Asymmetric lithosphere as the cause of rifting and magmatism in the Permo-Carboniferous Oslo Graben

Compared to other Permo-Carboniferous rift basins of NW Europe, the Oslo Graben has two distinct characteristics. First, it initiated inside cold and stable Precambrian lithosphere, whereas most Permo-Carboniferous basins developed in weaker Phanerozoic lithosphere, and second, it is characterized by large volumes of magmatic rocks despite relatively little extension. Seismic reflection surveys show that the crust thickens from southern Norway to southern Sweden, the most significant Moho deepening occurring from the Oslo Region eastwards. Deep seismic studies also suggest that the base of the lithosphere deepens markedly eastwards from the Oslo Region. Such a long-wavelength lithospberic geometry cannot be explained by the Permian or post-Permian evolution of the area, hence the Oslo Graben appears to have evolved at the transition between two lithospheric domains with contrasting thickness. Numerical thermo-mechanical modelling is applied to test if this transitional position can influence the dynamics of rifting. Different models with varying lithosphere thickness contrast are considered. Model results show that a crust and lithosphere thickness contrast comparable to the Oslo Region can explain rifting and focusing of magmatism in a narrow zone with minor thinning of the crust. Models also account for the major characteristics of the Oslo Graben in terms of location and strain distributions in the crust. © The Geological Society of London 2004.