Two large, seismically slow regions in the lower mantle beneath Africa and the Pacific Ocean are sometimes referred to as "superplumes". This name evokes images of large-scale active upwellings. However, it remains unclear whether these features are real or represent collections of multiple regular mantle plumes. Here, we investigate the implications of these upwellings for dynamic topography. We combine detailed measurements of oceanic residual topography from Hoggard et al. (2016) with continental constraints derived from CRUST1.0 to produce a global model expanded in spherical harmonics. Observed dynamic topography is subsequently compared to predictions derived from mantle flow following Steinberger (2016) using tomographic density models. Results yield relatively good overall agreement and amplitude spectra with similar slopes, except for degree two (i.e. > 10, 000 km wavelengths) where predicted amplitude is more than two times as large and is dominated by contributions from the lower mantle. Predictive models suggest two large-scale uplifted regions above the "superplumes" that are barely seen in the observed topography. We suggest that this mismatch can only partly be reconciled by altering the seismic velocity to density conversion factor or by including the effects of lower mantle chemical heterogeneity. In addition, it may be important to consider more significiant revisions to the lower mantle flow patterns, such as those possibly induced by different radial viscosity profiles and laterally-varying or anisotropic lower mantle viscosity.

Lithospheric plates move over the low‐viscosity asthenosphere balancing several forces, which generate plate motions. We use a global 3‐D lithosphere‐asthenosphere model (SLIM3D) with visco‐elasto‐plastic rheology coupled to a spectral model of mantle flow at 300 km depth to quantify the influence of intra‐plate friction and asthenospheric viscosity on plate velocities. We account for the brittle‐ductile deformation at plate boundaries (yield stress) using a plate boundary friction coefficient to predict the present‐day plate motion and net rotation of the lithospheric plates. Previous modeling studies have suggested that small friction coefficients ( μ<0.1, yield stress ∼ 100 MPa) can lead to plate tectonics in models of mantle convection. Here we show that in order to match the observed present‐day plate motion and net rotation, the frictional parameter must be less than 0.05. We obtain a good fit with the magnitude and orientation of the observed plate velocities (NUVEL‐1A) in a no‐net‐rotation (NNR) reference frame with μ<0.05 and a minimum asthenosphere viscosity of ∼ 5·1019 Pas to 1020 Pas. Our estimates of net rotation (NR) of the lithosphere suggest that amplitudes ∼ 0.1−0.2 ( °/Ma), similar to most observation‐based estimates, can be obtained with asthenosphere viscosity cutoff values of ∼ 1019 Pas to 5·1019 Pas and friction coefficients μ<0.05.

Abstract. The orientation and tectonic regime of the observed crustal/lithospheric stress field contribute to our knowledge of different deformation processes occurring within the Earth's crust and lithosphere. In this study, we analyze the influence of the thermal and density structure of the upper mantle on the lithospheric stress field and topography. We use a 3-D lithosphere-asthenosphere numerical model with power-law rheology, coupled to a spectral mantle flow code at 300 km depth. Our results are validated against the World Stress Map 2016 (WSM2016) and the observationbased residual topography. We derive the upper mantle thermal structure from either a heat flow model combined with a seafloor age model (TM1) or a global S-wave velocity model (TM2). We show that lateral density heterogeneities in the upper 300 km have a limited influence on the modeled horizontal stress field as opposed to the resulting dynamic topography that appears more sensitive to such heterogeneities. The modeled stress field directions, using only the mantle heterogeneities below 300 km, are not perturbed much when the effects of lithosphere and crust above 300 km are added. In contrast, modeled stress magnitudes and dynamic topography are to a greater extent controlled by the upper mantle density structure. After correction for the chemical depletion of continents, the TM2 model leads to a much better fit with the observed residual topography giving a good correlation of 0.51 in continents, but this correction leads to no significant improvement of the fit between the WSM2016 and the resulting lithosphere stresses. In continental regions with abundant heat flow data, TM1 results in relatively small angular misfits. For example, in western Europe the misfit between the modeled and observation-based stress is 18.3 • . Our findings emphasize that the relative contributions coming from shallow and deep mantle dynamic forces are quite different for the lithospheric stress field and dynamic topography.