1. High‐Resolution Mantle Flow Models Reveal Importance of Plate Boundary Geometry and Slab Pull Forces on Generating Tectonic Plate Motions.
- Author
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Saxena, Arushi, Dannberg, Juliane, Gassmöller, Rene, Fraters, Menno, Heister, Timo, and Styron, Richard
- Subjects
PLATE tectonics ,EARTH'S mantle ,OCEAN zoning ,ROTATION of the earth ,SHEAR zones ,DEFORMATION of surfaces ,SHALLOW-water equations ,FORCED convection - Abstract
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]
- Published
- 2023
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