Coupled flow and anisotropy in the UltraLow Velocity Zones

Abstract

Seismic observations reveal a patchwork of thin and dense structures, named UltraLow Velocity Zones (ULVZs) atop the Earth's core mantle boundary. The high width to height ratio of the ULVZs, their spatial correlation with the edges of Large Low Shear Velocity Provinces (LLSVPs), and their preservation as distinct structures in the convecting mantle remain an enigmatic problem. In this article, we carry out a series of numerical simulations using Fast Multipole Boundary Elements Method (FMBEM) to address these questions and study the internal deformation within the ULVZs. Our results demonstrate that coupled flow between dense, low viscosity ULVZ patches and the LLSVP accumulates the ULVZ into stable piles along LLSVP corners, while coalescence and gravitational drainage leads to thin and wide ULVZs away from the corners. Deformation of the matrix is localized within the weaker ULVZ and the LLSVP edges, while the strain in the interior of the LLSVP remains uniform and low, explaining the observed localized anisotropy near LLSVP edges.