithosphere and mantle (8160);
- 7218 Lithosphere and upper mantle;
- 7255 Surface waves and free oscillations;
- 8121 Dynamics;
- convection currents and mantle plumes;
- 8180 Tomography