Upper mantle viscosity from continuous GPS baselines in Fennoscandia

The permanently recording BIFROST (Baseline Inferences for Fennoscandian Rebound Observations, Sea Level and Tectonics) GPS network was set into operation during the early to mid-1990s to monitor the three-dimensional crustal deformation field in Fennoscandia. We have employed 2500 days of BIFROST GPS data to estimate rates of baseline component change (length, transverse and up). Baselines are less sensitive than single-site position estimates to perturbations in satellite orbits and reference frame realizations. Hence, the baseline time series represent a particularly robust data set for model parameter estimation. We use this data set to outline an evaluation scheme for GPS observations of intraplate horizontal movements. Our results show that the data can be used to infer parameters in a model that simulates the glacial isostatic adjustment (GIA) of the region. This process is widely believed to dominate the present-day deformation field, and analysis of the length and transverse components support this view. The data prefer an elastic lithosphere of thickness 120 km and an upper mantle viscosity in the range (3-10)×1020 Pa s (to 95% confidence). These results are consistent with those obtained in a previous analysis that considered single-site position estimates. This suggests that the problems associated with perturbations in satellite orbits and reference frame realizations have not significantly impacted the analysis of single-site positions from the BIFROST time series. Subtracting the best-fitting GIA model predictions from the observed baseline rates leaves a horizontal residual field of magnitude less than 1 mm year−1.