Structural control and depth clustering of extensive hydrothermal venting on the shelf of Milos Island

Abstract

Abstract High-resolution geophysical surveys, complemented by chemical, physical, and visual data collected via the ROV MARUM-SQUID, revealed a previously undocumented intermediate-depth hydrothermal vent field (30–230 m) on the shelf of Milos Island, Greece. Three primary vent areas—Aghia Kiriaki, Paleochori–Thiorychia, and Vani—are aligned with ENE–WSW and NW–SE trending active faults, which define the SE coast of Milos and the Milos Gulf–Fyriplaka graben, deepening the Milos shelf up to 230 m. All areas exhibit a distinct bimodal vent distribution, with peaks at ~ 130 m and ~ 180 m; the Paleochori–Thiorychia region additionally hosts a cluster near 210 m. Shallow vents occur on flat, sandy substrates with white microbial mats and gas emissions, whereas deeper vents feature extensive microbial mats, CO₂-degassing chimneys, and actively boiling fluids exceeding 180 °C, indicative of persistent hydrothermal activity. Although the cause of the bimodal —and, in one case, trimodal—distribution remains uncertain, our findings offer a comprehensive, high-resolution characterization of the spatial distribution, morphology, and fluid characteristics of intermediate-depth hydrothermal venting on the Milos shelf.