Influence of Preexisting Crustal Structures on the Emplacement of Eruptive Dikes at an Oblique Spreading Center, SW Iceland

The plate boundary on the Reykjanes Peninsula (RP) of SW Iceland has an overall trend of approximately 075 degrees, about 30 degrees oblique from the direction of absolute plate motion as calculated by DeMets et al,1994. Field, seismic, GPS, and InSAR data have suggested at least some partitioning of oblique spreading in time as extensional magmatic events and strike-slip dominated amagmatic interludes. Strike slip appears to be accommodated in the main along an E-W trending zone of seismicity and subtly expressed N-S faults. Accommodating extension are normal faults and individual volcanic fissure swarms arranged as en echelon, right-stepping structures having an average trend of 040 degrees, perpendicular to the expected direction of maximum horizontal extensional strain expected for a 30 degree oblique spreading center (Withjack and Jamison, 1986). Surface expressions of linear eruptive features on the RP occur typically as subglacially- erupted hyaloclastite ridges, and post-glacial crater rows and eruptive fissures oriented between 035 and 045 degrees. Such structures almost certainly reflect the emplacement of a dike or dikes of similar orientation at depth. In this work we have observed that significant deviations from the general fissure strike have taken place for some eruptive events where their activity intersected the strike slip zone. Many eruptive fissures take sharp and sudden changes in strike to follow a strike-slip orientation for distances greater than 1 km whereas others form large scoria cones elongated in a direction significantly different from the crater row as a whole. Topographic lineaments in certain sub-glacially erupted hyaloclastite ridges show significant deviations from the average trend as well, with notable N to NNE trends and subordinate E to ENE trends within the strike-slip belt. We suggest that in an oblique rift zone magma uses all favorably oriented pathways to find its way to the surface. With strike-slip structures present, both normal and strike-slip faults will open under extensional strain during magmatic rifting episodes and thus be used as pathways for eruption.