Unravelling the Post-glacial History of Faulting on Reykjanes Peninsula, SW Iceland

The Reykjanes Peninsula, in southwest Iceland, is a complex tectonic system which includes the transition zone from an off-shore to an on-shore spreading segment. At the same time it exhibits many characteristics of a transform-type plate boundary. As the ridge comes onshore at the Reykjanes Peninsula it bends sharply to the east, becoming approximately 30 degrees oblique to the direction of plate motion. Because of its highly oblique geometry with respect to the NUVEL 1A spreading direction, it experiences both extension and shear which may be partitioned in both time and space. The peninsula is characterized by arrays of eruptive fissures, spaced on average approximately 5 km apart, and having an average strike of N40E. These have been described in the literature as comprising either five or four distinct volcanic systems each with their own magma supply, high temperature geothermal system, and clusters of closely spaced fractures referred to as fissure swarms. The fissure swarms are comprised of shear fractures (mainly normal faults), extension fractures (mainly gaping fissures with no shear displacement) and hybrid fractures which exhibit components of both shear (vertical offset parallel to the fracture plane) and extension (opening perpendicular to the fracture plane). Still unclear are the dynamics of this zone, how faulting and magmatism are related in time and space on the peninsula. This study presents a preliminary map of faults and open fractures for all of Reykjanes Peninsula. The map is compiled from new field data and new mapping from high-resolution, digital orthophotos. The map is contained within a GIS and overlain on both air photos and digital geologic maps. Local bedrock geology consists of Plio-Pleistocene lavas and sub-glacially erupted hyaloclastites, as well as lava flows dating from earliest post-glacial times up until 725 ybp. This study examines in particular the development of fractures in post-glacial lavas.