The Krafla fissure swarm, Iceland, and its formation by rifting events

Abstract

Fissure swarms at divergent plate boundaries are activated in rifting events, during which intense fracturing occurs in the fissure swarm accompanied by intrusion of magma to form dikes that sometimes lead to eruptions. To study the evolution of fissure swarms and the behaviour of rifting events, detailed mapping was carried out on fractures and eruptive fissures within the Krafla fissure swarm (KFS). Fracture densities of dated lava flows ranging from 10,000 years bp to ~30 years old were studied, and the fracture pattern was compared with data on the historical Mývatn rifting episode (1724–1729) and the instrumentally recorded Krafla rifting episode (1975–1984). Additionally, the interaction of transform faults and fissure swarms was studied by analysing the influence of the Husavik transform faults on the KFS. During the historical rifting episodes, eruptions on the fissure swarm occurred within ~7 km from the Krafla central volcano, although faults and fractures were formed or activated at up to 60–70 km distance. This is consistent with earlier rifting patterns, as Holocene eruptive fissures within the KFS are most common closer to the central volcano. Most fractures within the central Krafla caldera are parallel to the overall orientation of the fissure swarm. This suggests that the regional stress field is governing in the Krafla central volcano, while the local stress field of the volcano is generally weak. A sudden widening of the graben in the northern KFS and a local maximum of fracture density at the junction of the KFS and the extrapolation of the Husavik transform fault zone indicates possible buried continuation of the Husavik transform fault zone which extends to the KFS. Eruptive fissures are found farther away from the Krafla central volcano in the southern KFS than in the northern KFS. This is either due to an additional magma source in the southern KFS (the Heiðarsporður volcanic system) or caused by the Husavik transform faults, transferring some of the plate extension in the northern part. Fracture density within particular lava flow fields increases with field age, indicating that repeated rifting events have occurred in the fissure swarm during the last 10,000 years bp. The fracture density in the KFS is also generally higher closer to the Krafla central volcano than at the ends of the fissure swarm. This suggests that rifting events are more common in the parts of the fissure swarm closer to the Krafla central volcano.