Étudier les dépôts volcaniques à terre et en mer pour reconstruire l'évolution géodynamique d'une zone de subduction : Application à la marge nord-andine

International audienceDes forages IODP réalisés dans le bassin de Panamá, au large de l'Équateur, ont mis en évidence une trentaine de niveaux de cendres volcaniques enregistrées dans les séquences sédimentaires marines. Ces cendres ont été émises lors d'éruptions explosives majeures qui ont eu lieu au cours des derniers 10 Ma. Dans la Cordillère nord andine, le plus ancien dépôt volcanique de l'arc actuel a été émis il y a 2.7 Ma, le matériel volcanique plus ancien ayant été soit érodé, soit recouvert par des produits plus récents. L'enregistrement des cendres retombées en mer et scellées par les sédiments marins est donc extrêmement précieux, puisqu'il nous permet d'avoir un accès unique aux produits des éruptions majeures passées, et fournit des informations essentielles pour quantifier la fréquence de ces éruptions et pour étudier l'évolution de la composition chimique des magmas sur une échelle de temps inédite. Grâce à la caractérisation d'une cinquantaine de niveaux de cendres identifiés dans 6 sites de forages, nous proposons un nouveau modèle de construction de l'arc volcanique nord andin depuis la fin du Miocène. Dans un deuxième temps, nous discutons du lien entre l'évolution long-terme de la fréquence éruptive et de la composition des magmas, et le contexte géodynamique régional (subduction d'une ride océanique, activité des failles crustales majeures)

Reconstruction de l'histoire des volcans de l’arc équatorien : contraintes pour l’évolution chronologique de l’arc andin et pour l'évaluation du risque volcanique

The Ecuadorian arc presents a large number of Quaternary volcanoes. Our ninety new K-Ar ages, carried out on products from twenty volcanic edifices, show that the development of the Quaternary arc began about 1 Ma in northern Ecuador and migrated southward since 600 ka. This migration could be induced by recent change of the slab geometry at depth and by the activation of crustal faults. The magmatic productivity rates are rather homogeneous at the arc scale, and range between 0.1 and 5.6 km³.ka⁻¹. These data highlights that volcanoes grew during sporadic activity pulses separated by quiescence periods. There is no clear correlation between the age of the volcanoes and their emission rates, but the subduction of the Carnegie ridge could favor the magma genesis and explained the increase of the volcanic activity in northern Ecuador during the Quaternary. Erosion rates range between 0.01 and 0.14 km³.ka⁻¹. Youngest volcanic edifices seem to be eroded by a chemical alteration process, while the dismantling of older volcanoes is mostly influenced by physical processes such as glacial abrasion and tectonic activity. Finally, we show that K-Ar ages performed on groundmass or pumice glass shards can be successfully applied to Quaternary volcanic products from the Ecuadorian arc.L’arc équatorien est caractérisé par un nombre particulièrement élevé de volcans quaternaires. Nos quatre-vingt-dix nouveaux âges K-Ar, réalisés pour une vingtaine d’édifices, montrent que son développement a commencé vers 1 Ma au nord et a migré vers le sud à partir de 600 ka. Ce déplacement pourrait être induit par un changement récent de la géométrie de la plaque Nazca en profondeur et l'activation des failles crustales. Les taux de production magmatique sont assez homogènes à l’échelle de l’arc, et varient entre 0,1 et 5,6 km³.ka⁻¹. Ces données montrent que les volcans se sont construits lors de phases d’activité brèves, séparées par des périodes de repos. Il n'y a pas de corrélation apparente entre l'âge des volcans et leur taux d'émission, mais la subduction de la ride de Carnegie pourrait favoriser la genèse du magma et expliquer la densité de volcans quaternaires en Équateur. Les taux d'érosion varient entre 0,01 et 0,14 km³.ka⁻¹. Les volcans les plus jeunes semblent être érodés par des processus d'altération chimique, tandis que les volcans plus anciens sont principalement démantelés par des processus physiques tels que l'abrasion glaciaire et l'activité tectonique. Enfin, nous montrons que la méthode de datation K-Ar réalisées sur la mésostase ou les esquilles de verre des ponces est bien adaptée pour dater les produits volcaniques quaternaires de l'arc équatorien

Reconstruction of the eruptive history of Ecuadorian volcanoes : constraints on the evolution of the Andean arc and for the assessment of volcanic hazards

L’arc équatorien est caractérisé par un nombre particulièrement élevé de volcans quaternaires. Nos quatre-vingt-dix nouveaux âges K-Ar, réalisés pour une vingtaine d’édifices, montrent que son développement a commencé vers 1 Ma au nord et a migré vers le sud à partir de 600 ka. Ce déplacement pourrait être induit par un changement récent de la géométrie de la plaque Nazca en profondeur et l'activation des failles crustales. Les taux de production magmatique sont assez homogènes à l’échelle de l’arc, et varient entre 0,1 et 5,6 km³.ka⁻¹. Ces données montrent que les volcans se sont construits lors de phases d’activité brèves, séparées par des périodes de repos. Il n'y a pas de corrélation apparente entre l'âge des volcans et leur taux d'émission, mais la subduction de la ride de Carnegie pourrait favoriser la genèse du magma et expliquer la densité de volcans quaternaires en Équateur. Les taux d'érosion varient entre 0,01 et 0,14 km³.ka⁻¹. Les volcans les plus jeunes semblent être érodés par des processus d'altération chimique, tandis que les volcans plus anciens sont principalement démantelés par des processus physiques tels que l'abrasion glaciaire et l'activité tectonique. Enfin, nous montrons que la méthode de datation K-Ar réalisées sur la mésostase ou les esquilles de verre des ponces est bien adaptée pour dater les produits volcaniques quaternaires de l'arc équatorien.The Ecuadorian arc presents a large number of Quaternary volcanoes. Our ninety new K-Ar ages, carried out on products from twenty volcanic edifices, show that the development of the Quaternary arc began about 1 Ma in northern Ecuador and migrated southward since 600 ka. This migration could be induced by recent change of the slab geometry at depth and by the activation of crustal faults. The magmatic productivity rates are rather homogeneous at the arc scale, and range between 0.1 and 5.6 km³.ka⁻¹. These data highlights that volcanoes grew during sporadic activity pulses separated by quiescence periods. There is no clear correlation between the age of the volcanoes and their emission rates, but the subduction of the Carnegie ridge could favor the magma genesis and explained the increase of the volcanic activity in northern Ecuador during the Quaternary. Erosion rates range between 0.01 and 0.14 km³.ka⁻¹. Youngest volcanic edifices seem to be eroded by a chemical alteration process, while the dismantling of older volcanoes is mostly influenced by physical processes such as glacial abrasion and tectonic activity. Finally, we show that K-Ar ages performed on groundmass or pumice glass shards can be successfully applied to Quaternary volcanic products from the Ecuadorian arc

Reconstruction de l'histoire des volcans de l’arc équatorien : contraintes pour l’évolution chronologique de l’arc andin et pour l'évaluation du risque volcanique

The Ecuadorian arc presents a large number of Quaternary volcanoes. Our ninety new K-Ar ages, carried out on products from twenty volcanic edifices, show that the development of the Quaternary arc began about 1 Ma in northern Ecuador and migrated southward since 600 ka. This migration could be induced by recent change of the slab geometry at depth and by the activation of crustal faults. The magmatic productivity rates are rather homogeneous at the arc scale, and range between 0.1 and 5.6 km³.ka⁻¹. These data highlights that volcanoes grew during sporadic activity pulses separated by quiescence periods. There is no clear correlation between the age of the volcanoes and their emission rates, but the subduction of the Carnegie ridge could favor the magma genesis and explained the increase of the volcanic activity in northern Ecuador during the Quaternary. Erosion rates range between 0.01 and 0.14 km³.ka⁻¹. Youngest volcanic edifices seem to be eroded by a chemical alteration process, while the dismantling of older volcanoes is mostly influenced by physical processes such as glacial abrasion and tectonic activity. Finally, we show that K-Ar ages performed on groundmass or pumice glass shards can be successfully applied to Quaternary volcanic products from the Ecuadorian arc.L’arc équatorien est caractérisé par un nombre particulièrement élevé de volcans quaternaires. Nos quatre-vingt-dix nouveaux âges K-Ar, réalisés pour une vingtaine d’édifices, montrent que son développement a commencé vers 1 Ma au nord et a migré vers le sud à partir de 600 ka. Ce déplacement pourrait être induit par un changement récent de la géométrie de la plaque Nazca en profondeur et l'activation des failles crustales. Les taux de production magmatique sont assez homogènes à l’échelle de l’arc, et varient entre 0,1 et 5,6 km³.ka⁻¹. Ces données montrent que les volcans se sont construits lors de phases d’activité brèves, séparées par des périodes de repos. Il n'y a pas de corrélation apparente entre l'âge des volcans et leur taux d'émission, mais la subduction de la ride de Carnegie pourrait favoriser la genèse du magma et expliquer la densité de volcans quaternaires en Équateur. Les taux d'érosion varient entre 0,01 et 0,14 km³.ka⁻¹. Les volcans les plus jeunes semblent être érodés par des processus d'altération chimique, tandis que les volcans plus anciens sont principalement démantelés par des processus physiques tels que l'abrasion glaciaire et l'activité tectonique. Enfin, nous montrons que la méthode de datation K-Ar réalisées sur la mésostase ou les esquilles de verre des ponces est bien adaptée pour dater les produits volcaniques quaternaires de l'arc équatorien

Preservation of inherited argon in plagioclase crystals and implication for residence time after reservoir remobilization

International audienceWe compare K-Ar ages obtained on groundmass and plagioclase from lava domes that erupted after flank collapse events in the Lesser Antilles and Ecuador. All samples contain plagioclase with distinct zoning patterns, as well as inclusion-rich zones, that reveal one or more crystal resorption events due to rapid temperature changes. In these samples, plagioclase crystals yield ages 2 to 3 times older than the groundmass due to a partial retention of inherited 40Ar. We investigate textural and compositional zoning in plagioclase phenocrysts using backscattered electron images, electron microprobe and scanning electron microscope analysis of major and trace elements. Age and zoning patterns are coupled to modelling of Ar and Sr diffusion to calculate residence time of crystals at magmatic temperature after reservoir remobilization. Combined data suggest that crystals were remobilized after a magma mixing event related to a flank collapse event. In order to account for the age difference, we have modeled the residence time of plagioclase using magma temperature conditions and possible inherited crystal initial ages. We have calculated that the age differences observed require residence times of tens to a few hundred years. This suggests that eruptions studied here have been triggered by reservoir remobilization in less than 100 years. This can be related to changes in the volcano’s morphology due to large scale flank collapse having affected the plumbing system relatively quickly. Based on similar features observed in different settings, it can be proposed that similar processes are common at arc volcanoes. Our approach should help us to better constrain the timing between magmatic intrusion, mixing, flank collapse and eruptions

New unspiked K-Ar ages of Holocene lava flows and pumices from the Ecuadorian arc

International audienceIn the northern Andes, the Ecuadorian arc presents more than 80 Quaternary volcanoes, including 25 active during the Holocene. Reconstruction of the past eruptive history of these volcanoes is essential to understand their activity frequency and eruptive dynamicsevolution, and for hazard assessment. The groundmass unspiked K-Ar dating method is particularly suitable for dating young subduction lava flows, which contain low K and high Ca contents. Our youngest ages obtained for Holocene lava flows from Tungurahua and Chimborazo volcanoes and Calpi cones range between 8 ± 2 and 4 ± 2 ka, with a K content of about 1.5-2%, and a radiogenic argon content varying between 0.06 and 0.18%, for a narrow density range groundmass. These new ages are consistent with stratigraphy and morphologic observations, and are in agreement with previous published 14C age determinations. We also performed K-Ar measurement on pumice glass shards from the Chalupas ignimbrite, which originate from an ultra-plinian eruption and constitute a major stratigraphic marker in Ecuador. The pumice blocks were gently crushed by hand in an agate mortar, sieved, then grains were separated using heavy liquids. Such procedure allows to preserve massive structure of the glass shards and remove altered or vesicular shards. We obtained the age of 216 ± 5 ka, which agrees with a previous unpublished 40Ar/39Ar age (211 ± 14 ka), and is in agreement with age models of Pacific marine cores, where tephra of this eruption were founded. Our results therefore show that the K-Ar dating method on groundmass can be successfully applied for fresh Holocene lava flows, and for pumice glass shards of the most explosive eruptions, which cannot be dated by the 40Ar/39Ar technique due to 39Ar recoil occurring during irradiation

Evolution of the Ecuadorian arc: construction periods, magmatic productivity and erosion rates

International audienceIn the northern volcanic zone of the Andes, the Ecuadorian arc presents a large number of Quaternary volcanoes, spread over a rather restricted and populated surface area. The activity of the southern termination of the arc seems partly related to the tectonic activity, while the northern part is located above the inland prolongation of the oceanic Carnegie ridge. In this study, we focus on two E-W transects, in the northern and the southern termination of the arc. The former includes Mojanda, Imbabura and Cusín, and the later Igualata, Huisla and Tungurahua volcanoes. We performed numerical reconstructions of the volcanoes flanks morphology before their erosion by using the available DEM. These reconstructions allow us to estimate the total volume of magma emitted, and the amount of eroded material due to abundant precipitations and glacial-interglacial alternating periods.Together with new groundmass K-Ar ages, these volumes are used to quantify the rates of volcanic emission of these volcanoes, as well as their erosion rates during quiescence periods. The volcanic emission rates obtained here are rather similar for both segments, rangingbetween 0.1 and 5.6 km3.ka-1, while the erosion rates vary between 0.01 and 0.14 km3.ka-1. Both rates are not related to the geodynamic or tectonic settings. The highest emission rates are obtained for volcanoes constructed over less than 100 ka, and could represent sporadic eruptive pulses, whereas rates calculated over longer time periods include quiescence phases and do not exceed 1 km3.ka-1. The highest erosion rates are obtained for young volcanic edifices, whose activity ended recently. These results highlight that the volcano morphology is carved by chemical and physical erosion processes that mainly depend on the age and degree of weathering of rocks

New constraints on the geological and chronological evolution of the Cotacachi-Cuicocha Volcanic Complex (Ecuador)

International audienceThe Cotacachi-Cuicocha volcanic complex (CCVC), located to 60-70 km from Quito, is part of the volcanic front (Western Cordillera) of the Ecuadorian Andes. This complex is located inside the new Imbabura UNESCO Geopark, and it is surrounded by several highly populated cities like Cotacachi and Quiroga.New geochemical, petrological and geochronological data (groundmass K – Ar and radiocarbon ages) were obtained for this volcanic complex, allowing to a better understanding of its evolution. The CCVC is constituted by at least four major eruptive stages. The main older and eroded Cotacachi edifice is formed by lava sequences of basaltic andesitic to andesitic compositions: Cotacachi I (Ol + Cpx + Opx + Pl) and Cotacachi II (Pl + Cpx + Opx + Amph ± Ol), as well as several satellite dacitic domes (Muyurcu, Loma Negra, Piribuela and Cuicocha) (Pl + Amph + Bio ± Cpx ± Opx ± Ol). In contrast, the Cuicocha volcanic center is a siliceous andesitic dome complex, characterized by an explosion caldera, whosepseudo-elliptical shape has an average diameter of 3 km. That was formed during an estimated VEI 5 explosive eruption. The CCVC rocks belong to a medium-K calc-alkaline series, varying from 54.7 to 64.8 wt. % of silica. Cuicocha rocks are compositionally depleted of the main trend of Cotacachi rocks, especially in potassium and other trace elements like: La, Th, Rb and Ba.K-Ar dating yield ages between 173 to 108 ka for Cotacachi I and II edifices. During the formation of Cotacachi I, the most primitive products (133 to 113 ka) were erupted around the eastern flank. The satellite domes yield ages between 138 to 65 ka. Finally, one Cuicocha pre-caldera dome shows a Late Holocene age of around 4 ka. The caldera formation has been dated by radiocarbon at 2980 a BP, and the last activity of this volcanic center corresponds to the extrusion of the intra-caldera domes. Cotacachi volcano suffered at least two flank collapses, whose deposits were identified in the present study. The northeastern avalanche (Estimated minimum volume: 0.18 km3) is distributed along the Ambiriver valley. Its occurrence is interpreted between 108 ka and 65 ka and is possibly the youngest and smallest event attributed to Cotacachi II. The northwestern avalanche (Estimated minimum volume: 0.72 km3) is distributed along the Intag river valley. We do not have conclusive data on its age, but its litho-facies suggest it belongs to Cotacachi I structural evolution

Geocronología y evolución del arco volcánico Cuaternario del Ecuador : nuevos avances por el método K-Ar (técnica Cassignol-Gillot)

National audienceDespite the multiple efforts to understand the temporal evolution of the Ecuadorian Volcanic Arc, the evolutionary history of the oldest volcanic edifices remained poorly studied. The Paris-Saclay University, the Instituto Geofísico of the Escuela Politécnica Nacional and the French Instituto de Investigación para el Desarrollo carried out an ambitious research project where more than 140 K-Ar dates were obtained for more than 30 volcanoes of the arc. These high quality ages showed that most of the volcanoes are younger than previously thought, with formation ages of less than 600 ka. This contribution summarizes the K-Ar dating technique employed, the sampling strategies and laboratory processes that ensure the quality of the results, as well as our key geochronological findings of the last decade

Geocronología y evolución del arco volcánico Cuaternario del Ecuador : nuevos avances por el método K-Ar (técnica Cassignol-Gillot)

National audienceDespite the multiple efforts to understand the temporal evolution of the Ecuadorian Volcanic Arc, the evolutionary history of the oldest volcanic edifices remained poorly studied. The Paris-Saclay University, the Instituto Geofísico of the Escuela Politécnica Nacional and the French Instituto de Investigación para el Desarrollo carried out an ambitious research project where more than 140 K-Ar dates were obtained for more than 30 volcanoes of the arc. These high quality ages showed that most of the volcanoes are younger than previously thought, with formation ages of less than 600 ka. This contribution summarizes the K-Ar dating technique employed, the sampling strategies and laboratory processes that ensure the quality of the results, as well as our key geochronological findings of the last decade