Abstract

During the past 12 yr, a major volcano-tectonic episode occurred in the Krafla fissure swarm at the divergent plate boundary in north-east Iceland. This swarm is an 80-km-long and as much as 10-km-wide zone of tension fractures, normal faults, and volcanic fissures. The average length of 1,083 measured tectonic fractures is about 350 m, the maximum length being 3.5 km, and the average estimated depth is of the order of 102 m. Most fractures strike north to north-northeast, with widths as much as 40 m and throws of as much as 42 m. Pure tension fractures are most common, but as they grow they commonly change into normal faults. Most fractures gradually thin out at their ends, but several exceptionally wide tension fractures end in tectonic caves, several tens of meters long, only a few meters beneath the surface. The total dilation measured in 5 profiles across the Krafla swarm reaches a maximum of at least 80 m and decreases from south to north along the swarm. Some 20 intrusive events and 9 eruptive events occurred during this volcano-tectonic episode. New lavas covered many old fractures, but several new fractures were also formed and many old ones grew. New lava flowed into some of the major fractures in the area, presumably forming pseudodikes. Locally, magma used a part of a pre-existing fracture as a pathway to the surface. Small width: length ratios of the normal faults, as compared with such ratios of the tension fractures, are attributed to the tendency of tension fractures to close as they develop into normal faults. It is concluded that divergent plate movements with dike intrusions, or pressure changes in a deep-seated changes in a deep-seated magma reservoir, are viable models for formation of the fractures.

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