Geometry and mode of emplacement of dike swarms around the Birnudalstindur igneous centre, SE Iceland

Abstract

Dike geometries around the well-exposed periphery of the Birnudalstindur igneous centre (SE Iceland) are constrained by moving averages of strike, dip, thickness and dilation by 775 mafic dikes, mapped along three strategically placed transects. On the basis of spatial analysis of dike strikes, a rift-parallel swarm is distinguished from a cross-cutting tri-axial swarm pattern of ‘brown dolerites’ that clearly post-dates the volcano's cone sheet swarm. Dikes are on average orientated at right angles to the lava pile and consequently used to constrain the ‘flexured’ geometry of the host lava pile, subsequently back-rotated to horizontal. This produces two end-member scenarios, which can be tentatively used to evaluate the dynamic formation of Icelandic crust. Dike dilations above a prominent stratigraphical transition into hyaloclastite breccias are markedly lower than in the underlying plateau lava pile, suggesting that vertical dike propagations were inhibited along this density/stress boundary. Lined up with the Birnudalstindur igneous centre, average dike thicknesses decrease towards the axes of both the rift-parallel dike swarm and the rift-perpendicular branch of the tri-axial swarm. This arguably links all dike swarms to the Birnudalstindur igneous centre, even if it remains inconclusive whether rift-parallel dikes fed and/or were injected laterally away from its sub-volcanic magma chamber. It seems more likely that the slightly offset tri-axial swarm of brown dolerites was preferentially emplaced along a peripheral bulge that was created around the ‘down-sagging’ volcano.