We present 2-D thermo-mechanical and numerical modeling at lithospheric scale to assess the impact of pre-existing lithosphere thickness contrast on rift localization. The results of the modeling suggest that thickness heterogeneity was responsible for focussing rifting and passive asthenospheric upwelling in the Permo-Carboniferous Oslo Graben at the western margin of the Fennoscandian Shield. These findings are consistent with inferences from recent studies showing that both the crust-mantle boundary and the base of the lithosphere are and were deeper at the eastern margin of the Oslo Graben, resulting in contrasting crust and lithosphere thickness.