The geometries of the Devonian basins of northern Scotland and southern Norway are analysed to
determine the regional tectonic control on their formation. The Orcadian Basin in northern Scotland
consists of distinct half-graben sub-basins controlled by separate, closely spaced, sub-parallel arcuate
extensional fault elements which are parti y con tro lied by the Caledonian crustal fabric. The unconformity
at the base of the middle Devonian in this area is interpreted as being formed by a sudden increase in
the rate of extension. In southern Norway, in contrast, extensional structures are more widely spaced
and larger in scale. Very deep (>8 km) half-graben basins were developed of which only the bottom
parts are now preserved. In the Devonian of Norway the Jack of a major stratigraphic break suggests a
constant extension rate. The observed differences in style are thought to be due to different initial crustal
thicknesses.