The Evolution of Dense ULVZs Originating Outside LLSVPs and Implications for Dynamics at LLSVP Margins

Abstract

Interactions between multiple-scale thermochemical heterogeneities in the lowermost mantle, specifically ultralow velocity zones (ULVZs) and large low shear velocity provinces (LLSVPs), are critical in lower mantle dynamics. However, the evolution of ULVZs formed outside LLSVPs has not been thoroughly explored. Here we perform two-dimensional numerical experiments to examine the evolution of highly dense ULVZs originating beneath cold downwellings and their interactions with the LLSVP. We find that ULVZs with an intrinsic density anomaly more than 500 kg/m³ compared with the ambient lowermost mantle cannot fully enter the LLSVP and would dwell at LLSVP margins for an indefinitely long time. This suggests that dense ULVZ within LLSVPs might have different sources from those outside LLSVPs. The buoyancy number and compositional viscosity of ULVZs are controlling factors on their dynamics and imprints on the core-mantle boundary (CMB), such as how much the ULVZ protrudes into the LLSVP and the CMB topography beneath the ULVZ. The excess density of ULVZs dictates their width but not their thickness. The oscillations of ULVZ morphology suggest that various types of plumes occur at the LLSVP margin. The mobility of ULVZ implies that the bottom margin of the LLSVP moves much faster than its center. Hot zones exist within the LLSVP near its margins, which may affect the evolution of ULVZs and subducted material nearby. The CMB topography under dense ULVZs are positive unless the buoyancy number of ULVZs exceeds 6.0. These results have intriguing implications for the distribution of ULVZs as well as thermochemical evolution in the lowermost mantle.