Deformation partitioning inside a fissure swarm of the northern Icelandic rift

Extension in an oceanic rift domain is accommodated by fissuring and faulting processes. The fissuring, including dike injection, involves extension and an increase in crustal volume, consistent with magma supply, as in oceanic ridge and hot spot contexts. The faulting mainly involves thinning and extension. The relationship between these two deformation modes is analysed in the northern rift of Iceland. In the active rift zone, deformation and volcanism occur within 2–3-km-wide fissure swarms separated by 10-km-wide zones where deformation is minor. Based on topographic and geological mapping, the local structure in a major fissure swarm was analysed. The fracture density increases near the faults, but also beyond their tips. The geometrical analysis of structures revealed that for the last 10,000 years, block faulting and tilting accommodated about one half of the extension, the other half resulting from fissuring. This distribution shows that even in an active volcanic zone with widespread magmatism, the stretching resulting from normal shear can play an important role. As estimated from the ages of eroded surfaces and lava flows, the deformation rates range between 1.5 and 15 cm/year. These values are compared with other rates estimated for different time intervals in the same area.