Using thermo-mechanical models of subduction to constrain effective mantle viscosity

Abstract

Mantle convection and plate dynamics transfer and deform solid material on scales of hundreds to thousands of km. However, viscoplastic deformation of rocks arises from motions of defects at subcrystal scale, such as vacancies or dislocations. In this study, results from numerical experiments of dislocation dynamics in olivine for temperatures and stresses relevant for both lithospheric and asthenospheric mantle (800-1700 K and 50-500 MPa; Gouriet et al., 2019) are used to derive three sigmoid parameterizations (erf, tanh, algebraic), which express stress evolution as a function of temperature and strain rate. The three parameterizations fit well the results of dislocation dynamics models and may be easily incorporated into geodynamical models. Here, they are used in an upper mantle thermo-mechanical model of subduction, in association with diffusion creep and pseudo-brittle flow laws. Simulations using different dislocation creep parameterizations exhibit distinct dynamics, from unrealistically fast-sinking slabs in the erf case to very slowly-sinking slabs in the algebraic case. These differences could not have been predicted a priori from comparison with experimentally determined mechanical data, since they principally arise from feedbacks between slab sinking velocity, temperature, drag, and buoyancy, which are controlled by the strain rate dependence of the effective asthenosphere viscosity. Comparison of model predictions to geophysical observations and to uppermantle viscosity inferred from glacial isostatic adjustment shows that the tanh parameterization best fits both crystal-scale and Earth-scale constraints. However, the parameterization of diffusion creep is also important for subduction bulk dynamics since it sets the viscosity of slowly deforming domains in the convecting mantle. Within the range of uncertainties of experimental data and, most importantly, of the actual rheological parameters prevailing in the upper mantle (e.g. grain size, chemistry), viscosity enabling realistic mantle properties and plate dynamics may be reproduced by several combinations of parameterizations for different deformation mechanisms. Deriving mantle rheology cannot therefore rely solely on the extrapolation of semi-empirical flow laws. The present study shows that thermo-mechanical models of plate and mantle dynamics can be used to constrain the effective rheology of Earth's mantle in the presence of multiple deformation mechanisms.

Effects of basal drag on subduction dynamics from 2D numerical models

“…The parameterisation of diffusion and dislocation creep mechanisms are consistent with the experimental range derived from olivine deformation (e.g. Hirth and Kohlstedt, 2003) and predict realistic bulk uppermantle viscosities consistent with large-scale observations (Garel et al, 2020). The parameterisation of the Peierls creep is simplified (Garel et al, 2014) but is consistent with firstorder approximation of the dependency on temperature and strain rate (e.g.…”

How Slab Age and Width Combine to Dictate the Dynamics and Evolution of Subduction Systems: A 3‐D Spherical Study

“…Their imaged morphologies are therefore far from uniform (e.g., Fukao & Obayashi, 2013; Goes et al., 2017; Karato et al., 2001; Li et al., 2008; van der Meer et al., 2018). The dominant controls on such variations remain unclear, and likely vary between different subduction zones, due to complexities arising from non‐linear and multi‐scale interactions between several aspects of the mantle system, including downgoing and overriding plate properties, global mantle flow, mineral phase changes and material rheology (e.g., Agrusta et al., 2017; Alsaif et al., 2020; Capitanio et al., 2007; Cerpa et al., 2022; Čížková et al., 2002; Garel et al., 2014, 2020; Goes et al., 2008, 2017; Holt & Royden, 2020; Karato et al., 2001; Schellart et al., 2007; Stegman, Farrington, et al., 2010; Suchoy et al., 2021).…”

How slab age and width combine to dictate the dynamics and evolution of subduction systems: a 3-D spherical study

“…Their imaged morphologies are therefore far from uniform (e.g., Karato et al, 2001;Li et al, 2008;Fukao & Obayashi, 2013a;Goes et al, 2017;van der Meer et al, 2018). The dominant controls on such variations remain unclear, and likely vary between different subduction zones, due to complexities arising from non-linear and multi-scale interactions between several aspects of the mantle system, including downgoing and overriding plate properties, global mantle flow, mineral phase changes and material rheology (e.g., Karato et al, 2001;Čížková et al, 2002;Capitanio et al, 2007;Schellart et al, 2007;Goes et al, 2008;Stegman, Farrington, et al, 2010;Garel et al, 2014;Goes et al, 2017;Agrusta et al, 2017;Alsaif et al, 2020;Holt & Royden, 2020;Garel et al, 2020;Suchoy et al, 2021).…”