Fluid Dynamics in a Thrust Fault Inferred from Petrology and Geochemistry of Calcite Veins: An Example from the Southern Pyrenees

Petrographic and geochemical analyses (δ18O, δ13C, 87Sr/86Sr, clumped isotopes, and elemental composition) coupled with field structural data of synkinematic calcite veins, fault rocks, and host rocks are used to reconstruct the episodic evolution of an outstanding exposed thrust zone in the Southern Pyrenees and to evaluate the fault behavior as a conduit or barrier to fluid migration. The selected thrust displaces the steeply dipping southern limb of the Sant Corneli-Bóixols anticline, juxtaposing a Cenomanian-Turonian carbonate unit against a Coniacian carbonate sequence. Successive deformation events are recorded by distinct fracture systems and related calcite veins, highlighting (i) an episodic evolution of the thrust zone, resulting from an upward migration of the fault tip (process zone development) before growth of the fault (thrust slip plane propagation), and (ii) compartmentalization of the thrust fault zone, leading to different structural and fluid flow histories in the footwall and hanging wall. Fractures within the footwall comprise three systematically oriented fracture sets (F1, F2, and F3), each sealed by a separate generation calcite cement, and a randomly oriented fracture system (mosaic to chaotic breccia), cemented by the same cements as fracture sets F1 and F2. The formation of fractures F1 and F2 and the mosaic to chaotic breccia is consistent with dilatant fracturing within the process zone (around the fault tip) during initial fault growth, whereas the formation of the latest fracture system points to hybrid shear-dilational failure during propagation of the fault. The continuous formation of different fracture systems and related calcite cementation phases evidences that the structural permeability in the footwall was transient and that the fluid pathways and regime evolved due to successive events of fracture opening and calcite cementation. Clumped isotopes evidence a progressive increase in precipitation temperatures from around 50°C to 117°C approximately, interpreted as burial increase linked to thrust sheet emplacement. During this period, the source of fluid changed from meteoric fluids to evolved meteoric fluids due to the water-rock interaction at increasing depths and temperatures. Contrary to the footwall, within the hanging wall, only randomly oriented fractures are recognized and the resulting crackle proto-breccia is sealed by a later and different calcite cement, which is also observed in the main fault plane and in the fault core. This cement precipitated from formation fluids, at around 95°C, that circulated along the fault core and in the hanging wall block, again supporting the interpretation of compartmentalization of the thrust structure. The integration of these data reveals that the studied thrust fault acted as a transverse barrier, dividing the thrust zone into two separate fluid compartments, and a longitudinal drain for migration of fluids. This study also highlights the similarity in deformation processes and mechanisms linked to the evolution of fault zones in compressional and extensional regimes involving carbonate rocks

Significance of Fracture-Filling Rose-Like Calcite Crystal Clusters in the SE Pyrenees

Fracture-filling rose-like clusters of bladed calcite crystals are found in the northern sector of the Cadí thrust sheet (SE Pyrenees). This unusual calcite crystal morphology has been characterized by using optical and electron microscope, X-ray diffraction, Raman spectroscopy, 18O, 13C, 87Sr/86Sr, clumped isotopes, and major and rare earth elements + yttrium (REEs + Y) analysis. Petrographic observations and powder X-ray diffraction measurements indicate that these bladed crystals are mainly made of massive rhombic crystals with the conventional (104) faces, as well as of possibly younger, less abundant, and smaller laminar crystals displaying (108) and/or (108) rhombic faces. Raman analysis of liquid fluid inclusions indicates the presence of aromatic hydrocarbons and occasionally alkanes. Clumped isotopes thermometry reflects that bladed calcite precipitated from meteoric fluids at ~60-65 C. The 87Sr/86Sr ratios and major elements and REEs content of calcite indicate that these fluids interacted with Eocene marine carbonates. The presence of younger 'nailhead' calcite indicates later migration of shallow fresh groundwater. The results reveal that rose-like calcite clusters precipitated, at least in the studied area, due to a CO2 release by boiling of meteoric waters that mixed with benzene and aromatic hydrocarbons. This mixing decreased the boiling temperature at ~60-65 C. The results also suggest that the high Sr content in calcite, and probably the presence of proteins within hydrocarbons trapped in fluid inclusions, controlled the precipitation of bladed crystals with (104) rhombohedral faces

Fluid composition changes in crystalline basement rocks from ductile to brittle regimes

The relationships between deformation and fluid flow have been investigated in the Paleozoic basement of an isolated horst of the Catalan Coastal Ranges. A structural, petrological and geochemical study has been performed in a complex fracture network that resulted from a long-lived tectonic history (from Carboniferous to Miocene). Nine fracture types, developed from a ductile regime in the greenschist facies to a shallow brittle regime, have been characterized in order to establish P, T and fluid compositions during the evolution of the horst. Syncleavage and late-cleavage quartz veins (qtz1-chl1 ± mu and late qtz2-chl2-dol1) formed during the Hercynian ductile deformation. These minerals precipitated from metamorphic fluids, possibly evolved from seawater, at temperatures between 240 and 340 °C. En-echelon albite vein arrays (ab-qtz3-chl3 ± ti-an) and NE-SW normal faults generating breccias mark the change from ductile to brittle, from compression to extension and from a closed to an open hydrologic regime. This paragenesis precipitated from the mixing of metamorphic and magmatic fluids at temperatures between 180 and 290 °C during the early Permian extension. Dolomite veins (dol2-chl4-qtz4), precipitated at 210-280 °C from the mixing of previous fluids with hypersaline oxidizing external brines, possibly during the late stage of the early Permian extension. Reverse faults and calcite veins (Cc1-ba) formed either during the Paleogene compression or during the Langhian to early Serravallian minor compression. Calcite and barite precipitated from meteoric or marine waters in an open hydrological system at temperatures below 50 °C. The Miocene extension is represented by NE-SW normal faults with fault gouges and NNW-SSE normal faults cemented by calcite 2 that precipitated at temperatures below 50 °C from meteoric fluids in an open basin-scale hydrological system. The studied horst was part of a relay zone between two segments of the NNW-SE Llobregat fault during the early Permian, explaining the high fracture density and the fast upflowing of hydrothermal fluids at that time, thus controlling the development of albite veins exclusively in this area

Crestal graben fluid evolution during growth of the Puig-reig anticline (South Pyrenean fold and thrust belt)

tThe Puig-reig anticline, located in the South Pyrenean fold and thrust belt, developed during the Alpinecompression, which affected the upper Eocene-lower Oligocene sediments of the Solsona and BergaFormations. In this study, we highlight the controls on formation of joints and reverse, strike-slip andnormal faults developed in the crest domain of the Puig-reig anticline as well as the relationships betweenfluids and these fractures. We integrated structural, petrographic and geochemical studies, using for thefirst time in the SE Pyrenees the clumped isotopes thermometry to obtain reliable temperatures of calciteprecipitation.Structural and microstructural analysis demonstrate that at outcrop scale fracturing was controlledby rigidity contrasts between layers, diagenesis and structural position within the anticline, whereasgrain size, cementation and porosity controlled deformation at the microscopic scale. Petrographic andgeochemical studies of calcite precipitated in host rock porosity and fault planes reveal the presence oftwo migrating fluids, which represents two different stages of evolution of the Puig-reig anticline. Duringthe layer-parallel shortening, hydrothermal fluids with temperatures between 92 and 130◦C circulatedthrough the main thrusts to the permeable host rocks, reverse and most of strike-slip faults precipitatingas cement Cc1. During the fold growth, meteoric waters circulated downwards through normal and somestrike-slip faults and mixed at depth with the previous hydrothermal fluid, precipitating as cement Cc2at temperatures between 77 and 93◦C.Integration of the results from the Puig-reig anticline in this work and the El Guix anticline indicatesthat hydrothermal fluids did not reach the El Guix anticline, in which only meteoric and evolved meteoricwaters circulated along the fold

Influence of basement rocks on fluid evolution during multiphase deformation: the example of the Estamariu thrust in the Pyrenean Axial Zone

Calcite veins precipitated in the Estamariu thrust during two tectonic events decipher the temporal and spatial relationships between deformation and fluid migration in a long-lived thrust and determine the influence of basement rocks on the fluid chemistry during deformation. Structural and petrological observations constrain the timing of fluid migration and vein formation, whilst geochemical analyses (δ13C, δ18O, 87Sr/86Sr, clumped isotope thermometry and elemental composition) 15 of 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 related to the Alpine reactivation of the Estamariu thrust, and is characterized by the migration of meteoric fluids, heated at depth (temperatures between 56 and 98 ºC) and interacted with crystalline basement rocks before upflowing through the thrust zone. During the Neogene extension, the Estamariu thrust was reactivated and normal faults and shear fractures with calcite cements Cc3, Cc4 and Cc5 developed. Cc3 and Cc4 precipitated 20 from hydrothermal fluids (temperatures between 127 and 208 ºC and between 102 and 167 ºC, respectively) derived from crystalline basement rocks and expelled through fault zones during deformation. Cc5 precipitated from low temperature meteoric waters percolating from the surface through small shear fractures. The comparison between our results and already published data in other structures from the 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 25 precipitated from fluids interacted with crystalline 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)

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

Quantifying deformation processes in the SE Pyrenees using U-Pb dating of fracture-filling calcites

It is difficult to quantify the timing of the deformation processes in brittle fold-thrust belts because minerals suitable for dating and well-preserved growth strata sediments are scarce or absent. Here, we quantify the duration of thrust sheet emplacement and shortening rates in the SE Pyrenean thrust sequence using U-Pb dating of fracture-filling calcites. The obtained U-Pb dates reveal a minimum duration for the emplacement of each thrust unit (18.7 Ma for the Bóixols-Upper Pedraforca, 11.6 Ma for the Lower Pedraforca and 14.3 Ma for the Cadí thrust sheets) and show that piggy-back thrusting was accompanied by post-emplacement deformation of the upper thrust sheets above the lower sheets during their south-directed tectonic transport. We calculated shortening rates of 0.6, 3.1 and 1.1 mm a−1 from the older to younger emplaced thrust sheets. Our results also reveal the formation of local normal faults during the late Oligocene as a result of the late stages of compression and exhumation in the SE Pyrenees. We observed that temperatures >110 °C could be a limiting factor when applying the U-Pb dating method

Petrological, petrophysical and petrothermal study of a folded sedimentary succession: the Oliana anticline (Southern Pyrenees), outcrop analogue of a geothermal reservoir

The Oliana anticline (Southern Pyrenees) has been characterized as an outcrop analogue of a geothermal reservoir using field data (stratigraphy and fracturing) and petrological, petrophysical and petrothermal analyses. Five lithofacies were established including conglomerates, hybrid arenites, lithic arenites, carbonates and evaporites. Petrophysical measurements indicate widely dispersed values of bulk density, connected porosity, permeability and velocity of compressional acoustic waves. Connected porosity is the factor that mostly influences bulk density, compressional wave velocity and permeability. In turn, diagenetic processes (such as dissolution and cementation) and fracturing, coupled with petrological features such as mineral composition, matrix content and grain size, are the most critical factors controlling rock porosity along the Oliana anticline. Thermal conductivity measures reveal a compositional control on the thermal properties of rocks. Thermal characterization of the structure reveals a low conductive area that matches the carbonate and evaporite succession of the anticline core and a highly conductive zone associated with the detrital succession of the fold limbs. The Oliana anticline has been classified as a petrothermal system due to the low permeability values of the studied sedimentary succession. Despite such classification, this contribution provides a useful exploration tool for future studies of non-conventional geothermal and CO2 storage sites located in folded sedimentary successions in the proximal domain of foreland basins

Petrophysical and petrothermal dataset of the sedimentary succession in the Oliana anticline (Southern Pyrenees)

The petrophysical and petrothermal characterization of the sedimentary succession of the Oliana anticline in the Southern Pyrenees has been performed on the basis of mineral density, connected porosity, permeability, P-wave velocity and thermal conductivity measurements of rock samples distributed along this anticline. This dataset was used to explain: (I) the variability of petrophysical rock properties along the Oliana anticline, (II) the distribution of thermal conductivity along the sedimentary units of the anticline, (III) the relationships between the fold and petrology concerning the mineral density, connected porosity, permeability, P-wave velocity and thermal conductivity of rocks and (IV) the tectonic and diagenetic controls underlying the observed relationships, as described in the research article: 'Petrological, petrophysical and petrothermal study of a folded sedimentary succession: the Oliana anticline (Southern Pyrenees), outcrop analogue of a geothermal reservoir - Global and Planetary Change Journal (2023)". This contribution presents here the raw and statistical datasets used to discuss the potential of the Oliana anticline as a geothermal reservoir analogue and also includes an extended methodological section that proposes a new procedure to measure the thermal conductivity of highly heterogeneous coarse-grained sedimentary rocks using the Modified Transient Source Plane (MTPS) method on a TCi C-Therm thermal analyzer. These complete datasets can be used to better discuss and understand the principal limitations of outcrop analogue studies applied to unconventional geothermal reservoirs in foreland basins on the basis of the analysis of rock petrophysical and petrothermal properties. Furthermore, the data obtained in the Oliana anticline can be used to understand the structural, diagenetic and petrological factors that can modify the petrophysical and petrothermal properties of rocks and to discuss the potential of foreland basin margins to be used as geothermal reservoirs, comparing the data obtained in Oliana with studies developed in similar geological settings worldwide.</p

Geochronological and geochemical data from fracture-filling calcites from the Lower Pedraforca thrust sheet (SE Pyrenees)

U-Pb dating using laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS), δ13 C, δ18 O, clumped isotopes and 87 Sr/ 86 Sr analysis, and electron microprobe have been applied to fracture-filling calcites and host carbonates from the Lower Pedraforca thrust sheet, in the SE Pyrenees. These data are used to determine the type and origin of migrat- ing fluids, the evolution of the palaeohydrological system and timing of fracturing during the emplacement of this thrust sheet, as described in the article 'From hydroplastic to brit- tle deformation: controls on fluid flow in fold and thrust belts. Insights from the Lower Pedraforca thrust sheet (SE Pyrenees)'-Marine and Petroleum Geology (2020). The in- tegration of these data is also used to compare the fluid flow evolution of the Southern Pyrenees with that of other oro- gens worldwide and to generate a fluid flow model in fold and thrust belts. At a more local scale, the U-Pb dataset pro- vides new absolute ages recording the deformation in the Lower Pedraforca thrust sheet, which was previously dated by means of indirect methods such as biostratigraphy of ma- rine sediments and magnetostratigraphy of continental de- posits