Effect of anisotropy on oceanic upper mantle temperatures, structure, and dynamics

Abstract

Olivine and orthopyroxene crystals composing the oceanic upper mantle align under progressive simple shear strain. Because the thermal diffusivities (κ) and viscosities of these minerals are anisotropic, mineral alignment affects vertical heat flow and upper mantle dynamics. The vertical thermal diffusivity of upper mantle peridotite decreases with progressive simple shear strain, leading to higher temperatures in the shallow upper mantle than predicted by an isotropic half‐space cooling model. This, in turn, causes higher surface heat flow, shallower ocean basins, weaker asthenosphere, and slightly thinner lithosphere. Viscosity associated with an oriented simple shear strain (ηsh), such as that caused by plate motion, also evolves with progressive strain, though ηsh at high strains has not been characterized. Regardless of the evolution of ηsh with strain, the effects of thermal diffusivity anisotropy on upper mantle temperatures, surface heat flow, lithosphere thickness, asthenosphere viscosity and shear stress at the plate bottom remain evident. Shear heating elevates the geotherm by more than κ anisotropy except in the youngest ocean and in cases with significant shear weakening or a slowly moving plate (less than ∼3 cm/yr). The magnitude of shear heating effects depends on how ηsh increases or decreases at high strains. For models in which ηsh increases as a function of strain, shear heating elevates temperatures beyond reasonable upper mantle estimates (i.e., above 1400°C), suggesting that such strain induced viscosity increases are not likely. We present a model for ocean upper mantle cooling and deformation that includes upper mantle thermal diffusivity anisotropy and shear heating. This model predicts heat flow, shear wave split times, lithosphere thicknesses, and basin depths which are consistent with observations but suggests that basal tractions on ocean plates are lower than previously thought.