BSGF - Earth Sciences Bulletin最新文献

{"title":"Short-lived active margin magmatism preceding Variscan collision in the Western French Massif Central","authors":"Julien Berger, Léa Beau-Hurdebourcq, Julien Serrano, Mathieu Benoit, Michel Grégoire, Anissa Benmammar, Stéphanie Duchene, O. Bruguier, J. Baele","doi":"10.1051/bsgf/2024003","DOIUrl":"https://doi.org/10.1051/bsgf/2024003","url":null,"abstract":"This paper presents and discusses new geochronological and petrological data on a suite of calc-alkaline plutons composed predominantly of diorites and tonalites from the West Massif Central. Their petrochemical fingerprints are compatible with partial melting of a hydrous mantle wedge followed by fractional crystallization of amphibole and plagioclase before final emplacement between 5 and 8 kbar within the continental upper plate of a subduction system. In situ U-Pb zircon dating on tonalites yields a fairly narrow age range of 362-356 Ma for igneous crystallization. These calc-alkaline plutons imply active margin magmatism near the Devonian-Carboniferous boundary and are contemporaneous with the back-arc magmatism and HP metamorphism as dated by recent studies. However, such isolated igneous bodies do not form a transcrustal magmatic arc but rather represent dispersed plutons emplaced within less than 20 Myr when all data from the Variscan belt of France are considered. In Limousin, they intrude migmatitic paragneisses and retrogressed eclogites from the Upper Gneiss Unit (UGU), suggesting that the high pressure rocks were already exhumed at 19-30 km depth before 362 Ma. Moreover, the diorites and tonalites are never found within units below the UGU. It therefore proves that these tectono-metamorphic units of the Western French Massif Central were piled up after 356 Ma. Altogether these results support the monocyclic model for Variscan geodynamics in the French Massif Central, with the transition between oceanic subduction and continental collision taking place between Upper Devonian and Lower Carboniferous.","PeriodicalId":202681,"journal":{"name":"BSGF - Earth Sciences Bulletin","volume":"98 ","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-02-15","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139834833","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Did subduction in the western Mediterranean drive Neogene alpine dynamics? Insights from analogue modeling","authors":"J. Martinod, Ayend-Christ Daou, Laurent Métral, Christian Sue","doi":"10.1051/bsgf/2024001","DOIUrl":"https://doi.org/10.1051/bsgf/2024001","url":null,"abstract":"We present analogue models simulating the Neogene subduction that occurred in the Western Mediterranean region, in order to understand how it impacted the regional tectonics. Although models do not include the lithospheric plate overriding the subduction zone, their surface deformations share many similarities with the Neogene tectonics of Western Europe and Iberia. We observe that the tectonic evolution is largely controlled by the roll-back of the slab, that occurred much faster than the Africa-Eurasia convergence. Models reproduce the opening of the Western Mediterranean Basins and the dispersion of the AlKaPeCa continental fragments. They also show that oceanic subduction favors the counterclockwise rotation of Adria. In more elaborated models, we introduced a pre-existing weakness along the Africa and Adria margins, to reproduce the break-off of the oceanic slab that followed the beginning of continental subduction both in Northern Africa and Italia. Slab break-off is followed by the exhumation of the subducted continent. We observe that the influence of subduction on the kinematics of Adria largely decreases following slab break-off. In models, the total counterclockwise rotation of Adria varies between 7° and more than 30°, depending on the timing of slab break-off. Since the process of subduction modifies the displacement of Adria, it also impacts the tectonic evolution of the regions that bound this plate, especially in the Alpine belt: in the Western Alps, an older Late Cretaceous to Eocene “Pyrenean-Provençal” tectonic phase accommodating N-S shortening is classically described resulting from the convergence between Africa and Eurasia. It is followed by the Neogene “Alpine phase” accommodating E-W shortening. Since this major tectonic change is not explained by a modification of the global Africa-Eurasia convergence, it should be explained instead by more local causes. Our models show that during slab-roll back and before slab break-off, the azimuth of convergence between Adria and Europe shifts from ~N-S to ~ENE-WSW. Hence, they suggest that the oceanic subduction in the Western Mediterranean may explain the “Oligocene revolution” described by Dumont et al. (2011), leading to E-W shortening in the Western Alps and to the activation of the Periadriatic right-lateral shear zones in the Central Alps. We conclude that the western Mediterranean region is a spectacular example showing how the tectonics of mountain ranges and plate boundaries may be controlled by distant subduction processes.","PeriodicalId":202681,"journal":{"name":"BSGF - Earth Sciences Bulletin","volume":"6 12","pages":""},"PeriodicalIF":0.0,"publicationDate":"2024-01-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139616561","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Impact of alteration corridors on karst reservoir organisation and evolution of groundwater flow path: An example from the southern border of the Larzac Causse, southern France","authors":"Céline Baral, Michel Séranne, H. Camus, Johan Jouves","doi":"10.1051/bsgf/2023017","DOIUrl":"https://doi.org/10.1051/bsgf/2023017","url":null,"abstract":"All the features of karstic reservoirs result from the chemical and/or mechanical erosion of an initial rock volume, which modifies the initial petrophysical properties (i.e., porosity and permeability). The spatial distribution and organisation of the karst system in carbonate massif are often overlooked in studies on karst hydrological functioning. However, these parameters are key to understand and accurately model dynamic flow. This contribution aims at characterising the nature and impact of alteration corridors on the organisation and evolution of the karstic reservoir and its present-day functioning. We focus on characterising the specific impact of the late onset of pocket valley regressive erosion on the organisation of the present-day drainage system. We used a 3D approach to correlate field observations on the surface and in caves, with remote sensing. The expression of alteration corridors is analysed in a 40 km² area on the southern border of the Larzac Causse. The Jurassic carbonate massif is affected on its entire thickness by a network of vertically elongated alteration corridors containing dissolution-collapse breccia, mainly oriented in an N-S direction. Ghost-rock karstification played a significant part in the karstic reservoir structure and evolution. Alteration corridors result from the in-situ dissolution of the bedrock along an initial jointing pattern. The dissolution-collapse breccia corridors correspond to ghost-rock corridors selectively emptied of their alterite, under the effect of a hydraulic gradient. The vertically elongated structure of alteration corridors that cross-cut the Jurassic sequence enables fluid circulation from an upper to a lower aquifer, initially separated by a Toarcian marly seal unit. The later initiation of pocket valleys led to the rapid evacuation of residual alterite contained in ghost-rock corridors, under the influence of gravity. These alteration corridors determine the position of present-day pocket valley springs and enhance the regressive erosion dynamics, that progressively capture a north-east flowing watershed (Vis River). Last, residual alterite are evacuated below the overflow spring height during high-flow events. Such mechanisms of alterite evacuation provide insight into the part of the karstic reservoir supplying spring and the associated flow dynamics. This study suggests that corridor networks constitute a substantial volume of porous and permeable materials that play a major role in the present-day groundwater flow path. Such features should be considered valuable drilling prospects for water exploration when located below the piezometric level.","PeriodicalId":202681,"journal":{"name":"BSGF - Earth Sciences Bulletin","volume":" 1289","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-12-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"138960198","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Landslide susceptibility mapping using GIS Matrix Method and Frequency Ratio, application in the marly context of Moulay Yacoub Region, Morocco","authors":"Ilias Obda, Elkharim Younes, Obda Oussama, Sahran Reda, Lahrach Abderrahim, Ahniche Mohamed","doi":"10.1051/bsgf/2023016","DOIUrl":"https://doi.org/10.1051/bsgf/2023016","url":null,"abstract":"In the recent decades, the growth of population, man-made facilities, infrastructures, and lifelines at the expense of landslides prone areas has been responsible for an exponential increase in human and economic losses in many parts of the world. In the Moulay Yacoub region, where marly hills dominate, the interaction of the semi-urban and rural socioeconomic development and landslides significantly increases, which urges identifying and prioritizing areas of risk in order to maximize harm reduction and to avoid the disastrous outcomes as is the case of Moulay Yacoub town. This paper aims to develop a landslide susceptibility map in a highly affected sector of the province, where no previous landslide data have been produced, and to find the most involved parameters. This goal will be attained using two robust methods, the Frequency Ratio and the GIS Matrix Method. Before that, the correlation of 11 factors was tested. The results show that the anthropogenic factors, particularly the agricultural practices, were highly involved, and the field investigation proved that cereal farming slopes were the most affected. The success rate was about 0.75 (75%) for both models showing good quality results for the two susceptibility maps. Therefore, the two models could be efficiently used, and the new agricultural projects located in landslide-prone areas of the province must include such reliable methods of landslide risk analysis to minimize the triggering probabilities, which would put human lives, ecosystems, food production, and infrastructure at risk.","PeriodicalId":202681,"journal":{"name":"BSGF - Earth Sciences Bulletin","volume":"67 2","pages":""},"PeriodicalIF":0.0,"publicationDate":"2023-11-17","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"139265845","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Late extension of a passive margin coeval with subduction of the adjacent slab: TheWestern Alps and Maghrebides file","authors":"A. Michard, A. Farah, Moulley Charaf Chabou, O. Saddiqi","doi":"10.1051/bsgf/2023010","DOIUrl":"https://doi.org/10.1051/bsgf/2023010","url":null,"abstract":"The evolution of the Alpine Tethys margins during the beginning of the African-Eurasian convergence (Upper Cretaceous) was little studied compared to their evolution during the post-Pangea rifting and oceanic expansion, i.e., from the Early Jurassic to the Early Cretaceous. The aim of the present work is first to make up for this shortcoming in the case of the distal European margin of the Alpine Tethys, namely the Briançonnais domain of the Western Alps. We show that this magma-poor passive margin was affected by a systemic extension in Late Cretaceous-Paleocene times. Remarkably, this extensional tectonics shortly preceded Lutetian times, when Briançonnais margin encroached the SE-dipping subduction zone under the Adria microplate (“Alpine subduction”). Secondly, we aim to assess the Late Cretaceous-Paleocene evolution of the north-Tethyan paleomargin in the Maghrebides transects, i.e., south-west of the Briançonnais transect along the same European-Iberian margin. For this purpose, we consider the Triassic-Eocene series of the \"Dorsale Calcaire\" in the Alboran, Kabylias and Peloritan terranes that constitute with Calabria the Alkapeca blocks formerly located along the southeastern border of Iberia until the Eocene. Reinterpretation of the literature allows us to assert that the Tethyan margin of these blocks was extending like the Briançonnais during the Late Cretaceous-Paleocene, when Africa-Eurasia-Iberia convergence and then subduction of the intervening Tethyan slab were active. We propose here for the first time that the subduction of the Ligurian-Maghrebian slab occurred under the North African margin at that time in the southward continuation of the Alpine subduction. In the Alboran transect, the Rif-Betic Dorsale Calcaire can be seen as the detached cover of the thinned crust of the Alpujarrides-Sebtides Complex. In the same transect, the oceanic domain may have included a continental allochthon of African origin (Ketama Unit). Contrary to some assertions, the North African margin did not experience significant compression during the Cretaceous. During the Eocene, a Subduction Polarity Reversal occurred, which was associated with the relocation of the subduction zone along the Alkapeca block. This was the beginning of the \"Apenninic subduction\", which triggered the back-arc opening of the Mediterranean basins and corresponds to the backthrusting tectonic phase in the Western Alps.","PeriodicalId":202681,"journal":{"name":"BSGF - Earth Sciences Bulletin","volume":"33 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-09-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"127427640","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Questioning carbonate facies model definition with reference to the Lower Cretaceous Urgonian platform (SE France Basin)","authors":"Julien Michel, C. Lanteaume, G. Massonnat, J. Borgomano, A. Tendil, F. Bastide, C. Frau, P. Léonide, M. Rebelle, M. Barbier, C. Danquigny, J. Rolando","doi":"10.1051/bsgf/2023009","DOIUrl":"https://doi.org/10.1051/bsgf/2023009","url":null,"abstract":"A facies model, which is generally used to fill data gap, is based on multiple data integration and correlation, interpretation and concepts. Main uncertainties of model build up reside in chronostratigraphic interpretations, synchronous strata correlations, palaeoecology of extinct ecosystems, poorly recorded palaeotopographic profiles and contemporaneous palaeoecological successions. The precise recording and tracking of data and concepts that were used to support model interpretation and definition represents a great challenge for the geologists who build a facies model and is close to impossible for the user of a facies model. The present study explores a methodological workflow to define a well-supported facies model. An exhaustive literature review is presented on sedimentary facies and profiles of Urgonian carbonate platforms from SE France and Switzerland (Barremian–Aptian interval). The study provides a thorough analysis of available data to propose a harmonised and integrated facies model. The conceptual evolution of the Urgonian model through time is investigated to help comparing published models. The structure of the conceptual model and table of depositional facies shows a consistent organisation of specific elementary facies, facies associations and carbonate system; the model promotes the study and understanding of sedimentary processes. Quantitative analyses of facies belt extent and palaeobathymetric estimation are provided. The resulting sedimentary profile is based on general palaeoecological and sedimentological concepts, facies distribution on palaeogeographic maps and consistent stacking trends. In very rare locations, direct lateral facies belt transitions are recorded. Proximal depositional facies are rarely observed; most outcrops record the (i) rudist facies association, which corresponds to distal parts of the inner platform, (ii) coral, ooidal and bioclastic facies associations, which are interpreted to occur on the outer platform, and (iii) calcisiltite, slope and basinal deposits. Key data and concepts allow for the building up of a robust, harmonised facies model that can be used to properly interpret palaeoenvironmental changes, stacking trends and stratigraphic sequence evolution, the resolution of which depends on the available chronostratigraphic framework.","PeriodicalId":202681,"journal":{"name":"BSGF - Earth Sciences Bulletin","volume":"25 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-07-31","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"125353583","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Gravity-driven sliding and associated deformations along complex submarine slopes: a laboratory modeling approach based on constraints observed offshore Martinique Island (Lesser Antilles)","authors":"M. Brunet, T. Nalpas, Erwan Hallot, A. Le Friant, G. Boudon, J. Kermarrec","doi":"10.1051/bsgf/2023008","DOIUrl":"https://doi.org/10.1051/bsgf/2023008","url":null,"abstract":"Submarine gravity-driven sliding of sediments are common processes in the vicinity of volcanic islands. In the Lesser Antilles arc, the Montagne Pelée volcano on Martinique Island underwent several flank-collapse events during its long-term eruptive history, resulting in debris avalanches. When the debris avalanches entered into the seawater, they were emplaced over the unstable slope of the volcano, triggering a seafloor sediment failure and massive landslides downstream. Using a laboratory modeling approach, we simulated the gravity-driven sliding of a sand layer lying above a silicone layer. The experiments were performed using various slope geometries (slope lengths and number of slope breaks separating the slopes with different angles), under both dry and aqueous conditions, and while varying the amount of additional sand inputs upstream. The resulting deformations were characterized in each experiment in order to compare the obtained structures with those shown by the seismic lines offshore to the west of Martinique Island. During all the experiments, a compressive frontal deformation zone made of several reverse faults formed downstream, often near the slope breaks. Downstream, a portion of the sediments was mostly displaced and poorly deformed in a damping zone, while an extensional deformation zone formed upstream. The displacements of the surficial markers were measured through time to characterize the sliding dynamics. Our study demonstrates that the slope geometry and additional sand inputs primarily favor and increase the sliding deformation, whereas the hydrostatic pressure plays a secondary catalytic role over time. These results provide new constraints on the driving factors and their consequences on gravity-driven sliding in terms of deformations and runout distance over time. This may have a significant impact on the associated hazard assessment related to offshore infrastructures, in a region known for its seismic and volcanic risks.","PeriodicalId":202681,"journal":{"name":"BSGF - Earth Sciences Bulletin","volume":"17 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-06-08","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"114905179","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"A tectonic model for the juxtaposition of granulite- and amphibolite-facies rocks in the Eburnean collision in the orogenic belt (Sassandra-Cavally domain, Côte d’Ivoire)","authors":"Augustin Yao Koffi, L. Baratoux, P. Pitra, Alain Nicaise Kouamelan, O. Vanderhaeghe, N. Thébaud, O. Bruguier, S. Block, Hervé Jean-Luc Fossou Kouadio, J. Koné","doi":"10.1051/bsgf/2023007","DOIUrl":"https://doi.org/10.1051/bsgf/2023007","url":null,"abstract":"The Sassandra-Cavally (SASCA) domain (SW Côte d’Ivoire) marks the transition between the Archaean Kenema-Man craton and the Palaeoproterozoic (Rhyacian) Baoule-Mossi terranes. It is characterized by the tectonic juxtaposition of granulite-facies and amphibolite-facies rocks. Migmatitic grey gneisses, garnet-cordierite-sillimanite migmatitic paragneisses and garnet-staurolite-bearing micaschists reached peak pressure conditions ranging from ~6.6 kbar at 620°C to ~10 kbar at 820°C. These conditions are associated with the first recorded deformation D1 and correspond to a Barrovian geothermal gradient of ~25°C/km. Subsequent exhumation, associated with a second deformation D2, was marked by decompression followed by cooling along apparent geothermal gradients of ~40°C/km. A D3 deformation phase is marked by folding and local transposition of the regional S1/S2 foliation by E-W trending shear zones. LA-ICP-MS U-Pb dating of monazite, which displays complex internal structures, reveals four age groups as a function of their textural position: 1) Rare relictual zones yield dates at the Archaean-Palaeoproterozoic transition (c. 2400–2600 Ma); 2) a cluster of dates centered at c. 2037 Ma. This cluster includes dates from crystals located in the S2 foliation of the migmatitic grey gneiss, which suggests that this is the age for the D2 exhumation; 3) a cluster of dates centered at c. 2000 Ma, and 4) dates spreading from c. 1978 to 1913 Ma, documented for the first time in the West African Craton monazites. The ages of the latter two groups are similar to the ones identified in the Guiana Shield, and could be attributed to a disturbance by fluids, to a periodic opening of U-Pb system or to an episodic crystallization of monazite during slow cooling lasting several tens of Myrs. These data bring new petro-tectonic and geochronological constraints on the contact between the Rhyacian Baoule-Mossi terrains and the Archaean Kenema-Man nucleus, and confirm the collisional character of the polyphase tectono-metamorphic evolution of the SASCA domain during the Eburnean orogeny. They suggest a transitional regime between the predominance of gravitational instabilities and modern plate tectonics. The exhumation of the granulite- and amphibolite-facies rocks results from a combination of crustal-scale folding associated with lateral flow and regional transcurrent shear zones, which would explain the juxtaposition of rocks that reached significantly different P–T conditions.","PeriodicalId":202681,"journal":{"name":"BSGF - Earth Sciences Bulletin","volume":"55 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-06-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"126446001","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Deciphering protoliths nature, protoliths age and peak P-T conditions in retrogressed mafic eclogites: An attempt in the metabasites from Maures-Tannneron Massif (SE France) and consequences for the southern European Variscides","authors":"","doi":"10.1051/bsgf/2023006","DOIUrl":"https://doi.org/10.1051/bsgf/2023006","url":null,"abstract":"We present new constraints on age, nature and tectonic setting of mafic eclogites protoliths from the Maures-Tanneron Massif, southern Variscan belt. Whole-rock major and trace elements geochemistry was combined with 206Pb/238U zircon dating to improve the understanding of this key-target of the European Southern Variscides. Geochemical data show that protoliths of the mafic eclogites are typical MORBs, while REE and HFSE patterns suggest an E-MORB affinity. However, the geochemical study shows several signs of crustal contamination, increasing with degree of retrogression. A comparison with Sardinian eclogites, which belong to the same Variscan micro-plate “MECS” (Maures-Estérel-Corsica-Sardinia), demonstrate that the eclogites included in migmatites, which is the case of the studied samples, are the most contaminated. Anyway, the Maures-Tanneron mafic eclogites represent the witness of oceanic basaltic crust. Zircon cores display homogeneous Th/U ratios (0.3-0.4), consistent with a magmatic origin, and define an age peak at ca. 500 Ma interpreted as the most plausible emplacement age of the basaltic protolith. This age suggests that this protolith was part of an oceanic floor older than the Rheic Ocean and located north of the Gondwana active continental margin as predicted by recent unified full plates reconstruction models. Although the eclogites studied are retrogressed the study of frozen mineral inclusions trapped in garnets combined with thermodynamic modelling yields a P-T range of 16.8-18.5 kbar for 630-660°C, coherent with standard subduction paleo-geotherm. These new data suggest that mafic eclogites recognized in the “MECS” Variscan micro-plate represent the closure of oceanic domains of different ages.","PeriodicalId":202681,"journal":{"name":"BSGF - Earth Sciences Bulletin","volume":"81 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-03-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"132039930","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}

{"title":"Natural hydrogen potential and basaltic alteration in the Asal–Ghoubbet rift, Republic of Djibouti.","authors":"Gabriel Pasquet, Amin Mohamed Idriss, Lou Ronjon-magand, M. Ranchou-Peyruse, Marion Guignard, M. Duttine, Anthony Ranchou-Peyruse, I. Moretti","doi":"10.1051/bsgf/2023004","DOIUrl":"https://doi.org/10.1051/bsgf/2023004","url":null,"abstract":"The Asal–Ghoubbet active rift in the Republic of Djibouti is a site of interest for geothermal energy and natural hydrogen, and previous studies have indicated that dihydrogen (H2) emanates from this rift. However, the well-known serpentinization reaction does not appear to be the main mechanism generating H2 at this site. Rather, the H2 is generated as follows: (1) by alteration of basaltic lava at depth via reaction with seawater flowing from Ghoubbet Bay towards Lake Asal; (2) by simple degassing of the volcanic chamber located a few kilometers below the Fiale Caldera in the rift axis; or (3) as a result of pyritization processes via the oxidation of H2S. Analysis of microorganisms did not indicate any production or consumption of H2, CO2, or CH4; therefore, it is unlikely that microorganisms affected H2 gas contents measured at the surface. However, air contamination at fumaroles is typically considerable and may limit interpretation of such processes. \u0000Drill cuttings from the Fiale 1 (F1) and Gale le Goma 1 (Glc1) wells (located on the inner and outer rift margins, respectively) were analyzed to determine where H2 is generated. Total rock analyses indicated distinct zones at depths of 464 m and 280 m for F1 and Glc1, respectively, representing the boundary between the Asal and Stratoïd Basalts. 57Fe Mössbauer analyses show a decrease in the percentage of Fe3+ at depth, indicating that Fe2+-rich material, particularly in the Stratoïd Basalts, may be a source of H2. \u0000Based on well data from the rift center and the outer rift margin, it is evident that H2 is present at the surface in the rift axis and that this area offers good remnant potential because of the presence of Fe-rich chlorite. Conversely, few H2 emissions were measured at the surface on the outer rift margins, although well data showed some H2 (~0.25%) at depth. The presence of a cap rock in the rift axis has not yet been proven; however, the high loss on ignition and the mineralogy in well Glc1 may indicate that the rocks are sufficiently altered to offer potential as a seal. If so, the rift margins would offer greater exploration potential than the rift center. \u0000","PeriodicalId":202681,"journal":{"name":"BSGF - Earth Sciences Bulletin","volume":"2007 1","pages":"0"},"PeriodicalIF":0.0,"publicationDate":"2023-03-28","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"127301090","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":0,"RegionCategory":"","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}