3:30 PM - 3:45 PM
[SIT20-07] Anisotropic thermal conductivity and seismic anisotropy in the oceanic lithosphere-asthenosphere system
★Invited Papers
Keywords:lithosphere-asthenosphere system, anisotropy, thermal conductivity, seismic wave velocity, thermal structure
It is widely accepted that the crystallographic-preferred orientation (CPO) of mantle minerals leads to anisotropic material properties. In this presentation I will focus on anisotropy of thermal conductivity and seismic wave velocity of CPO origin in the oceanic lithosphere-asthenosphere system predicted by using a two-dimensional thermomechanical modeling near a mid-ocean ridge. The CPO of mantle minerals is predicted using D-Rex, a code based on a theory of CPO development by plastic deformation and dynamic recrystallization.
Previous numerical studies have investigated the thermal structure of the oceanic plate by considering complex thermal conductivity, although it was assumed to be isotropic. I conducted calculations including anisotropic thermal conductivity, assuming that the thermal conductivity of olivine single crystal is 5.0, 3.0, and 4.3 W/m/K along the [100], [010], and [001] axis, respectively. The results show that the temperature of the oceanic plate increases by up to 23-38 K depending on the assumed half-spreading rate at the age of 60 Myr compared to cases assuming isotropic thermal conductivity. This increase occurs because the vertical component of thermal conductivity plays a major role in determining the thermal structure of the oceanic plate and olivine [010] axis tends to align vertically due to deformation associated with plate motion. The results suggest that previous studies assuming isotropic thermal conductivity may have underestimated the temperature of the oceanic plate. The effects of anisotropic thermal conductivity on surface heat flow are minor.
When dehydration associated with partial melting and melt extraction occurs beneath a mid-ocean ridge, the viscosity of the mantle at a fixed differential stress may increase by more than an order of magnitude, which will greatly affect mantle deformation and the resultant CPO. I conducted calculations incorporating the viscosity increase within the upper 60 km of the mantle and found that the effects on seismic anisotropy are significant. Both azimuthal and radial anisotropy show a rapid increase near 60 km depth, and these features are largely age independent. This is because rock deformation is localized just beneath 60 km depth due to the assumed viscosity change. This may explain why some seismological observations of the lithosphere-asthenosphere system show weakly age-dependent features.
Previous numerical studies have investigated the thermal structure of the oceanic plate by considering complex thermal conductivity, although it was assumed to be isotropic. I conducted calculations including anisotropic thermal conductivity, assuming that the thermal conductivity of olivine single crystal is 5.0, 3.0, and 4.3 W/m/K along the [100], [010], and [001] axis, respectively. The results show that the temperature of the oceanic plate increases by up to 23-38 K depending on the assumed half-spreading rate at the age of 60 Myr compared to cases assuming isotropic thermal conductivity. This increase occurs because the vertical component of thermal conductivity plays a major role in determining the thermal structure of the oceanic plate and olivine [010] axis tends to align vertically due to deformation associated with plate motion. The results suggest that previous studies assuming isotropic thermal conductivity may have underestimated the temperature of the oceanic plate. The effects of anisotropic thermal conductivity on surface heat flow are minor.
When dehydration associated with partial melting and melt extraction occurs beneath a mid-ocean ridge, the viscosity of the mantle at a fixed differential stress may increase by more than an order of magnitude, which will greatly affect mantle deformation and the resultant CPO. I conducted calculations incorporating the viscosity increase within the upper 60 km of the mantle and found that the effects on seismic anisotropy are significant. Both azimuthal and radial anisotropy show a rapid increase near 60 km depth, and these features are largely age independent. This is because rock deformation is localized just beneath 60 km depth due to the assumed viscosity change. This may explain why some seismological observations of the lithosphere-asthenosphere system show weakly age-dependent features.