Rift-Parallel Strike Slip Faulting in the Karahnjukar area of Eastern Iceland

Spreading centers in Iceland consist of a series of linked rift zones subdivided into volcanic systems with rift- parallel normal faults, dike swarms and central volcanoes. Pleistocene basaltic lavas and hyaloclastites in the Kárahnjúkar area of northeast Iceland are located on the eastern flank of the active Northern Rift Zone. Recent investigations and excavations in this area reveal a major rift-parallel strike-slip fault zone. A high fault density found by previous investigators was confirmed and more than 300 faults were mapped in outcrops, gorges, quarries, and a tunnel system near the Kárahnjúkar area. Mesoscopic structures show that most faults have strike-slip displacements; oblique-slip, normal and reverse faults are less common. Many of the faults, especially the normal faults, have very large exposed fault surfaces, up to tens of square meters or more with excellent kinematic indicators. Overall, the faults are sinuous striking 030 - 045 with steep dips. Dextral faults strike about 035 whereas sinistral faults strike slightly more to the north, about 030. Although offset markers are rare, the larger strike-slip faults may have had substantial displacements (tens of meters) based on their lateral continuity and gouge zone widths. There are no systematic crosscutting relationships between these strike-slip faults. The normal faults strike about 045, dip moderately to steeply and have down- to-west and -east displacements, one with as much as 15 m of vertical Pleistocene separation. The normal faults cut the strike-slip faults and displace Holocene deposits by up to 2.5 m. The kinematics and crosscutting relations of the faults suggest that the Kárahnjúkar area has undergone a change from initial strike-slip to extensional deformation. Overall, the strike-slip faults in this area may accommodate extension at a lateral offset between two parallel rift zones, one to the west which is still actively spreading (Kverkfjööll Grímsvötn) the other to the east which is dormant or extinct (Sn\ae fell Esjufjöll) (bookshelf faulting similar to that of Icelandic transform fault zones?). They could also be related to unidentified rift propagation or ridge jumps in this part of the spreading system. The normal faults have been related to a late Pleistocene invasion of the Kverkfjöll fissure swarm across the Kárahnjúkar area.