3D Viscosity affects GIA-induced Uplift Rate Patterns in North America and Greenland

Abstract:

The present-day uplift rate pattern in GIA areas is sometimes called the footprint of the past ice sheet. However, if the relaxation properties vary within or close to the footprint it might not resemble the shape of the past ice sheet. In North America seismic models indicate larger than average viscosity around Hudson Bay down to 300 km and lower viscosities towards the West and East coast, and a narrow zone of low viscosity between North America and Greenland that could influence the strength of the forebulge in Greenland. Here we study the relation between maximum ice height and uplift rate in North America with a GIA model with 3D viscosity, and we investigate the effect of Laurentide glaciation on the forebulge in Greenland.

We use a new global model of 3D temperature in the lithosphere and upper mantle (WINTERC5.4) which is based on seismic and gravity data and petrology of mantle rocks. Mantle rheology is based on flows laws for diffusion and dislocation creep with varying grain size and water content, as well as grain boundary sliding. For grain size and water content values that correspond to lower average viscosity the maximum uplift rate pattern changes, with the peak uplift rate in Western Canada becoming stronger. This indicates that ice models inferred from geodetic data can be biased by 3D viscosity. The lower viscosities also enhance the forebulge in Greenland by several mm/year. Although the model is not yet well constrained, it suggests that there is potentially significant uncertainty in the contribution of Laurentide glaciation on uplift rates in Greenland.