Fluid circulation related to post-Messinian extension, Thassos Island, North Aegean P. BOULVAIS 1 , J.-P. BRUN 1 AND D. SOKOUTIS 2 1 Ge ´osciences UMR 6118, University of Rennes 1, Rennes Cedex, France; 2 Faculty of Earth and Life Sciences, Netherlands Centre for Integrated Solid Earth Sciences, Vrije Universiteit, Amsterdam, The Netherlands 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 de ´ collement located at depth within a marble unit. Numerous minerali- zations 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 100–200°C) resulted in dolomitization of marbles near the base of the Plio- Pleistocene basin. The dolomites are characterized by low d 18 O 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 d 18 O values (about 13&). This shows that high-temperature flu- ids moved up from the de ´ collement 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-Pleis- tocene basin. These veins have O isotope values relatively constant at about 23–25& and C isotope values inter- mediate between the high d 13 C value of the carbonate host rock (about 1–3& versus Peedee Belemnite) and the low d 13 C value of soil-derived carbon ()10&). The calcites associated with the oxidative remobilization of pri- mary sulphide Zn–Pb mineralization of Thassos carbonates have comparable O and C isotope compositions. Hot fluids, within the 100–200°C temperature range, have likely contributed to the weakening of the lower marble unit of Thassos and, thus, to the process of de ´ collement. Key words: de ´ collement, extension, fluid flow, North Aegean, stable isotopes Received 28 March 2006; accepted 21 December 2006 Corresponding author: Philippe Boulvais, Ge ´ osciences UMR 6118, University of Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France. Email: philippe.boulvais@univ-rennes1.fr. Tel: +33 2 23 23 61 79. Fax: +33 2 23 23 60 97. Geofluids (2007) 7, 159–170 INTRODUCTION The development of low-dipping extensional faults is prob- lematic as it is at variance with the classical Andersonian theory of faulting (Wernicke 1995; Wills & Buck 1997). It requires the existence of a very weak zone that may corre- spond to low strength rocks such as salt, or be a conse- quence of fluid circulation. The presence of fluids in fault zones is recorded by hydrothermal alteration (e.g. crystalli- zation of chlorite, serpentine), by lowering P- and S-wave velocities (Floyd et al. 2001) as well as by specific electro- magnetic signatures (Bedrosian et al. 2004). Both phyllo- silicate crystallization and the development of high pore fluid pressure reduce the frictional stress significantly (Axen 1992; Parnell et al. 1998; Sibson 2000; Floyd et al. 2001). The question that arises now is how high fluid pressure may be maintained (Wills & Buck 1997). Knowing that fluid mobility, as demonstrated by the various mineraliza- tions, is often associated with faults, a major fluid reservoir connected with the fault region is required to obtain high fluid pressure. Fluid migration is promoted by enhance- ment of permeability in the fault zone. Fault–fluid relations have been intensively studied in the seismically active Aegean region of the Cyclades (Patriat & Jolivet 1998), the Gulf of Corinth (Labaume et al. 2004) and western Turkey (Verhaert et al. 2004). Thassos Island, located close to the North Aegean Trough and to the western end of the North Anatolian Fault (Fig. 1), Geofluids (2007) 7, 159–170 doi: 10.1111/j.1468-8123.2007.00171.x Ó 2007 The Authors Journal compilation Ó 2007 Blackwell Publishing Ltd