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).