The Effects of Non-Newtonian Rheology in the Upper Mantle on GIA Observables

Abstract:

Studies of glacial isostatic adjustment (GIA) provide important constraints on Earth’s mantle viscosity. Most GIA models assume Newtonian viscosity, but both laboratory experiments and studies of mantle dynamics show that in the upper mantle non-Newtonian viscosity (i.e., stress-dependent dislocation creep) may be important. A stress-dependent viscosity reflects a non-linear stress-strain rate relationship such that the viscosity is reduced (increased) when stress is high (low), leading to mantle viscosity that may vary not only spatially but also temporally during the GIA process. This study explores non-Newtonian effects on GIA observables including surface uplift and relative sea level (RSL) changes. We employ the recently updated and fully benchmarked CitcomSVE software package for GIA simulations. We adopt the ICE-6G ice history, VM5a lower mantle and lithospheric viscosities, and a composite rheology that combines linear and non-linear viscosities for the upper mantle. Our results show that: 1) The RSL and surface uplift at glaciated regions (e.g., Churchill and Richmond) change much more rapidly with time during deglaciation in the non-Newtonian models than those in the Newtonian models (e.g., VM5a), but after deglaciation, the trend is reversed. In the far field such as Barbados, the non-Newtonian effects on the RSL and uplift are relatively small. RSL and uplift at the periphery of glaciated regions show markedly different temporal variations between the non-Newtonian and Newtonian models. 2) The mantle stress beneath glaciated regions increases significantly during deglaciation, leading to regionally reduced upper mantle viscosity by more than an order of magnitude. Such effects can be rather localized at the periphery of glaciated regions. Due to ocean loading, upper mantle stress at the periphery of continents is also increased, causing local viscosity reduction. However, non-Newtonian effects on far-field mantle viscosity are negligibly small. GIA induced stress is also significant in the lithosphere and lower mantle. These results have significant implications for interpreting the observations of RSL and uplift associated with GIA process and for understanding mantle rheology.