Two subparallel rift zones in the South Aegean Sea hosting the Santorini and Nisyros volcanic centres

The South Aegean Sea hosts two ENE-WSW rift zones with sedimentary basins of 500-700 m depth: the Christiana-Santorini-Kolumbo-Amorgos (CSKA) and the Astypalaea-Nisyros-Kos (ANK) zones. These form discontinuous graben structures with sediment thickness of 600-700 m from the Late Pliocene-Quaternary period. Both rift zones comprise active volcanic centers-Santorini-Kolumbo in CSKA and Nisyros in ANK. Segmented by transverse NW-SE fault lines, these zones produce 25-40 km fault blocks, limiting maximum earthquake sizes per segment. High seismic activity includes the 7.4 magnitude 1956 Amorgos earthquake in CSKA and several ∼6.5 events near Kos in ANK (1933, 2017). Volcanic activity is stronger in the southwestern CSKA, with eruptions in Santorini continuing from the Minoan event (∼3.6 Ka) to 1950, while Nisyros last erupted in 1878-1885. The CSKA rift is also marked by a dike system along the Santorini caldera and aligned submarine domes extending NE from Kolumbo. Both rifts are slightly oblique to the Aegean volcanic arc, with NW-SE crustal extension confirmed by GPS, showing 4-5 mm/yr extension between the eastern Cycladic and southern Dodekanese islands. These rift geometries reflect deep tectonic processes related to magma generation and shallow crustal deformation above the East Mediterranean subducted slab.