Anisotropy of the thermal conductivity, thermal expansion coefficient, and seismic wave velocity in the oceanic lithosphere–asthenosphere system

Abstract

Previous numerical studies have considered complex material properties in investigations of the thermal structure of oceanic plates, although the properties are assumed to be isotropic. This study focused on anisotropic thermal conductivity and expansion coefficients related to the crystallographic-preferred orientation of mantle minerals, and the effects of this anisotropy on the thermal structure of oceanic plates and associated surface heat flow, seafloor subsidence, and seismic anisotropy. A steady-state temperature and rock flow velocity are obtained in a two-dimensional model domain near a mid-ocean ridge. The results show that anisotropic thermal conductivity increases the mantle temperature by up to 18–28 K for oceanic crust with an age of 60 Myr, depending on the assumed half-spreading rate, because the vertical component of the conductivity is lower than the isotropic value. The effects on surface heat flow are minor. Anisotropy of the thermal expansion coefficient results in a small increase in seafloor subsidence, as the vertical component of the thermal expansion coefficient is higher than the isotropic case. Incorporating the effects of enstatite leads to a reduced anisotropy for the thermal conductivity and expansion coefficient. The increase in mantle viscosity due to dehydration associated with partial melting has a large effect on the seismic anisotropy. This leads to an abrupt, largely age-independent increase in azimuthal and radial anisotropy at $\sim$60 km depth, which may explain some seismic observations.