The Gulbahce Geothermal Field is located on the eastern margin of the Karaburun Peninsula, about 45 km from the city of Izmir, western Anatolia, Turkey. The stratigraphy of the study area is represented by a Miocene volcano-sedimentary succession, including several sedimentary and volcanic units. These units overlie the basement rocks of the Karaburun Platform and Bornova Flysch Zone which consist of sand-stones, shales and carbonate blocks. These rock units are cut and deformed by a series of NW-SE- to NE-SW-trending faults, extending from Sigacik Bay to Gulbahce Bay. Structural studies suggest that while most of the geothermal systems in western Anatolia are controlled by normal faults, the geothermal system at Gulbahce is controlled by a strike-slip dominated shear zone, previously named the Izmir-Balikesir Transfer Zone. Along the fault zone, associations of active fault segments accommodate deep circulation of hydrothermally modified sea water, and thus the resulting negative flower structure is the primary control mechanism for the geothermal system. Hydrogeochemical properties of the field show that surface temperature of fluid ranges from 30 to 34 degrees C. Geothermal fluids in Gulbahce have high salinity (EC>34 mS/cm) and low enthalpy. Piper and Schoeller diagrams indicate that geothermal fluid is in the NaCI fades. Chemical geothermometers suggest that the reservoir temperature is around 53-136 degrees C. The isotopic data (oxygen-18, deuterium and tritium) suggest that geothermal fluids are formed by local recharge and deep circulation of sea water. (C) 2017 Elsevier Ltd. All rights reserved.