HR: 0800h
AN: U21A-0001    [Abstracts]
TI: Seismic Evidence of Subducting Slab for Whole Mantle Convection Versus Layered Convection
AU: * Tajima, F
EM: fumiko@geol.sci.hiroshima-u.ac.jp
AF: Hiroshima University, Graduate School of Science, Kagamiyama 1-3-1, Higashi Hiroshima, 739-8526, Japan
AB: Recent seismic tomography models provide variable implications for mantle convection, i.e., the subducted slab flattens in the upper mantle and broadens that was visualized as high velocity anomaly (HVA) with a lateral extent of over a few thousand km beneath the northwestern Pacific ocean while a HVA zone appears to penetrate into the lower mantle associated with the Java subduction zone. Our analysis of broadband body waveforms has shown fairly broad regions where subducting plate is lying flat or piling up within the transition zone in the northwestern Pacific that is in agreement with tomography studies. However, we also found that the total volume of stagnant slab is considerably less than the image in the tomography model and much smaller than that of subducted slab during the known subduction history. The results imply that the stagnant slab moves somewhere else out of the transition zone or further descends into the lower mantle. Thus, the production of stagnant slab does not preclude the possibility of whole mantle convection. Features implied from stagnant slab along with other seismic evidence for subducted oceanic plates in the lower mantle suggest that convection is taking place in the whole mantle. On the other hand variation of the discontinuity depth at the bottom of the transition zone has been interpreted as variable distribution of geochemical properties and fluids associated with stagnant slab. Mantle materials descend into the lower mantle through the 660 km phase boundary only given gravitational instability due to thermo-chemical anomalies presumably associated with cold slab. The descending subducted slab into the lower mantle does not sufficiently support the convection patterns that are taking place in the entire mantle. The volume that involves in whole mantle convection may be a fraction of the entire mantle. Otherwise the layered structure is distinct in the mantle and there may be separate convection cells in the upper and lower mantle.
DE: 6982 Tomography and imaging (7270, 8180)
DE: 7203 Body waves
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 7270 Tomography (6982, 8180)
SC: Union [U]
MN: 2007 Fall Meeting