On the relations between morphotectonics, seismicity, perennial and sporadic hydrothermalism on the volcanic island complex of Milos (Greece)

The volcanic island complex of Milos, situated ∼200 km north of the Hellenic subduction zone, records the interplay of volcanic, tectonic, and gravitational processes within multidirectional, polyphased rift systems that developed throughout the Mio–Plio–Quaternary. Its active and seismogenic fault network hosts both perennial and transient hydrothermal venting zones, offering a natural laboratory to examine fluid circulation within a volcanic arc built on stretched continental crust. This study refines the relationships between magmatism, seismicity, and hydrothermal activity, with implications for local geological hazards. Two new morpho-tectonic sketches are presented. The first places the Milos volcanic center within the structural framework of the Aegean microplate; the second illustrates the spatial organization of the Milos tectono-hydrothermal-volcanic system. These reconstructions integrate detailed physiographic analyses (relief, slope), field observations, and multidisciplinary datasets on seismicity, hydrothermalism, and mineralization. At the scale of the Aegean microplate, (i) NE–SW faults are attributed to extensional deformation along the Mid-Cycladic Lineament; (ii) E–W faults relate to the opening of the Milos, Cretan, and Christiana basins, parallel to the mid-Miocene West Cycladic Detachments; (iii) NW–SE faults correspond to the Myrtoon Basin and Gulf of Milos openings, the latter being closed to the south by the Fyriplaka volcano; and (iv) N–S faults controlling the Zephyria graben and the eastern margin of Milos are continuous with Cretan fault systems and are parallel both to the Eocene trans-Cycladic thrust in the region and to magnetic anomalies in the subducted Tethyan oceanic crust south of the Hellenic subduction zone. At the archipelago scale, seismicity, hydrothermal venting, alteration zones, and phreatic explosions are concentrated in the hanging wall of the Achivadolimni fault, along or near the intersections of the N–S Zephyria and NW–SE Fyriplaka grabens, directly above the inferred magma chamber. Microearthquake hypocentres, likely linked to fluid-induced fault dilation, occur beneath the Gulf of Milos (∼7 km) and Fyriplaka volcano (∼5 km). Their distribution suggests a genetic link between the intersecting grabens, cooling of isotherms beneath the gulf by descending seawater, surface hydrothermal manifestations, and tectonic earthquakes (e.g., Mw 5.3 Milos, 1992). A shallow hydrothermal convection loop (<3 km) probably overlies a magmatic reservoir where fluids accumulate beneath a self-sealed, low-permeability cap, possibly corresponding to a thermal brittle-ductile transition (370–450 °C). Co-seismic ruptures may locally breach this barrier, inducing “arterial fault” behaviour along the Achivadolimni fault. Pulsating hydrothermal activity and historical phreatic eruptions are interpreted as surface expressions of transient injections of over-pressurized magmatic fluids, leading to decompression, phase transitions, and fluid expulsion during seismic events. These coupled magmatic-hydrothermal-tectonic processes should be integrated into future hazard assessments for Milos.