Regional Scale, Fault-Related Fluid Circulation in the Ionian Zone of the External Hellenides Fold-And-Thrust Belt, Western Greece: Clues for Fluid Flow in Fractured Carbonate Reservoirs

We combined field mapping, structural and microstructural analyses, stable-clumped isotope geochemistry, and U-Pb dating of calcite veins and syn-tectonic slickenfibres, to assess the regional scale fault-related fluid flow during the evolution of the External Hellenides fold-and-thrust belt. We show that fluid circulation during forebulge uplift was characterized by cold meteoric water-derived fluids, from which calcite precipitated and sealed bed-perpendicular joints. Fluid circulation during foreland flexuring and early layer-parallel shortening was characterized by warm fluids buffered by the carbonate host rock, which circulated through normal faults and bed-parallel veins. Mixing with meteoric-derived fluids also occurred at this stage of tectonic evolution. Fluid circulation during the late stage of thrust wedge accretion and post-orogenic extension at 1.6 ± 1 Ma was characterized by increasing dominance of cold meteoric water circulating in strike-slip and normal faults. The ingress of meteoric-derived fluids was controlled by throughgoing fault conduits, while host rock-buffered fluids were confined in isolated structures such as minor faults and veins. We developed a conceptual model of fault-related fluid circulation, which invokes a transition from an open fluid system during forebulge uplift, to a semi-closed fluid system during foreland flexuring and early layer-parallel shortening, and to an open system during late thrust wedge accretion and post-orogenic extension. This type of fluid circulation may have impacted fluid migration/leakage, including hydrocarbons, into or outside potential reservoirs in the highly prospective Hellenides-Albanides fold-and-thrust belt, a renovated frontier for hydrocarbon exploration in the Mediterranean area.