GPS-derived Horizontal Velocities Constrain Viscosity Contrast Beneath the Transantarctic Mountains

Abstract:

The response of the solid Earth to the melting of ice sheets, or Glacial isostatic adjustment (GIA), is controlled by thickness of the lithosphere and the viscosity of the mantle, and as such it is also sensitive to lateral changes in these parameters. However it has proved difficult to uniquely constrain lateral changes in Earth parameters using observations of GIA. Here we investigate whether horizontal velocities in Antarctica can constrain a viscosity contrast beneath the Transantarctic mountains.

A large ice sheet that existed in the Ross sea at the last glacial maximum has since melted. In GIA models in which viscosity only varies with depth, the removal of an ice sheet leads to horizontal motion that is directed away from the former ice sheet (outward). Seismic imaging indicates thicker crust and colder mantle beneath East Antarctica which translates to a viscosity contrast at depth. Simulations with a simple, axisymmetric model show that horizontal velocities are reversed (inward motion) following ice sheet removal, if there is a viscosity contrast of at least 3 orders of magnitude with the stiffer mantle a viscosity of at least 10²² Pa s. Simulations with a 3D finite element model and viscosity maps based on seismic models show a more complicated velocity pattern with mostly inward motion directly outside the area of largest ice melt. The 3D models provide a better fit to GPS derived horizontal velocities in the north-western shore of the Ross Sea. There, the direction of horizontal motion can only be reproduced if the model includes a lateral viscosity contrast.