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

Abstract

The El Guix anticline is the southernmost structure of the south-Pyrenean fold-and-thrust belt (NE Spain). Compressional activity in the area represents the latest stages of Alpine compressional tectonics and affects the Upper Eocene–Oligocene fluvio-lacustrine deposits overlying an evaporite sequence. The evolution pattern of the fractures containing calcite cement consists of three stages of microfractures which reflect the evolution of the structure and the relationships between fluids and thrust development. In each fracture, deformation started with a network of discontinuous microfractures which acted as traps for local meteoric fluids (stage 1). During the second stage, dilatant thrust faults serving as the conduit for both evolved meteoric ascending and local meteoric fluids were developed. Migration of fluids through the thrusts was multiepisodic. After the precipitation of cements in microfracture stage 2, the thrusts were practically occluded by calcite cements, acting as fluid barriers and dividing the structure into different hydrological compartments. The last stage of microfracture (stage 3) is 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, enriched with 13C and with low 87Sr/ 86Sr 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 lower Fe/Mn and Fe/Mg ratios, with Na and Sr, depleted in 13C and with high 87Sr/ 86Sr with respect to their host rock were only present during thrust faults development (stage 2) within a relatively closed palaeohydrological system. The underlying evaporites acted as the lower boundary of the aquifer. Comparison with older thrust fronts of the same system reveals that the Pyrenean fold-and-thrust belt and its deformed southern foreland basin were compartmentalised 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 basin were basically formation fluids derived from Eocene marine waters trapped in the underlying Eocene marls, although influences of meteoric waters were also present. During the middle Eocene, coeval with the Gavarnie thrust emplacement, the thrust fault was dominated by a medium scale fluid flow. The fluid was basically a hypersaline Sr-rich brine stored within Triassic redbeds. No evidence of a significant input of either surface or metamorphic fluids during thrusting was found. During the same period, in the crystalline basement of the central Pyrenees the thrust faults were dominated by a large scale fluid flow mainly derived from the underlying silicate rocks as a result of metamorphic devolatilisation reactions. During the Oligocene, the most external part of the fold-and-thrust belt developed on top of a salt detachment horizon. The thrust front affected cemented continental materials of late Eocene–Oligocene age. At this moment, the thrusts were conduits for meteoric fluids arriving from the surface and also for evolved meteoric fluids migrating a short distance upwards after being in contact with the underlying evaporitic beds.