Sensitivity of crustal velocities in Fennoscandia to radial and lateral viscosity variations in the mantle

We investigate the sensitivity kernels of the present-day velocities in Fennoscandia induced by the retreat of the Late Pleistocene ice sheets with a 3D Finite-element model having compressible, viscoelastic material properties and a realistic ice load history of the Fennoscandian ice sheet. The model is subdivided into blocks of variable size, which results in a large number of kernels to interpret. Thus, we introduce a simple approach to calculate the “averaged” kernel of a block by averaging the perturbed predictions of all surface nodes of this block to one value for this block.Our results show that the present-day uplift velocity is mostly sensitive to upper-mantle layers between 220 and 540 km depth, independent of the block size. Velocities in blocks located inside the former ice sheet area are more sensitive to viscosity variations than velocities in blocks located outside the former ice sheet. The largest effects are found for blocks located below the former ice maximum on the surface. The uplift velocity in the smaller blocks is more sensitive to viscosity changes than in larger blocks.For the present-day horizontal velocity, the sensitivity depends on the block size and the location of this block in relation to the former ice sheet. In general, lateral viscosity variations in the transition zone of the mantle have a strong influence on the tangential motion. A comparison of the results of smaller and larger blocks also indicates higher sensitivities for the horizontal velocities of larger blocks.