In this talk, we first examine how microphysical models of transient deformation might manifest at the macroscale which may be summarized as a distinct frequency dependence of dissipation that extends beyond typical models used within geophysics (e.g., Maxwell and Burger viscoelastic models). We will explore several seismological, geodetic, and geological observations that show a strong indication of the activation of transient deformation. These examples include seismic normal modes and tides that span minutes to decades, and observations of viscoelastic rebound in the response to melting ice sheets at time scales from weeks to thousands of years. For these, we will feature examples from Antarctica and Greenland. In all our cases we demonstrate that laboratory derived constitutive laws describing a full spectrum of deformation mechanisms can help to reconcile different inferences of viscoelastic structure.To arrive at these conclusions, theoretical and observational insights from a wide range of spatio-temporal data (the microphysical and planetary-scale) and disciplines (rock physics, seismology and geodynamics) have been combined. The talk will end with implications of the importance of considering transient deformation in other examples of Earth dynamics and associated challenges that remain.

Studies of glacial isostatic adjustment (GIA) often use paleoshorelines and present-day deformation to constrain the viscosity of the mantle and the thickness of the elastic lithosphere. However, different studies focused on similar locations have resulted in different estimates of these physical properties. We argue that these different estimates infer apparent viscosities and apparent lithospheric elastic thicknesses, dependent on the timescale of deformation. We use recently derived relationships between these frequency dependent apparent quantities and the underlying thermodynamic conditions to produce predictions of mantle viscosity and lithospheric thickness across a broad spectrum of geophysical timescales for two Antarctic locations (Amundsen Sea and the Antarctic Peninsula). Our predictions are constrained by input from seismic tomography, require the self-consistent consideration of elastic, viscous, and transient rheological behavior and also include non-linear steady state viscosity, which have been determined by several laboratories. We demonstrate that these frequency dependent predictions of lithospheric thickness and apparent viscosity display a significant range and that they align to first order with estimates from GIA studies on different timescales. We suggest that observational studies could move towards a framework of determining the frequency dependence of apparent quantities – rather than single, frequency independent values of viscosity – to gain deeper insight into the rheological behavior of Earth.

Studies of glacial isostatic adjustment (GIA) often use paleoshorelines and present-day deformation to constrain the viscosity of the mantle and the thickness of the lithosphere. However, different studies focused on similar locations have resulted in different estimates of these physical properties even when considering the same model of viscoelastic deformation. We argue that these different estimates infer apparent viscosities and apparent lithospheric thicknesses, dependent on the timescale of deformation. We use recently derived relationships between these frequency dependent apparent quantities and the underlying thermodynamic conditions to produce predictions of mantle viscosity and lithospheric thickness across a broad spectrum of geophysical timescales for three locations (Western North America, Amundsen Sea, and the Antarctic Peninsula). Our predictions require the self-consistent consideration of elastic, viscous, and transient deformation and also include non-linear steady state deformation, which have been determined by several laboraties. We demonstrate that these frequency dependent predictions of apparent lithospheric thickness and viscosity display a significant range and that they align to first order with estimates from GIA studies on different timescales. Looking forward, we suggest that observationally based studies could move towards a framework of determining the frequency trend in apparent quantities – rather than single, frequency independent values of viscosity – to gain deeper insight into the rheological behavior of Earth materials.