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3. Geology of Selvagens

Author

Massironi, M., Sauro, Francesco, Meyzen, Christine, Pozzobon, Riccardo, Tomasi, Ilaria, Miller, A. Z., Massironi, M., Sauro, Francesco, Meyzen, Christine, Pozzobon, Riccardo, Tomasi, Ilaria, and Miller, A. Z.

Abstract

Together with the Canary and Madeira archipelagos, the Selvagens islands belong to the east North Atlantic volcanic province which insists on the slow moving African plate. They are constituted by two groups of islands and islets pertaining to two central volcanic edifices separated by a NE-SW ridge produced by fissural activity [1]. Thought to be part of the Canary hot spot track, the Selvagens islands are characterized by a very long lived magmatic activity with vigorous stages of volcanism interleaved by long periods of quiescence [2]. In particular, the subaerial part of the NE volcano, corresponding to Selvagem Grande island, records most of the geological evolution of the group which is subdivided into several phases [2]. The shield stage, dated back to the Oligocene (24-26Ma), gave rise to the entire basal complex. It is constituted by phonolites emplaced as lava flows, lava domes, peperites and minor pyroclastic sequence. At 80 m from the sea level a continuous bank of fossiliferous limestones testifies a submarine sedimentation in the early Miocene (13-24 Ma) during a volcanic stasis which was most probably accompanied by an active subsidence proven by numerous sedimentary dykes. A second volcanic pulse is testified by basaltic dykes of the Late Miocene (12-8Ma) crosscutting the calcareous horizon on both islands and often exploiting the sedimentary ones in the Selvagem Grande. In the same island a second magmatic stasis was accompanied by a conglomerates unconformly lying over the previous sequence and was followed by the last volcanic activity at 3.4 Ma which formed the upper volcanic complex characterized by basaltic lavas and breccia erupted from at least three volcanic centers. Caves on Selvagem Grande were developed through the alteration of the peperites of the basal complex and karst action on the sedimentary dykes formed during the early Miocene magmatic stasis. Overall the geological analogies with other planetary contexts, a part for the volcanic

Published
2023

11. Assessing Origins of End-Triassic Tholeiites From Eastern North America Using Hafnium Isotopes

Author

Elkins, Lynne J., Meyzen, Christine M., Callegaro, Sara, Marzoli, Andrea, Bizimis, Michael, Elkins, Lynne J., Meyzen, Christine M., Callegaro, Sara, Marzoli, Andrea, and Bizimis, Michael

Abstract

The driving processes responsible for producing the Central Atlantic Magmatic Province, the Large Igneous Province associated with end-Triassic rifting of Pangea, remain largely debated. Because their compositions encompass most of the Central Atlantic basalt spectrum, tholeiites from southern Eastern North America are considered pivotal for identifying magma origins. New(176)Hf/Hf-177 measurements for 201 Ma Eastern North American tholeiites dominantly record a local petrogenetic history. Their epsilon(Hf)ratios, corrected to an emplacement age of 201 Ma (-7.85 to +5.86), form a positive but shallowly sloped array slightly deviating from the terrestrial array on a epsilon(Hf)versus epsilon(Nd)diagram. Comparison of(176)Hf/Hf-177 to other isotope ratios and trace elements helps to rule out several petrogenetic scenarios, particularly mixing of melts from global depleted or enriched mantle components. In contrast, partial melting of subduction-metasomatized mantle can explain the parental magma composition for southern Eastern North America. Such metasomatism likely occurred during Paleozoic subduction around Pangea and may have been dominated by sediment-derived fluid reactions. The observed(176)Hf/Hf-177 versus(143)Nd/Nd-144 array may reflect subsequent assimilation of lower continental crust, perhaps together with limited direct melting of recycled continental crust in the asthenosphere. The proposed recycling scenario does not specifically support or preclude a mantle plume origin for the Central Atlantic Magmatic Province but instead points toward the presence of a distinct local mantle source and crustal assimilation processes during magma transport. Detailed understanding of these local effects is needed in order to more accurately understand the origins of Large Igneous Provinces.

Published
2020
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17. FLY-SPEC prototype: UV-VIS-NIR reflectometry and laser induced breakdown spectroscopy

Author

Zuppella, Paola, primary, Da Deppo, Vania, additional, Zehra, Syedah Sadaf, additional, Chioetto, Paolo, additional, Slemer, Alessandra, additional, Fedel, Massimo, additional, Nicolosi, Piergiorgio, additional, Ferrari, Sabrina, additional, Meyzen, Christine, additional, Nava, Jacopo, additional, Nestola, Fabrizio, additional, and Massironi, Matteo, additional

Published
2020
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20. How huge lava tubes form?

Author

Tomasi, Ilaria, Massironi, Matteo, Meyzen, CHRISTINE MARIE, Pozzobon, Riccardo, Luca, Penasa, Francesco, Sauro, Tommaso, Santagata, Matteo, Tonello, Jesús, Martínez-Frías, and Elena Mateo Medero

Published
2019

24. The Central Atlantic Magmatic Province (CAMP) in Morocco

Author

Marzoli, Andrea, primary, Bertrand, Hervé, additional, Youbi, Nasrrddine, additional, Callegaro, Sara, additional, Merle, Renaud, additional, Reisberg, Laurie, additional, Chiaradia, Massimo, additional, Brownlee, Sarah I, additional, Jourdan, Fred, additional, Zanetti, Alberto, additional, Davies, Joshua H F L, additional, Cuppone, Tiberio, additional, Mahmoudi, Abdelkader, additional, Medina, Fida, additional, Renne, Paul R, additional, Bellieni, Giuliano, additional, Crivellari, Stefano, additional, El Hachimi, Hind, additional, Bensalah, Mohamed Khalil, additional, Meyzen, Christine M, additional, and Tegner, Christian, additional

Published
2019
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31. Central Atlantic Magmatic Province (CAMP) in Morocco.

Author

Marzoli, Andrea, Bertrand, Hervé, Youbi, Nasrrddine, Callegaro, Sara, Merle, Renaud, Reisberg, Laurie, Chiaradia, Massimo, Brownlee, Sarah I, Jourdan, Fred, Zanetti, Alberto, Davies, Joshua H F L, Cuppone, Tiberio, Mahmoudi, Abdelkader, Medina, Fida, Renne, Paul R, Bellieni, Giuliano, Crivellari, Stefano, Hachimi, Hind El, Bensalah, Mohamed Khalil, and Meyzen, Christine M

Subjects
TRIASSIC Period, TRACE elements, IGNEOUS provinces, LAVA flows, GEOCHEMISTRY, MINERALOGY, CAMPS
Abstract

The Central Atlantic Magmatic Province (CAMP) is a large igneous province (LIP) composed of basic dykes, sills, layered intrusions and lava flows emplaced before Pangea break-up and currently distributed on the four continents surrounding the Atlantic Ocean. One of the oldest, best preserved and most complete sub-provinces of the CAMP is located in Morocco. Geochemical, geochronologic, petrographic and magnetostratigraphic data obtained in previous studies allowed identification of four strato-chemical magmatic units, i.e. the Lower, Intermediate, Upper and Recurrent units. For this study, we completed a detailed sampling of the CAMP in Morocco, from the Anti Atlas in the south to the Meseta in the north. We provide a complete mineralogical, petrologic (major and trace elements on whole-rocks and minerals), geochronologic (40Ar/39Ar and U–Pb ages) and geochemical set of data (including Sr–Nd–Pb–Os isotope systematics) for basaltic and basaltic–andesitic lava flow piles and for their presumed feeder dykes and sills. Combined with field observations, these data suggest a very rapid (<0·3 Ma) emplacement of over 95% of the preserved magmatic rocks. In particular, new and previously published data for the Lower to Upper unit samples yielded indistinguishable 40Ar/39Ar (mean age = 201·2 ± 0·8 Ma) and U–Pb ages (201·57 ± 0·04 Ma), suggesting emplacement coincident with the main phase of the end-Triassic biotic turnover (c.201·5 to 201·3 Ma). Eruptions are suggested to have been pulsed with rates in excess of 10 km3/year during five main volcanic pulses, each pulse possibly lasting only a few centuries. Such high eruption rates reinforce the likelihood that CAMP magmatism triggered the end-Triassic climate change and mass extinction. Only the Recurrent unit may have been younger but by no more than 1 Ma. Whole-rock and mineral geochemistry constrain the petrogenesis of the CAMP basalts. The Moroccan magmas evolved in mid-crustal reservoirs (7–20 km deep) where most of the differentiation occurred. However, a previous stage of crystallization probably occurred at even greater depths. The four units cannot be linked by closed-system fractional crystallization processes, but require distinct parental magmas and/or distinct crustal assimilation processes. EC-AFC modeling shows that limited crustal assimilation (maximum c.5–8% assimilation of e.g. Eburnean or Pan-African granites) could explain some, but not all the observed geochemical variations. Intermediate unit magmas are apparently the most contaminated and may have been derived from parental magmas similar to the Upper basalts (as attested by indistinguishable trace element contents in the augites analysed for these units). Chemical differences between Central High Atlas and Middle Atlas samples in the Intermediate unit could be explained by distinct crustal contaminants (lower crustal rocks or Pan-African granites for the former and Eburnean granites for the latter). The CAMP units in Morocco are likely derived from 5–10% melting of enriched peridotite sources. The differences observed in REE ratios for the four units are attributed to variations in both source mineralogy and melting degree. In particular, the Lower basalts require a garnet peridotite source, while the Upper basalts were probably formed from a shallower melting region straddling the garnet–spinel transition. Recurrent basalts instead are relatively shallow-level melts generated mainly from spinel peridotites. Sr–Nd–Pb–Os isotopic ratios in the CAMP units from Morocco are similar to those of other CAMP sub-provinces and suggest a significant enrichment of the mantle-source regions by subducted crustal components. The enriched signature is attributed to involvement of about 5–10% recycled crustal materials introduced into an ambient depleted or PREMA-type mantle, while involvement of mantle-plume components like those sampled by present-day Central Atlantic Ocean Island Basalts (OIB, e.g. Cape Verde and Canary Islands) is not supported by the observed compositions. Only Recurrent basalts may possibly reflect a Central Atlantic plume-like signature similar to the Common or FOZO components. [ABSTRACT FROM AUTHOR]

Published
2019
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41. Spreading rate, spreading obliquity, and melt supply at the ultraslow spreading Southwest Indian Ridge

Author

Cannat, C, Sauter, D, Bezos, A, Meyzen, CHRISTINE MARIE, Humler, E., and LE RIGOLEUR, M.

Subjects
melt supply, mantle melting, mid-ocean ridges, magmatic segmentation
Abstract

We use bathymetry, gravimetry, and basalt composition to examine the relationship between spreading rate, spreading obliquity, and the melt supply at the ultraslow spreading Southwest Indian Ridge (SWIR). We find that at regional scales (more than 200 km), melt supply reflects variations in mantle melting that are primarily controlled by large-scale heterogeneities in mantle temperature and/or composition. Focusing on adjacent SWIR regions with contrasted obliquity, we find that the effect of obliquity on melt production is significant (about 1.5 km less melt produced for a decrease of 7 mm/a to 4 mm/a in effective spreading rates, ESR) but not enough to produce near-amagmatic spreading in the most oblique regions of the ridge, unless associated with an anomalously cold and/or depleted mantle source. Our observations lead us to support models in which mantle upwelling beneath slow and ultraslow ridges is somewhat focused and accelerated, thereby reducing the effect of spreading rate and obliquity on upper mantle cooling and melt supply. To explain why very oblique SWIR regions nonetheless have large outcrops of mantle-derived ultramafic rocks and, in many cases, no evidence for axial volcanism [Cannat et al., 2006; Dick et al., 2003], we develop a model which combines melt migration along axis to more volcanically robust areas, melt trapping in the lithospheric mantle, and melt transport in dikes that may only form where enough melt has gathered to build sufficient overpressure. These dikes would open perpendicularly to the direction of the least compressive stress and favor the formation of orthogonal ridge sections. The resulting segmentation pattern, with prominent orthogonal volcanic centers and long intervening avolcanic or nearly avolcanic ridge sections, is not specific to oblique ridge regions. It is also observed along the SWIR and the arctic Gakkel Ridge in orthogonal regions underlain by cold and/or depleted mantle.

Published
2008

45. Post-Laramide, Eocene magmatic activity in Sierra de Catorce, San Luis Potosí, México

Author

Mascuñano, Eduardo, Levresse, Gilles, Cardellach López, Esteve, Tritlla Cambra, Jordi, Corona Esquivel, Rodolfo, Meyzen, Christine, Mascuñano, Eduardo, Levresse, Gilles, Cardellach López, Esteve, Tritlla Cambra, Jordi, Corona Esquivel, Rodolfo, and Meyzen, Christine

Abstract

The Sierra de Catorce is a N-S-trending mountain range located in the northeastern portion of the Mesa Central physiographic province, Mexico. The Sierra lies at the northern end of the Taxco-San Miguel de Allende fault system, which is interpreted as the surface expression of a major crustal discontinuity. A Triassic to upper Cretaceous sedimentary succession, deformed during the Laramide orogeny, crops out in the range. From the structural point of view, the Sierra de Catorce is a double plunging anticlinorium bounded in its western side by a post-Laramide N-S-trending and west dipping normal fault. We report geochronologic and geochemical data for several post-tectonic granitoids cropping out in the range. The intrusive bodies are I-type continental arc granitoids with Eocene instrusion ages (ca. 38 to 45 Ma, U-Pb determined by LA-ICPMS on individual zircon crystals). Epithermal argentiferous ore deposits are spatially associated with the granitoids. The geochemical signature of the granitoids indicates that magmas derived from a metasomatized mantle, and were highly contaminated with crustal materials. Despite the inferred position of the subduction zone during the Eocene (over 600 km westward), an influence of the subducted slab is needed to explain the metasomatization of the mantle wedge and the high-Ba content observed in all the Sierra de Catorce granitoids. Geochemical features of the granitic rocks are consistent with the regional and local geological framework that allowed rapid emplacement of magma pulses in a post-orogenic extensional geodynamic setting., La Sierra de Catorce se ubica en el límite nororiental de la provincia fisiográfica de la Mesa Central. La sierra está constituida principalmente por formaciones sedimentarias de edad mesozoica, las cuales están plegadas formando un doble anticlinal orientado N-S y recumbente hacia el E, formado durante la deformación laramídica. El límite occidental es una falla normal que forma parte del sistema Taxco - San Miguel de Allende, el cual se considera la expresión superficial de una discontinuidad de la corteza continental de México. Varias intrusiones magmáticas se emplazaron durante eventos extensivos posorogénicos. Los cuerpos intrusivos están relacionados espacialmente con depósitos argentíferos epitermales encajados en calizas mesozoicas. Las edades isotópicas de los intrusivos graníticos varían entre 38 y 45 Ma (U-Pb en circones, LA-ICPMS). La firma geoquímica de estos granitoides es de arco continental de tipo I, formados a partir de un magma derivado del manto metasomatizado, altamente contaminado por materiales corticales. A pesar de la posición de la zona de subducción durante el Eoceno (más de 600 km hacia el W), es preciso recurrir a una subducción plana de más de 600 km para explicar tanto la metasomatización del manto como el alto contenido de Ba observado en todas las muestras. Las características geoquímicas de los granitoides son coherentes con la geología regional y local, la cual controló el emplazamiento rápido de pulsos de magmas relacionados con el ambiente geodinámico extensional poscolisión.

Published
2013

46. Petrogenesis of MORB in the ' cold ' areas of the upper Indian mantle : the Southwest Indian Ridge and the Australian-Antarctic Discordance

Author

Meyzen, Christine, UL, Thèses, Centre de Recherches Pétrographiques et Géochimiques (CRPG), Institut national des sciences de l'Univers (INSU - CNRS)-Université de Lorraine (UL)-Centre National de la Recherche Scientifique (CNRS), Institut National Polytechnique de Lorraine, John Ludden, and M. J. Toplis

Subjects
Dorsales océaniques -- Indien, Majeurs, Isotopes, Traces, [SDU.STU] Sciences of the Universe [physics]/Earth Sciences, [SDU.STU]Sciences of the Universe [physics]/Earth Sciences, Croûte océanique-Indien, Manteau, Terre -- Manteau supérieur-Indien, MORB, Ride ultra-lente, Océan, Basalte-Indien
Abstract

Non disponible / Not available, La ride sud-ouest indienne (SWIR) est une des rides les plus lentes (taux d'expansion : 1.6 cm.an(-1)) et les plus profondes du réseau mondial de rides médio-océaniques. La mission EDUL a permis d'échantillonner des verres basaltiques localisés entre 69°E, près du point triple de Rodriguez et de 49°E, à 300 km de la zone de fracture Gallieni. Les compositions géochimiques des verres basaltiques permettent de définir trois grandes provinces magmatiques, séparées par les zones de fracture Malville (ZFM) et Gallieni. A l'est de la ZFM (60°45'E), les verres ont des compositions géochimiques qui diffèrent par leurs Na8.0 élevés et leurs faibles Fe8.0, Ti8.0 et CaO/Al2O3 de celles des autres rides médio-océaniques. Les indicateurs de degré de fusion partielle tels que le Na8.0 et le (Sm/Yb)n y sont découplés. Les variations des teneurs en éléments en traces très incompatibles et modérément incompatibles, bien que le Na8.0 élevé et le Fe8.0 bas reflètent la présence d'une température potentielle du manteau faible, ne peuvent être expliquées par de faibles taux de fusion partielle. Quelques unes de ces caractéristiques ont aussi été identifiées sur une autre section " froide ", la discordance australo-antarctique. A l'ouest de la ZFM, les échantillons suivent majoritairement les tendances globales définies par les autres MORB. Les compositions anormales observées dans la section la plus orientale pourraient résulter d'une refusion du manteau, en relation avec la propagation, au cours du temps de la ride vers le nord-est, s'accompagnant d'un épisode de metasomatisme. Cette hypothèse est étayée par les variations de compositions isotoniques du Nd et à un moindre degré du Sr qui semblent indiquer l'échantillonnage durant la fusion partielle de volumes de manteau distincts en composition et aussi d'un stade de métabolisme.

Published
2002

49. Seismic structure across the rift valley of the Mid-Atlantic Ridge at 23°20′ (MARK area): Implications for crustal accretion processes at slow spreading ridges

Author

Cann, J., Blackman, D., Smith, D. K., McAllister, E., Janssen, B., Mello, S., Avgerinos, E., Pascoe, A., Russell, S.M., Slootweg, P.A., Tanaka, T., Mitchell, Neil, Hirth, Greg, Escartin, J., Cowie, P.A., Searle, R.C., Mitchell, N.C., Macleod, C.J., Slootweg, A., Cannat, Mathilde, Briais, Anne, Deplus, Christine, Escartin, Javier, Georgen, Jennifer, Lin, Jian, Mercouriev, Serguei, Meyzen, Christine, Muller, Mark, Pouliquen, Gaud, Rabain, Aline, DA SILVA, Pedro, Allerton, Simon, Searle, Roger, Canales, J. Pablo, Collins, John, Detrick, Robert, School of Earth and Environment [Leeds] (SEE), University of Leeds, Institut de Physique du Globe de Paris (IPGP), Institut national des sciences de l'Univers (INSU - CNRS)-Université Paris Diderot - Paris 7 (UPD7)-Université de La Réunion (UR)-Institut de Physique du Globe de Paris (IPG Paris)-Centre National de la Recherche Scientifique (CNRS), Department of Earth Science [Bergen] (UiB), University of Bergen (UiB), Department of Earth Sciences [Durham], Durham University, School of Earth and Ocean Sciences [Cardiff], Cardiff University, Géosciences Environnement Toulouse (GET), Institut de Recherche pour le Développement (IRD)-Université Toulouse III - Paul Sabatier (UT3), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Observatoire Midi-Pyrénées (OMP), Université de Toulouse (UT)-Université de Toulouse (UT)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France -Institut de Recherche pour le Développement (IRD)-Institut national des sciences de l'Univers (INSU - CNRS)-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Météo-France -Centre National de la Recherche Scientifique (CNRS), Laboratoire des sciences de l'ingénieur, de l'informatique et de l'imagerie (ICube), École Nationale du Génie de l'Eau et de l'Environnement de Strasbourg (ENGEES)-Université de Strasbourg (UNISTRA)-Institut National des Sciences Appliquées - Strasbourg (INSA Strasbourg), Institut National des Sciences Appliquées (INSA)-Institut National des Sciences Appliquées (INSA)-Institut National de Recherche en Informatique et en Automatique (Inria)-Les Hôpitaux Universitaires de Strasbourg (HUS)-Centre National de la Recherche Scientifique (CNRS)-Matériaux et Nanosciences Grand-Est (MNGE), Université de Strasbourg (UNISTRA)-Université de Haute-Alsace (UHA) Mulhouse - Colmar (Université de Haute-Alsace (UHA))-Institut National de la Santé et de la Recherche Médicale (INSERM)-Institut de Chimie du CNRS (INC)-Centre National de la Recherche Scientifique (CNRS)-Université de Strasbourg (UNISTRA)-Université de Haute-Alsace (UHA) Mulhouse - Colmar (Université de Haute-Alsace (UHA))-Institut National de la Santé et de la Recherche Médicale (INSERM)-Institut de Chimie du CNRS (INC)-Centre National de la Recherche Scientifique (CNRS)-Réseau nanophotonique et optique, Université de Strasbourg (UNISTRA)-Université de Haute-Alsace (UHA) Mulhouse - Colmar (Université de Haute-Alsace (UHA))-Centre National de la Recherche Scientifique (CNRS)-Université de Strasbourg (UNISTRA)-Centre National de la Recherche Scientifique (CNRS), Centre de Recherches Pétrographiques et Géochimiques (CRPG), Institut national des sciences de l'Univers (INSU - CNRS)-Université de Lorraine (UL)-Centre National de la Recherche Scientifique (CNRS), Woods Hole Oceanographic Institution, Department Geology and Geophysics, Woods Hole Oceanographic Institution (WHOI), Institut national des sciences de l'Univers (INSU - CNRS)-IPG PARIS-Université Paris Diderot - Paris 7 (UPD7)-Université de La Réunion (UR)-Centre National de la Recherche Scientifique (CNRS), Institut national des sciences de l'Univers (INSU - CNRS)-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Université Toulouse III - Paul Sabatier (UT3), Université Fédérale Toulouse Midi-Pyrénées-Université Fédérale Toulouse Midi-Pyrénées-Observatoire Midi-Pyrénées (OMP), Météo France-Centre National d'Études Spatiales [Toulouse] (CNES)-Université Fédérale Toulouse Midi-Pyrénées-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD)-Météo France-Centre National d'Études Spatiales [Toulouse] (CNES)-Centre National de la Recherche Scientifique (CNRS)-Institut de Recherche pour le Développement (IRD), Institut National des Sciences Appliquées (INSA)-Institut National des Sciences Appliquées (INSA)-Centre National de la Recherche Scientifique (CNRS)-Matériaux et nanosciences d'Alsace (FMNGE), Institut de Chimie du CNRS (INC)-Université de Strasbourg (UNISTRA)-Université de Haute-Alsace (UHA) Mulhouse - Colmar (Université de Haute-Alsace (UHA))-Institut National de la Santé et de la Recherche Médicale (INSERM)-Centre National de la Recherche Scientifique (CNRS)-Institut de Chimie du CNRS (INC)-Université de Strasbourg (UNISTRA)-Université de Haute-Alsace (UHA) Mulhouse - Colmar (Université de Haute-Alsace (UHA))-Institut National de la Santé et de la Recherche Médicale (INSERM)-Centre National de la Recherche Scientifique (CNRS)-Réseau nanophotonique et optique, Centre National de la Recherche Scientifique (CNRS)-Université de Strasbourg (UNISTRA)-Université de Haute-Alsace (UHA) Mulhouse - Colmar (Université de Haute-Alsace (UHA))-Centre National de la Recherche Scientifique (CNRS)-Université de Strasbourg (UNISTRA), Centre National de la Recherche Scientifique (CNRS)-Université de La Réunion (UR)-Université Paris Diderot - Paris 7 (UPD7)-IPG PARIS-Institut national des sciences de l'Univers (INSU - CNRS), Department of Earth Science [Bergen], University of Bergen (UIB), Centre National de la Recherche Scientifique (CNRS)-Observatoire Midi-Pyrénées (OMP), Université Fédérale Toulouse Midi-Pyrénées-Université Fédérale Toulouse Midi-Pyrénées-Université Toulouse III - Paul Sabatier (UT3), Université Fédérale Toulouse Midi-Pyrénées-Institut de Recherche pour le Développement (IRD)-Centre National d'Études Spatiales [Toulouse] (CNES), Institut National des Sciences Appliquées - Strasbourg (INSA Strasbourg), Institut National des Sciences Appliquées (INSA)-Institut National des Sciences Appliquées (INSA)-Université de Strasbourg (UNISTRA)-Centre National de la Recherche Scientifique (CNRS)-École Nationale du Génie de l'Eau et de l'Environnement de Strasbourg (ENGEES)-Réseau nanophotonique et optique, Université de Strasbourg (UNISTRA)-Université de Haute-Alsace (UHA) Mulhouse - Colmar (Université de Haute-Alsace (UHA))-Centre National de la Recherche Scientifique (CNRS)-Université de Strasbourg (UNISTRA)-Université de Haute-Alsace (UHA) Mulhouse - Colmar (Université de Haute-Alsace (UHA))-Centre National de la Recherche Scientifique (CNRS)-Matériaux et nanosciences d'Alsace, Université de Strasbourg (UNISTRA)-Université de Haute-Alsace (UHA) Mulhouse - Colmar (Université de Haute-Alsace (UHA))-Institut National de la Santé et de la Recherche Médicale (INSERM)-Centre National de la Recherche Scientifique (CNRS)-Université de Strasbourg (UNISTRA)-Institut National de la Santé et de la Recherche Médicale (INSERM), and Université de Lorraine (UL)-Centre National de la Recherche Scientifique (CNRS)

Subjects
Atmospheric Science, 010504 meteorology & atmospheric sciences, Soil Science, Mid-Atlantic Ridge, Aquatic Science, 010502 geochemistry & geophysics, Oceanography, 01 natural sciences, Geochemistry and Petrology, Earth and Planetary Sciences (miscellaneous), Petrology, ComputingMilieux_MISCELLANEOUS, 0105 earth and related environmental sciences, Earth-Surface Processes, Water Science and Technology, [SDU.STU.TE]Sciences of the Universe [physics]/Earth Sciences/Tectonics, geography, Rift, geography.geographical_feature_category, Ecology, Paleontology, Forestry, Mid-ocean ridge, Crust, Igneous rock, Geophysics, 13. Climate action, Space and Planetary Science, Ridge, Rift zone, Geology, Seismology, Rift valley
Abstract

The results from a 53-km-long, wide-angle seismic profile across the rift valley of the Mid-Atlantic Ridge south of the Kane transform (near 23°20′N, MARK area) provide new constraints on models of tectonic extension and magmatic accretion along slow spreading mid- ocean ridges. Anomalously low middle and lower-crustal P wave velocities beneath the neovolcanic Snake Pit ridge are consistent with elevated axial temperatures and with the presence of 4±1% partial melt evenly distributed within the lower crust in preferentially oriented, elongated thin films. If the melt inclusions have larger aspect ratios, melt fractions can be up to 17±3%. This and other geological observations suggest that the study area is presently in a magmatically active period. The igneous crust is anomalously thin beneath both flanks of the median valley (≤2.3–2.5 km). Thus the mantle rocks observed along the western rift valley wall at Pink Hill were probably emplaced at shallow levels within the valley floor during a period of very low magma supply and were later exposed on the valley walls by normal faulting. The crust within the eastern rift valley and flanking rift mountains is seismically heterogeneous, with igneous crustal thickness variations of ≥2.2 km over horizontal distances of ∼5 km. This heterogeneity indicates that the magma supply in the area has fluctuated during the last ∼2 m.y. Thus magmatic and amagmatic periods at slow spreading ridges may alternate over much shorter temporal scales that previously inferred from sea surface gravity data.

Published
2000

50. Spreading rate, spreading obliquity, and melt supply at the ultraslow spreading Southwest Indian Ridge

Author

Cannat, Mathilde, Sauter, Daniel, Bezos, Antoine, Meyzen, Christine, Humler, Eric, Le Rigoleur, Marion, Cannat, Mathilde, Sauter, Daniel, Bezos, Antoine, Meyzen, Christine, Humler, Eric, and Le Rigoleur, Marion

Abstract

We use bathymetry, gravimetry, and basalt composition to examine the relationship between spreading rate, spreading obliquity, and the melt supply at the ultraslow spreading Southwest Indian Ridge (SWIR). We find that at regional scales (more than 200 km), melt supply reflects variations in mantle melting that are primarily controlled by large-scale heterogeneities in mantle temperature and/or composition. Focusing on adjacent SWIR regions with contrasted obliquity, we find that the effect of obliquity on melt production is significant (about 1.5 km less melt produced for a decrease of 7 mm/a to 4 mm/a in effective spreading rates, ESR) but not enough to produce near-amagmatic spreading in the most oblique regions of the ridge, unless associated with an anomalously cold and/or depleted mantle source. Our observations lead us to support models in which mantle upwelling beneath slow and ultraslow ridges is somewhat focused and accelerated, thereby reducing the effect of spreading rate and obliquity on upper mantle cooling and melt supply. To explain why very oblique SWIR regions nonetheless have large outcrops of mantle-derived ultramafic rocks and, in many cases, no evidence for axial volcanism [Cannat et al., 2006; Dick et al., 2003], we develop a model which combines melt migration along axis to more volcanically robust areas, melt trapping in the lithospheric mantle, and melt transport in dikes that may only form where enough melt has gathered to build sufficient overpressure. These dikes would open perpendicularly to the direction of the least compressive stress and favor the formation of orthogonal ridge sections. The resulting segmentation pattern, with prominent orthogonal volcanic centers and long intervening avolcanic or nearly avolcanic ridge sections, is not specific to oblique ridge regions. It is also observed along the SWIR and the arctic Gakkel Ridge in orthogonal regions underlain by cold and/or depleted mantle.

Published
2008
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