Modelling the thermal evolution of slow-spreading ridge segments and their off-axis geophysical signature

International audienceSystematic studies conducted between 15°N and 40°N over ridge segments along the slow-spreading Mid-Atlantic Ridge (MAR) have shown that segment characteristics are related to the thermal state of the segments and gradually vary with their length. This paper presents further developments of a 3-D model, based on the presence of a hot zone located under the segment centres (Gac et al. 2003), to (1) quantify the thermal structures and the geophysical signatures of segments of various lengths, considered as representative of the various MAR segments; (2) test if a simple and single model of thermal evolution can account for the characteristics of all segments and (3) explain the past evolution of the segmentation, as is observed off-axis along the MAR. The modelled thermal structure and three simulated geophysical outputs [crustal structure , along-axis variations of the earthquake maximum depth and the mantle Bouguer gravity anomalies (MBA)] are found to be directly controlled by the shape (geometry and dimensions) of the hot zone. A consistent fit between model outputs and along-axis variations of the geophysical observables over the various segments is obtained by varying solely the length of the hot zone. This result shows that segments of different length may in fact constitute the different stages of a single evolution process: the axial geophysical characteristics of the segment would progressively evolve from those of shorter segments to those of longer ones, as the hot zone lengthens along-axis. A subsequent shortening of the segment would result from a simultaneous shortening of the hot zone, segment characteristics reverting back from those of longer segments to those of shorter ones. Three geophysical fields (topography, gravity and magnetic anomalies) are simulated as the results of the thermal evolution of aligned and offset segments the length of which evolves through time. These simulations succeed in fitting observations for the entire range of observed axis offsets between adjacent MAR segments. The segment evolution produces peculiar off-axis isostatic topography and gravity anomaly (MBA), the rhomb-shaped patterns. Our simulations, which model adjacent offset segments having evolved through several cycles of lengthening and shortening, yield a good fit to the isostatic topography and MBA patterns observed in the off-axis region. Finally, the distribution of magnetization depends on the magnetic properties of each type of rocks and on the petrological structure of the lithosphere, which, in turn, results from its thermal structure and evolution. Modelled magnetic anomalies are shown to be in good agreement with off-axis observations along the N21°40' segment (TAMMAR) of the MAR

Evolution of the accretion processes along the Mid-Atlantic Ridge north of the Azores since 5.5 Ma: An insight into the interactions between the ridge and the plume

International audienceHigh-resolution bathymetry and gravity data north of the Azores Plateau show that this part of the North Mid-Atlantic Ridge is presently undergoing a phase of weak crustal production and magmatism. Most of the ridge segments are small and short-lived, suggesting a disrupted and highly variable accretion regime since anomaly 3A. The influence of the nearby plume appears to be relatively minor and corresponds more to a weak thermal signal than to any major input of plume material and increased crustal production at the axis. A period of increased magmatism was identified at the southern limit of the study area (near 40°N) around anomaly 5. This magmatic "pulse" caused the emplacement of a topographic high, probably underlain by a thickened crust. This pulse probably marks the northernmost and last significant arrival of material from the Azores plume to the MAR axi

Axial magnetic anomalies over slow-spreading ridge segments: insights from numerical 3-D thermal and physical modelling

International audienceThe axial magnetic anomaly amplitude along Mid-Atlantic Ridge segments is systematically twice as high at segment ends compared with segment centres. Various processes have been proposed to account for such observations, either directly or indirectly related to the thermal structure of the segments: (1) shallower Curie isotherm at segment centres, (2) higher Fe-Ti content at segment ends, (3) serpentinized peridotites at segment ends or (4) a combination of these processes. In this paper the contribution of each of these processes to the axial magnetic anomaly amplitude is quantitatively evaluated by achieving a 3-D numerical modelling of the magnetization distribution and a magnetic anomaly over a medium-sized, 50 km long segment. The magnetization distribution depends on the thermal structure and thermal evolution of the lithosphere. The thermal structure is calculated considering the presence of a permanent hot zone beneath the segment centre. The 'best-fitting' thermal structure is determined by adjusting the parameters (shape, size, depth, etc.) of this hot zone, to fit the modelled geophysical outputs (Mantle Bouguer anomaly, maximum earthquake depths and crustal thickness) to the observations. Both the thermoremanent magnetization, acquired during the thermal evolution, and the induced magnetization, which depends on the present thermal structure, are modelled. The resulting magnetic anomalies are then computed and compared with the observed ones. This modelling exercise suggests that, in the case of aligned and slightly offset segments, a combination of higher Fe-Ti content and the presence of serpentinized peridotites at segment ends will produce the observed higher axial magnetic anomaly amplitudes over the segment ends. In the case of greater offsets, the presence of serpentinized peridotites at segment ends is sufficient to account for the observations

Sounds from airguns and fin whales recorded in the mid-Atlantic Ocean, 1999-2009

Between 1999 and 2009, autonomous hydrophones were deployed to monitor seismic activity from 16° N to 50° N along the Mid-Atlantic Ridge. These data were examined for airgun sounds produced during offshore surveys for oil and gas deposits, as well as the 20 Hz pulse sounds from fin whales, which may be masked by airgun noise. An automatic detection algorithm was used to identify airgun sound patterns, and fin whale calling levels were summarized via long-term spectral analysis. Both airgun and fin whale sounds were recorded at all sites. Fin whale calling rates were higher at sites north of 32° N, increased during the late summer and fall months at all sites, and peaked during the winter months, a time when airgun noise was often prevalent. Seismic survey vessels were acoustically located off the coasts of three major areas: Newfoundland, northeast Brazil, and Senegal and Mauritania in West Africa. In some cases, airgun sounds were recorded almost 4000 km from the survey vessel in areas that are likely occupied by fin whales, and at some locations airgun sounds were recorded more than 80% days/month for more than 12 consecutive months

Long-term seismicity of the Reykjanes Ridge (North Atlantic) recorded by a regional hydrophone array

The seismicity of the northern Mid-Atlantic Ridge was recorded by two hydrophone networks moored in the sound fixing and ranging (SOFAR) channel, on the flanks of the Mid-Atlantic Ridge, north and south of the Azores. During its period of operation (05/2002-09/2003), the northern 'SIRENA' network, deployed between latitudes 40 degrees 20'N and 50 degrees 30'N, recorded acoustic signals generated by 809 earthquakes on the hotspot-influenced Reykjanes Ridge. This activity was distributed between five spatio-temporal event clusters, each initiated by a moderate-to-large magnitude (4.0-5.6 M) earthquake. The rate of earthquake occurrence within the initial portion of the largest sequence (which began on 2002 October 6) is described adequately by a modified Omori law aftershock model. Although this is consistent with triggering by tectonic processes, none of the Reykjanes Ridge sequences are dominated by a single large-magnitude earthquake, and they appear to be of relatively short duration (0.35-4.5 d) when compared to previously described mid-ocean ridge aftershock sequences. The occurrence of several near-equal magnitude events distributed throughout each sequence is inconsistent with the simple relaxation of main shock-induced stresses and may reflect the involvement of magmatic or fluid processes along this deep (>2000 m) section of the Reykjanes Ridge.info:eu-repo/semantics/publishedVersio

Ankyrin G restricts ion channel diffusion at the axonal initial segment before the establishment of the diffusion barrier

Ion channel immobilization by ankyrin G is regulated by casein kinase 2 in immature hippocampal neurons

Postnatal deamidation of 4E-BP2 in brain enhances its association with raptor and alters kinetics of excitatory synaptic transmission

The eIF4E-binding proteins (4E-BPs) repress translation initiation by preventing eIF4F complex formation. Of the three mammalian 4E-BPs, only 4E-BP2 is enriched in the mammalian brain and plays an important role in synaptic plasticity and learning and memory formation. Here we describe asparagine deamidation as brain-specific posttranslational modification of 4E-BP2. Deamidation is the spontaneous conversion of asparagines to aspartates. Two deamidation sites were mapped to an asparagine-rich sequence unique to 4E-BP2. Deamidated 4E-BP2 exhibits increased binding to the mammalian Target of Rapamycin (mTOR)-binding protein raptor, which effects its reduced association with eIF4E. 4E-BP2 deamidation occurs during postnatal development, concomitant with the attenuation of the activity of the PI3K-Akt-mTOR signalling pathway. Expression of deamidated 4E-BP2 in 4E-BP2−/− neurons yielded mEPSCs exhibiting increased charge transfer with slower rise and decay kinetics, relative to the wild type form. 4E-BP2 deamidation may represent a compensatory mechanism for the developmental reduction of PI3K-Akt-mTOR signalling

Conversion sismo-acoustique au passage du fond océanique

Depuis plus de quinze ans, l enregistrement des signaux hydroacoustiques par des hydrophones dans le canal SOFAR (SOund Fixing And Ranging) a permis la détection et la localisation de nombreux séismes de faible magnitude dans l océan. Cependant, l interprétation de ces signaux hydroacoustiques appelés ondes T ne permet pas de fournir une information directe sur les magnitudes, les mécanismes aux foyers et les profondeurs focales des séismes en cause. Cette limitation vient en partie du fait que le mécanisme de conversion des ondes sismiques en ondes acoustiques au passage du tond océanique n est pas totalement compris à ce jour. Pour tenter de résoudre ce problème, nous avons développé un code mécanique 2D solide-fluide capable de modéliser le processus de conversion sismo-acoustique. En calculant de manière exacte la solution du champ de vitesses de l onde acoustique produite, notre modèle montre qu une source en double couple dans la croûte océanique produit des ondes T dans la colonne d eau avec des angles permettant de se propager dans le SOFAR. Notre modèle confirme également l importance relative des ondes P et S dans l amplitude de l onde T produite. Des développements sont en cours pour modéliser l effet des caractéristiques topographiques régionales ou locales du fond océanique sur la production des ondes T. A terme, les sorties de ce modèle, utilisées comme entrées d un code de propagation acoustique longue distancé, pourraient permettre de modéliser l onde T sur l ensemble de son trajet, de la source sismique à l hydrophone.For more than 15 yr, the recording of hydroacoustic signals with hydrophones moored in a minimum sound-velocity channel, called the SOFAR (Sound Fixing And Ranging) channel, has allowed for detection and localization of many small-magnitude earthquakes in oceanic areas. However, the interpretation of these hydroacoustic signals termed T-waves fails to provide direct information on the magnitudes, focal mechanisms, or focal depths of the causative earthquakes. These limitations result, in part, from an incomplete understanding of the physics of the conversion, across the seafloor interface, from seismic waves generated by subseafloor earthquakes to hydroacoustic T-waves. To try and overcome some of these limitations, we have developed a 2-D finite-element mechanical model of the conversion process. By computing an exact solution of the velocity field of the waterborne T-waves, our model shows that a double-couple source mechanism of a subseafloor earthquake generates T-waves, whose take-off angles are adequate to allow penetration into the SOFAR channel and efficient trapping by this waveguide. In addition, our model confirms the relative importance of P and S seismic waves in the amplitude of me produced T-wave. Further developments are in progress with a view to modelising the effects of regional or local topographic characteristics of seafloor on T-wave production. In the long term, the use of our code s outputs as inputs to a long range acoustic propagation code could allow T-wave modelling along its whole path, from seismic source to hydrophone.BREST-BU Droit-Sciences-Sports (290192103) / SudocPLOUZANE-Bibl.La Pérouse (290195209) / SudocRENNES-Géosciences (352382209) / SudocSudocFranceF