Other
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.u11a..03d&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #U11A-03
Other
7208 Mantle (1212, 1213, 8124), 7230 Seismicity And Tectonics (1207, 1217, 1240, 1242), 7270 Tomography (6982, 8180), 8120 Dynamics Of Lithosphere And Mantle: General (1213), 8121 Dynamics: Convection Currents, And Mantle Plumes
Scientific paper
Three dimensional Earth models (Ritsema et al., 2004; Panning and Romanowicz, 2006; Kustowski et al., 2006) derived using data that have good control on the structure in the transition zone (body-wave waveforms or overtone measurements) all show a discontinuous or very rapid change in the spectrum of lateral heterogeneity at the boundary between the upper and lower mantle. This was first pointed out by Woodward et al. (1994) and discussed in detail by Gu et al. (2001). All models have a strong chemical/thermal/mechanical boundary layer beginning just under the Moho and extending to 200-250 km; this heterogeneity is dominated by degrees five and six caused by the distribution of cratons and mid-oceanic ridges. The power spectrum decreases rapidly below that depth, but then begins to increase at about 400 km. It is dominated by degree two down to the 660 km discontinuity. There, it changes from very red to white at the top of the lower mantle. This is a planetary scale phenomenon that which must affect the flow of material and mixing in the mantle. Similar spectra have been generated in mantle convection models that considered the effect of an endothermic phase change (Tackley et al., 1994). The existence of this boundary layer is supported by the pattern of large scale positive velocity anomalies in the transition zone in places where subduction occurs (thus indicating ponding of slabs), large-wavelength variations in the topography of the 660 km discontinuity, variations in thickness of the transition zone and deep earthquakes outside the main Wadati-Benioff zones as well as changes in their mechanisms. During the last decade, the debate on the scale of mantle convection was strongly affected by images of slabs appearing to penetrate into the lower mantle. Relatively detailed mapping of velocity anomalies in the vicinity of slabs is feasible, because of illumination of these regions by earthquakes; these models do not tell much aboutt velocity anomalies in other places. However, the power spectra estimates are global and tell us about the behavior of the mantle as the whole. The reported pattern of the power spectra does not exclude an exchange of material between the upper and lower mantle; there may be local penetration of subducted material or there may be episodic events such as "avalanches". However, successful models of mantle convection should be able to explain the change in the spectra of lateral heterogeneity such as revealed by global tomographic models, obtained with the requisite data sets.
Dziewonski Adam M.
Kustowski Bogdan
Lekic Ved
Romanowicz Barbara A.
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