Infrastructure and mechanics of volcanic systems in Iceland

Abstract

The divergent plate boundary in Iceland consists of more than two dozen systems where most of the volcano-tectonic activity takes place. At the surface the volcanic systems are characterised by 5–20-km-wide and 40–100 km-long swarms of tension fractures (~10 2 m long), normal faults (~10 3 m long) and volcanic fissures. The Holocene fissure swarms are confined to less than 10,000-year-old basaltic lava flows, mostly pahoehoe, occurring near the centre of the active rift zone. In any particular swarm, the number of tension fractures exceeds that of normal faults. All tension fractures and normal faults are vertical at the surface, indicating that the surface parts were generated by an absolute tension. In addition to a fissure swarm, many volcanic systems have a central volcano some of which have developed collapse calderas. In the late Tertiary and Pleistocene lava pile of Iceland, extinct volcanic systems are represented by local sheet swarms and regional dyke swarms. The sheet swarms are normally circular or slightly elliptical, several kilometres in radius, and are confined to the extinct central volcanoes. Many swarms are associated with large plutons, exposed at 1–2 km depth beneath the initial top of the rift zone and presumably the uppermost parts of extinct crustal magma chambers, and in short traverses up to 90% of the rock may consist of sheets. The sheets have a very variable strike, dip on average 45–65 °, mostly towards the centre of the associated volcano, and have an average thickness of about 1 m. The regional dykes occur outside central volcanoes in swarms that are commonly 50 km long and 5–10 km wide. In several-kilometre-long traverses, commonly 1–5% of the rock consist of dykes but occasionally as much as 15–28%. Most regional dykes are subparallel and subvertical. The average dyke thickness in the Pleistocene swarms is less than 2 m but 4–6 m in the Tertiary swarms. While active, the volcanic systems in the rift zone are supplied with magma from reservoirs located at the depth of 8–12 km at the boundary between the crust and upper mantle. The reservoirs are partially molten, with totally molten top regions, and of cross-sectional areas similar to those of the volcanic systems that they feed. Some active volcanic systems, especially those that develop central volcanoes, high-temperature areas and calderas, have, in addition to the deep-seated magma reservoirs, shallow crustal magma chambers, located at 1–3 km depth, which, in turn, are fed by the deeper magma reservoirs. It is proposed that the regional dyke swarms are supplied with magma largely from the deep-seated reservoirs, whereas the local sheet swarms are mainly fed from the associated crustal magma chambers. Because the volume of a crustal chamber is less than that of its deeper source reservoir, a single magma flow (dyke intrusion) from the reservoir may trigger tens of magma flows (sheet intrusions) from the chamber, which is one explanation for the enormous number of sheets associated with many central volcanoes. The sheets follow the stress trajectories of the local stress fields around the source chambers, whereas the regional stress field associated with the divergent plate movements controls the emplacement of the regional dykes. It is suggested that many dykes develop as self-affine (some as self-similar) structures. When applied to the Krafla volcanic system in northern Iceland, model calculations suggest that a magma flow from the Krafla reservoir, with regional dyke formation, should occur, on average, once every several hundred years, and that tens of sheets might be injected from the shallow Krafla chamber and triggered by a single magma flow from the Krafla reservoir. These results are in broad agreement with the available data.