Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
10121249 | Physics of the Earth and Planetary Interiors | 2005 | 17 Pages |
Abstract
Partially molten regions in the lowermost mantle have been inferred to exist by the discovery of the ultralow velocity zone (ULVZ). We consider a small-scale stagnant-lid convection in the Dâ²â²layer, following [Solomatov, V.S., Moresi, L.N., 2002. Small-scale convection in the Dâ²â²layer. J. Geophys. Res., 107, doi:10.1029/2000JB000063.], to investigate the relationship between partial melt and seismic anisotropies often found in the Dâ²â²layer. Such convection can bring melts in the Dâ²â²layer from the ULVZ, deform the melt inclusions, and accordingly would profoundly affect seismic structures including anisotropies. We therefore calculate the deformation history of partially molten regions at the base of the Dâ²â²layer which is heated from below, using a 2D model with a strongly temperature dependent viscosity. An initially isotropic partial melt is strongly deformed by the viscous stress caused by thermal convection, and becomes anisotropic by shape preferred orientation (SPO) of melt inclusions, whose aspect ratios are of the order of 101 to 102 at the base of the plume and become as large as 103 to 104 in the plume head. We calculate the effective elastic constants for such anisotropic media which contain deformed melt inclusions, and obtain the seismic velocity for a horizontal ray path. We find that the horizontally averaged velocity profile can be correlated with convective patterns. If melting occurs not only in the ULVZ but extends to the top of hot regions of the Dâ²â²layer, the vertical seismic profile consists of three layers corresponding to the base, conduit and head of a rising plume. The lowermost layer, which corresponds to the ULVZ, becomes strongly anisotropic with VSH>VSV. The deformation and alignment of the melt, rather than the melting itself, is primarily responsible for reducing the seismic velocity. On the other hand, in the conduit, the anisotropy is of VSV>VSH type because of vertical alignment. In the plume head, the anisotropy is of VSH>VSV type with a magnitude of about 2%. We discuss how shear wave anisotropy may be used to infer the temporal evolution of the Dâ²â²layer.
Related Topics
Physical Sciences and Engineering
Earth and Planetary Sciences
Geophysics
Authors
Tatsuto Okamoto, Ikuro Sumita, Tomoeki Nakakuki, Shigeo Yoshida,