Volcanic systems and calderas in the Vatnajökull region, central Iceland: Constraints on crustal structure from gravity data

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Abstract A Bouguer anomaly map is presented of southern central Iceland, including the western part of Vatnajokull and adjacent areas. A complete Bouguer reduction for both ice surface and bedrock topography is carried out for the glaciated regions. Parts of the volcanic systems of Vonarskarð-Hagongur, Barðarbunga-Veiðivotn, Grimsvotn-Laki, and to a lesser extent Kverkfjoll, show up as distinct features on the gravity map. The large central volcanoes with calderas: Vonarskarð, Barðarbunga, Kverkfjoll and Grimsvotn, are associated with 15–20 mGal gravity highs caused by high density bodies in the uppermost 5 km of the crust. Each of these bodies is thought to be composed of several hundred km3 of gabbros that have probably accumulated over the lifetime of the volcano. The Skaftarkatlar subglacial geothermal areas are not associated with major anomalous bodies in the upper crust. The central volcanoes of Vonarskarð and Hagongur belong to the same volcanic system; this also applies to Barðarbunga and Hamarinn, and Grimsvotn and Þorðarhyrna. None of the smaller of the two volcanoes sharing a system (Hagongur, Hamarinn and Þorðarhyrna) is associated with distinct gravity anomalies and clear caldera structures have not been identified. However, ridges in the gravity field extend between each pair of central volcanoes, indicating that they are connected by dense dyke swarms. This suggests that when two central volcanoes share the same system, one becomes the main pathway for magma, forming a long-lived crustal magma chamber, a caldera and large volume basic intrusive bodies in the upper crust. Short residence times of magma in the crust beneath these centres favour essentially basaltic volcanism. In the case of the second, auxillary central volcano, magma supply is limited and occurs only sporadically. This setting may lead to longer residence times of magma in the smaller central volcanoes, favouring evolution of the magma and occasional eruption of rhyolites. The eastern margin of the Eastern Volcanic Zone is marked by a NE–SW lineation in the gravity field, probably caused by accumulation of low density, subglacially erupted volcanics within the volcanic zone. This lineation lies 5–10 km to the east of Grimsvotn.