Seawater-dominated, tectonically controlled and volcanic related geothermal systems: the case of the geothermal area in the northwest of the island of Euboea (Evia), Greece

Abstract

The northwest of the island of Euboea is located in a back-arc geological position, at the western extremity of the North Anatolian Fault. In that area, several hot springs occur in three locations (Ilia, Gialtra, Aedipsos; including newly found offshore-springs) with temperatures up to 84 °C, depositing ore-grade thermogenic travertine. The geothermal system is seawater-dominated and under pressure, using the local fault systems and is related to the Plio-Pleistocene Lichades volcanic centre. The whole area could be characterized as the lateral tips of a major fault segment, with the presence of complex networks of additional fault systems leading to fault intersections. That conclusion is also supported by the travitonic data. The geothermal fluids are near neutral pH, sodium-chloride and their chemistry is controlled by: (i) high seawater participation, (ii) a deep magmatic source and (iii) chemical composition of the bedrocks. Based on all the available data, including drilling and temperature logging data, the bedrock hosting the upflow circulation of the geothermal fluid is not in hydraulic connection with cold aquifers or permeable geological formations of the area. The local metamorphic rock formations are impermeable and work as a geothermal cap. Also, Aedipsos’ vast deposit of thermogenic travertine probably acts as a second geothermal cap formation. However, at the same time, it presents serious thermal anomalies, since major geothermal fluid circulation has been identified inside its fractures. According to chemical geothermometers, the temperature of the geothermal reservoir is 140–164 °C. The typical geothermal gradients in the area are from 7.8 °C/100 m to 18.7 °C/100 m. In one case, an anomalous high geothermal gradient (53.9 °C/100 m) was found, most probably due to spatial shape diversity of the geothermal reservoir, a suggestion also supported by the estimated circulation depth of the geothermal fluid, which varies from area to area (~ 300–1800 m) and the fluid residence time (by 226Ra–222Rn method), which is around 80–100 years.