What Are LLSVPs? Geodynamic Insights into Lower Mantle Chemistry and Structure

The Large Low Shear Velocity Provinces (LLSVPs) are antipodal, continent-sized low-velocity zones that sit at the base of the Earth's mantle. Since their discovery in the 1980s, an array of models has been proposed to explain their origin and dynamics, invoking varying amounts of compositional heterogeneity, viscosity variation and intrinsic stability. Although mineral physics experiments have narrowed the range of possible compositions and thermal states that might explain the LLSVPs' seismic characteristics, parameter trade-offs introduce significant non-uniqueness that has frustrated efforts to discriminate between candidate models on the basis of seismology alone. Fortunately, a range of geodynamic observations exist that help cut through this ambiguity. Earth's long-wavelength geoid and core-mantle boundary (CMB) excess ellipticity are the most well-established constraints but more recently, new observations of long-wavelength dynamic topography and body tide deformation have been added to the mix. Here, we develop mantle flow models that simultaneously satisfy these geodynamic and seismological constraints, we find that the upper bound on the height of LLSVPs above the CMB is ~2000 km and that a thin, ~200 km-thick layer of anomalously dense material (~+1%) is likely present at the base of these structures. The apparent density of this basal layer is consistent with the presence of basaltic material, implying that it may represent the remnants of ancient oceanic crust. This model can explain the apparent stability and sharp sides of LLSVPs whilst ensuring that the structures are, on average, positively buoyant, as necessitated by seismological and long-wavelength geodynamic constraints.