NEW_Accomplishments.indd - IRIS

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UPWELLING AND DOWNWELLING 2006 <strong>IRIS</strong> 5-YEAR PROPOSAL Plume Heat Flux Estimates Guust Nolet, Raffaella Montelli • Princeton University Shun Karato • Yale University The global coverage with broad-band seismic stations is now such that our resolving power in selected locations is good enough to make quantitative estimates of plume flux. This figure shows an example of a resolution test for a cross-section through the Reunion plume at a depth of 1100 km. The left image shows the temperature anomaly (degrees K), the center column the rise velocity in cm/yr computed for a viscosity at background temperature of 1.6 × 10 23 Pa.s and an iron enrichment ∆XFe of 0.3%. On the right we show the associated heat flux in W/m 2 . These quantities were computed for three different models, from top to bottom: the model that was input to generate data for the resolution test, the model resulting from the tomographic inversion of these synthetic data, and the model resulting from the tomographic inversion of the actual data. In this case the heat flux (0.12 TW) calculated from the tomographic image of the synthetic model was slightly higher than the input heat flux of 0.10 TW. The ʻobservedʼ heat flux calculated for the same values of viscosity and iron enrichment is 0.29 TW (biased) or 0.24 TW (corrected for bias). These estimates become lower when the effect of anelasticity is taken into account. The units are K (left), cm/yr (center) and W/m 2 (right). 136
2006 <strong>IRIS</strong> 5-YEAR PROPOSAL UPWELLING AND DOWNWELLING Detection of Upper Mantle Flow Associated with the African Superplume Mark D. Behn • Woods Hole Oceanographic Institute Clinton P. Conrad • University of Michigan Paul G. Silver • Carnegie Institution of Washington A continental-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 a large-scale upwelling is predicted to affect the regional horizontal flow field in the upper mantle. To test this model we compared seismic anisotropy inferred from shear wave splitting measurements with instantaneous flow calculations that incorporate mantle density structure determined from seismic tomography. We calculated splitting parameters at 13 ocean-island GSN and Geoscope stations surrounding Africa. Splitting measurements from island stations are ideal for interpreting anisotropy induced by asthenospheric flow because they lack a thick overlying lithosphere that may also contribute to the observed anisotropy. We tested for a possible lithospheric contribution by comparing the splitting measurements with the fossil spreading directions. We found that although the fossil l poor fit to the data at stations located farther off-axis. Thus, we conclude that far from a ridge axis the observed anisotropy is dominated by asthenospheric flow. To test for an active component of mantle upwelling, we considered several models with net-rotation (NNR) or hotspot reference frames, 2) driven by plate motions at the Earthʼs surface, or 3) driven by a combination of plate-motion and mantle density heterogeneity inferred from either seismic tomography or the history of subduction. We found that the best-fitting flow field is one generated by plate motions and density heterogeneity associated with largescale upwelling originating in the lower-mantle beneath southern Africa and is manifest as a radial pattern of flow at the base of the asthenosphere. This model provides a significantly better fit to the observed anisotropy than a model in which mantle flow is driven through a passive response to subduction. A.) Comparison of splitting observations to anisotropy predicted from global mantle flow. Black symbols indicate the orientation of the fast polarization direction from shear-wave splitting measurements at GSN and Geoscope stations. Red bars show horizontal projection of maximum shear calculated at the center of the asthenosphere for plate+density-driven mantle flow. Flow is calculated using the S20RTS tomography model (Ritsema et al., 1999) and the NUVEL-1A plate motions (DeMets et al., 1994). B.) Predicted flow field illustrating upwelling from lower mantle beneath southern Africa. Behn, M.D., C.P. Conrad, and P.G. Silver, Detection of upper mantle flow associated with the African Superplume, Earth Planet. Sci. Lett., 224, 259–274, 2004. 137
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Cornerstone Facilities for Seismolo
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Cornerstone Facilities for Seismolo
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2006 IRIS 5-YEAR PROPOSAL ACCOMPLIS
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SCIENCE SUMMARIES 2006 IRIS 5-YEAR
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2006 IRIS 5-YEAR PROPOSAL EARTHQUAK
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2006 IRIS 5-YEAR PROPOSAL MONITORIN
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2006 IRIS 5-YEAR PROPOSAL INNER COR
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2006 IRIS 5-YEAR PROPOSAL EDUCATION
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