Reversal of the Direction of Horizontal Velocities Induced by GIA as a Function of Mantle Viscosity

Abstract

In regions undergoing glacial isostatic adjustment present-day horizontal surface motion is observed to point mostly, but not always, away from the former ice load. To interpret these observations, we investigate the direction of horizontal velocities using glacial isostatic adjustment models. The direction is controlled by the opposing actions of inward mantle flow and outward lithosphere motion. In contrast with the prevailing idea that glacial isostatic adjustment-induced horizontal velocities point outward, we show that velocities can be either outward or inward. Immediately after deglaciation velocities point inward but change direction to outward after a time that is controlled by mantle viscosity. Present-day horizontal velocities point outward for a uniform mantle viscosity below 10 20 Pa s and inward for above 10 22 Pa s, with a combination of outward and inward in between. Our results help to interpret GPS-observed horizontal velocities in areas with varying mantle viscosity.Plain Language Summary The rebound of the Earth following the disappearance of large ice sheets leads to vertical and horizontal movements of the Earth's surface that can be observed with GPS. To explain GPS observations of postglacial rebound with models, it is important to understand how deformation rates depend on the internal structure of the Earth. Here we investigate how the direction of horizontal velocities depends on the viscosity of the mantle using numerical models. The horizontal velocities result from the opposite movements of different layers inside of the Earth. After melting velocities initially point toward the previously glaciated area, but their direction changes after a time that depends on mantle viscosity. Present-day horizontal velocities at the surface point toward the former ice load for a relatively high mantle viscosity, and point away from the former ice load for relatively low viscosities. Our results show that the direction of horizontal velocities derived from GPS observations can provide important information about the Earth's interior.

On the Origin of Holocene Sea-Level Transgressions in Formerly Glaciated Regions

“…However, at that point the rates of uplift and subsidence will likely be small and possibly undetectable. It is also worth noting that horizontal motions are affected by the mantle dynamics discussed here (Hermans et al, 2018), but are not the focus of this study.…”

An introduction to the geochemistry and geophysics of the Antarctic mantle

“…Horizontal GPS velocities are dominated by plate rotation, but neotectonic deformation and GIA also play a role (King et al 2015;Vardićet al 2022). Even though the GIA contribution is small relative to other parameters, its sensitivity is such that even the sign is dependent on the mantle viscosity (Hermans et al 2018). In some regions, post-seismic deformation can be detected (King and Santamaría-Gómez 2016) but the following over-arching question remains: What mantle processes contribute to observed present-day vertical and horizontal motion, and at which temporal and spatial scales?…”