Crustal structure and composition of the Oslo Graben, Norway

Although more than 250 my old, the Oslo Graben has retained a distinctive distribution of P-wave velocities associated with rifting, intrusion and underplating of the thinned crustal section. We calculate Poissonʹs ratio values from crustal P and S-wave velocities along an ~ 400 km long profile across the southern Scandinavian Peninsula, with a focus on the Oslo Graben. Plutonic rocks are now exposed at the surface in the graben due to post rifting erosion and the corresponding low (5.5 km/s) P-wave velocities extend to depths of ~ 3 km. The P-wave velocity and Poissonʹs ratio between depths of 6 and 34 km are 5% higher inside than outside the rift boundaries which, together with thickening of the > 7 km/s lower crust beneath the rift, indicate magmatic intrusions. Poissonʹs ratio of 0.27 and high 7.1–7.4 km/s velocities indicate a heterogeneous composition lower crust. The thickened high velocity lower crust is 1.5 times wider than the surface expression of the rift and extends to both the east and west of the main rift axis. Depth dependant extension may have occurred within the rift and may account for the significant volumes of volcanic output despite the small surface extension observed. The added volume of intrusive material may explain why the Moho up warp is small (as little as 2 km) at the rift. Based on the new P-wave velocities, Poissonʹs ratio measurements and the thickness of the magmatic underplate, apparent β values for the rift are estimated to be 1.5 but, when intrusion within the crust is included, could be as high at 2.3.