Widening of the Krafla fissure swarm during the 1975–1981 volcano-tectonic episode

Abstract

Frequent distance measurements across the Krafla fissure swarm, North Iceland, recorded the extension accompanying the sequence of rifting events which started in December, 1975, and lasted for 6 years. An 80 to 90 km long section of the fissure swarm extended during this sequence of rifting and volcanic events. Maximum widening of about 8 m occurred 10 to 12 km north of Leirhnjukur. which is located above the center of the Krafla magma reservoir. From that location, the amount of widening decreased north-wards and is estimated to exceed 2 m where the seismicity indicated the northern termination of the present rifting, off the north coast, about 70 km north of Leirhnjukur. The amount of widening also decreased southwards and approached zero at 15 to 20 km south of Leirhnjukur. The ground deformation associated with these rifting events can be summarised as: A narrow strip, 1 to 2 km wide, along the fissure swarm is heavily fractured with numerous open cracks parallel to the fissure swarm. This fractured strip has subsided 2 to 3 m relative to its flanks. The flanks of the fractured zone have been uplifted relative to regions farther away. The uplift is not well constrained, but tilt observations at several locations indicate about 1 m uplift. The flanks of the rift zone have contracted, perpendicular to the fissure swarm. The maximum contractional strain exceeds 300 mm per km. The amount of areal expansion (windening of the fissure swarm times the length of the fractured zone) associated with these rifting events is estimated to be about 0.30 km2. For individual events, the area of expansion has been roungly proportional to the volume of subsidence above the Krafla magma reservoir. If the width of a new dike is equal widening of surface fissures, the ratio of the subsidence volume to the area of expansion for the best observed events indicated a height of a new dike system as 2.4 to 2.8 km. This ratio is significantly less for events of large lava production, but even during these events, the majority of magma leaving the Krafla reservoir was apparently emplaced in subsurface fissures.