Tectono-magmatic Segmentation on the Reykjanes Peninsula

The Mid-Atlantic Ridge (MAR) is highly segmented as it passes through Iceland. Although spreading at 2 cm per year, the magma budget is higher than normal for a slow-spreading ridge.Segmentation of the plate boundary in Iceland occurs at scales similar to those described for the Reykjanes Ridge (RR) and elsewhere on the MAR. Second order segmentation can describe the 4 currently active rift zones, while volcanic systems within the zones are 3rd order segments.Each rift zone is structurally distinct due to differing obliquity with respect to the direction of absolute plate motion.Differences also result from variations in magma budget with distance from the Iceland mantle plume.Additional structural complexity is seen in the pattern of faulting at segment ends.Data from Reykjanes Peninsula (RP), the first on-shore segment in SW Iceland, shows third order segmentation in the distribution of post-glacial eruptive fissures and faults.These have previously been described as either 4 volcanic systems or 5 volcanic fissure swarms striking N40E, in a right-stepping en echelon pattern along a N75E- trending plate boundary zone.Each system is said to have its own center of magmatic production, high temperature geothermal field and set of faults and fissures. Closer examination reveals systematic differences from west to east in the geometry and distribution of post-glacial eruptive fissures within these pre-defined systems. The first on-shore system displays widely spaced en echelon fissures in a zone 12 km wide.Further eastward, overlapping, anastomosing fissures occur in a 4 km wide zone, followed by tightly spaced fissures in a 1 km wide zone.Overall, this progression reflects the evolution from axial volcanic ridges on the RR to more focused magmatic activity in well-developed central volcanoes towards the center of Iceland, and correlates with increases in crustal thickness and magma budget.Individual eruptive fissures are themselves highly segmented.At the 10 to 100 m scale they vary widely in strike and appear to have used pre-existing faults as pathways to the surface.The tip-to-tip strike of a fissure swarm can vary by more than 10 degrees from the average strike of fissure segments within a given swarm.This is attributed to a temporally and spatially inhomogeneous strain field on the RP rift segment.