HR: 15:10h
AN: V43G-06    [Abstracts]
TI: The Boundary Between the Upper and Lower Mantle
AU: * Dziewonski, A M
EM: dziewons@eps.harvard.edu
AF: Deapartment of Earth and Planetary Sciences, Harvard University, 20 Oxford St. , Cambridge, MA 02138 United States
AB: The debate on the style of mantle convection continues partly because information from seismic tomography appears to be contradictory. There are strong arguments in favor of a major obstacle to the flow across the 650 km discontinuity. These include a change in the spectrum of lateral heterogeneity above and below the boundary, with the "red" spectrum of the velocity anomalies in the transition zone changing, either abruptly or very rapidly, to the white spectrum in the middle mantle. There are large wavelength (2,000-3,000 km) perturbations in the topography of the 650 km discontinuity with an amplitude of up to 20 km; the pattern of these perturbations matches well that of the velocity anomalies in the transition zone, with fast velocities being underlaid by depressions. The topographies of the 410 km and 650 km discontinuities are uncorrelated, indicating that they are not caused by a large scale mantle flow. Ponding of the subducted material in the transition zone seems to be consistent with all these facts. In addition, there is information on the deep earthquakes that supports the hypothesis of slab accumulation. The most outstanding example is a 500-600 km long zone of earthquakes under the Fiji Plateau with focal depths greater than 600 km. Isolated events, distant by 200-300 km from the main Wadati-Benioff zone, are also observed in South America, Tonga, Kuriles, Izu-Bonin and Banda Sea. These events are rare on the decadal time scale, but may be widespread over thousands or millions of years. Also, there is a frequent change in the orientation of the principal stress axes as the slab approaches the 650 km boundary. On the other hand, the tomographic images of the lowermost mantle bear resemblance to the near-surface tectonics. The circum-Pacific ring of fast velocities correlates well with the past locations of the subduction zones. There is also a significant concentration of the hot-spots in the regions of slow velocities in the lowermost mantle: in particular, above the Equatorial Pacific Plume Group and the Great African Plume (Dziewonski {\it et al.}, 1993). It appears that some connection between the upper and lower mantle must exist; whether it is direct or indirect cannot be determined from seismic tomography alone.
DE: 8121 Dynamics, convection currents and mantle plumes
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8125 Evolution of the Earth
DE: 7207 Core and mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting