The NE Atlantic broke up in a disorderly way. North of the Greenland-Iceland-Faroe Ridge the new rift propagated south along the Caledonian suture. South of the Greenland-Iceland-Faroe Ridge it propagated north through the North Atlantic Craton along an axis ~150 km west of the northern rift. Both propagators stalled where the confluence of the Nagssugtoqidian and Caledonian orogens formed a ~300-km-wide transverse barrier.

Thereafter, the ~150 x 300-km block of continental crust between the rift tips–the Iceland Microcontinent–extended in a distributed, unstable manner along multiple axes of extension. These frequently migrated laterally with shearing occurring between them in diffuse transfer zones. Such deformation continues to the present day in Iceland. It is enabled by underlying magma-assisted stretching of ductile continental crust from the Iceland Microplate and flanking continental areas.

The Icelandic-type crust which underlies the Greenland-Iceland-Faroe Ridge is thus not anomalously thick oceanic crust as often assumed. Upper Icelandic-type crust comprises magma flows and dikes and lower Icelandic-type crust comprises magma-inflated continental lower crust. Magma production in Iceland thus has a total thickness of only 10-15 km, much less than the 30-40 km usually assumed. Numerical modeling confirms that ductile extension of continental crust can continue for tens of Ma by lower-crustal flow.

A largely continental composition of Icelandic-type lower crust can account for multiple seismic, gravity, bathymetric, topographic, petrological and geochemical data that are inconsistent with a gabbroic Icelandic-type lower crust. It also offers a solution to difficulties in numerical models for melt-production, the chronic tectonic disequilibrium of adjacent rifted margins, the chevron ridges of thick crust that flank the Reykjanes Ridge and the tectonic decoupling of the oceans to the north and south. Such a model fits other regions, e.g., the Davis Strait, South Atlantic and West Indian Ocean.