Edges of thermochemical structures in the lower mantle

Abstract

Global seismic tomography consistently identifies two large low shear velocity provinces (LLSVPs) beneath Africa and the Pacific in the lower mantle. These structures are generally hypothesized to have a thermochemical origin with a higher bulk modulus ($K$) than ambient mantle. Regional high-resolution seismic studies have revealed that LLSVPs exhibit diverse edge morphologies, though the factors controlling these variations remain unclear. Here we quantitatively investigate the evolution of LLSVP boundary topographies through high-$K$ thermochemical convection models. The calculations show that the boundary morphology of a thermochemical pile is primarily controlled by its density and viscosity. Comparison with observed boundary shapes suggests that the African LLSVP may be less dense and thus less stable than the Pacific LLSVP, potentially reflecting differences in their compositions and evolutions. Additionally, the observed boundary complexity indicates that the viscosity of LLSVPs is likely no more than an order of magnitude higher than that of the surrounding mantle.