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.

There exist two large (~1000 km) dome-like structures rising from the base of the mantle in global tomographic models that exhibit low seismic wave-speeds, known as “Large Low Velocity Provinces” (LLVPs). The LLVP beneath Africa is also known as Tuzo, while the LLVP beneath the Pacific Ocean is known as Jason – two early pioneers of plate tectonics and plume theory (Wilson and Morgan, respectively). The source of these anomalously low wave-speeds is debated as both thermal and chemical heterogeneities can lower wave-speeds. These heterogeneities, however, lead to dynamically different modes of convection, where the former results in buoyant LLVPs and the latter relatively dense LLVPs. Recent studies that include dynamical modeling, Stoneley modes, and body tide constraints, are beginning to converge towards a picture where dense, chemical heterogeneities may be stored in the deepest part of the LLVPs. Many of these analyses, however, have not treated each LLVP distinctly. Considering the interdisciplinary focus of this session, we combine the unique geodetic observation of Earth’s solid body tide with methods that combine Backus-Gilbert estimation with theoretical advances in adjoint methodology, to constrain the density of each LLVP separately. This process results in the enhancement of the combined data sensitivity to a chosen region (e.g., one LLVP) while diminishing the combined data sensitivity elsewhere. We discuss the results in the context of geochemical observations unique to Tuzo and Jason and implications for the dynamics of mantle evolution.

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.

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 1D models are considered to compute sensitivity kernels. When using model PREM (Dziewonski and Anderson, 1981) 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”.

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.