The effects of non-Newtonian rheology in the upper mantle on relative sea level change and geodetic observables induced by glacial isostatic adjustment process

SUMMARY Studies of glacial isostatic adjustment (GIA) provide important constraints on the Earth's mantle viscosity. Most GIA models assume Newtonian viscosity through the mantle, but laboratory experimental studies of rock deformation, observational studies of seismic anisotropy, and modelling studies of mantle dynamics show that in the upper mantle non-Newtonian viscosity may be important. This study explores the non-Newtonian effects on the GIA induced variations in mantle stress and viscosity and on surface observables including vertical displacement, relative sea level (RSL) and gravity change. The recently updated and fully benchmarked software package CitcomSVE is used for GIA simulations. We adopt the ICE-6G ice deglaciation history, VM5a lower mantle and lithospheric viscosities, and a composite rheology that combines Newtonian and non-Newtonian viscosities for the upper mantle. Our results show that: (1) 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. However, non-Newtonian effects on far-field mantle viscosity are negligibly small. GIA induced stress is also significant in the lithosphere (∼30 MPa) and lower mantle (∼2 MPa). (2) The predicted RSL changes from non-Newtonian models display distinct features in comparison with the Newtonian model, including more rapid sea level falls associated with the rapid deglaciation at ∼14 000 yr ago followed by a more gradual sea level variation for sites near the centres of formerly glaciated regions, and an additional phase of sea level falls for the last ∼8000 yr for sites at the ice margins. Similar time-dependence associated with the deglaciation is also seen for rate of vertical displacement, suggesting a relatively slow present-day rates of vertical displacement and gravity change. These features can be explained by the non-Newtonian effects associated with a loading event which manifest a fast relaxation stage followed by a relative slow relaxation stage. Our results may provide GIA diagnoses for distinguishing non-Newtonian and Newtonian rheology.