STRUCTURAL INFLUENCES ON AN ERUPTIVE FISSURE ON THE REYKJANES PENINSULA, SW ICELAND

The Reykjanes Peninsula (RP) in Southwest Iceland is an oblique segment of the mid-Atlantic ridge system. The rift segment is oriented 060 - 075°, which is ~30° clockwise to the direction of absolute plate motion. The dominant tectonic structures on the peninsula are a NE trending, en echelon eruptive fissures spaced 5 km apart, and normal faults striking ~20° clockwise to the rift axis. Volcanic activity on the RP has been dominated by fissure eruptions during the last 8,000 years, with a proposed recurrence time of 1,000 years.

The 2,000 year old Sundhnukur eruptive fissure in the western part of the Reykjanes Peninsula developed within an overall left-handed strike-slip shear zone. A new GIS-based map of the crater row shows 22 segments, varying in length and azimuth. Azimuths for the segments range from 6° - 52°, with a mean direction of 33°. Azimuths for the individual craters ranged from 0° - 179°, with a mean direction of 62°. Crater lengths range from 2 –109m, with a mean length of 20m. From southwest to northeast the first 18 segments step right, and then the last 4 segments step left. These left-stepping segments occur along an open fracture, which could indicate that magma used a pre-existing fault to reach the surface. Preliminary mapping showed 152 tectonic fractures mapped within proximity to the eruptive fissure, including 29 strike-slip and 9 normal faults, as well as many fractures for which kinematics can not be determined. The mean strike of all the fractures is 46°. Strike-slip faulting occurs in left-stepping series of northerly striking, right handed strike-slip faults, and compressional push-ups resulting from NW trending thrust faults. The mapped normal faults have are dominantly NE trending, although some follow an E-W orientation. Kinematic analysis of these fractures allows for evaluation of the interactions between the fractures and eruptive fissures. Structural controls on the surface geometry of the crater row are determined, which is used to determine if magma used pre-existing fractures as pathways to the surface.