HR: 16:00h
AN: T24B-01    [Abstracts]
TI: Three-Dimensional Thermal and Chemical Structure of the Subduction Zone Upper Mantle Beneath the Philippine Sea
AU: * Shito, A
EM: azusas@eri.u-tokyo.ac.jp
AF: Ocean Hemisphere Research Center, Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tok 113-0032 Japan
AU: Matsukage, K N
EM: kyoko.nishihara@yale.edu
AF: Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, CT 06511 United States
AU: Nishihara, Y
EM: yu.nishihara@yale.edu
AF: Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, CT 06511 United States
AU: Karato, S
EM: shun-ichiro.karato@yale.edu
AF: Department of Geology and Geophysics, Yale University, 210 Whitney Avenue, New Haven, CT 06511 United States
AB: Three-dimensional thermal and chemical structure of the upper mantle beneath the Philippine Sea is investigated using recent seismic velocity [Gorbatov and Kennett, 2003] and attenuation [Shito and Shibutani, 2004] tomography data together with mineral physics model. In order to separately determine the thermal and chemical anomalies, one needs to have a multiple data set (such as Vp, Vs, and Q anomalies) for each point and a relationship between these physical/chemical anomalies and seismological observables. In this study, we use the latest knowledge of mineral physics to derive such a relationship. The mineral physics observations indicate: (1) the major element chemistry has only small effects on Q but has some effects on seismic wave velocities, (2) temperature has important effects both on velocities and Q, and (3) water content has a large effect on attenuation and a modest influence on velocities. Our recent analysis [Matsukage et al., 2004] shows that the influence of major element chemistry is only modest and can be described by a single parameter such as the concentration of magnesium or opx. In this scheme, three unknowns (temperature, water content and major element chemistry) are related to three observables (Vp, Vs, and Q) through a matrix that contains physical parameters. This non-linear equation is solved by iteration to determine the three unknowns. The error analysis shows that temperature anomalies and water contents are well-resolved in most regions, but the major element chemistry is poorly resolved because the velocities are only weakly sensitive to the major element chemistry. High temperature anomalies are found in shallow regions (< 200 km) and the wedge mantle beneath the active volcanic chain. And the maximum high temperature anomaly is +300 degree. Regions of high water content are found in the deep upper mantle (and perhaps in the shallow upper mantle near the slab). The maximum content of water in the deep upper mantle is estimated to be ~10 times higher than normal mantle. This suggests that some water is carried into this depth range (perhaps carried by lawsonite). The inferred thermal and chemical structure is largely consistent with the tectonic history of this region involving a long history of subduction and active magmatism and indicates that the effects of subduction are not limited to the source region of most of arc volcanism and extends to far deeper regions.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 7218 Lithosphere and upper mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting