Volcanogenic mass flow deposits and seafloor diapirism following the largest insular Quaternary eruption of the eastern Mediterranean at Nisyros island, Aegean volcanic arc

Abstract

Nisyros volcanic island, at the eastern end of the Aegean Volcanic Arc, was mainly constructed after the ultra-Plinian Kos Plateau Tuff (KPT) eruption at 161 ka. The surrounding submarine area consists mostly of marginal mass-transport deposits that commonly characterize insular stratovolcanoes. This study aims at providing insights into the deposition and volume of such deposits by elucidating the products of a mega-eruption and subsequent paroxysmal volcanicity, typical of subduction zone-island arc systems. Seismic reflection and multibeam bathymetry around Nisyros identify pyroclastic and debris flow deposits covering areas several orders of magnitude greater than the island. The entire seabed between the basins of East Kos and Tilos is underlain by a ~7 km3 volcaniclastic unit that defines a ubiquitous basal unconformity at an average depth of 30 m below the seafloor. Southwest of Nisyros, two groups of avalanche deposits from the same unit are bathymetrically identified on the slopes of the northernmost Karpathos basin apron. Seismic stratigraphic markers that trace the unit on the shelf of Tilos suggest it is concomitant with the KPT eruption. This formerly unknown submarine expansion of the KPT deposits into three neighboring basins, when added to the volume of this massive unit to the rest of the Nisyros surroundings, gives it an explosivity index (VEI) >6. As such, it represents the largest Quaternary subaqueous volcanogenic mass flow deposit of the eastern Mediterranean. From this dataset, it can be inferred that the submarine advance of the erupted material occurred mainly through erosive gravity flows, envisioned as the main dispersion mechanism of large-scale subaqueous pyroclastic deposits of Plinian eruptions. East of Nisyros, unique small-scale diapiric domes and seafloor mounds are attributed to loading by the KPT in combination with regional faulting. Subsequent Nisyros effusive activity produced pyroclastic units around the northeast as well as debris avalanche and lava flows south of the island. These have been correlated to known onshore formations. The present study documents the intensity and impacts of a volcanic island mega-eruption and paroxysmal epilogue over a cycle of 100 kyr.