Comparison of mantle convective structures with seismic structures and its implication for the geoid

Seismic observations indicate significant accumulation of subducted slabs above the 670-km discontinuity in many subduction zones and support that the large low shear-wave velocity provinces (LLSVP) in the African and Pacific lower mantle be associated with chemical heterogeneity. Global mantle convection models with realistic plate motion history reproduce most of these structures. Especially our recent study showed that the primary characteristics of slab structures including the stagnant slabs in the western Pacific mantle transition zone and linear slab structures in the lower mantle beneath Americas and Asia can be explained in a global convection model that incorporates the spinel-to-post-spinel phase change at the 670 km depth and a weak layer below the phase boundary. However, it remains unclear how the convection models compare with seismic models at different spatial wavelengths and depths. By conducting quantitative analysis between mantle convection and seismic models, we found that mantle convective structures show significant correlations with seismic structure at <1000 km for wavelengths up to spherical harmonic degree 20. However, the global correlation is weak at intermediate- to short-wavelengths (for degrees 4 and higher) in the lower mantle below ~1000 km depth. Models with a weak layer beneath the spinel-to-post-spinel phase change, or with viscosity increase at 1000 km depth and the 670 km depth phase change, help consistently reproduce stagnant slabs in the western Pacific, while having insignificant effects elsewhere, e.g., the LLSVP structures. The cold slab structures and their correlations with the seismically fast anomalies are nearly identical for our convection models with and without the plumes, indicating that seismically fast anomalies in the mantle mainly result from the subducted slabs.

In addition, we seek to constrain the mantle viscosity structure, not only the relative viscosity variations but also the absolute values, by using mantle convection models and the resulting geoid. Compared with previous geoid models using mantle buoyancy converted from seismic tomographic models, our convection models produce similarly significant geoid correlations at the long- to intermediate-wavelengths (l=2-12).