Bathymetric data, stratigraphic correlations, and distribution of volcanism indicate that the Tjörnes fracture zone is composed of several subsiding troughs and volcanic chains having a north trend and thus differing structurally from the undivided axial rifting zone in northern Iceland. The troughs developed successively as spreading axes across the Tjörnes fracture zone. Thus a spreading axis shifted from a position at long. 18° W. to a position at long. 17° W. during late Matuyama time. The southern margin of the Tjörnes fracture zone is marked by west-northwest–trending oblique-slip faults observed on land south of the Tjörnes peninsula and traceable west-northwest on the sea bottom for 70 km. The strike slip along these faults amounts to possibly 60 km, most of which was accomplished prior to the development of the currently active spreading axis of the Axarfjördur trough. The southern boundary of the Tjörnes fracture zone cannot be traced as distinct faults across the areas of plateau basalt east of the axial rifting zone. A former position of the fracture zone is indicated by juxtaposed rock sequences of widely contrasting ages, however.
Available K-Ar ages and paleomagnetic data indicate a gap in the lava-flow succession in eastern Iceland between ∼8 and ∼4 m.y. ago. The gap is correlated with a major shift and reorganization of the axial rifting zone that affected the Tjörnes fracture zone as well. Formation of the present-day rifting zone in northern Iceland ∼4 m.y. ago was preceded by the tilting and subsidence of the Tertiary plateau basalts and by the accumulation of sediment in the resulting trough. Before this event, the Reykjanes-Langjökull rifting zone in southwest Iceland extended up to Skagi on the north coast to join more or less directly with the Kolbeinsey Ridge. The Tertiary plateau basalts of northern and eastern Iceland formed in this western rifting zone, the northern part of which is now extinct. Accreting plate margins in the Iceland region moving west-northwest relative to a stationary plume or hot spot in the mantle could explain the shift of a ridge segment to a new position east of the formerly active zone.