Geology and structure of the Reykjanes volcanic system, Iceland

The Reykjanes Peninsula is a trans-tensional plate boundary with several volcanic systems from the centers of which fissure swarms extend to the NE and SW. The fissure swarms accommodate the extensional component, whereas north-south trending strike-slip faults accomodate the transform component. The fissure swarms release stress during volcano-tectonic episodes that occur at intervals of several hundred years. Stress is released by the strike-slip faults by microearthquake swarms at intervals of a few decades.

The fissure swarms are segmented along their trend. The segments include a volcanic center with a clustering of fissure eruptions and extensional faults. A proximal zone of 20-40 km follows were faults become dominant with distance from the center. Linear anomalies of a high geothermal gradient extend further still, into the marginal area of Early Quaternary to Miocene rocks indicating dyke propagation for another 30-40 km beyond visible faults.

The Reykjanes volcanic center is nested in a 5-6 km wide rift zone with boundary faults of just over 20 m visible throw. Volcanic production keeps pace with extension and subsidence to fill the rift. The last three volcano-tectonic episodes occurred at Reykjanes in the 13th century, about 2000 years ago and about 3200 years ago. The lavas from these three fissure eruptions cover >2/3 of Reykjanes by area, largely smoothing over faults of the rift floor. During the Weichselian glacial maximum the ice margin may have reached 75-100 km beyond Reykjanes. It had become ice free 14,500 years ago. There is evidence of at least 10 eruptions on the Reykjanes volcanic system since, of which the first ones were of lava shield type. A similar eruption frequency may have prevailed at Reykjanes during its postglacial time interval.

There are three volcanic fissure zones within the Reykjanes center. The middle zone of 1.5 km² hosts the main geothermal resource of the system. Recently the western zone proved to host an exploitable resource also. It may be restricted to a narrow zone of dykes. The reservoir temperatures of these two lie in the range 280-310 °C. The eastern zone has not proved productive so far. It has erupted olivine rich shield lavas on most of its trace whereas the others have erupted slightly evolved tholeiite. Feed zones in the main production zone have been correlated partly with the axis of the eastern of two tindars. They cluster also in near horizontal intervals which may suggest that density controlled offshots also play a role.