Inference of the Timescale‐Dependent Apparent Viscosity Structure in the Upper Mantle Beneath Greenland

Abstract

Contemporary crustal uplift and relative sea level (RSL) change in Greenland is caused by the response of the solid Earth to ongoing and historical ice mass change. Glacial isostatic adjustment (GIA) models, which seek to match patterns of land surface displacement and RSL change, typically employ a linear Maxwell viscoelastic model for the Earth's mantle. In Greenland, however, upper mantle viscosities inferred from ice load changes and other geophysical phenomena occurring over a range of timescales vary by up to two orders of magnitude. Here, we use full‐spectrum rheological models to examine the influence of transient deformation within the Greenland upper mantle, which may account for these differing viscosity estimates. We use observations of shear wave velocity combined with constitutive rheological models to self‐consistently calculate mechanical properties including the apparent upper mantle viscosity and lithosphere thickness across a broad spectrum of frequencies. We find that the contribution of transient behavior is most significant over loading timescales of 10 ²–10 ³ years, which corresponds to the timeframe of ice mass loss over recent centuries. Predicted apparent lithosphere thicknesses are also in good agreement with inferences made across seismic, GIA, and flexural timescales. Our results indicate that full‐spectrum constitutive models that more fully capture broadband mantle relaxation provide a means of reconciling seemingly contradictory estimates of Greenland's upper mantle viscosity and lithosphere thickness made from observations spanning a range of timescales.

Plain Language Summary

Glaciated regions such as Greenland experience fluctuations in ice mass on timescales ranging from years to hundreds of thousands of years. Because the Earth is deformable, its surface subsides and rebounds in response to the growth and decay of ice. Studies that focus on the ice‐induced deformation of the Earth beneath Greenland over long timescales typically consider a constant viscosity, which governs the rate at which deformation occurs. However, the way the Earth deforms is complex, and includes transient processes that occur over shorter timescales. In this study, we use equations that link the temperature and composition of the Greenland mantle to its mechanical properties at different timescales to estimate the magnitude of these transient effects. We find that the contribution of transient deformation is greatest in response to load changes over hundreds to thousands of years, which may account for seemingly contradictory estimates of viscosity made from observations of deformation over these timescales. We conclude that it is important to consider transient mantle behavior in Greenland to predict crustal deformation and relative sea level change more accurately across a broad range of timescales.

Key Points

• We use constitutive models including transient deformation to infer the full‐spectrum mechanical behavior of the Greenland upper mantle

• Contribution of transient deformation is most significant over centennial to millennial timescales relating to historical ice mass change

• Transient effects may explain conflicting mantle viscosity and lithosphere thickness estimates from geodetic and geological observations