Fluid history related to the Alpine compression at the margin of the south-Pyrenean Foreland basin: the El Guix anticline

The El Guix anticline is the southernmost structure of the south-Pyrenean f old-and-thrust belt (NE Spain). Compressional activity in the area represen ts the latest stages of Alpine compressional tectonics and affects the Uppe r Eocene-Oligocene fluvio-lacustrine deposits overlying an evaporite sequen ce. The evolution pattern of the fractures containing calcite cement consis ts of three stages of microfractures which reflect the evolution of the str ucture and the relationships between fluids and thrust development. In each fracture, deformation started with a network of discontinuous microfractur es which acted as traps for local meteoric fluids (stage 1). During the sec ond stage, dilatant thrust faults serving as the conduit for both evolved m eteoric ascending and local meteoric fluids were developed. Migration of fl uids through the thrusts was multiepisodic. After the precipitation of ceme nts in microfracture stage 2, the thrusts were practically occluded by calc ite cements, acting as fluid barriers and dividing the structure into diffe rent hydrological compartments. The last stage of microfracture (stage 3) i s attributed to a younger phase, where the circulation of fluids concerned only meteoric fluids and the previously formed fractures acted as barriers to fluid circulation. Meteoric fluids, with high Fe/Mn and Fe/Mg ratios, without Na and Sr, enric hed with C-13 and with low Sr-87/Sr-86 with respect to their host rock were widely distributed in the structure throughout all its evolution, within a relatively open palaeohydrological system. Evolved meteoric fluids, with l ower Fe/Mn and Fe/Mg ratios, with Na and Sr, depicted in C-13 and with high Sr-87/Sr-86 With respect to their host rock were only present during thrus t faults development (stage 2) within a relatively closed palaeohydrologica l system, The underlying evaporites acted as the lower boundary of the aqui fer, Comparison with older thrust fronts of the same system reveals that the Pyr enean fold-and-thrust belt and its deformed southern foreland basin were co mpartmentalised hydrologically in time and space. During the early Eocene, when the thrust front affected soft-sediment in the Ainsa basin, the thrust Faults were dominated by a medium scale fluid flow. The fluids in the basi n were basically formation fluids derived from Eocene marine waters trapped in the underlying Eocene marls, although influences of meteoric waters wer e also present. During the middle Eocene; coeval with the Gavarnie thrust e mplacement, the thrust fault was dominated by a medium scale fluid flow. Th e fluid was basically a hypersaline Sr-rich brine stored within Triassic re dbeds. No evidence of a significant input of either surface or metamorphic fluids during thrusting was found. During the same period, in the crystalli ne basement of the central Pyrenees the thrust faults were dominated by a l arge scale fluid flow mainly derived from the underlying silicate rocks as a result of metamorphic devolatilisation reactions. During the Oligocene, t he most external part of the fold-and-thrust belt developed on top of a sal t detachment horizon. The thrust front affected cemented continental materi als of late Eocene-Oligocene age. At this moment, the thrusts were conduits for meteoric fluids arriving from the surface and also for evolved meteori c fluids migrating a short distance upwards after being in contact with the underlying evaporitic beds. (C) 2000 Elsevier Science B.V. All rights rese rved.