Contrasting volcanic-tectonic processes during the past 2 Ma on the Mid-Atlantic Ridge: submersible mapping,petrological and magnetic results at lat. 34°52′?N and 33°55′?N |
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Authors: | Bideau Daniel Hékinian Roger Sichler Bertrand Gràcia Eulàlia Bollinger Claire Constantin Marc Guivel Christelle |
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Institution: | (1) Département Géosciences Marines, Institut Français de Recherche et d'Exploitation de la Mer, IFREMER, BP 70, 29280, Plouzané cedex, France;(2) Challenger Division for Seafloor Processes, Southampton Oceanography Centre Empress Dock, Southampton, SO14 3ZH, UK;(3) Département Géosciences Marines, Institut Français de Recherche et d'Exploitation de la Mer, CNRS, BP 70, 29280, Plouzané cedex, France;(4) MERC / Department of Earth Sciences, Laurentian University, Sudbury, Ontario, Canada, P3E 2C6;(5) Faculté des Sciences, UMR 6538 Domaines Océaniques, 6, Avenue Le Gorgeu, BP 809, 29285 Brest Cédex, France |
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Abstract: | In August–September of 1995, 20 Nautile dives and detailed magnetic surveys (spaced every 1.8 km) were undertaken on two segments of the Mid-Atlantic Ridge between the Oceanographer and Hayes fractures zones. These two segments are only 65 km apart and show strong morphology and gravity contrasts. OH1 is shallower and has a large mantle Bouguer anomaly (MBA) bull's eye, whereas OH3 is deeper and has a smaller MBA bull's eye.Thirteen dives were devoted to segment OH1. The Median Ridge (MR) located on the central high (1700 m deep) is topped by 100 to 300 m high circular volcanoes. The volcanics consists mainly of porphyritic and/or vesicular pillows and volcaniclastics. The NVZ (2200 m deep), located in the valley floor east of the MR, consists of near aphyric fluid lava flows. A chain of off-axis volcanoes, displaying a magnetic continuity with surroundings, extends on both sides of the axis. Three volcanoes on the east side and one on the west side of the axis were explored and sampled by submersible. The off-axis increase of weathering, Fe-Mn coating and magnetic signature suggest that the volcanoes were built at or near the ridge axis. The spacing of NS elongated hills bearing circular volcanoes and separated large magnetic signature (2 to 4 km) depressions suggests that several similar volcanic events occured during the past 2 Ma. The last 1 Ma episode involves (1) the construction of an axial ridge (MR) by fissure eruptions and the formation of circular summit volcanoes by focused volcanism, and (2) the extrusion of fluid magma in the depressions formed by further fissuring and faulting of the MR. |
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Keywords: | Mid-Atlantic Ridge magnetic anomalies ocean crust basalts peridotites volcanic ejecta |
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