Improving upper mantle viscosity estimates: Constraints from seismic and magnetotelluric data, and impacts on asthenospheric flow

A simplified Earth’s structure consists of three geological layers: a stiff lithosphere (plates), a mantle and a core. Like for an example your favorite yogurt, the rock in the solid mantle can be flowing, but it requires an enormous amount of force to deform it and a time scale over thousands to millions of years! To quantify how fast the deformation process would be for a certain applied force, the physical quantity “viscosity” is used. For instance, water has lower viscosity than yogurt, which enables you to stir the water faster than the yogurt. For the mantle, viscosity affects how fast the plates can move laterally and vertically. An example for this is Scandinavia (including Norway), which is continuously uplifting as a response to melting of past ice sheet from “siste istid” with up to 3000 meters ice caps, thereby influencing climate and sea level. Unfortunately, mantle viscosity cannot be measured directly, but is commonly estimated from surface deformations. This doctoral study introduces a method for estimating viscosity using geophysical observations (seismic and magnetotelluric), which reflect realistic mantle conditions such as temperature, water content, and partial melt that control viscosity. The method provides useful results for Scandinavia and can be applied in other places as well. Resulting viscosity models can aid in geodynamic studies such as modelling flow patterns below the moving oceanic plates.