Éditeur(s) : HAL CCSD
Résumé : East African Rift (EAR) is the divergent plate boundary. EAR exposes different stages of extension, from early stage rifting in Tanzania to oceanic accretion in Afar (Ethiopia). Manyara basin is the southernmost rift system of the east branch of EAR with recent volcanism (< 1.5 Ma) and a seismic swarm in the lower crust (20 – 40 km). Due to its location and tectonic setting, the Manyara basin offers the opportunity to study the earliest stage of rift initiation. Manyara volcanism is composed of several types of hyper-alkaline lavas as Mg-nephelinites (Mg# > 55) (Labait, Kwaraha), calciocarbonatite (Kwaraha) and evolved nephelinites (Mg# < 35) (Hanang).Mg-nephelinites (Labait and Kwaraha) are primary lavas mainly composed of olivine and clinopyroxene (cpx). Geochemical modelling from trace elements suggests that these primary magmas result from a degree of partial melting < 1 % from a CO2-garnet-phlogopite-bearing peridotite. These magmas have an asthenospheric source at depth > 120 km (lava carries xenoliths with equilibrium conditions > 4 GPa). The minerals were crystallized from a magma with a low H2O content (0.1 and 0.5 wt% H2O). The calciocarbonatite and evolved nephelinites are derived from Mg-nephelinites by fractional crystallization and immiscibility processes. Hanang nephelinites are silica- and alkaline-rich lavas (44.2 – 46.7 wt % SiO2, 9.5 –12.1 wt % Na2O+K2O, respectively) composed by cpx, Ti-garnet, nepheline, apatite and titanite. Complex zonation of cpx (e.g. abrupt change of Mg#, Nb/Ta, and H2O) and trace element patterns of nephelinites record magmatic differentiation involving open system with carbonate-silicate immiscibility and primary melt replenishment. The low H2O content of cpx (3 – 25 ppm wt. H2O) indicates that at least 0.3 wt % H2O was present at depth during carbonate-rich nephelinite crystallization at 340 – 640 MPa and 1050 – 1100 °C. The study of hosted-nepheline melt inclusions from Hanang allows constraining the late magmatic evolution of nephelinites during storage and magma ascent. Melt inclusions are composed by a silicate trachytic glass, a carbonate phase and a shrinkage bubble. Trachytic glass contains high content in CO2 (0.43 wt %, SIMS analyses), sulfur (0.21 – 0.92 wt % S), chlorine (0.28 –0.84 wt % Cl) and H2O low content (< 0.1 wt %, Raman analyses). Immiscibility process leading to the formation of carbonate occurs in a closed system during rapid magma ascent between 200 – 500 MPa. The carbonate phase is a Ca-Na-K-S-rich and anhydrous carbonate (33 wt % CaO, 20 wt % Na2O, 3 wt % K2O, and 3 wt % S). The pre-immiscible liquid has a phonolitic composition with 6 ± 1.5 wt % CO2 at 700 MPa. A preliminary study of melt inclusions by XANES spectroscopy and whole rocks by Mössbauer spectroscopy was used to determine these Manyara lavas were formed at oxidizing conditions (~ ΔFMQ +1.5).The early stage rifting volcanism (Manyara Basin) is characterized by CO2-rich and H2O-poor magmas from at least 120 km below the rift escarpment. The presence of CO2-rich magmas and the small amount of volcanic rocks erupted at the surface may indicate that the storage and percolation of these magmas at depth is a potential trigger for deep seismic swarms.
Le rift Est africain (REA) est une frontière de plaque en extension. Ce rift présente plusieurs stades d’extension, de l’initiation du rift en Tanzanie jusqu’à l’accrétion océanique en Afar. Le bassin de Manyara se situe le plus au sud de branche Est du REA. Il est caractérisé par la présence de volcanisme récent (< 1,5 Ma) et d’un essaim sismique dans la croûte inférieure (20 – 40 km). De par sa localisation et son contexte tectonique, le bassin de Manyara offre l’opportunité d’étudier le stade le plus précoce de l’initiation du rift. Le bassin de Manyara est composé de plusieurs types de laves hyperalcalines, les néphélinites magnésiennes (Mg# > 55) (Labait, Kwaraha), de calciocarbonatite (Kwaraha) et des néphélinites différenciées (Mg# < 35) (Hanang).Les néphélinites magnésiennes (Labait et Kwaraha) sont des laves primaires composées d’olivines et de clinopyroxènes (cpx). La modélisation géochimique des éléments en trace suggère que ces magmas primaires résultent d'un degré de fusion partielle ≤ 1 % à partir d'une péridotite à grenat et phlogopite. Ces magmas proviennent d’une profondeur > 120 km (présence de xénolites avec des conditions d’équilibre > 4 GPa). Les minéraux ont cristallisés à partir d’un magma pauvre en eau (0,1 et 0,5 pds % H2O). La calciocarbonatite et les néphélinites différenciés sont issues des néphélinites magnésiennes par cristallisation fractionnée et processus d’immiscibilité. Les néphélinites du Hanang sont riches en éléments alcalins (9,5 – 12,1 pds % Na2O+K2O) et en silice (44,2 – 46,7 pds% SiO2) et sont composés de cpx, grenat, néphéline, titanite et apatite. La zonation complexe dans les cpx (par exemple, changement brusque de Mg#, Nb/Ta, et H2O) implique une différenciation magmatique en système ouvert avec immiscibilité de liquide carbonaté et silicaté ainsi qu’un remplissage de la chambre magmatique avec des liquides primaires. La faible teneur en eau des cpx (3 – 25 ppm H2O) indique la présence d’un magma pauvre en eau (0,3 pds % H2O) lors de la cristallisation des cpx à des conditions crustales (340 – 640 MPa et 1050 – 1100 °C). L’étude des inclusions vitreuses dans les néphélines de Hanang permet de contraindre l'évolution magmatique tardive des néphélinites et le comportement des éléments volatils (CO2, H2O, S, F, Cl) lors du stockage et de la remontée du magma. Les inclusions vitreuses sont composées d’un verre trachytique, d’une phase carbonatée et d’une bulle de rétraction. Le verre trachytique contient du CO2 (0,43 pds % CO2, analyses SIMS), du soufre (0,21 à 0,92 pds% S), du chlore (0,28 – 0,84 pds % Cl) et très peu d’H2O (< 0,1 pds % H2O, analyses Raman). Le processus d’immiscibilité conduisant à la formation du carbonate se produit dans un système fermé pendant l'ascension rapide du magma, entre 200 – 500 MPa. La phase carbonatée est un carbonate anhydre et riche en Ca-Na-K-S (33 pds % CaO, 20 pds % Na2O, 3 pds % K2O, et 3 pds % S). Le liquide pré-immiscible a une composition phonolitique avec 6 ± 1,5 pds % CO2 à une pression de 700 MPa. Une étude préliminaire des inclusions par spectroscopie XANES et des roches par spectroscopie Mössbauer a permis de déterminer que les laves de Manyara se sont formées à conditions oxydantes (~ ∆FMQ +1,5).À l’initiation du rift, le volcanisme dans le bassin de Manyara est caractérisé par des magmas riches en CO2 et pauvres en H2O issus d’au moins 120 km de profondeur sous l'escarpement du rift. La présence de ces magmas riches en CO2 et la faible quantité de roches volcaniques émises à la surface peuvent indiquer que le piégeage et la percolation de ces magmas en profondeur est un déclencheur potentiel des essaims sismiques profonds.
https://tel.archives-ouvertes.fr/tel-01563231

NNT : 2016MONTT114
tel-01563231
https://tel.archives-ouvertes.fr/tel-01563231
https://tel.archives-ouvertes.fr/tel-01563231v2/document
https://tel.archives-ouvertes.fr/tel-01563231/file/2016_BAUDOUIN_archivage.pdf

Éditeur(s) : HAL CCSD
Résumé : Both short-term and long-term morphodynamics of sand barrier systems forced by moderate storm conditions have been studied significantly in the literature. However, the dynamics of the emerged beach -- from a seaside water depth of ~ 1.5 metres below the lowest tide to the back-barrier -- impacted by typhoons remain scarcely documented. This study focuses on the analysis and a better understanding of the morphologic changes of such an emerged beach, when it is forced by one typhoon, several typhoons or the combination of a summer season of typhoons and a winter season of Monsoon wave/ wind conditions. The monitoring of near-shore hydrodynamics and beach morphodynamics was performed in the Wan-Tzu-Liao sand barrier from November 2011 to August 2012. Waves, currents and water level were measured at an offshore buoy (Cigu buoy; 18 m of water depth; 1.5 km seaward the studied zone), a current profiler (4 m of water depth), and a serie of pressure sensors deployed along a cross-shore section from the lower intertidal zone up to the eolian dune. Tide was obtained from a tide gauge inside the lagoon and wind conditions were obtained respectively from an inland station and the offshore Cigu buoy. Morphodynamic changes were monitoring thanks to DGDPS surveys performed on a grid including a 300 meters long portion of sand barrier extending from the subtidal zone to the back-barrier, as well as along two cross-shore and two long-shore profiles located within the grid. Surveys were performed each week during winter and just before and after each storm during the typhoons summer season. This field methodology provided a new and very robust dataset in order to quantify sand barrier dynamics exposed to high-energy events at various time scales. Amongst the eight typhoons recorded, TALIM is the most significant and representative. Offshore wave height reached 10.34 m (Tp = 14.6 s) at Cigu buoy and 2.3 m in 4 m of water depth. Morphological changes recorded 6 m of retreat for the dune front, a 20 m large dune breaching and the occurence of a wash-over fan in the lagoon. These changes form a well-known morphodynamic responses to moderate storms. More surprisingly, TALIM drove a significant nourishment of the supratidal zone, as well as a positive sand balance on the whole sand barrier. Five other typhoons resulted in the same type of morphological responses while two typhoons resulted in a strong erosion of the whole emerged beach. During winter and spring (moderate wave conditions and strong Monsoon winds), sand moved from the beach front to the back-barrier. For the whole year, the sand barrier recorded a 12 m landward migration concomitantly with a 12 m widening without abrasion of the dune elevation.
American Geophysical Union, Fall Meeting 2013

hal-01012312
https://hal.archives-ouvertes.fr/hal-01012312

Éditeur(s) : HAL CCSD American Geophysical Union - AGU
Résumé : IODP Expedition 335 "Superfast Spreading Rate Crust 4" returned to ODP Hole 1256D with the intent of deepening this reference penetration of intact ocean crust several hundred meters into cumulate gabbros. This was the fourth cruise of the superfast campaign to understand the formation of oceanic crust accreted at fast spreading ridges, by exploiting the inverse relationship between spreading rate and the depth to low velocity zones seismically imaged at active mid-ocean zones, thought to be magma chambers. Site 1256 is located on 15-million-year-old crust formed at the East Pacific Rise during an episode of superfast ocean spreading (>200 mm/yr full rate). Three earlier cruises to Hole 1256D have drilled through the sediments, lavas and dikes and 100 m into a complex dike-gabbro transition zone. The specific objectives of IODP Expedition 335 were to: (1) test models of magmatic accretion at fast spreading ocean ridges; (2) quantify the vigor of hydrothermal cooling of the lower crust; (3) establish the geological meaning of the seismic Layer 2-3 boundary at Site 1256; and (4) estimate the contribution of lower crustal gabbros to marine magnetic anomalies. It was anticipated that even a shortened IODP Expedition could deepen Hole 1256D a significant distance (300 m) into cumulate gabbros. Operations on IODP Expedition 335 proved challenging from the outset with almost three weeks spent re-opening and securing unstable sections of the Hole. When coring commenced, the destruction of a hard-formation C9 rotary coring bit at the bottom of the hole required further remedial operations to remove junk and huge volumes of accumulated drill cuttings. Hole-cleaning operations using junk baskets returned large samples of a contact-metamorphic aureole between the sheeted dikes and a major heat source below. These large (up to 3.5 kg) irregular samples preserve magmatic, hydrothermal and structural relationships hitherto unseen because of the narrow diameter of drill core and previous poor core recovery. Including the ~60 m-thick zone of granoblastic dikes overlying the uppermost gabbro, the dike-gabbro transition zone at Site 1256 is over 170 m thick, of which more than 100 m are recrystallized granoblastic basalts. This zone records a dynamically evolving thermal boundary layer between the principally hydrothermal domain of the upper crust and a deeper zone of intrusive magmatism. The recovered samples document a sequence of evolving geological conditions and the intimate coupling between temporally and spatially intercalated intrusive, hydrothermal, contact-metamorphic, partial melting and retrogressive processes. Despite the operational challenges, we achieved a minor depth advance to 1522 m, but this was insufficient penetration to complete any of the primary objectives. However, Hole 1256D has been thoroughly cleared of junk and drill cuttings that have hampered operations during this and previous Expeditions. At the end of Expedition 335, we briefly resumed coring and stabilized problematic intervals with cement. Hole 1256D is open to its full depth and ready for further deepening in the near future.
InterRidge News

hal-00688821
https://hal.archives-ouvertes.fr/hal-00688821

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Talc is a hydrous magnesium rich layered silicate that is widely disseminated in the Earth from the seafloor to over 100 km depth, in ultra-high pressure metamorphism of oceanic crust. In this paper we determine the single crystal elastic constants at pressures from 0 to 12 GPa of talc triclinic (C 1¯) and monoclinic (C2/c) polytypes using ab initio methods. We find that talc has an extraordinarily high elastic anisotropy at zero pressure that reduces with increasing pressure. The exceptional anisotropy is complemented by a negative Poisson's ratio for many directions in crystal space. Calculations show that talc is not only one of very few common minerals to exhibit auxetic behaviour, but the magnitude of this effect may be the largest reported so far for a mineral. The compression (Vp) and shear (Vs) wave velocity anisotropy is 80% and 85% for the triclinic polytype. At pressures where talc is known be stable in the Earth (up to 5 GPa) the Vp and Vs anisotropy is reduced to about 40% for both velocities, which is still a very high value. Vp is slow parallel to the c-axis and fast perpendicular to it. This remains unchanged with increasing pressure and is observed in both polytypes. The shear wave splitting (difference between fast and slow S-wave velocities) at low pressure has high values in the plane normal to the c-axis, with a maximum near the alow asterisk-axis in the triclinic and the b-axis in the monoclinic polytype. The c-axis is the direction of minimum splitting. The pattern of shear wave splitting does not change significantly with pressure. The volume fraction of talc varies between 11 and 41% for hydrated mantle rocks, but the lack of data on the crystallographic preferred orientation (CPO) precludes a detailed analysis of the impact of talc on seismic anisotropy in subduction zones. However, it is highly likely that CPO can easily develop in zones of deformation due to the platy habit of talc crystals. For random aggregates of talc, the isotropic Vp, Vs and Vp/Vs ratio have significantly lower values than those of antigorite and may explain low-velocity regions in the mantle wedge. Vp/Vs ratios are more complex in anisotropic media because there are fast and slow S-waves, resulting in Vp/Vs1 and Vp/Vs2 ratios for every propagation direction, making interpretation difficult in deformed polycrystalline talc with a CPO. Talc on the subduction plate boundary can strongly influence guided wave velocity as CPO would develop in this region of intense shearing. The very low coefficient of friction (< 0.1) of talc above 100 °C could also explain silent earthquakes at shallow depths (ca 30 km) along the subduction plate boundaries, frequently responsible for tsunami.
ISSN: 0012-821X

hal-00411481
https://hal.archives-ouvertes.fr/hal-00411481
DOI : 10.1016/j.epsl.2008.07.047

Éditeur(s) : Université Nice Sophia Antipolis
Résumé : The Lesser Antilles is a case study of a very slow subduction (~2 cm/yr) of an old oceanic lithosphere (~84-100 Ma). The region presents a relatively low seismic activity, especially along the interplate contact, and the seismic hazard associated with a possible mega-thrust earthquake is still poorly known. This PhD thesis is a first step toward assessing the ability of the Lesser Antilles subduction zone to produce such a large subduction event. To do so, it aims at constraining the downdip width of the interplate's seismogenic zone. The lack of coverage of permanent seismological stations is a major limitation in the exploration of the Lesser Antilles subduction zone. It is due to the presence of only small aligned islands at far distances from the potentially seismogenic interplate area. Several oceanographic cruises were therefore planned that notably allowed the repeated deployment of ocean bottom seismometers; some of them being left for a few months of background seismicity recording. This thesis specifically focuses on two sets of wide-angle seismic data acquired offshore the Dominica and Martinique islands. From their analysis 3D and 2D tomographic models were produced respectively over the forearc region and across the whole subduction complex. These models constrain the plates' seismic structure as well as their geometry. They allow the discussion of how the imaged structures affect the subduction processes and give a first estimation of the downdip width of the seimogenic zone, defined as the segment of the interplate between the backstop and the upper plate's Moho. The joint interpretation of seismic models and earthquake localizations then refine this first assessment. Epicenter distribution from height months of seismic recording shows indeed that seismicity concentrates within the inner forearc region. The strong velocity gradient that characterize its basement suggests it is denser and more rigid than the more deformable outer forearc basement. The updip limit of the seismogenic zone could then lie arcward of the backstop at the contact of the interplate and the seaward end of the inner forearc crust. At depth, interplate earthquake mechanisms are observed between 35 and 45 km depth and interpreted to have occurred at the downdip limit of the seismogenic zone. The latter could reach a depth over 10,km deeper than the contact of the upper plate's Moho with the interplate, and therefore lie within the mantle wedge. All together, these results imply a large downdip width of the seismogenic zone (~70 km) offshore the Dominica and Martinique islands. Further work is, however, needed in order to fully comprehend the ability of the Lesser Antilles subduction zone to produce a possible mega-thrust earthquake. This would necessitate the evaluation of seismic coupling at the interplate contact and the possible segmentation of the seismogenic zone, for instance, due to the subduction of oceanic ridges.
Les Petites Antilles présentent un contexte géodynamique caractérisé par la subduction à très faible vitesse (~2 cm/an) d'une lithosphère océanique âgée (~84-100 Ma). L'activité sismique y est relativement faible, notamment à l'interplaque, où l'aléa sismique lié à un éventuel séisme de méga-chevauchement reste encore mal contraint. Cette thèse se veut être une première étape dans l'évaluation de la capacité de la zone de subduction des Petites Antilles à générer un tel évènement. Dans ce but, ces travaux tentent d'appréhender l'extension en profondeur du domaine sismogène de l'interplaque. Le manque de couverture des stations sismologiques permanentes est un inconvénient majeur dans l'exploration des Petites Antilles. Il s'explique en raison du peu de terres émergées et de leur éloignement de la zone potentiellement sismogène de l’interplaque. La région a donc fait l'objet de plusieurs campagnes océanographiques qui ont permis, notamment, le déploiement de sismomètres fond de mer (OBS); certains instruments étant restés immergés plusieurs mois afin de procéder à une écoute de la sismicité. Les travaux réalisés au cours de cette thèse se sont focalisés sur deux jeux de données de sismique grand-angle acquis au large des îles de la Dominique et de la Martinique. Leur analyse a permis la construction de modèles tomographiques 3D et 2D respectivement à l'échelle de l'avant-arc et de l'ensemble de la subduction. Ces modèles renseignent sur la structure sismique des plaques en convergence ainsi que sur leur géométrie. Ils permettent de discuter le rôle de la structure dans le fonctionnement de la subduction et d’obtenir une première estimation de l'extension en profondeur de la zone sismogène en considérant la portion de l'interplaque comprise entre le butoir et le Moho de la plaque supérieure. Dans un second temps, l'interprétation conjointe des modèles tomographiques et des localisations des séismes locaux a permis d'affiner cette estimation. Huit mois d'enregistrement de la sismicité montrent en effet une concentration des épicentres dans la région interne de l’avant-arc. Celle-ci présente un socle épais, à fort gradient de vitesse interprété comme plus dense et rigide que le socle de la région externe de l'avant-arc, plus déformable. La limite amont de la zone sismogène pourrait donc se situer en retrait de la position du butoir au contact de l'interplaque et de la limite entre ces deux zones de l'avant-arc. En profondeur, des mécanismes interplaques sont observés entre 35 et 45 km et interprétés comme des marqueurs de la limite aval de la zone sismogène. Cette dernière pourrait donc atteindre une profondeur jusqu'à 10 km supérieure à la limite précédemment évoquée et se trouver, par conséquent, au contact du manteau lithosphérique. L'ensemble de ces résultats suggèrent une extension en profondeur de la zone sismogène (i.e une largeur) de près de 70 km face aux îles de la Dominique et de la Martinique. Ces travaux doivent cependant être poursuivis afin d’évaluer pleinement la capacité de la zone de subduction des Petites Antilles à générer un éventuel séisme de méga-chevauchement. Le taux de couplage à l'interplaque doit être précisé ainsi que sa possible segmentation en lien, par exemple, avec à l'entrée en subduction des rides océaniques.

Droits : info:eu-repo/semantics/openAccess
http://archimer.ifremer.fr/doc/00172/28335/26628.pdf
http://archimer.ifremer.fr/doc/00172/28335/

Éditeur(s) : Elsevier
Résumé : A study of organic carbon mineralization from the Congo continental shelf to the abyssal plain through the Congo submarine channel and Angola Margin was undertaken using in situ measurements of sediment oxygen demand as a tracer of benthic carbon recycling. Two measurement techniques were coupled on a single autonomous platform: in situ benthic chambers and microelectrodes, which provided total and diffusive oxygen uptake as well as oxygen microdistributions in porewaters. In addition, sediment trap fluxes, sediment composition (Org-C, Tot-N, CaCO3, porosity) and radionuclide profiles provided measurements of, respectively input fluxes and burial rate of organic and inorganic compounds. The in situ results show that the oxygen consumption on this margin close to the Congo River is high with values of total oxygen uptake (TOU) of 4 +/- 0.6, 3.6 +/- 0.5 mmol m(-2) d(-1) at 1300 and 3100m depth, respectively, and between 1.9 +/- 0.3 and 2.4 +/- 0.2 mmol m(-2) d(-1) at 4000 m depth. Diffusive oxygen uptakes (DOU) were 2.8 +/- 1.1, 2.3 +/- 0.8, 0.8 +/- 0.3 and 1.2 +/- 0.1 mmol m(-2) d(-1), respectively at the same depths. The magnitude of the oxygen demands on the slope is correlated with water depth but is not correlated with the proximity of the submarine channel-levee system, which indicates that cross-slope transport processes are active over the entire margin. Comparison of the vertical flux of organic carbon with its mineralization and burial reveal that this lateral input is very important since the sum of recycling and burial in the sediments is 5-8 times larger than the vertical flux recorded in traps. Transfer of material from the Congo River occurs through turbidity currents channelled in the Congo valley, which are subsequently deposited in the Lobe zone in the Congo fan below 4800 m. Ship board measurements of oxygen profiles indicate large mineralization rates of organic carbon in this zone, which agrees with the high organic carbon content (3%) and the large sedimentation rate (19 mm y(-1)) found on this site. The Lobe region could receive as high as 19 mol C m(-2) y(-1), 1/3 being mineralized and 2/3 being buried and could constitute the largest depocenter of organic carbon in the South Atlantic.
Deep Sea Research Part II: Topical Studies in Oceanography (0967-0645) (Elsevier), 2009-11 , Vol. 56 , N. 23 , P. 2223-2238

Droits : 2009 Elsevier B.V. All rights reserved.
http://archimer.ifremer.fr/doc/2009/publication-7316.pdf
DOI:10.1016/j.dsr2.2009.04.005
http://archimer.ifremer.fr/doc/00000/7316/

Éditeur(s) : HAL CCSD
Résumé : International audience
Mylonite bodies are exhumed extensions at depth of shallow structural level brittle faults. They are characterised by intense strain localisation involving large volumes of middle to lower continental crust. The analysis of shear zone geometry and microstructures is a useful tool to decipher deformation kinematics, though finite strain estimation is still relatively qualitative. Timing of the deformation has also been the focus of numerous studies using the complete spectrum of geochronometers. However, only rare examples provide geochronological data that can be unambiguously interpreted and cooling, recrystallisation and neocrystallisation are difficult to tell from each other. Thus, time-scales over which major mylonite zones develop and remain active under ductile conditions as well as their strain rate often remain poorly documented. We investigated two sections across the South-Armorican Shear Zone (SASZ), a crustal-scale pluri-km thick dextral shear zone active during a single tectonic event of late-Variscan age and associated with syntectonic granite emplacement.. Finite strain profiles on these cross-sections, quantified from average Quartz grain-size in 34 samples, show a strong deformation partitioning towards the center of the SASZ, with a decrease, for each section, from 150µm in slightly deformed zones to less than 7-5 µm in the core of the ultramylonite. Furthermore, toward the core of the SASZ, development of a penetrative shear band network progressively eliminates lenses of less deformed rocks and results in homogenisation of the Quartz grain sizes. In parallel, we performed in-situ UV-laser Ar/Ar dating on the fabric-forming white micas. White micas population can be decomposed into inherited muscovites carried by the folation and newly-formed, syn-tectonic substituted phengites located along the shear bands yielding 2 age groups for each sample. Along the profiles, the difference between these two ages evolves from 10-15 Ma in weakly deformed domains to zero toward the SASZ core. In addition, mylonite-forming phengites yielded the same age irrespectively of their position on the section or of the rock strain.. On the regional point of view, this study brings deformation-ages for the late-orogenic evolution of the South Armorican domain. Strain accumulated along the SASZ in a time-span extending to 300-298 Ma, i.e. contemporaneously with the activity of nearby major extensional shear zones. These results support a new model of crustal-scale shear zone activity, where the whole body of the shear zone is active throughout its history (defined as the largest age difference through the section), i.e. the actively deforming zone does not get progressively narrower. The strain distribution nevertheless evolves with time, with an increasingly large fraction partitioned in the ultramylonitic core.
EGU General Assembly
Vienne, Austria

insu-00839320
https://hal-insu.archives-ouvertes.fr/insu-00839320
BIBCODE : 2010EGUGA..12.9881A

Éditeur(s) : HAL CCSD
Résumé : Karst systems are the most important groundwater reservoirs in Mediterranean area. Beyond saturated zone, unsaturated zone constitute an entity where water storage and transfer play a major role in a global behavior of spring water. This structure and functioning are complex and not well known because of the high heterogeneity created and organized by groundwater flow. In this study, we used gravimetric and hydrogeologic method to directly observe and quantify water storage and transfer processes in unsaturated zone of karst system. Surface to depth gravity measurements on several karst system allow quantifying seasonal water storage changes in unsaturated zone. We show that these variations occur in the first tens meters of unsaturated zone. Furthermore, water storage capacity of unsaturated zone seems to be influenced by lithology (limestone or dolomite). Finally, direct observations of water flow in unsaturated zone are done using caves access. We show that some characteristics of unsaturated water flow such as bipolar slow-fast flow can be widespread for whatever depth of measurement and recharge area. Model of this flow, using a simple modeling scheme, show the complexity of water transfer processes in unsaturated zone of karst system.
Les aquifères karstiques constituent l'essentiel des ressources en eau du pourtour Méditerranéen. Au-delà de la zone saturée de ces systèmes, la zone non saturée constitue une entité importante quant au transfert et au stockage temporaire de l'eau. La structure et le fonctionnement de cette entité du karst sont complexes et mal connus à cause du processus de karstification qui hiérarchise la circulation hydrodynamique en son sein. Dans cette étude nous utilisons des méthodes géodésiques et hydrogéologiques afin d'observer de façon directe et de quantifier les processus de transfert et de stockage de la zone non saturée. Les mesures gravimétriques en surface et profondeur sur différents systèmes karstiques ont permis de quantifier les variations du stockage en eau saisonnier dans la zone non saturée. La gravimétrie différentielle montre que ces variations sont réparties dans les quelques premiers mètres de la zone non saturée. Il semble, de plus, que la capacité de stockage de la zone non saturée soit fonction de la lithologie de l'encaissant (calcaire et dolomie). Enfin, des observations directes du flux d'eau en zone non saturée ont été effectuées par le biais de mesures dans différentes cavités. Nous montrons que certaines caractéristiques du flux d'eau en zone non saturée, comme la présence d'un flux bipolaire lent-rapide, peuvent être généralisées quelque soit la profondeur de mesure et la surface d'impluvium. La modélisation de ce flux, à partir d'un schéma simple, a montré la complexité des processus de transfert engendrés au sein de la zone non saturée.
https://tel.archives-ouvertes.fr/tel-00829346

tel-00829346
https://tel.archives-ouvertes.fr/tel-00829346
https://tel.archives-ouvertes.fr/tel-00829346/document
https://tel.archives-ouvertes.fr/tel-00829346/file/manuscrit_deville_finish.pdf

Éditeur(s) : OMICS Publishing Group
Résumé : Coastal tropical shrimp farming may impact the adjacent ecosystems through the release of large quantities of effluents rich in nutrients. In New Caledonia, mangroves are considered as a natural biofilter to reduce impacts on the surrounding World Heritage listed lagoon. Our main objective was to understand the influence of effluent discharge on the biogeochemistry of mangrove sediments. A monitoring of the physico-chemical parameters of mangrove sediments was carried out during a whole year, including active and non active periods of the farm. The parameters studied were: i) benthic primary production (Chl-a concentrations), ii) physico-chemical parameters of sediments (redox potential, pH, salinity, TOC, TN, TS, δ13C and δ15N), iii) concentrations of dissolved nitrogen, iron and phosphorus. A mangrove developing in the same physiographic conditions, presenting the same zonation, and free of anthropogenic input was used as reference. The concentration of benthic Chl-a measured at sediment surface in the effluent receiving mangrove was twice to three times that measured in the control zone whatever the season. We thus suggest that nutrients inputs significantly increased the phytobenthic production in the effluent receiving mangrove during the whole year, even after the cessation of discharges and because of natural seasonal dynamic of phytobenthos. Although the flow of surface OM was increased, the OM content at depth was not higher than in the control mangrove. However, the contribution of mangrove detritus to the sedimentary organic pool was higher probably as a result of higher density and much greater individual size of the mangrove trees. Unlike the control mangrove sediment, the effluent receiving mangrove sediment was not stratified, redox potential values were high and presence of Fe3+ was detected down to 50 cm depth, probably as a result of a larger root system, allowing a better sediment oxygenation and accentuated OM decomposition processes, and thus limiting ecosystem saturation.
Journal of Aquaculture Research & Development (21559546) (OMICS Publishing Group), 2014-07-03 , Vol. 5 , N. 5 , P. 1-13

Droits : 2014 Marchand C, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
http://archimer.ifremer.fr/doc/00251/36253/34801.pdf
DOI:10.4172/2155-9546.1000262
http://archimer.ifremer.fr/doc/00251/36253/

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
We herein focus on understanding what controls the detachment and migration of oceanic crustal slices along subduction zones. We provide evidence for the coeval Middle to Late Cretaceous exhumation of oceanic blueschists in the same Neotethyan subduction zone, across more than 3000 km, from depths around 30-40 km, over a short time interval (similar to 25 Ma) compared to the duration of subduction (>100 Ma). We stress the importance of such a geodynamic process and suggest that this exhumation was promoted by a regional-scale change in the long-term interplate mechanical coupling. On the basis of geological and geophysical evidence, we propose that the upward migration of oceanic fragments in subduction zones is normally inhibited and only occurs discontinuously, as the subduction channel opens up in response to a major change in plate geodynamics, slab geometry and dynamics, mantle wedge and slab hydration, or a combination of these.
ISSN: 1525-2027

hal-00420898
https://hal.archives-ouvertes.fr/hal-00420898
DOI : 10.1029/2009GC002428

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
B-type olivine fabrics are pervasive within highly depleted dunites of the small-sized Imono peridotite body located within the subduction-type Sanbagawa metamorphic belt of the southwest Japan arc. The dunites contain various microstructures, ranging from porphyroclastic to fine-grained intensely sheared textures. The Mg/(Mg+Fe) atomic ratios (Fo number) of olivine within these dunites are consistently around 0.9, as are the Cr/(Cr+Al) atomic ratios (Cr number) of chromian spinel, suggesting their evolution from a highly depleted magma (boninite). These data provide strong thermal constraints on the formation of the highly depleted dunites,as their formation requires hot, hydrous, shallow mantle (> 1250 degrees C at < 30 km depth) in the mantle wedge. Because the Sanbagawa metamorphic belt finally entrained these peridotites during progressive retrogression, B-type olivine fabrics probably developed in the fore-arc side of the subduction zone, above or along the subducting slab, possibly in association with dehydration fluids derived from the slab. The previously documented small magnitude of S-wave splitting can be explained by the seismic properties of B-type peridotites within an anisotropic layer of approximately several kilometers in thickness, oriented by flow parallel to the subducting slab, under maximum temperatures of 880-1030 degrees C depending on the flow stress. These findings indicate that such a B-type layer could constitute a dominant source of seismic S- and P-wave anisotropy in mantle wedge regions. (c) 2008 Elsevier B.V. All rights reserved.
ISSN: 0012-821X

hal-00412594
https://hal.archives-ouvertes.fr/hal-00412594
DOI : 10.1016/j.epsl.2008.06.014

Éditeur(s) : HAL CCSD
Résumé : International audience
Mylonite-bearing crustal shear zones are exhumed extensions at depth of (potentially seismogenic) brittle faultsat shallow structural level. By concentrating deformation, shear zones play a major role in the rheology and mechanicalbehavior of the continental crust. Analysis of shear zone geometry and microstructures is a useful toolto decipher deformation kinematics, though finite strain estimation is still relatively qualitative. Timing of thedeformation has also been the focus of numerous studies using the complete spectrum of geochronometers. However,only rare examples provide geochronological data that can be unambiguously linked with deformation. Thus,time-scales over which major mylonite zones develop and remain active under ductile conditions as well as theirstrain rates often remain poorly documented. Several sections across the South Armorican Shear Zone (SASZ),a crustal-scale, several km-thick dextral shear zone were investigated by structural, petrological and Ar/Ar radiochronologicalmethods. Finite strain profiles on these sections showed a strong deformation partitioning towardsthe ultramylonite core of the SASZ, with a decrease, of both the C’/S mean angle (from ca. 45 to less than5) and the Quartz grain-size (from ca. 150 to 5 m). In parallel, conventional and in-situ Ar/Ar datings on thefabric-forming white micas were performed on compositionally complex white-micas. Inherited magmatic muscovitescarried by the foliation and newly-formed, syn-tectonic substituted phengites located along the shear bandsyielded 2 age groups for the less deformed samples. Along the shear zone cross-section, the difference betweenthese two ages evolves from 10-15 Ma in weakly deformed domains to zero toward the SASZ core. In addition, thevery fine-grained syn-tectonic phengites yielded the same ca. 300-298 Ma age irrespectively of their distance to thecore of the shear zone, even in slightly deformed rocks distant by several km. These results support a new modelof crustal-scale shear zone activity, where the whole body of the shear zone is active throughout its history, i.e.the actively deforming zone does not get progressively narrower. The strain distribution nevertheless evolves withtime, with an increasingly larger fraction partitioned in the ultramylonitic core. Besides, combination of the finitestrain estimations and the Ar/Ar ages enables us to assess the range of strain rates associated with mylonitization.
EGU General Assembly 2014
Vienne, Austria

insu-01565057
https://hal-insu.archives-ouvertes.fr/insu-01565057

Éditeur(s) : Soc Geol France
Résumé : The Lesser Antilles subduction zone has produced no recent strong thrust earthquakes, making it difficult to quantify the seismic hazard from such events. The Lesser Antilles arc has a low subduction rate and an accretionary wedge that is very wide at its southern end. To investigate the effect of the wedge on seismogenesis, numerical models of forearc thermal structure were constructed along six transects perpendicular to the arc in order to determine the thermally predicted width of the seismogenic zone. The geometry of each section is constrained by published seismic profiles and crustal models derived from gravity and seismic data and by earthquake hypocenters at depth. A major constraint on the deep part of the model is that mantle temperature beneath the volcanic arc should achieve a temperature of 1,100 degrees C to generate partial melts. Predicted surface heat flow is compared to the available heat flow observations. Thermal modeling results indicate a systematic southward increase in the width of the seismogenic zone, more than doubling in width from north to south and corresponding to a dramatic southward increase in forearc width (distance from the arc to the deformation front of the accretionary wedge). The minimum width of the seismogenic zone (distance between the intersections of the subduction interface with the 150 degrees C and 350 degrees C isotherms) increases from about 80 km, north of 16 degrees N, to 230 km, at 13 degrees N. The maximum width (between the 100 degrees C and 450 degrees C isotherms) ranges from about 150 km in the north to up to 320 km in the south. This large variation in the width of the seismogenic zone is a consequence of the increasing width of the accretionary wedge to the south, caused by the increased thickness of sediment on the subducting plate. There is good agreement between the thermally predicted seismogenic limits and the sparse distribution of recorded thrust earthquakes, which are observed only in the northern portion of the arc. Possible scenarios for mega-thrust earthquakes are discussed. Depending on the segment length (along-strike) of the rupture plane, the occurrence of an event of magnitude 8-9 cannot be excluded.
L’absence de grands séismes récents à mécanismes chevauchants dans la zone de subduction des Petites Antilles rend difficile l’évaluation de l’aléa sismique lié à de tels événements. L’arc des Petites Antilles est caractérisé par une faible vitesse de subduction et par la présence d’un prisme d’accrétion très développé à son extrémité méridionale. Afin d’évaluer les effets de la largeur de ce prisme sur la genèse des séismes, nous avons étudié six sections perpendiculaires à l’arc, du nord au sud de celui-ci, pour déterminer la largeur de la zone sismogène prédite par les modèles thermiques appliqués à chacune de ces coupes. La géométrie de ces dernières est contrainte par les profils sismiques publiés, par les modèles de structure crustale déduits des données gravitaires et sismiques, et enfin par la distribution des hypocentres des séismes. Un contrôle important permettant de tester la validité des modèles thermiques en profondeur est qu’une température minimale de 1 100oC, compatible avec la fusion partielle du manteau hydraté, doit être atteinte sous l’arc volcanique actif. Par ailleurs, le flux thermique en surface prédit par ces modèles doit être compatible avec les mesures de flux de chaleur. Les modèles thermiques retenus d’après ces critères montrent une augmentation du simple au double vers le sud de la largeur de la zone sismogène, qui correspond à un élargissement considérable de la taille du domaine avant-arc. En effet, la largeur minimale de la zone sismogène (définie comme la distance entre les intersections de l’interface des plaques avec les isothermes 150o et 350oC) augmente d’environ 80 km au nord de 16oN jusqu’à 230 km à 13oN. La largeur maximale de cette zone (définie par les intersections de l’interface avec les isothermes 100o et 450oC) augmente, quant à elle, d’environ 150 km au nord jusqu’à 320 km au sud de l’arc. Cette variation considérable est la conséquence de l’augmentation de la largeur du prisme d’accrétion, elle-même causée par l’accumulation croissante des sédiments déposés sur la plaque plongeante. Les largeurs de la zone sismogène prédites à l’aide des modèles thermiques sont en bon accord avec les rares données disponibles sur les séismes à mécanismes chevauchants dans la partie nord de l’arc. Les scénarios possibles relatifs à des méga-séismes de ce type n’excluent pas de futurs événements atteignant des magnitudes de 8 à 9.
Bulletin De La Societe Geologique De France (0037-9409) (Soc Geol France), 2013 , Vol. 184 , N. 1-2 , P. 47-59

Droits : 2013 Societe Geologique de France
http://archimer.ifremer.fr/doc/00140/25140/29432.pdf
DOI:10.2113/gssgfbull.184.1-2.47
http://archimer.ifremer.fr/doc/00140/25140/

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
The brittle to ductile transition (BDT) in rocks may strongly influence their transport properties (i.e., permeability, porosity topology ...) and the maximum depth and temperature where hydrothermal fluids may circulate. To examine this transition in the context of Icelandic crust, we conducted deformation experiments on a glassy basalt (GB) and a glass-free basalt (GFB) under oceanic crust conditions. Mechanical and micro-structural observations at a constant strain rate of 10(-5) s(-1) and at confining pressure of 100-300 MPa indicate that the rocks are brittle and dilatant up to 700-800 degrees C. At higher temperatures and effective pressures the deformation mode becomes macroscopically ductile, i.e., deformation is distributed throughout the sample and no localized shear rupture plane develops. The presence of glass is a key component reducing the sample strength and lowering the pressure of the BDT. In the brittle field, strength is consistent with a Mohr-Coulomb failure criterion with an internal coefficient of friction of 0.42 for both samples. In the ductile field, strength is strain rate-and temperature-dependent and both samples were characterized by the same stress exponent in the range 3 < n < 4.2 but by very different activation energy Q(GB) = 59 +/- 15 KJ/mol and Q(GFB) = 456 +/- 4 KJ/mol. Extrapolation of these results to the Iceland oceanic crust conditions predicts a BDT at similar to 100 degrees C for a glassy basalt, whereas the BDT might occur in non-glassy basalts at deeper conditions, i.e., temperatures higher than 550 +/- 100 degrees C, in agreement with the Icelandic seismogenic zone.
ISSN: 0148-0227

hal-00760905
https://hal.archives-ouvertes.fr/hal-00760905
DOI : 10.1029/2011JB008884

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In Western Mediterranean, the Rif belt in Morocco is part of the Gibraltar Arc built during the Tertiary in the framework of Eurasia-Africa convergence. The structural and metamorphic evolution of the internal units of this belt as well as their timing, crucial to constrain the geodynamic evolution of the Alboran Sea, is still largely debated.Our study on the Ceuta Peninsula (Northern Rif) provides new structural, petrological and geochronological data (U-Th-Pb, Ar-Ar), which allow to precise the tectono-metamorphic evolution of the Lower Sebtides metamorphic units with:(1) a syn-metamorphic thrusting event developed under granulite facies conditions (7–10 kbar and 780–820 °C). A major thrust zone, the Ceuta Shear Zone, drove the emplacement of metapelites and peridotitic lenses from the Ceuta Upper Unit over the orthogneisses of the Monte Hacho Lower Unit. This compressional event ended during the Upper Oligocene.(2) an extensional event developed at the boundary between amphibolite and greenschist facies conditions (400–550 °C and 1–3 kbar). During this event, the Ceuta Shear Zone has been reactivated as a normal fault. Normal ductile shear zones contributed to the final exhumation of the metamorphic units during the Early Miocene.We propose that the compressional event is related to the formation of an orogenic wedge located in the upper plate, in a backward position, of the subduction zone driving the geodynamic evolution of the Alboran domain. In this context, the episode of lithospheric thinning could be related to the opening of the Alboran basin in a back-arc position.Furthermore, unlike the previous models proposed for the Rif belt, the tectonic coupling between mantle peridotites and crustal metamorphic rocks occurred in Ceuta Peninsula at a depth of 20–30 km under high temperature conditions, before the extensional event, and thus cannot be related to the back-arc extension.
ISSN: 0040-1951

insu-01682810
https://hal-insu.archives-ouvertes.fr/insu-01682810
DOI : 10.1016/j.tecto.2017.11.028

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Phase equilibrium data pertaining to melting of simplified carbonated peridotite in the systems CaO-MgO-SiO2-CO2 and CaO-MgO-Al2O3-SiO2-CO2 at pressures of 10-26 GPa, corresponding to similar to 300-750 km depths in the Earth, are presented. In both the studied systems, liquid compositions, with changing crystalline phase assemblage, are carbonatitic throughout the studied pressure range. In the system CMS-CO2, melting phase relations are isobarically invariant; liquid is in equilibrium with forsterite + clinoenstatite + clinopyroxene + magnesite, forsterite + majorite + clinopyroxene + magnesite, wadsleyite + majorite + clinopyroxene + magnesite, ringwoodite + majorite + calcium-silicate perovskite + magnesite, magnesium-silicate perovskite + periclase + calcium-silicate perovskite + magnesite at 12, 14, 16, 20, and 26 GPa, respectively. In the system CMAS-CO2, a phase assemblage consisting of forsterite + orthopyroxene + clinopyroxene + magnesite + garnet + melt from 10 to 14 GPa is isobarically invariant. However, owing to the disappearance of orthopyroxene at pressures greater than 14 GPa, from 16 and up to at least 26 GPa, the solidus of simplified carbonated peridotite spans a divariant surface in pressure-temperature space. The liquid coexists with wadsleyite + clinopyroxene + garnet + magnesite, ringwoodite + calcium-silicate perovskite + garnet + magnesite, and magnesium-silicate perovskite + periclase + calcium-silicate perovskite + magnesite at 16, 20, and 26 GPa, respectively. A curious, and as yet unexplained, feature of our study is an abrupt drop in the solidus temperature between 14 and 16 GPa that causes a small amount of melting of carbonated mantle in the Transition Zone of the Earth. In the systems CMS-CO2 and CMAS-CO2 liquid compositions at 16 and 20 GPa are highly calcic bona fide carbonatites; however, these liquids revert to being magnesiocarbonatites at 10-14 and 26 GPa. In the system CMS-CO2, at 16 GPa we locate an isobaric invariant point consisting of wadsleyite + clinopyroxene + anhydrous B + magnesite + melt. The presence of anhydrous B at 16 GPa and 1475 degrees C is interesting, as it lies outside the composition space of the mantle peridotite analog we have studied. However, despite the presence of two highly magnesian silicate crystalline phases, wadsleyite and anhydrous B, at 16 GPa and 1475 degrees C, the liquid composition remains calcic with molar Ca-number [Ca/(Ca + Mg) x 100] of about 63. The melting reactions at 16 and 20 GPa (with or without anhydrous B) show that lime-bearing crystalline silicates play a fairly large part in generating and controlling the composition of the liquids. At 16 GPa, in the system CMS-CO2, we also report an experimental run at 1575 degrees C, in which liquid coexists with only wadsleyite and majorite. The liquid composition is less calcic (Ca-number 54) than that for other runs at lower temperatures, but is still more calcic than liquids at 10-14 and 26 GPa in both the studied systems. At present, the likely cause for these changes in the reported phase relations is not known. For normally assumed mantle temperatures, melting in the Transition Zone of the Earth, owing to the presence of carbonate, is probably unavoidable. The depth range of the drop in the carbonated peridotite solidus closely matches that of commonly observed low seismic velocities at similar to 400-600 km depth in the Earth.
ISSN: 0022-3530

hal-00669945
https://hal.archives-ouvertes.fr/hal-00669945
DOI : 10.1093/petrology/egr048

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Rifting in a cratonic lithosphere is strongly controlled by several interacting processes including crust/mantle rheology, magmatism, inherited structure and stress regime. In order to better understand how these physical parameters interact, a 2 yr long seismological experiment has been carried out in the North Tanzanian Divergence (NTD), at the southern tip of the eastern magmatic branch of the East African rift, where the southward-propagating continental rift is at its earliest stage. We analyse teleseismic data from 38 broad-band stations ca. 25 km spaced and present here results from their receiver function (RF) analysis. The crustal thickness and Vp/Vs ratio are retrieved over a ca. 200 × 200 km2 area encompassing the South Kenya magmatic rift, the NTD and the Ngorongoro-Kilimanjaro transverse volcanic chain. Cratonic nature of the lithosphere is clearly evinced through thick (up to ca. 40 km) homogeneous crust beneath the rift shoulders. Where rifting is present, Moho rises up to 27 km depth and the crust is strongly layered with clear velocity contrasts in the RF signal. The Vp/Vs ratio reaches its highest values (ca. 1.9) beneath volcanic edifices location and thinner crust, advocating for melting within the crust. We also clearly identify two major low-velocity zones (LVZs) within the NTD, one in the lower crust and the second in the upper part of the mantle. The first one starts at 15–18 km depth and correlates well with recent tomographic models. This LVZ does not always coexist with high Vp/Vs ratio, pleading for a supplementary source of velocity decrease, such as temperature or composition. At a greater depth of ca. 60 km, a mid-lithospheric discontinuity roughly mimics the step-like and symmetrically outward-dipping geometry of the Moho but with a more slanting direction (NE–SW) compared to the NS rift. By comparison with synthetic RF, we estimate the associated velocity reduction to be 8–9 per cent. We relate this interface to melt ponding, possibly favouring here deformation process such as grain-boundary sliding (EAGBS) due to lithospheric strain. Its geometry might have been controlled by inherited lithospheric fabrics and heterogeneous upper mantle structure. We evidence that crustal and mantle magmatic processes represent first order mechanisms to ease and locate the deformation during the first stage of a cratonic lithospheric breakup.
ISSN: 0956-540X

hal-01622078
https://hal.archives-ouvertes.fr/hal-01622078
https://hal.archives-ouvertes.fr/hal-01622078/document
https://hal.archives-ouvertes.fr/hal-01622078/file/plasmanGJI2017.pdf
DOI : 10.1093/gji/ggx177

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Cycladic Blueschist Unit (CBU) represents the northern passive margin of the Adria continental block and ophiolites that are the remnants of the Pindos Ocean, which were affected by high pressure-low temperature metamorphism in the blueschist and eclogite facies during the Eocene. Prior to the Lutetian, the ophiolitic and margin units were thrust towards the SW inside a NE-dipping subduction zone. Two subsequent exhumation stages are characterized by top-to-the-NE senses of shear: i) from mantle depths to lower crustal levels, before 37 Ma, which was accommodated by an extensional inversion of the Vardar suture zone, and ii) from deep crustal levels to the near-surface, between 30 and 20 Ma, which was accommodated by the North Cycladic Detachment. It is during this late stage of ductile exhumation that the central Cyclades Core Complex was exhumed. Segmentation of the CBU by normal faults started at 12 Ma, as recorded by the low temperature thermochronometers, and was coeval with block rotation and folding with NS-trending axes, which were predominantly controlled by the Myrthes-Ikaria strike-slip fault in the centre of the Cyclades. A map restoration of the CBU geometry, prior to segmentation, has been carried out using available structural and paleomagnetic data. The restoration shows that, following the CBU exhumation from mantle depths to crustal levels, a single core complex was exhumed along a single NE dipping detachment.
ISSN: 0040-1951

insu-00672320
https://hal-insu.archives-ouvertes.fr/insu-00672320
DOI : 10.1016/j.tecto.2011.12.025

Éditeur(s) : American Geophysical Union
Résumé : We present four companion digital models of the age, age uncertainty, spreading rates, and spreading asymmetries of the world's ocean basins as geographic and Mercator grids with 2 arc min resolution. The grids include data from all the major ocean basins as well as detailed reconstructions of back-arc basins. The age, spreading rate, and asymmetry at each grid node are determined by linear interpolation between adjacent seafloor isochrons in the direction of spreading. Ages for ocean floor between the oldest identified magnetic anomalies and continental crust are interpolated by geological estimates of the ages of passive continental margin segments. The age uncertainties for grid cells coinciding with marine magnetic anomaly identifications, observed or rotated to their conjugate ridge flanks, are based on the difference between gridded age and observed age. The uncertainties are also a function of the distance of a given grid cell to the nearest age observation and the proximity to fracture zones or other age discontinuities. Asymmetries in crustal accretion appear to be frequently related to asthenospheric flow from mantle plumes to spreading ridges, resulting in ridge jumps toward hot spots. We also use the new age grid to compute global residual basement depth grids from the difference between observed oceanic basement depth and predicted depth using three alternative age-depth relationships. The new set of grids helps to investigate prominent negative depth anomalies, which may be alternatively related to subducted slab material descending in the mantle or to asthenospheric flow. A combination of our digital grids and the associated relative and absolute plate motion model with seismic tomography and mantle convection model outputs represents a valuable set of tools to investigate geodynamic problems.
Geochemistry Geophysics Geosystems - G3 (1525-2027) (American Geophysical Union), 2008-04 , Vol. 9 , P. NIL_18-NIL_36

Droits : 2008 American Geophysical Union
http://archimer.ifremer.fr/doc/2008/publication-3900.pdf
DOI:10.1029/2007GC001743
http://archimer.ifremer.fr/doc/00000/3900/

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
1] Recent statistical analysis by Lallemand et al. (2008) of subduction zone parameters revealed that the back-arc deformation mode depends on the combination between the subducting (nu(sub)) and upper (nu(up)) plate velocities. No significant strain is recorded in the arc area if plate kinematics verifies nu(up) = 0.5 vsub - 2.3 (cm/a) in the HS3 reference frame. Arc spreading ( shortening) occurs if nu(up) is greater ( lower) than the preceding relationship. We test this statistical law with numerical models of subduction, by applying constant plate velocities far away from the subduction zone. The subducting lithosphere is free to deform at all depths. We quantify the force applied on the two converging plates to sustain constant surface velocities. The simulated rheology combined viscous (non-Newtonian) and brittle behaviors, and depends on water content. The influence of subduction rate vs is first studied for a fixed upper plate. After 950 km of convergence ( steady state slab pull), the transition from extensional to compressive stresses in the upper plate occurs for vs similar to 1.4 cm/a. The effect of upper plate velocity is then tested at constant subduction rate. Upper plate retreat ( advance) with respect to the trench increases extension ( compression) in the arc lithosphere and increases ( decreases) the subducting plate dip. Our modeling confirms the statistical kinematic relationship between vsub and nu(up) that describes the transition from extensional to compressive stresses in the arc lithosphere, even if the modeled law is shifted toward higher rates of upper plate retreat, using our set of physical parameters ( e. g., 100 km thick subducting oceanic plate) and short- term simulations. Our results make valid the choice of the HS3 reference frame for assessing plate velocity influence on arc tectonic regime. The subduction model suggests that friction along the interplate contact and the mantle Stokes reaction could be the two main forces competing against slab pull for upper mantle subductions. Besides, our simulations show that the arc deformation mode is strongly time dependent.
ISSN: 1525-2027

hal-00412328
https://hal.archives-ouvertes.fr/hal-00412328
https://hal.archives-ouvertes.fr/hal-00412328/document
https://hal.archives-ouvertes.fr/hal-00412328/file/arcayG32008.pdf
DOI : 10.1029/2007GC001875