Deciphering the nature of LLSVPs using constraints from geoid and dynamic topography

Abstract:

The two large low shear velocity provinces (LLSVPs) beneath Africa and Pacific, which are important structures of the lower mantle, play a significant role in representing mantle compositional heterogeneities and affecting Earth’s convection behaviors. While seismic studies outline the geometry and point to a possible compositional origin of LLSVPs, both the depth-distribution and effective density anomalies within these structures are still unclear. Traditional geoid models assume pure-thermal LLSVPs that also lead to large positive dynamic topography above. Both these model properties have been questioned recently. Here, we revisit geoid calculations and dynamic topography estimates by considering the thermo-chemical nature of the LLSVPs, with a particular focus on the height and density of the compositional LLSVPs.

The observed long-wavelength geoid is sensitive to both the dynamic topography and the deep mantle density structures. With purely thermal LLSVPs, the geoid highs at the surface result from the compensation between the negative geoid caused by the buoyant LLSVPs and the positive geoid resulted from the positive dynamic topography. On the other hand, when the geoid highs are predominantly associated with the high-density compositional LLSVPs, dynamic topography contributes little to the geoid, which may correspond to small magnitudes of mantle density anomalies beyond the LLSVPs. The dynamic topography in the latter scenario can also better match the low-amplitude long-wavelength residual topography within oceans, as recently proposed (e.g., Hoggard et al., 2016). This model with dense thermo-chemical LLSVPs may provide new insights on the nature of the lower mantle and on the style of mantle convection.