Physics
Scientific paper
Dec 2003
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003agufm.t51b..05b&link_type=abstract
American Geophysical Union, Fall Meeting 2003, abstract #T51B-05
Physics
3210 Modeling, 7218 Lithosphere And Upper Mantle, 8121 Dynamics, Convection Currents And Mantle Plumes, 8147 Planetary Interiors (5430, 5724)
Scientific paper
The presence of a large-scale, low seismic velocity anomaly in the mid to lower mantle beneath Africa is a robust feature of global tomographic models. Assuming the low velocities are associated with warm, less dense material, the African seismic anomaly has been ascribed to a long-lived thermal upwelling from the lower mantle. Such large-scale upwelling should exert significant driving forces on the surrounding plates and affect the regional horizontal mantle flow field. To test this model we compare seismic anisotropy inferred from shear wave splitting measurements with instantaneous flow calculations that incorporate mantle density structure derived from seismic tomography. We calculate splitting parameters at 12 ocean island stations surrounding Africa. Splitting measurements from island stations are ideal for interpreting anisotropy induced by asthenospheric flow due to the lack a thick overlying lithosphere. The observed fast polarization directions are used to investigate the following hypotheses regarding mantle flow. First, we tested for a possible lithospheric contribution by comparing the splitting measurements to fossil spreading directions and found that they are a poor fit to the data at stations located >500 km from the ridge axis. Thus, we conclude that the observed anisotropy is dominated by asthenospheric flow. We then considered several models with varying assumptions about the velocity at the base of the asthenosphere: that it is 1) stationary below plates moving in the no-net-rotation (NNR) and hotspot reference frames, 2) driven by plate motion at the Earth's surface, and 3) driven by a combination of plate-motion and mantle density heterogeneity inferred from seismic tomography. We find that the best-fitting flow field is one that includes density heterogeneity associated with large-scale upwelling originating in the lower-mantle beneath southern Africa, and is manifest as a radial pattern of flow at the base of the asthenosphere. The resulting sub-asthenospheric flow field is characterized by velocities of 0-3 cm/yr, and an asthenospheric viscosity of 1-3ṡ1019 Paṡs is found to be most consistent with the regional anisotropy, geoid height, and dynamic topography.
Behn Mark D.
Conrad Clinton P.
Silver Paul G.
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