Rift-transform junction in North Iceland: rigid blocks and narrow accommodation zones revealed by GPS 1997-1999-2002

The current tectonics of North Iceland are characterized by rifting episodes along fissure swarms and by M-L 6-7 earthquakes in the transform zone between the northern rift zone and the Kolbeinsey Ridge north of Iceland. The last rifting period (1975-1984) was associated with an average opening of 5 m along the Krafla fissure swarm. Post-rifting deformation has been revealed by GPS investigations in Northern Iceland. A GPS network was occupied in 1997, 1999 and 2002 to quantify present-day displacements and their variation with time, on both sides of the on-land part of the Husavik-Flatey fault and around the Krafla fissure swarm. The main deformational features observed are: (1) decrease in spreading velocities between the two timespans 1997-1999 and 1999-2002, (2) the existence, south of the transform zone, of a well-defined similar to 20-30-km-wide plate boundary where extension dominates, (3) directly adjacent east and west rift shoulders where displacements are close to those observed on the Eurasian and North American plates and (4) a displacement gradient on Tjornes that could correspond either to elastic deformation related to a currently locked dextral strike-slip fault or to an attenuated post-rifting effect northwest of Krafla.