Lateral, radial and temporal variations in upper mantle viscosity and rheology under Scandinavia

Abstract

The viscosity of the upper mantle has a large control on the dynamics of plate tectonic processes or the response of the Earth's crust after a period of glaciation. Temperature variations within the upper mantle, time-dependent stress changes due to glaciations, and/or variations in the microstructural characteristics of upper mantle rocks (grain size, water content) will result in orders of magnitude variations in upper mantle viscosity. In this study we have taken a microphysical approach to determine variations in viscosity under Scandinavia. We combined experimentally determined flow laws for olivine, data on olivine grain size from Scandinavian xenoliths and peridotites, and stress changes within the upper mantle over the last glaciation period (30 kyr B.P.–present) with two data sets of the temperature distribution within the upper mantle under Europe from Goes et al. (2000) derived from seismic tomography and Artemieva (2006) derived from heat flow measurements. Modeling of olivine viscosity under Scandinavia shows large lateral, radial, and temporal variations in upper mantle viscosity. Lateral temperature variations cause up to four orders of magnitude lateral viscosity variations. Glaciation-induced time-dependent changes of stress cause two orders of magnitude variations in viscosity. Mean viscosity values for a dry upper mantle under Scandinavia are expected to be larger than 1022 Pa s, whereas for a wet upper mantle lower viscosity values in the range of 1019–1022 Pa s are predicted. Estimates of the upper mantle viscosity under Scandinavia from glacial isostatic adjustment studies (1020–1021 Pa s) would indicate that a wet upper mantle below Scandinavia is most likely present. The viscosity modeling furthermore shows that the type of rheology of the upper mantle (linear versus nonlinear rheology) is very sensitive to the microstructural state of the upper mantle. Both mechanisms are active in the upper mantle under Scandinavia on timescales of glacial isostatic adjustment, and their relative contribution varies radially, laterally, and temporally.