Main Geologic Features of the Oslo Graben

Abstract

En echelon faults cut a late Carboniferous/early Permian peneplain in South-Eastern Norway to begin subsidence of a complex graben. Volcanic eruptions filled the graben as it gradually subsided and most likely also spilled over the sides. Latitic rhomb porphyries and basalts dominate in preserved areas of volcanic rocks. A lava plateau comprising basalt B1, rhomb porphyries RP1-RP12, and basalts B2 and B3 formed. During the B3 phase at least seven caldera subsidences started. In all 15 to 18 cauldrons formed. Younger flows RP13-26 with several trachytes and additional basalts are now preserved only in the south (Vestfold area) and in some cauldrons. Late in and after the stage of plateau volcanism, monzonitic, syenitic, and granitic magmas stoped upwards, in some cases even penetrating the volcanic rocks to form composite plutonic bodies, in present exposures covering 5000 km² of the total 8500 km² of the graben. Especially well-known rocks are nordmarkite (aegerite syenite) and larvikite (augite monzonite), the cryptoperthite of which is a moonstone, in places with a strong blue schiller. Rb-Sr ages range from 294 (early lavas) to 274 (granite) m.y. Gravity anomaly and explosion seismic studies suggest an early Permian basalt magma pillow, 10 km thick at the base of a 20 km crust, as the major source of the magmatism. Trace element contents in lavas indicate that differentiation was far advanced at the start of surface volcanism.