Geometry and mode of emplacement of the Thverartindur cone sheet swarm, SE Iceland

The exposed interior of the Thverartindur igneous centre is dominated by a dense circumferential swarm of centrally inclined sheets. The swarm is hosted by a stratified lava–hyaloclastite sequence that is tectonically rotated into a rift-parallel monocline, shown by an increased degree of tilting (∼5–25°) towards the neovolcanic zone of Iceland. In this paper, measured orientations and thicknesses of a total of 745 mafic cone sheets from nine sampling locations are statistically constrained and spatially analysed in both their present configuration and corrected for a uniform 13°NW tilt of the swarm. Eight sheet dip trajectories, back-rotated to different levels of erosion, indicate a bowl- and slightly fan-shaped swarm geometry that converges onto a common magma source located ∼3 km below the pre-erosional surface; a geometry that bears striking resemblance to Gudmundssonʹs [J. Volc. Geotherm. Res. 35 (1998) 179–194] boundary element model of the maximum compressive stress field around a pressurised sphere. An estimated ∼7.2-km-wide and subcircular model source zone at such shallow crustal depth predicts a slightly flattened and 4.0±0.5-km-thick spheroidal magma chamber with a maximum volume of 140±50 km3. Marked declines in sheet densities along the inner and outer margins of the swarm suggest that most sheets originated from a relatively narrow source zone, compared to the swarmʹs estimated ∼2–3 km width at the surface. Improved statistical analysis, furthermore, reveals systematic spatial variations in sheet thickness distributions which behave in accordance with the bowl-shaped geometry for the Thverartindur swarm. Thus, a significant ∼0.1 m/km decrease in the average sheet thickness up through the swarm is thought to reflect an overall upward narrowing of sheets, while a relatively low number of <0.5-m-thick sheets in the uppermost part of the swarm arguably reflects the subsurface thermal arrest of more than a third of all sheets injected from the model source.