Fluid circulation related to post-messinian extension, Thassos Island, North Aegean
Abstract
In the North Aegean Domain, Thassos Island contains a Plio-Pleistocene basin controlled by a large-scale flat-ramp extensional system with a potential décollement located at depth within a marble unit. Numerous mineralizations associated with normal faults of Plio-Pleistocene age are the sign of fluid circulation during extension. Two main generations of fluid flow are recognized, related to Plio-Pleistocene extension. A first circulation under high-temperature conditions (about 100200°C) resulted in dolomitization of marbles near the base of the Plio-Pleistocene basin. The dolomites are characterized by low d18O values (down to 11 versus Standard Mean Ocean Water). Some cataclastic deformation affected the dolomites. Hydrothermal quartz that crystallized in extension veins above a blind ramp also has low d18O values (about 13). This shows that high-temperature fluids moved up from the décollement level toward the surface. A second downward circulation of continental waters at near-surface temperature is documented by calcite veins in fault zones and at the base of the Plio-Pleistocene basin. These veins have O isotope values relatively constant at about 2325 and C isotope values intermediate between the high d13C value of the carbonate host rock (about 13 versus Peedee Belemnite) and the low d13C value of soil-derived carbon (-10). The calcites associated with the oxidative remobilization of primary sulphide ZnPb mineralization of Thassos carbonates have comparable O and C isotope compositions. Hot fluids, within the 100200°C temperature range, have likely contributed to the weakening of the lower marble unit of Thassos and, thus, to the process of décollement.