Changes in fluid regime in syn-orogenic sediments during the growth of the south Pyrenean fold and thrust belt

The eastern sector of the south Pyrenean fold and thrust belt developed during the Alpine compression and affected Upper Cretaceous to lower Oligocene foreland basin deposits. In this study, we determine the changes in fluid regime and fluid composition during the growth of this fold and thrust belt, integrating petrographic and geochemical data obtained from fracture-filling cements. Hydrothermal fluids at temperatures up to 154 °C, migrated from the Axial zone to the foreland basin and mixed with connate fluids in equilibrium with Eocene sea-water during lower and middle Eocene (underfilled foreland basin). As the thrust front progressively emerged, low-temperature meteoric waters migrated downwards the foreland basin and mixed at depth with the hydrothermal fluids from middle Eocene to lower Oligocene (overfilled non-marine foreland basin). The comparison of the fluid flow models from the Southern Pyrenees with other orogens worldwide, seems to indicate that the presence or absence of thick evaporitic units highly control fluid composition during the development of fold and thrust belts. Whereas in thrusts not detached along thick evaporite units, mixed fluids are progressively more depleted in δ¹⁸O and have a lower temperature and lower Fe and Sr contents as the thrust front emerges, in thrust detachments through thick evaporite units, the mixed fluids are enriched in δ¹⁸O.