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Calcite veins precipitated in the Estamariu thrust during two tectonic events are studied in order to (i) decipher the temporal and spatial relationships between deformation and fluid migration in a long-lived thrust and (ii) determine the influence of basement rocks on the fluid chemistry during deformation. Structural and petrological observations constrain the relative timing of fluid migration and vein formation, whilst geochemical analyses (δ13C , δ18O , 87Sr/86Sr , clumped isotope thermometry, and elemental composition) applied to the related calcite cements and host rocks indicate the fluid origin, pathways, and extent of fluid–rock interaction. The first tectonic event, recorded by calcite cements Cc1a and Cc2, is attributed to the Alpine reactivation of the Estamariu thrust. Analytical data indicate that these cements precipitated from heated meteoric fluids (temperatures in the range of 50 to 100 ∘ C) that had interacted with basement rocks (87Sr/86Sr > 0.71) before upflowing through the thrust zone. The second tectonic event, attributed to the Neogene extension, is characterized by the reactivation of the Estamariu thrust and the formation of normal faults and shear fractures sealed by calcite cements Cc3, Cc4, and Cc5. Analytical data indicate that cements Cc3 and Cc4 precipitated from hydrothermal fluids (temperatures between 130 and 210 ∘ C and between 100 and 170 ∘ C, respectively) that had interacted with basement rocks (87Sr/86Sr > 0.71) and been expelled through fault zones during deformation. In contrast, cement Cc5 probably precipitated from meteoric waters that likely percolated from the surface through small shear fractures. The comparison between our results and already published data in other structures from the southern Pyrenees suggests that regardless of the origin of the fluids and the tectonic context, basement rocks have a significant influence on the fluid chemistry, particularly on the 87Sr/86Sr ratio. Accordingly, the cements precipitated from fluids that have interacted with basement rocks have significantly higher 87Sr/86Sr ratios (> 0.710) with respect to those precipitated from fluids that have interacted with the sedimentary cover (< 0.710), which involves younger and less radiogenic rocks. [ABSTRACT FROM AUTHOR]

Abstract 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. Highlights • Evolution of fluid regime in Southern Pyrenees from Lower Eocene to Oligocene. • Migration of hydrothermal fluids from the Axial zone to the foreland basin. • Increasingly meteoric influence as the fold and thrust belt evolves. • The presence or absence of evaporites highly controls fluid composition. [ABSTRACT FROM AUTHOR]

Sedimentary rocks of foreland fold-and-thrust belts typically undergo intensive fracturing as folds grow. The resulting fracture networks can present significant variations depending on the distribution of sedimentary facies and the complex structural characteristics of fold-and-thrust belts. The Puig-reig anticline, located in the south-eastern Pyrenees, mainly exposes proximal fluvial deposits in the north limb and medial fluvial deposits in the rest of the anticline. Thus, this anticline constitutes an excellent case study to investigate the main controls on the distribution of fracture networks in folded fluvial deposits, in terms of structural position and lithofacies variations. Outcrops were selected to be representative of different structural positions, from the fold hinge to its limbs, and of a variety of the main lithofacies, from proximal to medial fluvial deposits. Fracture data were acquired using the linear scanline method. The results indicate that the anticline rocks have been affected by four sets of fractures. The north limb is dominated by thick conglomerate bodies with interlayered sandstones deposited from unconfined flash floods and wide-shallow channel streams in the proximal fluvial fan, and presents large fracture spacing and low fracture intensity but relatively large fracture length and aperture. The crest and the crest-limb transition zones are mainly characterised by interlayered conglomerates, sandstones, siltstones and clays, deposited from braided channel streams and overbanks in the medial fluvial fan, and present fractures with relatively high fracture intensity and variable fracture length and aperture. The south limb, composed of channel filling sandstone layers and stable overbank fine deposits in the medial fluvial fan, is characterised by low fracture intensity and small fracture length and aperture. Based on multiple linear regression analysis, fracture intensity is mainly controlled by the structural position, bedding thickness and lithological associations, with relatively more intense fracturing in thin sandstone layers with multiple interlayers of fine deposits in the anticline crest. The fracture length mainly depends on bedding thickness and is affected lithological associations. The fracture apertures are mainly controlled by lithofacies, with relatively higher apertures affecting conglomerate bodies. The results of this study are relevant for characterising similar systems in the subsurface, where data is scarce. • Fracture data is collected from a folded fluvial succession using a linear scanline method. • Fracture intensity is significantly controlled by the structural position, bedding thickness and lithological associations. • Conglomerates present lower fracture intensity and larger fracture length and aperture than sandstones. • Among all the fluvial facies, channel-filling sandstones in high-strain zones present the highest potential to be effective reservoirs. [ABSTRACT FROM AUTHOR]