Massifs in which lower-crustal and upper-mantle rocks are exposed at the sea floor, formed at mid-ocean ridges through slip on detachment faults rooted below the spreading axis. The formation of these large, shallow sea-floor features appears to be an episodic outcome of plate rifting and accretion at slow spreading ridges. An outstanding example of an oceanic core complex has been mapped at the eastern intersection of the Mid-Atlantic Ridge and the Atlantis transform fault; see Planert et al. (2010) Geophys. J. Int. 181, 1, 113.