Fracturing and Seismicity at the Prestahnúkur Fissure Swarm in the Ultra-Slowly Spreading Western Volcanic Zone, Iceland

Abstract:

Extension across the mid-Atlantic plate boundary in south Iceland is taken up by two parallel zones, the Western and the Eastern Volcanic Zones. Spreading across the western zone is of the order of 1-7 mm/year, qualifying it as an ultra-slow rift. The Prestahnúkur fissure swarm in the Western Volcanic Zone offers an opportunity to study magma-tectonic interaction in an ultra-slow spreading setting. In this study, fractures and faults were mapped in detail from aerial photographs to determine the extent of the fissure swarm. The fissure swarm is about 60 km long and 10 km wide, although its extent to the south is uncertain due to its linkage with the adjacent Hengill fissure swarm. The fissure swarm has both open fractures, indicating postglacial activation, and normal faults with up to 45 m vertical offset. Hyaloclastite ridges („tindars“), produced by subglacial fissure eruptions during the Pleistocene, are found within the fissure swarm, as well as postglacial lava flows. Intermittent seismicity occurs in the area. Earthquakes during the last ~24 years were relocated to study their relation with the surface fractures. Relocation of these events indicate that some of the fractures seen on surface have been activated during these years. The small cumulative seismic moment and slow spreading measured by geodetic methods nevertheless show that no dike intrusions have occurred during this period. Pleistocene tindars and Holocene lavas are clear indications of magmatism, however. We therefore suggest that the Prestahnúkur fissure swarm is mainly formed by diking and rifting during rifting episodes, whereas small scale fracture movements can occur during inter-rifting periods. The present small-scale fracture movements may indicate lack of magma to drive dike intrusions.