HR: 0800h
AN: T11E-1326    [Abstracts]
TI: Deformation of a Partially Molten D$''$ Layer by Small-Scale Convection and the Resulting Seismic Anisotropy and Ultralow Velocity
AU: * Okamoto, T
EM: tatto@eps.nagoya-u.ac.jp
AF: Department of Earth and Planetary Sciences, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8602 Japan
AU: Sumita, I
EM: sumita@earth.s.kanazawa-u.ac.jp
AF: Department of Earth Science, Kanazawa University, Kakuma-machi, Kanazawa, Kanazawa, 920-1192 Japan
AU: Nakakuki, T
EM: nakakuki@geol.sci.hiroshima-u.ac.jp
AF: Department of Earth and Planetary Systems Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-hiroshima, 739-8526 Japan
AU: Yoshida, S
EM: yoshida@eps.nagoya-u.ac.jp
AF: Department of Earth and Planetary Sciences, Nagoya University, Furo-cho, Chikusa-ku, Nagoya, 464-8602 Japan
AB: Partially molten regions in the D$''$ layer have been demonstrated to exist by the discovery of the ultralow velocity zone (ULVZ). Following Solomatov and Moresi(2002), we regard the D$''$ layer as a thermal boundary layer within which small-scale stagnant-lid convection occurs. If the small-scale convection deforms partially molten regions, they would profoundly affect seismic structures including anisotropies. We therefore calculate the deformation history of partially molten regions at the base of the D$''$ layer which is heated from below, using a 2-D model with a strongly temperature dependent viscosity. The melt fraction is assumed to be proportional to temperature above solidus. An initially isotropic partial melt is strongly deformed by the viscous stress caused by the thermal instability, and becomes anisotropic by shape preferred orientation (SPO) of melt inclusions. The aspect ratio of the melt is of the order of $10^{1-2}$ at the base of the plume and becomes as large as $10^{3-4}$ in the plume head. We calculate the effective elastic constants for such anisotropic media which contain deformed partial melt, and obtain the seismic velocity for a horizontal ray path. We find that the horizontally averaged velocity profile consists of three layers corresponding to the base, conduit and head of a rising plume. An ultralow velocity zone (ULVZ) at the base and a negative shear wave gradient in the convective region form as soon as the thermal instability occurs. The deformation and alignment of the melt, rather than the melting itself, is primarily responsible for reducing the seismic velocity, and the lowermost ULVZ becomes strongly anisotropic (SH $>$ SV). On the other hand, in the conduit, the anisotropy is of SV $>$ SH type because of vertical alignment. In the plume head, the anisotropy is SH $>$ SV type with a magnitude of about 2%. We also show how shear wave anisotropy may be used to infer the temporal evolution of the instability at the D$''$ layer.
DE: 8147 Planetary interiors (5430, 5724)
DE: 5430 Interiors (8147)
DE: 5724 Interiors (8147)
DE: 7207 Core and mantle
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting