Sequential fluid migration along a fold and thrust belt: SE Pyrenees from late Cretaceous to Oligocene

The south eastern Pyrenees allowed us to study the relationships between fluid flow and deformation in a complete section of a well-preserved fold and thrust belt. Furthermore, this study enables us to decipher the main controls on fluid flow and to perform a conceptual model of fluid migration in fold and thrust belts by comparing the southern Pyrenees with other orogens worldwide. A combination of field-based and petrographic observations together with geochemical analyses was used to determine the origin of fluids from which these cements precipitated, the conditions of fluid migration and the fluid-rock relationships. These methods were applied to carbonate host rocks and calcite and dolomite cements precipitated in fractures and in intergranular and vug porosities. The integration of the methodology allowed us to define up to 20 fluid flow events for the Upper Pedraforca thrust sheet, Eight for the Lower Pedraforca thrust sheet, seven for the Vallfogona thrust, which is the southern margin of the Cadí thrust sheet, and two for both the Abocador thrust and the Puig-reig anticline, which are located in the foreland Ebro basin. During the late foreland stage of the south Pyrenean fold and thrust belt, the Puig-reig anticline formed. Structural and microstructural analysis developed in this fold demonstrate that at outcrop scale fracturing was controlled by rigidity contrasts between layers, diagenesis and structural position within the anticline, whereas grain size, cementation and porosity controlled deformation at the microscopic scale. Petrographic and geochemical studies of calcite precipitated in host rock porosity and fault planes reveal the presence of two migrating fluids, which represents two different stages of evolution of the Puig-reig anticline. During the layer- parallel shortening, hydrothermal fluids with temperatures between 92 and 130 ºC circulated through the main thrusts to the permeable host rocks, reverse and most of strike-slip faults precipitating as cement Cc1. During the fold growth, meteoric waters circulated downwards through normal and some strike-slip faults and mixed at depth with the previous hydrothermal fluid, precipitating as cement Cc2 at temperatures between 77 and 93 ºC. Integration of the results from the Puig-reig anticline in this work and the El Guix anticline indicates that hydrothermal fluids did not reach the El Guix anticline, in which only meteoric and evolved meteoric waters circulated along the fold. In the south Pyrenean foreland basin, 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. From U-Pb geochronology applied to calcite cements, 47 ages for the South Pyrenean fold and thrust belt are obtained. Results indicate that fluid migration took place during the Pyrenean compression and that deformation migrated from the upper thrust sheets to the lower thrust units and to the foreland from 70.5 ± 1.1 Ma to 25 ± 17 Ma. These U-Pb ages also indicate that each of the thrust sheets registers its own deformational history as well as the history of the underlying thrust units emplaced during tectonic stacking. For instance, the Upper Pedraforca thrust sheet records the entire compressional history of the SE Pyrenees. Likewise, the wide distribution of U- Pb ages within each tectonic unit indicates that deformation was continuous rather than episodic. Calcite veins with Neogene ages ranging from 18.9 ± 0.8 Ma to 2.6 ± 1.3 Ma are interpreted as having been formed during the Neogene rift and post-rift Western Mediterranean events stretching across NE Iberia. These ages are the first evidence demonstrating deformation within the SE Pyrenees during these post-compressional events. In the Lower Pedraforca thrust sheet, during syn-sedimentary hydroplastic normal faulting affecting poorly-consolidated Upper Cretaceous sediments and Eocene syn-orogenic sediments, calcite cements did not precipitate. During the burial and the layer-parallel shortening, however, calcite cements Cc1 to Cc4 precipitated from fluids in a relatively paleohydrological system. Cc3 precipitated from high-salinity fluids (~+5.4 ‰ VSMOW) with 87Sr/86Sr ratios of 0.707922 and at temperatures around 70 ºC. Contrarily, during folding and thrusting, calcite cements Cc5 to Cc8 precipitated in a more open paleohydrological system. Cc6 precipitated from high-salinity fluids (~+5 ‰ VSMOW) with 87Sr/86Sr ratios 0.707817 and at temperatures around 75 ºC. The controls of deformation on the paleohydrological system observed in the Lower Pedraforca thrust sheet have strong similarities with that observed other areas worldwide under both compressional and extensional regimes. In the Upper Pedraforca thrust sheet, brines at 125 and 145 ºC migrated through fractures during the Early Cretaceous extension. During the Late Cretaceous-Paleocene compression, formation waters at temperatures around 80 ºC and in equilibrium with Late Cretaceous seawater migrated through main thrust fault zones. As the Upper Pedraforca thrust sheet emplaced, the influence of meteoric waters increased, resulting in the slightly decrease of the salinity of migrating fluids. During the Eocene-Oligocene reactivation of this thrust unit, also formation waters at temperatures between 90 and 100 ºC migrated through main thrust fault zones. However, the influence of meteoric waters increased with respect to the Late Cretaceous-Paleocene compression, indicating exhumation of the Upper Pedraforca thrust sheet. Stable, clumped and strontium isotopes together with elemental composition and rare earths and yttrium analysis indicate that during the emplacement of the Upper and Lower Pedraforca thrust sheets, from Late Cretaceous to middle Eocene, the fluid system was dominated by high- salinity formation fluids and meteoric waters at temperatures ranging between 70 and 90 ºC. In these thrust sheets, fluids migrated above evaporite detachments that acted as barriers for the input of deep sourced fluids. Contrarily, during the emplacement of the Cadí thrust sheet and during the deformation affecting the northern side of the Ebro foreland basin from middle Eocene to Oligocene, high-salinity hydrothermal fluids derived from the deeper parts of the Axial zone and at temperatures between 100 and 177 ºC, migrated through fractures to the thrust front. Hydrothermal fluid flow induced the development of thermal anomalies in the Vallfogonala and Abocador thrusts and in the Puig-reig anticline, which are structures rooted at depth with the basement. These fluid flow patterns observed during the growth of the south eastern Pyrenean fold and thrust belt are similar to that observed in the western side of this orogen. The evolution of the fluid regime during the growth of the southern Pyrenees has strong similarities to that observed in other orogens worldwide such as the Sevier thrust belt, the western Alps, the Ionian zone in Albania, the Nuncios fold Complex in Mexico as some examples. From these similarities a conceptual model of fluid flow in fold and thrust belts in which the style of deformation is one of the main controlling parameters is performed. In this model, whereas in thin-skinned fold and thrust belts the fluid system is controlled by formation, marine and meteoric waters, in thick-skinned fold and thrust belts the system is controlled by the input of deep-sourced hydrothermal fluids, which induce the formation of thermal anomalies. In both situations, during the layer-parallel shortening stretching thrust sheets, the paleohydrological system was closed and the fluid-rock interaction was low. In contrast, during later folding and thrusting the system opened to the input external fluids and the interaction between fluids and their adjacent host rocks decreased progressively.