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Abstract

The Geitafell Volcano, an extinct Tertiary central volcano, was active in the Icelandic rift zone between 5 and 6 Ma. Because of deep glacial erosion, the interior of the volcano is exposed from its flanks to the top of the extinct crustal magma chamber represented by several gabbro plutons. Here we present the results of a detailed study of the infrastructure of the Geitafell Volcano. The study includes measurements of the shape, size and fracture pattern of the extinct crustal magma chamber and its remarkably sharp contact with a dense swarm of inclined sheets for which the chamber acted as a source. A comparison of the attitude of 1087 joints within the basaltic magma chamber (the pluton) with the attitude of 48 sheets cutting the chamber indicates that the injection of late-formed sheets from the still-molten inner part of the crustal magma chamber was structurally controlled by the cooling joints in the solidified outer part of the chamber. Similarly, the main trends of 408 mineral veins of the fossile geothermal system, ENE–WSW, NW–SE and NNE–SSW, coincide with those of cooling joints, suggesting that geothermal fluids circulated through the heat source of the geothermal system via cooling joints. Our study of 592 sheets from the dense swarm surrounding the crustal magma chamber suggests that most are inclined sheets whereas some are radial dykes. Their thickness distribution follows a negative exponential law with an average arithmetic thickness of 0.64 m. Most inclined sheets dip from 40 to 70° with the general dip decreasing with increasing distance from the magma chamber. The sheet swarm is generally bowl-shaped and concave upwards. Reconstruction of the magma chamber indicates that at its maximum size it was about 8 km in diameter, 1–2 km thick and with a top at 1.5 km depth below the surface. Numerical models using this magma-chamber geometry as a basis can formally account for the intensity and geometry of the sheet swarm of the Geitafell Volcano.

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