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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, https://doi.org/10.1177/8755293020931866), 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 5 × 1017 and 5 × 1018 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. Plain Language Summary: Plate tectonics can explain several geological and geophysical phenomena on Earth and is closely coupled to convection in the underlying mantle. To understand this plate–mantle coupling and quantify the forces contributing to plate motion, we develop high‐resolution three‐dimensional computational models of the Earth's present‐day mantle flow utilizing available geophysical constraints on density distribution and rheology. Additionally, we prescribe weak zones at the location of plate boundaries. We use different plate boundary geometries, forming either open or closed polygons, and we vary how easily the plate boundaries and the asthenosphere directly below the plates can be deformed to determine which model best fits observed plate motions. Our best‐fitting model features open plate boundaries that are weak (∼4 orders of magnitude weaker than the surrounding lithosphere) and traverse the whole plate in the oceans, but are shallower and more distributed within continents. The asthenosphere in these models is even weaker than the plate boundaries. Furthermore, we find that the downward force caused by subducted slabs contributes the most to the observed surface velocities. Our models suggest that plate boundaries are not uniformly weak everywhere and that their geometry has a strong influence on the direction and speed of plate motion. Key Points: We model plate motions in global instantaneous 3‐D mantle convection models with different plate boundary geometriesEarth's plate boundaries are not uniform and better described by discrete shear zones in the oceans and distributed faults within continentsSlab pull within the uppermost mantle (<300 km depth) contributes about 70% of the total plate driving force [ABSTRACT FROM AUTHOR]

The redistribution of past and present ice and ocean loading on Earth's surface causes solid Earth deformation and geoid changes, known as glacial isostatic adjustment. The deformation is controlled by elastic and viscous material parameters, which are inhomogeneous in the Earth. We present a new viscoelastic solid Earth deformation model in ASPECT (Advanced Solver for Problems in Earth's ConvecTion): a modern, massively parallel, open‐source finite element code originally designed to simulate convection in the Earth's mantle. We show the performance of solid Earth deformation in ASPECT and compare solutions to TABOO, a semianalytical code, and Abaqus, a commercial finite element code. The maximum deformation and deformation rates using ASPECT agree within 2.6% for the average percentage difference with TABOO and Abaqus on glacial cycle (∼100 kyr) and contemporary ice melt (∼100 years) timescales. This gives confidence in the performance of our new solid Earth deformation model. We also demonstrate the computational efficiency of using adaptively refined meshes, which is a great advantage for solid Earth deformation modeling. Furthermore, we demonstrate the model performance in the presence of lateral viscosity variations in the upper mantle and report on parallel scalability of the code. This benchmarked code can now be used to investigate regional solid Earth deformation rates from ice age and contemporary ice melt. This is especially interesting for low‐viscosity regions in the upper mantle beneath Antarctica and Greenland, where it is not fully understood how ice age and contemporary ice melting contribute to geodetic measurements of solid Earth deformation. Plain Language Summary: Mass changes on the Earth's surface, for example, from melting ice sheets or sea level rise, cause deflections of Earth's surface as interior rocks deform and flow. Scientists have developed models of the interior deformation resulting from loads applied to Earth's surface. Such models depend on the viscous and elastic properties of interior rocks, which quantify their capacity to deform and flow. However, because the Earth is heterogeneous, its viscoelastic properties exhibit large lateral variations that have proven difficult to accommodate within a (numerical) model. Here, we present and benchmark a new application of the open‐source code in ASPECT (Advanced Solver for Problems in Earth's ConvecTion), which was originally designed to model mantle convection occurring on timescales of millions of years or longer. The ASPECT code makes use of modern numerical methods, such as adaptive mesh refinement and advanced solver techniques. In particular, we show that this code is accurate and useful for modeling solid Earth deformation occurring on timescales relevant to contemporary (in response to climate change) and ice age melting (from decades to millennia). This code is especially useful for studying regions with both past and present ice melt and a heterogeneous Earth structure, such as Greenland and Antarctica. Key Points: The solid Earth is deforming in response to past and present ice loading changes at rates determined by elastic and viscous parametersWe benchmark a new viscoelastic solid Earth deformation model in the open‐source code ASPECT in combination with adaptive mesh refinementThis code can be used to study regional Earth deformation rates from ice age and contemporary ice melt on a laterally heterogeneous Earth [ABSTRACT FROM AUTHOR]

Problems arising in Earth's mantle convection involve finding the solution to Stokes systems with large viscosity contrasts. These systems contain localized features which, even with adaptive mesh refinement, result in linear systems that can be on the order of 109 or more unknowns. One common approach for preconditioning to the velocity block of these systems is to apply an Algebraic Multigrid (AMG) V‐cycle (as is done in the ASPECT software, for example), however, we find that AMG is lacking robustness with respect to problem size and number of parallel processes. Additionally, we see an increase in iteration counts with refinement when using AMG. In contrast, the Geometric Multigrid (GMG) method, by using information about the geometry of the problem, should offer a more robust option.Here we present a matrix‐free GMG V‐cycle which works on adaptively refined, distributed meshes, and we will compare it against the current AMG preconditioner (Trilinos ML) used in the ASPECT1 software. We will demonstrate the robustness of GMG with respect to problem size and show scaling up to 114,688 cores and 217 billion unknowns. All computations are run using the open‐source, finite element library deal.II.2 [ABSTRACT FROM AUTHOR]