Glacial isostatic adjustment (GIA) is the ongoing response of the solid earth and the geoid to changes in ice and ocean loading, and produces solid earth ground motion that can be measured using GNSS (Global Navigation Satellite Systems). Near areas of past or current ice cover change, it is commonly thought GIA displacements result from a combination of (a) a viscous response to historic ice load changes (i.e., ice age melting), and (b) an elastic response to contemporary ice load changes. Typically, the viscous response occurs over several thousand years, but recent studies have shown regions undergoing rapid viscous uplift on decadal or centennial timescales in response to contemporary ice melt in West Antarctica (Barletta et al., 2018;Nield et al., 2014) and southeast Greenland (Khan et al., 2016). Rapid uplift in these regions is commonly linked to low-viscosities in the upper mantle that accelerate the viscous response to recent melting. In this case, contemporary ice melt generates not only an instantaneous elastic response, but also a viscous response on short timescales. This rapid viscous response is mixed with the other deformation components of GIA (elastic and long-term viscous) that are measured using GNSS, which makes it difficult to distinguish between solid earth deformation due to historical and contemporary ice load changes (Whitehouse, 2018).There are indications that low-viscosity regions of the upper mantle are present beneath both Antarctica and Greenland. Here, we define low-viscosity regions as regions where the viscosity is considerably lower than surrounding mantle material, with a value that can result in deformation on decadal or centennial timescales (e.g., 5 ⋅ 10 19 Pa s or lower), as opposed to thousands of years. Seismic studies in Antarctica show slower velocity anomalies in West compared to East Antarctica (Heeszel et al., 2016;Lloyd et al., 2020), consistent with a colder cratonic region in East Antarctica, and a warmer tectonically active region in West Antarctica, possibly with a mantle plume (Bredow et al., 2021). Lateral variations in mantle temperature, derived from seismic velocity anomalies, suggest lateral variations in mantle rheology (Ivins & Sammis, 1995; van der Wal et al., 2013). Upper

Glacial isostatic adjustment (GIA) is the ongoing response of the solid Earth and the geoid to past and present changes in ice and ocean loading and produces solid Earth ground motion and mass redistributions. The solid Earth ground motion can be measured using GNSS (Global Navigation Satellite Systems) and the solid Earth mass displacements using ground-based gravimetry and satellite gravimetry, such as GRACE (Gravity Recovery and Climate Experiment). These geodetic measurements capture the ongoing response of the solid Earth to changes from both past (i.e., ice age) and contemporary ice load changes. Near areas of past and current ice cover, it is commonly thought that solid Earth ground motion results from a combination of (a) a viscous response to past ice load changes, and (b) an elastic response to contemporary ice load changes. Consequently, these

<p>Models of Glacial Isostatic Adjustment (GIA) processes are useful because they help us understand landscape evolution in past and current glaciated regions. Such models are sensitive to ice and ocean loading as well as to Earth material properties, such as viscosity. Many current GIA models assume radially-symmetric (layered) viscosity structures, but viscosity may vary laterally and these variations can have large effects on GIA modeling outputs. Here we present the potential of using ASPECT, an open-source finite element mantle-convection code that can handle lateral viscosity variations, for GIA modeling applications. ASPECT has the advantage of adaptive mesh refinement, making it computationally efficient, especially for problems such as GIA with large variations in strain rates. Furthermore, ASPECT is open-source, as will be the GIA extension, making it a valuable future tool for the GIA community.</p><p> </p><p>Our GIA extension is benchmarked using a similar case as in Martinec et al. (GJI, 2018), such that the performance of our GIA code can be compared to other GIA codes. In this case, a spherically symmetric, five-layer, incompressible, self-gravitating viscoelastic Earth model is used (Spada et al, GJI 2011). The surface load consists of a spherical ice cap centered at the North pole, and is applied as a Heaviside loading. The ice load remains constant with time, and thus we have not yet implemented the full sea level equation (SLE). Beyond this benchmark, we have incorporated lateral viscosity variations underneath the ice cap, to demonstrate the ability of efficiently implementing laterally-varying material properties in ASPECT.</p><p> </p><p>We show the possibilities, capabilities, and potential of ASPECT for GIA modeling. In the near future we will further develop the code with the sea level equation and an ocean basin, and will explore ASPECT’s current capability of using time-varying distributed surface loads. These functions will allow for modeling of GIA for realistic ice load scenarios imposed above potentially complex earth structures.</p>

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