Feedbacks between a non-Newtonian upper mantle, mantle viscosity structure and mantle dynamics

Abstract

Summary Previous studies have shown that a low viscosity upper mantle can impact the wavelength of mantle flow and the balance of plate driving to resisting forces. Those studies assumed that mantle viscosity is independent of mantle flow. We explore the potential that mantle flow is not only influenced by viscosity but can also feedback and alter mantle viscosity structure owing to a non-Newtonian upper mantle rheology. Our results indicate that the average viscosity of the upper mantle, and viscosity variations within it, are affected by the depth to which a non-Newtonian rheology holds. Changes in the wavelength of mantle flow, that occur when upper mantle viscosity drops below a critical value, alter flow velocities which, in turn, alter mantle viscosity. Those changes also affect flow profiles in the mantle and the degree to which mantle flow drives the motion of a plate analog above it. Enhanced upper mantle flow, due to an increasing degree of non-Newtonian behavior, decreases the ratio of upper to lower mantle viscosity. Whole layer mantle convection is maintained but upper and lower mantle flow take on different dynamic forms: fast and concentrated upper mantle flow; slow and diffuse lower mantle flow. Collectively, mantle viscosity, mantle flow wavelengths, upper to lower mantle velocities, and the degree to which the mantle can drive plate motions become connected to one another through coupled feedback loops. Under this view of mantle dynamics, depth-variable mantle viscosity is an emergent flow feature that both affects and is affected by the configuration of mantle and plate flow.

Characterizing the Complexity of Subduction Zone Flow With an Ensemble of Multiscale Global Convection Models

“…In our models incorporating dislocation creep, this process accommodates a substantial portion of upper-mantle strain, particularly in rapidly deforming regions around subduction zones, where it typically accommodates >75% of total strain (Figure S20 in Supporting Information S1). This near-slab and sub-plate weakening effect of dislocation creep has also been demonstrated in previous regional analytical subduction zone models (Tovish et al, 1978) and time-dependent subduction models (e.g., Billen & Hirth, 2005, and convection models (Becker, 2006;Christensen, 1984;Semple & Lenardic, 2021;Čadek et al, 1993). As a result of this localized weakening, our simulations with and without dislocation creep produce substantially different patterns of subduction zone flow (Figures 3 and 8; Figures S14-S16 in Supporting Information S1); the addition of dislocation creep increases lateral variation in viscosity and velocity (Figure 3 and Figure S14 in Supporting Information S1).…”

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

“…The weak asthenosphere underneath the lithosphere has ubiquitous effects on dynamic process of the mantle (e.g., Becker, 2017; Richards & Lenardic, 2018; Semple & Lenardic, 2020). To investigate its effects on the geoid, we formulate Cases B1–B7 with four viscosity layers by including an additional layer of asthenosphere that extends from the base of lithosphere to 300 km depth (Table 2).…”

The Mantle Viscosity Structure of Venus

“…For the latter hypothesis, it has been suggested that dissolved water in olivine could effectively reduce the viscosity (Hirth & Kohlstedt, 1996). Alternatively, low viscosities in the uppermost mantle could be caused by the predominance of dislocation creep, with larger depths being dominated by diffusion creep with higher viscosities (Semple & Lenardic, 2021; Van Den Berg & Yuen, 1996). Finally, viscosity interfaces in the mantle could be linked to mineralogic phase transitions (Meade & Jeanloz, 1990).…”