The morpho-tectonic structure of Kos-Nisyros-Tilos volcanic area based on onshore and offshore data

The tectonic structure of the volcanic area among the islands of Kos, Nisyros and Tilos is analyzed using as topographic base a digital map covering both onshore and offshore areas. The classification of faults in major and secondary structures separating blocks with Alpine basement outcrops, post-Alpine sedimentary sequences, present-day marine basins and volcanic structures permitted the distinction of neotectonic units. Thus, several tectonic horsts are described with considerable relative uplift of 1-2 km manifested by the outcrops of Alpine basement rocks at high altitudes (Dikeos, Kefalos, Kondellioussa, Tilos). Several tectonic grabens are distinguished by the subsidence of neotectonic blocks at about 600 m depth and the deposition of several hundreds of meters of Quaternary sediments (the basins of Eastern Kos, Western Kos, Western Nisyros, Southern Nisyros and Northern Tilos). Some intermediate transitional tectonic blocks show step-like structures with tilted post-Alpine strata in between the tectonic horsts and grabens (Antimachia plateau, Zipari and Kos – Knidos channel). The Quaternary volcanic structures occur at the central subsided area of the regional tectonic graben between Kos and Tilos forming a positive volcanic relief of more than 1.4 km around Nisyros. The maximum tectonic throw observed between the neotectonic blocks of the area is about 2.5 – 3.0 km based on the displacement of the top of the Alpine rocks.