A geochemical traverse along the “Sperchios Basin - Evoikos Gulf” graben (Central Greece): Origin and evolution of the emitted fluids

•The complex geodynamic structure of the area is reflected in the wide range of compositions in the emitted fluids.•Low salinity waters are found in the western part.•High salinity waters with a seawater component are found in the east.•Gases of crustal and atmospheric origin are the main components found in the collected samples.•A small but significant mantle component is found in the gases of the easternmost sites.

The studied area is a 130 km long fast spreading graben in Central Greece. Its complex geodynamical setting includes both the presence of a subduction slab at depth responsible for the recent (Quaternary) volcanic activity in the area and the western termination of a tectonic lineament of regional importance (the North-Anatolian fault). A high geothermal gradient is made evident by the presence of many thermal springs with temperatures from 19 to 82 °C, that discharge along the normal faults bordering the graben.In the period 2004-2012, 58 gas and 69 water samples were collected and their chemical and isotopic analysis revealed a wide range of compositions.Two main groups of thermal waters can be distinguished on the basis of their chemical composition. The first, represented by dilute waters (E.C. <0.6 mS/cm) of the westernmost sites, is characterised by the presence of CH4-rich and mixed N2-CH4 gases. The second displays higher salinities (E.C. from 12 to 56 mS/cm) due to mixing with a modified marine component. Reservoir temperatures of 150-160 °C were estimated with cationic geothermometers at the easternmost sites.Along the graben, from west to east, the gas composition changes from CH4- to CO2-dominated through mixed N2-CH4 and N2-CO2 compositions, while at the same time the He isotopic composition goes from typical crustal values (<0.1 R/RA) up to 0.87 R/RA, showing in the easternmost sites a small (3-11%) but significant mantle input. The δ13C values of the CO2-rich samples suggest a mixed origin (mantle and marine carbonates).