HR: 08:30h
AN: U11B-03 INVITED    [Abstracts]
TI: Seismic Tomography and Mantle Flow
AU: * Dziewonski, A M
EM: dziewons@eps.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, MA 02138, United States
AB: This year marks the 30th anniversary of the beginning of global seismic tomography. The hope was that mapping velocity heterogeneities in the Earth interior would answer some of the fundamental questions of geodynamics. This has turned out more difficult than originally thought. Nevertheless, seismic tomography reveals several features that should lead to constraining the range of allowable geodynamic models. One of such findings is the "redness" of the power spectrum of lateral heterogeneity. It is dominated by large wavelength features in the boundary layers, which are most heterogeneous and where spectra show a rapid decrease after degree 6, or earlier, while in the regions that separate them the spectra are white and have relatively low amplitude.. Consequently, these large wavelength features can be resolved globally without the danger of a significant bias from a truncated shorter wavelength field. Few, if any, of the existing geodynamic models can reproduce the strength of the boundary layer spectra. One of the relatively recent findings is the boundary layer property of the transition zone. It is characterized by a strong degree-2 signal, absent in the upper mantle above 400 km depth and at the top of the lower mantle. This fact indicates that the whole mantle circulation must be severely impeded, leading to a drastic change of the pattern of lateral heterogeneity across the 650 km discontinuity. Only models derived using overtone data can effectively resolve this feature. The source of dominating degree 2 and 3 signal in the lowermost mantle, with the African and Pacific mega- plumes and the circum-Pacific ring of fast velocities, remains a mystery. The spectrum of density anomalies in the lowermost mantle predicted by subduction reconstructions (i.e. Ricard et al., 1994) is white and is very different from that of seismic velocities. However, they correlate well for degrees 2 and 3 (Richards and Engerbretsen, 1991); the implication is that the processes in the lowermost mantle – whatever is their cause – may control the overall distribution of the subduction zones at the surface.
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7270 Tomography (6982, 8180)
DE: 8121 Dynamics: convection currents, and mantle plumes
DE: 8410 Geochemical modeling (1009, 3610)
SC: Union [U]
MN: 2007 Fall Meeting