A new tectonic scheme is proposed for the evolution of the southern Norwegian-Greenland Sea. The Jan Mayen fragment began to split from the East Greenland margin at the end of anomaly 20 times (43 Ma). Until anomaly 7 time (27 Ma), complementary "fan-shaped" spreading occurred about the newly formed Kolbeinsey axis to the west of the Jan Mayen block and about the Aegir axis in the Norway Basin to the east. The Jan Mayen block rotated 28° anticlockwise before spreading ceased in the Norway Basin and became regular in the Iceland Plateau. The primary cause of the shift of axis may have been the anticlockwise reorientation of spreading direction which followed the cessation of spreading in the Labrador Sea.

As defined by the presence of the relatively transparent basement reflector, "Horizon 0," the Jan Mayen block is about 90 km wide and extends from 70°N to at least as far south as 67°N. A reconstruction to the beginning of anomaly 24 time suggests that Horizon 0 is basaltic and represents either early phase oceanic crust formed by rapid subaerial accretion or plateau basalt overlying continental crust. During its separation from Greenland, the southern part of the block was sheared sinistrally, resulting in volcanism and en echelon graben formation. The Oligocene unconformity, "Horizon A" postdates the initial separation and was probably caused by the major mid-Oligocene drop in sea level.