Constraints on Mantle Viscosity From Intermediate‐Wavelength Geoid Anomalies in Mantle Convection Models With Plate Motion History

Abstract

The Earth's long‐ and intermediate‐wavelength geoid anomalies are surface expressions of mantle convection and are sensitive to mantle viscosity. While previous studies of the geoid provide important constraints on the mantle radial viscosity variations, the mantle buoyancy in these studies, as derived from either seismic tomography or slab density models, may suffer significant uncertainties. In this study, we formulate 3‐D spherical mantle convection models with plate motion history since the Cretaceous that generate dynamically self‐consistent mantle thermal and buoyancy structures, and for the first time, use the dynamically generated slab structures and the observed geoid to place important constraints on the mantle viscosity. We found that non‐uniform weak plate margins and strong plate interiors are critical in reproducing the observed geoid and surface plate motion, especially the net lithosphere rotation (i.e., degree‐1 toroidal plate motion). In the best‐fit model, which leads to correlation of 0.61 between the modeled and observed geoid at degrees 4–12, the lower mantle viscosity is ∼1.3–2.5 × 1022 Pa⋅s and is ∼30 and ∼600–1,000 times higher than that in the transition zone and asthenosphere, respectively. Slab structures and the geoid are also strongly affected by slab strength, and the observations prefer moderately strong slabs that are ∼10–100 times stronger than the ambient mantle. Finally, a thin weak layer below the 670‐km phase change on a regional scale only in subduction zones produces stagnant slabs in the mantle transition zone as effectively as a weak layer on a global scale.

Formation of Horizontally Deflected Slabs in the Mantle Transition Zone Caused by Spinel‐to‐Post‐Spinel Phase Transition, Its Associated Grainsize Reduction Effects, and Trench Retreat

“…The weak layer might only exist beneath subducted slabs as a result of the spinel‐to‐post‐spinel phase change (e.g., Panasyuk & Hager, 1998; Solomatov & Reese, 2008). Therefore, we also consider a regional weak layer model in which the weak layer is confined beneath slabs using the temperature criteria as in Mao and Zhong (2021) where and T ave are the nondimensional temperature anomaly and horizontally averaged temperature, respectively.…”

Constraints on Mantle Viscosity From Slab Dynamics

“…10.1029/2021JB022329 11 of 18 and high-accuracy residual topography. Moreover, Mao and Zhong (2021) conducted a global mantle convection model with plate motion history since the Cretaceous to produce mantle buoyancy and fit the observed geoid at degrees 4-12, supporting the hypothesis that the lower mantle viscosity is 30 times larger than that in the upper mantle. The longest wavelength (degrees 2-3) geoid is most sensitive to the viscosity contrasts and density anomalies in the lower mantle.…”

“…Although assuming the depths (but not the contrast) of viscosity layers in flow models with plate boundaries, Yang and Gurnis (2016) found good fit to the long wavelength (degrees 2–8) geoid, free‐air anomaly, gravity gradients, stress in the lithosphere and high‐accuracy residual topography. Moreover, Mao and Zhong (2021) conducted a global mantle convection model with plate motion history since the Cretaceous to produce mantle buoyancy and fit the observed geoid at degrees 4–12, supporting the hypothesis that the lower mantle viscosity is 30 times larger than that in the upper mantle. The longest wavelength (degrees 2–3) geoid is most sensitive to the viscosity contrasts and density anomalies in the lower mantle.…”

Characterization of Viscous Dissipative Heating in the Earth's Mantle Caused by Surface Forces

“…This implies that early Earth may have had a viscosity of only 10 19 Pa·s, or even smaller at shallower depths, due to pressure‐dependence of viscosity (Karato, 2008). To examine how surface loading‐induced dissipation processes may have evolved throughout Earth's history, the results for VS3 which most closely models the Earth's interior are rescaled for several reference viscosities (i.e., the lower mantle viscosity), ranging from 10 22 Pa·s, which is representative of Earth at present‐day (e.g., Mao & Zhong, 2021; Mitrovica & Forte, 2004), to 10 18 Pa·s, which may be representative of the early Earth.…”