40Ar-39Ar dating and new geological data to uncover the structure and evolution of the Kolumbo submarine volcanic field, Greece

The Kolumbo submarine volcanic field, aligned along NNE tectonic lineaments close to Santorini, comprises 25 monogenetic cones and the polygenetic active Kolumbo Central Volcano, which produced a VEI-5 rhyolitic explosive eruption in 1650 CE representing the most significant volcanic hazard in the region. Here we present the first 40Ar-39Ar age determinations of Kolumbo eruption deposits, a characterization of lithics collected in the 1650 CE tephra unit and a geologic map of the inside of Kolumbo crater. Together these new data elucidate the activity history of Kolumbo and the development of volcanism toward the monogenic cones. The obtained deposit ages are up to ten times younger than previously published estimates. At Kolumbo, nine categories of mainly fresh lava lithics have been distinguished and some of them may be derived from the mapped underlying geological units of the crater wall. The three dated samples from the andesite and crystal-rich andesite lithic categories have similar ages of 202.9 ± 36.9, 217.6 ± 24.9 and 247.1 ± 44.7 ka, so we assign this age range to their source lava unit, observed in the western inner crater wall beneath the 1650 CE tephra. A rhyolitic lava unit, visible along the eastern walls, lies above the andesitic unit, which has a biotite-bearing rhyolitic tuff unit at its base. The breccia lithics, apparently from a buried unit, allow us to indirectly estimate the onset of activity at around 340 ka. These geological and geochronological data constrain the chronostratigraphic context for Kolumbo region, including new age estimates that are much younger than previously thought for the Kolumbo volcanic field, confirmed by the young age obtained for the VC15 monogenetic cone (34.2 ± 15.7 ka). Finally, our results imply higher rates of volcanism than previously inferred, an important result for the hazard assessment in the region.