Poly-phased fluid flow in the giant fossil pockmark of Beauvoisin, SE basin of France

The giant Jurassic-aged pockmark field of Beauvoisin developed in a 800 m wide depression for over 3.4 Ma during the Oxfordian; it formed below about 600 m water depth. It is composed of sub-sites organized in clusters and forming vertically stacked carbonate lenses encased in marls . This fine-scale study is focused on a detailed analysis of petrographical organization and geochemical signatures of crystals that grew up in early to late fractures of carbonate lenses, surrounding nodules, and tubes that fed them. The isotopic signature (C, O and Sr) shows that at least three different episodes of fluid migration participated to the mineralization processes. Most of the carbonates precipitated when biogenic seepage was active in the shallow subsurface during the Oxfordian. The second phase occurred relatively soon after burial during early Cretaceous and thermogenic fluids came probably from underlying Pliensbachian, Late Toarcian or Bajocian levels. The third phase is a bitumen-rich fluid probably related to these levels reaching the oil window during Mio-Pliocene. The fluids migrated through faults induced by the emplacement of Triassic-salt diapir of Propiac during the Late Jurassic and that remained polyphased drain structures over time

Spatio-temporal evolution of hydrothermalism in the upper plate of the Lesser Antilles arc in Guadeloupe. : Applications to geothermal systems

Pour poursuivre le développement de la géothermie de haute-enthalpie dans l’arc insulaire des Petites Antilles il faut d’une part identifier de nouvelles zones potentielles et d’autre part mieux comprendre le fonctionnement des réservoirs géothermaux. Dans ce cadre, l’objectif de cette thèse est de caractériser l’évolution spatio-temporelle de l’hydrothermalisme au niveau de la plaque supérieure de l’arc en Guadeloupe au travers d’une analyse pluridisciplinaire (études structurales, géochimiques, minéralogiques, pétrologiques et géochronologiques) des complexes volcaniques les plus anciens de l’archipel. Au Nord de la Basse-Terre nous identifions un métamorphisme d’arc de type sub-Schistes Verts. L’état thermique standard de la croûte d’arc en Guadeloupe ainsi établit indique une transition fragile-ductile située à 3-4 km de profondeur. Un paléo-réservoir exhumé, identifié au Sud de l’île de la Basse-Terre, illustre un métamorphisme hydrothermal de type Schistes-Verts synchrone du développement de couloirs de schistosité et révèle la présence d’horizons verticaux et latéraux de transfert de fluides hydrothermaux situés à 2-3 km de profondeur. En comparant l’âge du volcanisme et la datation Ar-Ar des phases hydrothermales de haute-température, nous estimons une durée maximale de fonctionnement du paléo-réservoir à 650 ka. Enfin, la découverte, l’analyse et la datation K-Ar d’une nouvelle brèche hydrothermale, au coeur du système géothermal actif, met en évidence un système épithermal distal relié à l’activité volcanique de la Soufrière. Nos résultats conduisent à un modèle conceptuel de fonctionnement d’un réservoir géothermal de haute-énergie en contexte d’arc actif.To further develop high-enthalpy geothermal energy in Lesser Antilles arc, it is necessary to identify possible new key targets, and to better understand the modes of fluids and heat transfers in geothermal reservoirs. The objective of this work is thus to characterize the spatio-temporal evolution of hydrothermalism at the upper plate of the arc in Guadeloupe with a multidisciplinary approach (combined structural, geochemical, mineralogical, petrological and geochronological analyses) focussed on the oldest volcanic complexes of the archipelago. In the north of Basse-Terre Island reveals an arc metamorphism developed under sub-Greenschist facies. The standard thermal state of the arc crust in Guadeloupe thus established shows a brittle-ductile transition located at depths of 3 to 4 km. An exhumed geothermal paleo-reservoir, identified at the south of Basse-Terre Island, indicates a hydrothermal metamorphism developed under Greenschist facies synchronous with the development of schistose corridors and attests for both vertical and lateral hydrothermal fluid transfers at depths between 2 and 3 km. Comparison of the age of the volcanic activity and Ar-Ar dating of the high-temperature hydrothermal phases allows us to estimate a maximum operating time of the paleo-reservoir at 650 ka. Finally, the discovery, the analysis and the K-Ar dating of a new hydrothermal breccia, within the active geothermal system, reveals a link between current geothermal activity and volcanic activity of the Soufrière, interpreted as a distal epithermal system. Our results lead to a conceptual model for the operation of a high-energy geothermal reservoir in the context of an active arc

Evolution spatio-temporelle de l’hydrothermalisme dans la plaque supérieure de l’arc des Petites-Antilles en Guadeloupe. Applications aux systèmes géothermaux.

To further develop high-enthalpy geothermal energy in Lesser Antilles arc, it is necessary to identify possible new key targets, and to better understand the modes of fluids and heat transfers in geothermal reservoirs. The objective of this work is thus to characterize the spatio-temporalevolution of hydrothermalism at the upper plate of the arc in Guadeloupe with a multidisciplinary approach (combined structural, geochemical, mineralogical, petrological and geochronological analyses) focussed on the oldest volcanic complexes of the archipelago. In the north of Basse-Terre Island reveals an arc metamorphism developed under sub-Greenschist facies. The standard thermal state of the arc crust in Guadeloupe thus established shows a brittle-ductile transition located at depths of 3 to 4 km. An exhumed geothermal paleo-reservoir, identified at the south ofBasse-Terre Island, indicates a hydrothermal metamorphism developed under Greenschist facies synchronous with the development of schistose corridors and attests for both vertical and lateral hydrothermal fluid transfers at depths between 2 and 3 km. Comparison of the age of the volcanicactivity and Ar-Ar dating of the high-temperature hydrothermal phases allows us to estimate a maximum operating time of the paleo-reservoir at 650 ka. Finally, the discovery, the analysis and the K-Ar dating of a new hydrothermal breccia, within the active geothermal system, reveals a link between current geothermal activity and volcanic activity of the Soufrière, interpreted as a distal epithermal system. Our results lead to a conceptual model for the operation of a high-energy geothermal reservoir in the context of an active arc.Pour poursuivre le développement de la géothermie de haute-enthalpie dans l’arc insulaire des Petites Antilles il faut d’une part identifier de nouvelles zones potentielles et d’autre part mieux comprendre le fonctionnement des réservoirs géothermaux. Dans ce cadre, l’objectif de cette thèse est de caractériser l’évolution spatio-temporelle de l’hydrothermalisme au niveau de la plaque supérieure de l’arc en Guadeloupe au travers d’une analyse pluridisciplinaire (études structurales, géochimiques, minéralogiques, pétrologiques et géochronologiques) des complexes volcaniques les plus anciens de l’archipel. Au Nord de la Basse-Terre nous identifions un métamorphisme d’arc de type sub-Schistes Verts. L’état thermique standard de la croûte d’arc en Guadeloupe ainsi établit indique une transition fragile-ductile située à 3-4 km de profondeur. Un paléo-réservoir exhumé, identifié au Sud de l’île de la Basse-Terre, illustre un métamorphisme hydrothermal de type Schistes-Verts synchrone du développement de couloirs de schistosité et révèle la présence d’horizons verticaux et latéraux de transfert de fluides hydrothermaux situés à 2-3 km de profondeur. En comparant l’âge du volcanisme et la datation Ar-Ar des phases hydrothermales de haute-température, nous estimons une durée maximale de fonctionnement du paléo-réservoir à 650 ka. Enfin, la découverte, l’analyse et la datation K-Ar d’une nouvelle brèche hydrothermale, au cœur du système géothermal actif, met en évidence un système épithermal distal relié à l’activité volcanique de la Soufrière. Nos résultats conduisent à un modèle conceptuel de fonctionnement d’un réservoir géothermal de haute-énergie en contexte d’arc actif

The characterisation of an exhumed high-temperature paleo-geothermal system on Terre-de-Haut Island (the Les Saintes archipelago, Guadeloupe) in terms of clay minerals and petrophysics

Abstract The volcanic island of Terre-de-Haut (in the Les Saintes archipelago, Lesser Antilles arc) is described in the literature as an exhumed paleo-geothermal system. Outcropping conditions provide a good, nearly horizontal 2D view of the system. This present study focuses on clay minerals, as they are known to be good markers of hydrothermal alterations worldwide, and on flow pathways. At Terre-de-Haut, the distribution of the clay minerals has a ‘concentric’ zonation with smectite, illite and chlorite comprising the outer part to the inner part of the altered zone, respectively. In the active Bouillante geothermal system, located on Basse-Terre island, 35 km NNW of Terre-de-Haut, drillings have revealed a vertical succession of the same clay minerals: from bottom to top, chlorite above 240 °C, illite above 180 °C and smectite at lower temperatures. Both sites have the same clay minerals and can, therefore, be compared. Within those geothermal systems, pathways for the fluids are encountered at all scales, from the micrometre-scale (µm-scale; through microcracks within phenocrysts and at grain joints) to the metre- and kilometre-scale (m- and km-scale) due to fractures and faults. Petrophysical measurements indicate that fresh rock samples are porous and permeable, allowing hydrothermal alteration on a km-scale. The combination of 1D vertical data at Bouillante and this new 2D horizontal distribution of clay minerals and fractures on Terre-de-Haut allows for a 3D schematic representation of the geothermal systems of the Guadeloupe archipelago and suggests that they are of hectometre-size (hm-size)

Petrogenesis of the late Ediacaran A-type granites of the western part of the Moroccan Anti-Atlas belt: Insights from U–Pb dating, geochemistry and Sr–Nd isotopes of the Sidi El Houssein, Tafraout and Taourgha intrusions

International audienceThe U–Pb zircon age carried out on three distinct late Ediacaran granites through the Anti-Atlas yielded 207Pb/235U ages of 584 ± 2.29/579 ± 3.83 Ma for Sidi El Houssein granite (Zenaga Inlier), 581 ± 2.05 Ma for Tafraout granites (Kerdous inlier) and 580 ± 3.02 Ma for the Taourgha intrusion (Bas-Drâa inlier). These pink granites display high SiO2 contents that range from 69.19 to 77.3 wt %. They are mainly peraluminous to weak metaluminous and subalkaline and ferron granites with LILE (Rb, K, Th) and HFSE (Ta, Zr, Hf) enrichment, with depletions in Ba, Nb, Sr, and Ti. These rocks have relatively high zircon saturation temperatures (700–900 °C), high Zr + Nb + Ce + Y contents >350 ppm), and 10,000 × Ga/Al ratios> 2.6 that are typical of A-type granites. Isotopic compositions show that the granitoids have variable (87Sr/86Sr)i ratios (0.67–0.73), εNd(t) values (−2.9 to-7.63) and TDM model ages of 1.49–2.53 Ga Suggesting that they were derived from the partial melting of the continental crust with a moderate contribution of the mantle. The late Neoproterozoic granitoids of the Anti-Atlas belt were probably generated during a transtensional regime due to an asthenosphere upwelling that followed the lithospheric delamination related to the collapse of the Panafrican belts

The Concept of Exhumed Analogue for Characterization of High-Energy Geothermal Reservoir: an Example from Les Saintes Island (Guadeloupe Archipelago, Lesser Antilles) and Consequences for Exploration

International audienceThe recognition of a fossil equivalent of the active geothermal system of the Basse-Terre Island in Guadeloupe archipelago (Lesser Antilles arc) in Terre-de-Haut Island (Les Saintes archipelago) allow us to better constrain the structure, the mineralogy and the petrological characters of the deep parts of an active geothermal reservoir. First, we demonstrate the occurrence, in Terre-de-Haut Island, of an exhumed analogue of the deeper parts Bouillante geothermal field. In this exhumed paleo-geothermal reservoir we characterize the nature and the P-T evolutions of the hydrothermal mineral associations in relation with fluids transfers. Moreover, high-temperature hydrothermal activity is coeval with the development of ductile tectonic structures, particularly schistose corridors, and is dated at 2.59 ± 0.12 Ma on white micas, involving maximum activity of 600 ky. These data provide a conceptual model, including brittle-ductile transition located at a depth between 2.5 and 4 km. In this way, brittle and ductile tectonic structures are efficient drains to connected vertical and lateral fluids and heat transfers. This may provide new guides for exploration of geothermal resources in volcanic arcs

Arc-related metamorphism in the Guadeloupe archipelago (Lesser Antilles active island arc): First report and consequences

International audienceWe unravel for the first time the expression of a low-grade metamorphic event affecting the deepest exposures of the Guadeloupe Island, central Lesser Antilles, Caribbean plate. The work offers a novel insight for the presence of young (<3 Ma) sub-greenschist facies metamorphism in the Lesser Antilles active arc and is consistent with the elevated geothermal gradients measured for the arc axis regionally. Quantitative pressure-temperature constraint (i.e. pseudosection) for this metamorphism as well as fluid/rock ratios are discussed. This paper offers the first report of “ductile” deformation mechanisms (i.e. pressure-solution) operating at shallow crustal levels of the Antilles volcanic arc system

The Concept of Exhumed Analogue for Characterization of High-Energy Geothermal Reservoir: an Example from Les Saintes Island (Guadeloupe Archipelago, Lesser Antilles) and Consequences for Exploration

International audienceThe recognition of a fossil equivalent of the active geothermal system of the Basse-Terre Island in Guadeloupe archipelago (Lesser Antilles arc) in Terre-de-Haut Island (Les Saintes archipelago) allow us to better constrain the structure, the mineralogy and the petrological characters of the deep parts of an active geothermal reservoir. First, we demonstrate the occurrence, in Terre-de-Haut Island, of an exhumed analogue of the deeper parts Bouillante geothermal field. In this exhumed paleo-geothermal reservoir we characterize the nature and the P-T evolutions of the hydrothermal mineral associations in relation with fluids transfers. Moreover, high-temperature hydrothermal activity is coeval with the development of ductile tectonic structures, particularly schistose corridors, and is dated at 2.59 ± 0.12 Ma on white micas, involving maximum activity of 600 ky. These data provide a conceptual model, including brittle-ductile transition located at a depth between 2.5 and 4 km. In this way, brittle and ductile tectonic structures are efficient drains to connected vertical and lateral fluids and heat transfers. This may provide new guides for exploration of geothermal resources in volcanic arcs

3D morphology and timing of the giant fossil pockmark of Beauvoisin, SE Basin of France

International audienceThe resolution of data acquired over modern seafloors does not allow imaging of the inner features of a fluid seep structure, particularly in the shallow subsurface. In the SE Basin of France (Drôme), fossil cold seep structures comprising fossil-rich carbonate lenses were identified about 30 years ago within the Oxfordian (Late Jurassic) Terres Noires Formation. These structures were first interpreted as pseudo-bioherms related to hydrothermal activity, but comparison with active seep sites on modern margins, together with isotopic analyses, led to a reinterpretation involving cold fluids instead. To date, all seep sites have generally been studied individually without considering any link to neighbouring or more distant sites. Based on 23 high-resolution stratigraphic logs within the structure coupled to mosaicked aerial photographs from a drone survey, 19 fluid seep events were correlated in the area, including two new sites exposed as a result of weathering. We have shown that each identified sub-site is composed of subvertically stacked fossil-rich carbonate lenses interbedded with marls, which developed in smooth, 4-6 m deep depressions beneath the local seabed. The nodules present within the depressions are of primary importance as they mark the area of active seeping. This general organization is very similar to the modern Regab giant pockmark in the Lower Congo Basin where only a few sub-sites are active at the same time. A spatio-temporal 3D reconstruction of the position of these sub-sites shows that the carbonate lenses are organized into clusters comprising up to seven sub-sites grouped together in the same stratigraphic interval and the same geographical zone. A sandbox experiment where gas is injected at constant flow rate at the base of a box filled with a matrix of water-saturated grains displays a pattern consisting of disturbed sediments inside a parabolic-shaped area. This parabolic shape was also identified on a seismic profile across the Regab giant pockmark, suggesting that the processes are similar for the Beauvoisin and Regab seep areas. The laboratory experiments also show that the seeping conduit is stable during a given period of time and suddenly shifts laterally. This is mainly as a result of the collapse of the conduit, the lateral migration and the reopening at a new position. The general log obtained in the Beauvoisin seep area suggests a similar pattern with periods of seeping alternating with periods of quiescence, each of which is c. 200 kyr. Even if a pockmark seems to have been inactive for a long period of time, this could be due to the lateral shift of the feeder conduit, meaning that the sub-seafloor is still charged with gas. This is of primary importance for risk assessment, hydrocarbon exploration and general understanding of geobiology at seafloor seeps

40Ar/39Ar dating of high temperature geothermal systems: First attempt on hydrothermally altered pyroxenes from the Saintes archipelago (Lesser Antilles arc, Guadeloupe)

International audienceWe investigate the potential of 40Ar-39Ar systematics in secondary K-bearing minerals developed at the expense of hydrothermally altered volcanic pyroxenes from a fossil geothermal system located in the Saintes archipelago (Lesser Antilles magmatic arc).Our work reports the first example of dating hydrothermal muscovite crystallization developed during pseudomorphic transformation of pyroxenes within altered rhyodacites at temperatures above 300 °C. The white micas display a well-defined Ar/Ar plateau age of 2.59 ± 0.12 Ma, interpreted as the best age estimate for high temperature fluid circulation in the Saintes geothermal paleo-reservoir. A model of muscovite substitution by illite during cooling supports the interpretation that illitization perturbs the K-Ar and the Ar-Ar ages only when muscovite is highly illitized and/or if illitization is significantly younger than muscovite crystallization. The whole temporal dataset implies a fast cooling rate (>150 °C/200 ka) and a maximal lifetime of 650 ka for the Saintes hydrothermal system