Tectonics of the thingvellir fissure swarm, sw iceland

Abstract

The Thingvellir fissure swarm dissects 9000 year-old pahoehoe lava and contains about 100 fractures of average orientation N29.3E. The average length of fractures is 620 m, the minimum being 57 m and the maximum 7.7 km. The maximum width and throw on a single fracture are 68 m and 40 m, respectively. Most fractures are vertical at the surface and must be the result of an absolute tensile stress. The geometry and arrangement of the fractures indicate that they have grown by coalescence of initially offset small fractures. It is concluded that most fractures attain depths of the order of several hundred meters or less, but that the largest faults attain depths of many kilometers. Comparison with the Vogar fissure swarm on the Reykjanes Peninsula suggests that the Thingvellir swarm may have the greater rate of dilation; the total maximum postglacial dilation of the Vogar swarm is only 15 m, whereas the corresponding figure for the Thingvellir swarm is about 100 m.