Deformation partitioning inside a fissure swarm of the northern Icelandic rift

Extension in an oceanic rift domain is accommodated by fissuring and faulti ng processes. The fissuring, including dike injection, involves extension a nd an increase in crustal volume, consistent with magma supply, as in ocean ic ridge and hot spot contexts. The faulting mainly involves thinning and e xtension. The relationship between these two deformation modes is analysed in the northern rift of Iceland. In the active rift zone, deformation and v olcanism occur within 2-3-km wide fissure swarms separated by 10-km-wide zo nes where deformation is minor. Based on topographic and geological mapping , the Local structure in a major fissure swarm was analysed. The fracture d ensity increases near the faults, but also beyond their tips. The geometric al analysis of structures revealed that for the last 10,000 years, block fa ulting and tilling accommodated about one half of the extension, the other half resulting from fissuring. This distribution shows that even in an acti ve volcanic zone with widespread magmatism, the stretching resulting from n ormal shear can play an important role. As estimated from the ages of erode d surfaces and lava flows, the deformation rates range between 1.5 and 15 c m/year. These values are compared with other rates estimated for different time intervals in the same area. (C) 2001 Elsevier Science Ltd. All rights reserved.