Accretionary volcanic processes and the crystal structure of Iceland

Abstract

The geology of the Quaternary axial zones in Iceland is dominated by the occurrence of well-defined volcanic centres, such as Krafla (65°40'N, 16°40'W) marked by the occurrence of a caldera overlying a high-level magma chamber. The lateral injection of magma from the chamber, during active volcanism and crustal extension, results in the emplacement of a dyke swarm, occasional fissure eruptions, and associated normal faulting. Comparable relations are observable in the older, Tertiary sections of the lava pile, where well-defined swarms of basic dykes are focused on Tertiary volcanic centres, some of which have high-level plutonic complexes. The gently dipping Tertiary lava sequences are characterised by a dip at sea-level which is notably greater than the dip of the lavas at higher elevations. The features of both the Quaternary and Tertiary areas are interpreted in terms of a model for crustal generation at the axial zone involving fissure volcanism, dyke emplacement and listric faulting. Rotation of successive lavas along the curved fault surfaces produces the changes in the dips of the flows with elevation observed in the Tertiary areas, and a broad synclinal structure, centred about the spreading axis. The hypothesis that listric faulting is important in relation to crustal growth in Iceland, links that extensional regime with continental rift environments where listric faulting has been recognised as the dominant structural element.