Neotectonics and the Kos Plateau Tuff eruption of 161 ka, South Aegean arc

Abstract

A high-resolution seismic-reflection survey of the area between Kos and Tilos islands is used to constrain the nature of the Kos Plateau Tuff (KPT) eruption and post-eruptive subsidence. A unique acoustically incoherent unit tens of metres thick at a subbottom depth of 15–35 m is recognised throughout the West Kos basin, which lies between Pachia, Yali and Kos. It commonly unconformably overlies folded, stratified sediment, but in places is concordant with stratified sediment more than 100 m thick. In places south of Kos, the acoustically incoherent unit is overlain by an unconformity and irregularly stratified sediment interpreted as terrestrial or shallow marine. Southeast of Nisyros, a correlative acoustically incoherent unit overlies a planar marine transgression erosion surface that extends almost to Tilos. The stratigraphic level of this unit is dated by comparison with the global eustatic sea-level record and the presence of major transgressive erosion surfaces on adjacent continental shelves, constrained by regional sedimentation rates, and indicates that it is of similar age to the Kos Plateau Tuff eruption. The relationship of this unit to coastal erosion surfaces, and its absence in many areas where seismic-reflection profiles show continuous marine sedimentation, suggests that it is a pyroclastic deposit of subaerial, or at most very shallow marine, origin from the Kos Plateau Tuff eruption. This presence of transgressive unconformities implies that a coastal plain or shallow sea extended southeast of Nisyros to Tilos and the Datça peninsula, and thus it is unlikely that pyroclastic flows crossed large stretches of deep water towards Tilos, as proposed by Allen and Cas (Allen, S.R., Cas, R.A.F., 2001. Transport of pyroclastic flows across the sea during the explosive, rhyolitic eruption of the Kos Plateau Tuff, Greece. Bull. Volcanol. 62, 441-456). Late Pleistocene tectonic subsidence has taken place throughout the whole region between Kos and Tilos, in places at an average rate of at least 3 mm/a. The older dacites of Pyrgousa and Pachia and the post-KPT volcanics of Yali, Strongili and Nisyros lie on a NE–SW-trending lineament marked by major faults and abrupt changes in bathymetry. This NE–SW faulting was initiated in the early Pleistocene and parallels prominent lineaments in the east Cretan Sea. ENE–WSW sinistral strike-slip faulting initiated in the middle Pleistocene in the area from Santorini to Kos would have produced extension on this older lineament, playing a major role in channelling magma to the surface and permitting the ingress of water to the magma conduit.