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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 10²–10³ 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.

We apply the Backus‐Gilbert approach to normal mode center frequency data, to constrain jumps in P, S, bulk‐sound speed and density at the “660” discontinuity in the earth’s mantle (∼650–670 km depth). Different 1‐D models are considered to compute sensitivity kernels. When using model PREM (Dziewonski & Anderson, 1981, Physics of the Earth and Planetary Interiors, 25, 297–356. doi:10.1016/0031‐9201(81)90046‐7) as reference, with a “660” at 670 km depth, the best‐fitting jumps in density, P‐ and S‐wave speeds range from (5.1–8.2)%, (5.3–8.0)%, (5.0–7.0)%, respectively, so the PREM values lie outside the ranges of acceptable density and P wave speed jumps. When shifting the depth of “660” to 660 km, the density and S wave speed jumps increase, while the P‐wave speed jump decreases. Normal mode data do not support a global transition at 650 km depth. The density jumps are closer to those of pyrolite than PREM, while our bulk‐sound wave speed jumps suggest a larger garnet proportion at “660.”

Determining the thickness of the lithosphere in any given setting combines uncertainty in both the observational method and laboratory‐derived understanding of mantle rheology. The many observational and modeling criteria across geophysical subfields for plate thickness lead to significant differences in plate thickness estimates depending on the process of interest, be it seismic wave propagation or relaxation in response to changes in loads—from earthquakes, ice sheets to volcanoes—or convection. This paper proposes a framework in which to model and interpret upper mantle mechanical structure smoothly across the full spectrum of geophysical timescales. We integrate viscous, elastic, and linear anelastic constitutive models and calculate the mechanical response from convective to seismic wave timescales (i.e., 0 to infinite frequency or, in practice,10⁻¹⁵to 1 Hz). We apply these calculations to 1‐D thermal structures and determine the normalized complex viscosity, a quantity that shows clearly the role of transient creep in weakening rock relative to the associated Maxwell rheology. Using various criteria for the lithosphere‐asthenosphere boundary, we show that the apparent plate thickness will be thicker at higher frequencies than at lower frequencies. Additional calculations for nonlinear Maxwell behavior (dislocation mechanisms) demonstrate significant changes in the apparent plate structure, decreasing the long‐term plate thickness, consistent with observations. Other effects such as dislocation damping (associated with a steady‐state dislocation structure), melt, water, major element composition, and grain size are not included here but, when incorporated into this framework, will significantly change the full‐spectrum plate thickness predictions.

We develop a conceptual/quantitative framework whereby measurements of Earth's viscoelasticity may be assessed across the broad range of geophysical processes, spanning seismic wave propagation, postseismic relaxation, glacial isostatic adjustment, and mantle convection. Doing so requires overcoming three challenges: (A) separating spatial variations from intrinsic frequency dependence in mechanical properties; (B) reconciling different conceptual and constitutive viscoelastic models used to interpret observations at different frequencies; and (C) improving understanding of linear and nonlinear transient deformation mechanisms and their extrapolation from laboratory to earth conditions. We focus on (B), first demonstrating how different mechanical models lead to incompatible viscosity estimates from observations. We propose the determination of the “complex viscosity”—a frequency‐dependent parameter complementary to other measures of dissipation (including frequency‐dependent moduli and attenuation)—from such observations to reveal a single underlying broadband mechanical model. The complex viscosity illuminates transient creep in the vicinity of the Maxwell time, where most ambiguity lies.