Updating of the geological and geothermal research on Milos island

The oldest geologic formations outcropping in Milos are an Alpine age crystalline basement and a transgressive marine Neogene sequence. The island is mainly volcanic. It belongs to the Aegean Active Arc, within which the Milos archipelago shows the most important volcanism in terms of quantity, variety of products and duration of activity (3.5−0.8 M.a.). The volcanic products belong to the calc-alkaline suite characteristic of island arcs, with composition varying between basaltic andesites (rare) to rhyolites. There are no large central volcanic edifices but different, frequently coeval eruption centres. The initial and intermediate phases of activity were mainly pyroclastic and submarine, whereas the last one (0.1 M.a.) was subaerial and formed “tuff rings”, surge deposits and lava flows, all of homogenous rhyolitic composition. Recent detailed studies have addressed the mechanism of feeding and the type of magmatic chambers beneath Milos. Distention tectonics have two main phases: an earlier one (Pliocene) with NE-SW direction and a much more intense recent (quaternary) one, trending NW-SE. The geological, tectonic and magmatic activity favoured the formation of a high enthalpy geothermal field. Many fossil and active thermal manifestations exist: hot springs, fumaroles, hot grounds, phreatic explosion craters. The hydrothermal alteration of the volcanites produced, by self sealing, a perfect cover for the geothermal fluids. Geothermometry of the surface fluids indicated high values for the source temperatures and very high geothermal gradients (5–10°C/1Om) were measured in central and eastern Milos. Geoelectric soundings defined the structure at depth and a shallower zone with very low resistivities, corresponding to levels with saline and mainly hot water. Such geothermally anomalous zones, defined by two different methods, together with superficial geological and tectonic information, permitted the location of sites for deep drilling. Five exploratory wells 1000–1400m deep gave satisfactory results of flow rate (40–120 t/h), temperature (300–320°C) and enthalpy. The geophysical survey was extended later to the whole island and also included gravimetric measurements.