Origin of the Oslo Graben in relation to the Hercynian-Alleghenian orogeny and lithospheric rifting in the North Atlantic

The contemporaneous extrusion of basalts in the Oslo Graben and intrusion of dolentes in northern Britain and southern Sweden at ~ 295 Ma calls for a common explanation. Two hypotheses are investigated: 1. (1) the Graben and dykes resulted from extensional stresses associated with progressive lithosphere separation to the northwest of Europe in the late Carboniferous. 2. (2) the dykes, and therefore the Graben, were related in some way to oblique collision of plates in the Hercynian orogeny which was developing to the south. The first hypothesis is mechanically more satisfying, and makes a number of testable predictions. It states that in late Carboniferous times the lithosphere separated in two places along the orogenic grain to the west of Britain and Norway. The two embryonic oceanic rifts were divided by thick cold lithosphere with an Archaean crust to the north of Scotland. Extensional stresses were focussed in this region, fanning out in an arcuate zone to the south and east, causing failure where the lithosphere was relatively thin. In Norway the strain was restricted to a zone previously thinned and weakened in the Early Palaeozoic i.e. the Oslo Graben. In Britain the Caledonoid grain is oblique to the expected direction of extensional strain, and a dyke swarm, trending E-W and about 300 km wide, was formed. The first hypothesis predicts that the dykes should die out to the west but continue along an arc and widen to the east-northeast under the western North Sea. Interpretation of aeromagnetic maps shows that the dykes behave as predicted by hypothesis one, but that their trends and extent are at variance with the expectations of hypothesis two. The apparent contradiction of rifting to the northwest of Europe occurring at the same time as compression and oblique collision in the heart of Europe is resolved in principle by two plate tectonic reconstructions of Pangaea drawn for late Carboniferous and early Permian times. The Oslo Graben was conceived in mid-Carboniferous times and born towards the end of the Carboniferous as the thin lithosphere failed and allowed the extrusion of basalt magma. It was then abandoned as the two collinear oceanic rifts to the west and northwest joined up, but the Graben carried on a life of its own, as the mafic magma cushion caught in the density trap beneath the crust continued to differentiate, and alkali magma pods rose buoyantly to freeze in the upper crust or extrude on the Graben floor