New results on P and S-wave seismic velocity structure in southern Norway indicate that the crust has an average Poisson's ratio of 0.25, is predominantly of felsic-intermediate composition and lacks a significant mafic lower crust. A crustal scale refraction seismic study (Magnus-Rex — Mantle investigations of Norwegian uplift structure, refraction experiment) acquired data along three 300 to 400 km long active source seismic profiles across the Southwest Scandinavian Domain in southern Norway, the youngest section of the Fennoscandian shield. Moho depths in the Domain are 36–40 km, thinning towards the continental shelf and Oslo Graben. The high Vp lower crust beneath the Southwest Scandinavian Domain (Vp > 7 km/s) is around 4 km thick. Crustal structure in the adjacent Svecofennian Domain differs significantly; Moho depths reach ~ 50 km and an up to 24 km thick high Vp lower crust is present. Strong P and S-wave arrivals are recorded from Magnus-Rex, from which Poisson's ratio for the crust is calculated. The unusually strong S-wave arrivals allow rare insight into crustal Poisson's ratio structure that is not commonly available from active source data. The thinness of the mafic lower crust in southern Norway is in significant contrast to other parts of the Fennoscandian shield and delamination or convective removal of the lower crust is proposed. Tectonic events that may have triggered detachment of the lower crust include extensional collapse after the Caledonian orogeny and Permian extension.