THE VOGAR FISSURE SWARM, REYKJANES PENINSULA, ICELAND: ASEISMIC KINEMATICS OF AN OBLIQUE RIFT ZONE

Exposure of the Mid-Atlantic ridge in Iceland offers a unique opportunity to study hotspot-ridge interactions along an oblique rift zone. The Reykjanes Peninsula is a left lateral shear zone characterized by a series of NE striking en echelon fissure swarms and N-S trending dextral strike slip faults oblique to the rift axis. The peninsula is volcanically active and eruptive periods typically occur every 1000 years with the last ending in the 13^th century. Fissure swarms are structures made up of normal faults and arranged in a graben structure, and are typically active during eruptive episodes. The Vogar fissure swarm in southwest Iceland was studied to better constrain the kinematics of deformation within it. The swarm is 7km wide and 8 km long, with highly asymmetric graben walls, and a possible second graben in the southern end. Movement of the faults and fissures is thought to either be related to seismic events along the ridge, or to dike injections during aseismic eruptive episodes. GPS constrained fieldwork and GIS mapping were employed to examine a 7x3 km swath across the fissure swarm. The study area contains eight different lava flow units, but is made up primarily of the oldest early Holocene shield basalt and the youngest 13^th century A’a lava flow. Over 30 faults and fissures were identified. The faults range from meters to kilometers in length with throws ranging from 1 to 20 meters. Faults are prevalent in the shield basalt and all other younger flows, but absent in the youngest lava flow. At the termination of these faults along the contact of the historical lava, there often appears an apparent reversal in the hanging wall. The cooling/contractional properties of the lava flow as it flowed over pre-existing faults may account for the apparent reversal of slip. The termination of faults at the historical lava flow suggests that movement along the faults occurs during eruptive episodes in an aseismic mode.