Thermodynamic evidence of giant salt deposit formation by serpentinization: an alternative mechanism to solar evaporation

International audienceThe evaporation of seawater in arid climates is currently the main accepted driving mechanism for the formation of ancient and recent salt deposits in shallow basins. However, the deposition of huge amounts of marine salts, including the formation of tens of metres of highly soluble types (tachyhydrite and bischofite) during the Aptian in the South Atlantic and during the Messinian Salinity Crisis, are inconsistent with the wet and warm palaeoclimate conditions reconstructed for these periods. Recently, a debate has been developed that opposes the classic model of evaporite deposition and argues for the generation of salt by serpentinization. The products of the latter process can be called "dehydratites". The associated geochemical processes involve the consumption of massive amounts of pure water, leading to the production of concentrated brines. Here, we investigate thermodynamic calculations that account for high salinities and the production of soluble salts and MgCl2-rich brines through sub-seafloor serpentinization processes. Our results indicate that salt and brine formation occurs during serpentinization and that the brine composition and salt assemblages are dependent on the temperature and CO2 partial pressure. Our findings help explain the presence and sustainability of highly soluble salts that appear inconsistent with reconstructed climatic conditions and demonstrate that the presence of highly soluble salts probably has implications for global tectonics and palaeoclimate reconstructions

Evolution of a low convergence collisional orogen: a review of Pyrenean orogenesis

The Pyrenees is a collisional orogen built by inversion of an immature rift system during convergence of the Iberian and European plates from Late Cretaceous to late Cenozoic. The full mountain belt consists of the pro-foreland southern Pyrenees and the retro-foreland northern Pyrenees, where the inverted lower Cretaceous rift system is mainly preserved. Due to low overall convergence and absence of oceanic subduction, this orogen preserves one of the best geological records of early orogenesis, the transition from early convergence to main collision and the transition from collision to post-convergence. During these transitional periods major changes in orogen behavior reflect evolving lithospheric processes and tectonic drivers. Contributions by the OROGEN project have shed new light on these critical periods, on the evolution of the orogen as a whole, and in particular on the early convergence stage. By integrating results of OROGEN with those of other recent collaborative projects in the Pyrenean domain (e.g., PYRAMID, PYROPE, RGF-Pyrénées), this paper offers a synthesis of current knowledge and debate on the evolution of this immature orogen as recorded in the synorogenic basins and fold and thrust belts of both the upper European and lower Iberian plates. Expanding insight on the role of salt tectonics at local to regional scales is summarised and discussed. Uncertainties involved in data compilation across a whole orogen using different datasets are discussed, for example for deriving shortening values and distribution

Thermal record of hyperextended rifted margins: the fossil record of the Pyrenees

International audienceThe thermal architecture of late rifting to breakup along the deep passive margins is still poorly known. This is mostly because of the limited access to industry drillhole data that, anyway, calibrate topographic highs and rarely the deepest rift domains (and even less the basement). However, unravelling this evolution is a fundamental requirement to define the ultimate exploration potential of these frontier domains. An alternative way to document this thermal evolution is to describe fossil analogues onshore. In this study, we use the fossil hyperextension record of the Pyrenean belt that was sampled by orogenic deformation into the North Pyrenean None and Nappe des Marbres alpine units. Previous studies have shown that the rift came into hyperextension and recorded locally mantle exhumation. These rift domains are associated with a HT-LP metamorphism event that was shown to vary spatially within the rift basin as well as into the basement. In order to restore the late rift thermal architecture of the Pyrenean hyperextended rift, we use a new compilation of Raman Spectroscopy measurements on Carbonaceous Material (RSCM) and Vitrinite Reflectance data. This method allows to record the palaeo-maximum temperatures in the sedimentary basins spatially as well as vertically and can be superposed to geological sections. This method was applied in almost 200 samples collected all along the belt at different stratigraphic level as well as into the Paleozoic basement. When the base of the rift basin is exposed, RSCM Tmax range between 450 and 620°C below a <5km thick sedimentary pile. Western Pyrenees was shown to be an exception as RSCM Tmax are less than 300°C on the outcropping superficial part of the rift basin. However; Vitrinite Reflectance data from wells that are calibrating the deep basin demonstrate that the same thermal intensity was actually reached. These results discard any lateral variation in thermal regime and is pointing out that it is a burial function into a (very)high late rift thermal gradient that largely exceed 100°c/km. Far from being restricted to the Pyrenean case, such a thermal evolution with the same amplitude gradient within the same exhumed mantle domains were documented in the Northern Red Sea example

The role of segmented magma-rich rifted margins in convergent settings: insights from the Demerara Plateau and Guyana-Suriname margins

International audienc

The North Pyrenean Frontal Thrust: structure, timing and late fluid circulation inferred from seismic and thermal-geochemical analyses of well data

During orogenesis, large-scale thrusts as orogenic fronts can act as conduits and/or barriers for fluid flow. Unravelling the timing and modes of tectonic activation of large-scale faults is crucial to understanding the relationship between fluid flow and deformation. The North Pyrenean Frontal Thrust (NPFT) corresponds to a major basement-involved thrust responsible for the northward overthrust of the pre-orogenic sediments on top of the Aquitaine Foreland Basin. This study questions the timing of activation of this thrust, its geometry, the nature of the last fluids, which circulated there, and its role on the circulation of fluids. The structural study confronted to published thermochronology data led to determine the timing of the two tectonic activations during the NPFT compression phase and to relate them to the fluid circulations. We constrain the first activation at Campanian times and link it to the leak of the deep gas reservoir present in depth, as the NPFT acted as a conduit. Then the NPFT acted as a barrier, probably due to the breccia consolidation during the Paleocene quiescence period. Finally, the Eocene-Oligocene reactivation led to fluid circulation of high salinity fluids from the Triassic evaporites leaching. This latter event is associated with a fracturing event and the late generation of calcite veins studied here. This is the first study in the Pyrenees directly applied to the NPFT which uses the association between fluid inclusions study, seismic and thermochronological data. It highlights that the NPFT may be an important structure responsible of the leakage of deep hydrocarbons reservoirs. It also shows the importance of the determination of the activation steps of large-scale faults to decipher the origin of fluid circulations in space and time

The role of segmented magma-rich rifted margins in convergent settings: insights from the Demerara Plateau and Guyana-Suriname margins

International audienc

The role of segmented magma-rich rifted margins in convergent settings: insights from the Demerara Plateau and Guyana-Suriname margins

International audienc

Distribution and intensity of High-Temperature Low-Pressure metamorphism across the Pyrenean-Cantabrian belt: constraints on the thermal record of the pre-orogenic hyperextension rifting

International audienceWhereas a straightforward link between crustal thinning and geothermal gradients during rifting is now well established, the thermal structure of sedimentary basins within hyperextended domains remains poorly documented. For this purpose, we investigate the spatial distribution of rift-related High-Temperature Low-Pressure (H T /L P ) metamorphism recorded in the preserved hyperextended rift basins inverted and integrated in the Pyrenean-Cantabrian belt. Based on Vitrinite Reflectance (R o ) data measured in 169 boreholes and more than 200 peak-metamorphic temperatures ( T max ) data obtained by Raman Spectroscopy of Carbonaceous Material (RSCM) added to ∼425 previously published T max data, we propose a new map depicting the spatial distribution of the H T /L P metamorphism of the Pyrenean-Cantabrian belt. We also provide three regional-scale geological cross-sections associated with R o and T max data to constrain the distribution of paleo-isograds at depth. Based on these results, we show that the impact of rift-related metamorphism is restricted to the pre- and syn-rift sequence suggested by the depth profiles of R o values measured in different tectonostratigraphic intervals (pre-, syn- and post-rift and syn-convergence sediments). However, a small strip of early orogenic sediments (Santonian in age) appears also affected by high temperatures along the North Pyrenean Frontal Thrust and above the Grand Rieu ridge, which we attribute to the percolation of hot hydrothermal fluids sourced from the dehydration of underthrust basement and/or sedimentary rocks at depth during the early orogenic stage. The map shows that the H T /L P metamorphism (reaching ∼500 °C) is recorded with similar intensity along the Pyrenean-Cantabrian belt from the west in the Basque-Cantabrian Basin to the east in the Boucheville and Bas-Agly basins, for similar burial and rift-related structural settings. This thermal peak is also recorded underneath the northern border of the Mauléon Basin (calibrated by wells). It suggests that the high temperatures were recorded at the basement-sediment interface underneath the most distal part of the hyperextended domain. At basin-scale, we observe in the Basque-Cantabrian, Mauléon-Arzacq and Tarascon rift segments an asymmetry of the thermal structure revealed by different horizontal thermal gradients, supporting an asymmetry of the former hyperextended rift system. Using our results, we compare the Pyrénées to the Alps that also recorded hyperextension but no H T /L P metamorphic event and suggest that the high-temperature record within the basins depends on high sedimentation rate promoting a thermal blanketing effect and circulation of hydrothermal fluids.Bien que l’association de l’amincissement crustal et de gradients géothermiques élevés lors du rifting continental soit largement reconnue, la structure thermique des bassins sédimentaires dans la partie distale des systèmes de rift reste mal documentée. Pour cela, nous étudions la distribution spatiale du métamorphisme Haute-Température/Basse Pression (H T /B P ) enregistrée dans les bassins préservés du système de rift hyper-aminci, par la suite inversés et intégrés dans la chaîne Pyrénéo-Cantabrique. Basé sur la réflectance de la Vitrinite (R o ) mesurée dans 169 puits et plus de 200 données de pic de température lié au métamorphisme ( T max ) obtenues avec la méthode de Spectroscopie Raman de la Matière Carbonée (RSCM) ainsi que plus de 425 T max provenant d’études précédentes, nous proposons une nouvelle carte de la distribution spatiale du métamorphisme H T /B P de la chaîne Pyrénéo-Cantabrique. Nous proposons également trois coupes géologiques regionales, sur lesquelles nous avons placé les données de T max et de R o afin de contraindre la distribution des paléo-isogrades en profondeur. Basé sur ces résultats, nous montrons que l’impact du métamorphisme lié au rifting est restreint aux sédiments pré- et syn-rift, ce qui est suggéré par la tendance des profils des valeurs de R o en profondeur mesurées dans les différents intervalles tectonostratigraphiques (sédiments pré-, syn- et post-rift ainsi que syn-convergence). Cependant, une fine bande de sédiments syn-orogéniques (d’âge Santonien) est affectée par des températures relativement élevées au-dessus de la ride de Grand Rieu et le long du Chevauchement Frontal Nord Pyrénéen, que nous attribuons à la percolation de fluides hydrothermaux chauds provenant de la déshydratation du socle chevauché et/ou des sédiments profonds, lors du stade d’inversion précoce. La carte présentée montre que le métamorphisme (atteignant ∼500 °C) est enregistré avec la même intensité du bassin Basque-Cantabrique à l’ouest, aux bassins de Boucheville et du Bas-Agly à l’est, pour un enfouissement et un positionnement lors du rifting équivalents. Le pic thermique est également enregistré sous la bordure nord du bassin de Mauléon (calibré par des puits). Cela suggère que les hautes températures ont été enregistrées à l’interface socle-sédiments au niveau de la partie la plus distale du domaine hyper-aminci. À l’échelle des bassins, nous observons dans les segments Basque-Cantabrique, Mauléon-Arzacq et Tarascon une asymétrie de la structure thermique, révélée par différents gradients thermiques horizontaux, supportant une asymétrie de l’ancien système de rift hyper-aminci. En utilisant nos résultats, nous comparons les Pyrénées avec les Alpes qui ont également enregistré l’hyper-extension mais pas d’évènement métamorphique H T /B P , ce qui suggère que l’enregistrement des hautes températures dans les bassins dépend de taux de sédimentation élevés, favorisant un effet de couverture thermique et de circulations de fluides hydrothermaux

Fluids evolution, hydrocarbon generation and destruction in foreland basins: example of the petroleum systems of Southern Aquitaine (France)

International audienc

Rift and salt-related multi-phase dolomitization: example from the northwestern Pyrenees

The Meillon (Callovo-Oxfordian) and Mano (Tithonian) Formations are dolomitized carbonate reservoirs that actively produce oil and gas (Aquitaine Basin, France). In this study, the dolomitization conditions of their counterparts exhumed in the northwestern Pyrenees are detailed using a combination of field observations, petrography, fluid inclusion microthermometry, elemental and isotopic geochemistry, and carbonate U-Pb geochronology. Dolomitization occurred in several stages spanning from the Neocomian (pre-rift) to the Albian (syn-rift, associated with mantle exhumation and active salt tectonics). Both formations were first massively dolomitized in near-surface to shallow burial conditions during the Berriasian-Valanginian, likely triggered by the influx of marine-derived waters. Between the Barremian and the Albian, the Early Cretaceous rifting caused the upward influx of hot fluids associated with the partial to complete recrystallization of the initial dolomites. During the Albian, subsequent dolomites precipitated in both formations as high-temperature (T > 160 degrees C) vein and pore-filling cement. Distinct fluid inclusion chlorinities and rare earth element patterns between the Meillon and Mano Formations point to fluid compartmentalization during this stage. Whereas dolomite cements indicate the involvement of evaporite-derived brines in the Meillon Formation, precipitation was likely related to clay derived water in the Mano Formation. Lastly, a final episode of dolomite cementation occurred only in the vicinity of faults and volcanic intrusions during the Albian when the highest temperatures were recorded in both formations (T > 250 degrees C). These saddle dolomites precipitated from hydrothermal water with a mixture of mantle, crustal-, and evaporite-derived waters channeled by faults and active diapirs. Subsequent quartz and calcite cement precipitation reveals a temperature decrease in a post-rift to inversion setting (post-Cenomanian) and indicates fluid compartmentalization between both formations. This study highlights the major control exerted by rifting, combined with the presence of diapiric salt, on dolomitization, making carbonate platforms of modern salt-rich passive margins potential targets for exploration