The Hofsjökull fissure swarm – A rift system on the periphery of the main active plate boundary in Iceland

Abstract:

As the Mid-Atlantic Plate boundary crosses south Iceland, it splits in two branches: The Western and Eastern Volcanic Zones, with the Hreppar microplate between them. The total dilatation across both zones is about 2 cm/yr. Most of this deformation is taken up on the Eastern Volcanic Zone. The Hofsjökull fissure swarms are located on the northern border of the microplate. Therefore, little or no deformation is currently occurring along the fissure swarms. Nevertheless, small Holocene lava flows and indications of Holocene movements along fractures and faults show that the fissure swarms have been activated during the Holocene. In this study, fractures and faults were mapped from aerial photographs and the vertical offsets of faults measured from ArcticDEM Digital Elevation Models. Field trips were also conducted. The Hofsjökull central volcano has a 6-7 km wide caldera. Several fissure swarms extend from the Hofsjökull central volcano, both to the north and south. All of them show evidence of Holocene activity and two of them cut Holocene lava flows, although the vertical offsets of the faults in the Holocene lava flows are only on the order of 1-2 m, suggesting that one dike intrusion has occurred after the emplacement of the lava flows. The southern Hofsjökull fissure swarms pass the Kerlingafjöll rhyolitic massif on both sides, but no faults penetrate Kerlingafjöll. We suggest that Kerlingafjöll act as a density barrier, where basaltic dikes are trapped beneath the lower density rhyolitic material, instead of reaching up closer to the surface where they would cause significant faulting. While the southern Hofsjökull fissure swarms are oriented towards the NNE, which is close to being perpendicular to the overall plate spreading direction across Iceland, the northern Hofsjökull fissure swarms are highly oblique to it. The orientation of the northern Hofsjökull fissure swarms coincides with the crustal thickness gradient in the area, as the crust east of the fissure swarms is much thicker than the crust west of them. We suggest that during deglaciations, differential movements can occur along this boundary, as the thinner crust responds differently to the unloading than the thicker crust. In the absence of a plate boundary, we suggest that this factor can become dominant in determining the orientation of the fissure swarms.