Lateral and radial viscosity variations beneath Fennoscandia inferred from seismic and MT observations

Abstract

Fennoscandia is continuously uplifting in response to past deglaciation, a process known as glacial isostatic adjustment or GIA. One of the factors that controls the uplift rates is the viscosity of the upper mantle, which is difficult to constrain. Here, we reconstruct the upper mantle viscosity structure of Fennoscandia by inferring temperature and water content from seismic and magnetotelluric (MT) data. Using a 1-D MT model for Fennoscandian cratons together with a global seismic model, we infer an upper mantle viscosity range of ~1019 - 1024 Pa·s for 1 – 10 mm grain size, which encompasses the GIA-constrained viscosities of 1020 - 1021 Pa·s. The associated viscosity uncertainties of our calculation are attributed to the uncertainties associated with the geophysical data and unknown grain size. We can obtain tighter constraints if we assume that the Fennoscandian upper mantle is either a wet harzburgite (1019.2 - 1023.5 Pa·s) or a dry pyrolite (1020.0 - 1023.6 Pa·s) below 250 km, where pyrolite is ~10 times more viscous than harzburgite. Furthermore, assuming a constant grain size of either 1 mm or 10 mm reduces the viscosity range by approximately 2 orders of magnitude. In northwestern Fennoscandia, where a high-resolution 2-D resistivity model is available, the calculated viscosities are ~10 - 100  times lower than those for the Fennoscandian craton because the mantle has a higher water content, and both pyrolite and harzburgite must be wet. Overall, our calculated viscosities for Fennoscandia that are constrained from seismic and MT observations agree with the mantle viscosities constrained from GIA. This suggests that geophysical observations can usefully constrain upper mantle viscosity, and its lateral variations, for other parts of the world without GIA constraints.