The interaction of fissure swarms and monogenetic lava shields in the rift zones of Iceland

The divergent plate boundaries of Iceland are marked by rift zones that consist of fissure swarms, i.e. areas with a high density of fractures, faults and eruptive fissures, generally considered to be the surface expressions of dikes. Monogenetic lava shields are common within the rift zones. They are gently sloping, circular cones created by eruptions from a single vent. Many of the shields were emplaced in the early Holocene in response to increased mantle melting when the Pleistocene glacier load on the crust was reduced. In our studies of Icelandic fissure swarms we have established that in most cases where a fissure swarm transects a lava shield there is an inverse correlation between surface elevation and the density of fractures. The summit area of a shield is generally less fractured than the flanks. Fracture densities are also highest on one site of the lava shield, but much lower and even approaching zero values on the other side. This phenomenon is most clearly seen within early-Holocene lava shields (i.e. between ~. 4500 and ~. 13,600. years. BP of age). Lava shields younger than 4500. years. BP of age have low fracture densities or are not fractured at all. The lower fracture densities within Holocene lava shields suggest that dike intrusions stop or remain deeper in the crust when they approach a lava shield. Various mechanisms may explain this. The topographic load of the lava shield adds to the lithostatic stress, thus increasing the strength, and may affect the depth of a propagating dike. A long-lived eruption of a lava shield may also affect the stress field in the surrounding crust, increase the horizontal compressive stress and thus create a stress barrier for propagating dikes for some time period following the eruption.