STRUCTURAL RELATIONSHIPS AND MAGNETOSTRATIGRAPHY OF THE VOLCANIC SUCCESSION AND THE BREIDDALUR DYKE SWARM IN REYDARFJORDUR, EASTERN ICELAND

Abstract

The 1~9-km-thick sequence inter~ected by the IRDP 1978 drill hole at Reydarfjordur, eastern Iceland, is correlated with the westward dipping exposed volcanic succession on the basis of extensive field mapping and paleomagnetic studies. The base of the landward extension of sea floor magnetic anomaly 5 (epoch 9) and other exposed marker formations can also be identified in the core. Field observations, inclu4ing vertically discontinuous dykes, the lack of lava-feeder relationships, and the measured K-Ar age difference between dykes and the surrounding lavas suggest that, contrary to previous in~erpretations, dykes of the Breiddalur dyke swarm were intruded by lateral injection from the south. These dykes are considerably younger than, and thus could not have fed, the exposed lava succession in the vicinity of the drill site. The lowest 450 m of the core may represent lavas related to activity in the Reydarfjordur volcanic center 13.5 km to the east. It is proposed that about 10.3 Ma the Reydarfjordur volcanic center ceased to be active and volcanic activity shifted about 20 km to the west, where the Breiddalur and Thingmuli volcanic centers later developed. The above field observations allow a model for the upper crustal construction of eastern Iceland to be proposed. The model assumes that the lava succession of eastern Iceland was formed. in at least three successive volcanic zones that resulted from frequent shifting of the location of greatest volcanism. Contrary to what is generally assumed, no systematic decrease in magnetic intensity with depth was observed for a 0.9 km thick vertical lava section in Holmatindur, about 13.5 km east of the drill site. This suggests that previous generalizations based on regional studies may have little predictive value when applied to certain specific areas.