Chapter 9 The oslo rift

Publisher Summary This chapter focuses on the Oslo rift formed in response to a combination of regional stretching caused by dextral strike-slip movements along the Sorgenfrei-Tornquist Zone and a positive temperature anomaly in the asthenosphere. The earliest manifestation of the Oslo rifting event was the formation of a shallow depression in late Carboniferous time. Subsequent erosion removed the upper 1-3 km of rift related and prerift rocks in the northern, exposed part of the rift, whereas in the southern, submerged part of the rift, rift-related deposits are buried under younger sediments. Seismic, gravity and geochemical data imply that the rifting event caused significant modifications of the lithosphere. Petrological and geochemical data suggest that massive layer represents dense cumulates and gabbroic rocks formed by fractional crystallization of mantle-derived magmas in deep crustal. The storage of hot, mafic magmas in the deep crust also caused anatexis of Permian gabbros and, to a lesser extent, Precambrian country-rocks, and transport of light, syenitic and granitic components to the upper crust, whereas dense residues are left in the deep crust together with the dense cumulates. The mafic magmatism associated with the rift appears to have originated in different parts of a heterogeneous upper mantle, which before the rifting event, belonged to the subcontinental lithosphere. Important unsolved problems in the Oslo rift are the relative timing of tectonic activity and magmatism between the exposed Oslo Graben, the submerged Skagerrak Graben, and the intensity of magmatism in the Skagerrak Graben as compared to the Oslo Graben.