Kristján Sæmundsson

Evolution of the Axial Rifting Zone in Northern Iceland and the Tjörnes Fracture Zone

Bathymetric data, stratigraphic correlations, and distribution of volcanism indicate that the Tjornes 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 Tjornes 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 Tjornes fracture zone is marked by west-northwest–trending oblique-slip faults observed on land south of the Tjornes 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 Axarfjordur trough. The southern boundary of the Tjornes 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 Tjornes 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-Langjokull 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.

Extension of the geomagnetic polarity time scale to 6.5 m.y.: K-Ar dating, geological and paleomagnetic study of a 3,500-m lava succession in western Iceland

More than 400 successive lavas in Borgarfjordur, western Iceland, have been subjected to paleomagnetic and K-Ar age analysis. Volcanism in the region was virtually continuous between about 7.0 and 2 m.y. ago, during which time more than 3,500 m of volcanics and interbedded sediments accumulated. Regression analysis of the K-Ar age and aggregate thickness data demonstrates that the rate of growth of the lava pile was remarkably uniform at 730 m/m.y. throughout the whole period. The magnetostratigraphic and K-Ar data indicate a nearly complete record of the geomagnetic polarity history and extend the polarity time scale based upon data from subaerial volcanic rocks to about 6.5 m.y. ago. The boundaries of polarity epoch 5 are shown to be 5.34 and 5.83 m.y. Epoch 6 lies between about 6.54 and 5.83 m.y. ago, during which time three normal polarity events are recognized. The ages determined for the polarity-interval boundaries in this study confirm recent estimates derived from analyses of marine magnetic anomalies using sea-floor–spreading assumptions. The results show that it is possible to obtain reliable K-Ar ages on lavas that have undergone zeolite facies metamorphism at temperatures up to about 150°C by careful selection of samples that are well crystallized and have their original high-temperature mineralogy preserved. A revised estimate of 5.2 ± 0.1 m.y. is proposed for the age of the Miocene-Pliocene boundary.