Mantle convection models based on geophysical constraints have provided us with a basic understanding of the forces driving and resisting plate motions on Earth. However, existing studies computing the balance of underlying forces are contradicting, and the impact of plate boundary geometry on surface deformation remains unknown. We address these issues by developing global instantaneous 3-D mantle convection models with a heterogeneous density and viscosity distribution and weak plate boundaries prescribed using different geometries. We find that the plate boundary geometry of the Global Earthquake Model (GEM, Pagani et al., 2018), featuring open plate boundaries with discrete lithospheric-depth weak zones in the oceans and distributed crustal faults within continents, achieves the best fit to the observed GPS data with a directional correlation of 95.1% and a global point-wise velocity residual of 1.87 cm/year. A good fit also requires plate boundaries being 3 to 4 orders of magnitude weaker than the surrounding lithosphere and low asthenospheric viscosities between 5e17 and 5e18 Pa s. Models without asthenospheric and lower mantle heterogeneities retain on average 30% and 70% of the plate speeds, respectively. Our results show that Earth’s plate boundaries are not uniform and better described by more discrete plate boundaries within the oceans and distributed faults within continents. Furthermore, they emphasize the impact of plate boundary geometry on the direction and speed of plate motions and reaffirm the importance of slab pull in the uppermost mantle as a major plate driving force.

Mantle plumes are thought to recycle material from the Earth’s deep interior. One constraint on the nature and quantity of this recycled material comes from the observation of seismic discontinuities. The detection of the X-discontinuity beneath Hawaii, interpreted as the coesite-stishovite transition, requires the presence of at least 40% basalt. However, previous geodynamic models have predicted that the percentage of high-density basaltic material that mantle plumes can carry to the surface is no higher than 15–20%. We propose this contradiction can be resolved by taking into account the length scale of chemical heterogeneities. While previous modeling studies assumed mechanical mixing on length scales smaller than the model resolution, we here model basaltic heterogeneities with length scales of 30–40~km, allowing for their segregation relative to the pyrolitic background plume material. Our models show that larger basalt fractions than previously thought possible—exceeding 40%—can accumulate within plumes at the depth of the X-discontinuity. Two key mechanisms facilitate this process: (1) The random distribution of basaltic heterogeneities induces large temporal variations in the basalt fraction with cyclical highs and lows. (2) The high density contrast between basalt and pyrolite below the coesite-stishovite transition causes ponding and accumulation of basalt at that depth, an effect that only occurs for intermediate viscosities of pyrolite. These results further constrain the chemical composition of the Hawaiian plume. Beyond that, they provide a geodynamic mechanism that explains the seismologic detection of the X-discontinuity and highlights how recycled material is carried towards the surface.