Hydrothermal activity along the slow-spreading Lucky Strike ridge segment (Mid-Atlantic Ridge) : distribution, heatflux, and geological controls

Author Posting. © The Author(s), 2015. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution. The definitive version was published in Earth and Planetary Science Letters 431 (2015): 1730185, doi:10.1016/j.epsl.2015.09.025.We have reviewed available visual information from the seafloor, and recently acquired microbathymetry for several traverses across the Lucky Strike segment, to evaluate the distribution of hydrothermal activity. We have identified a new on-axis site with diffuse flow, Ewan, and anactive vent structure ~1.2 km from the axis, Capelinhos. These sites are minor relative to the Main field, and our total heatflux estimate for all active sites (200-1200 MW) is only slightly higher than previously published estimates. We also identify fossil sites W of the main Lucky Strike field. A circular feature ~200 m in diameter located on the flanks of a rifted off-axis central volcano, is likely a large and inactive hydrothermal edifice, named Grunnus. We find no indicator of focused hydrothermal activity elsewhere along the segment, suggesting that the enhanced melt supply and the associated melt lenses, required to form central volcanoes, also sustain hydrothermal circulation to form and maintain large and long-lived hydrothermal fields. Hydrothermal discharge to the seafloor occurs along fault traces, suggesting focusing of hydrothermal circulation in the shallow crust along permeable fault zones.This work has been partly financed by ANR (France) Mothseim Project NT05-3 42213 toJE, and by EU-RTN-MOMARNET to MC. The French Ministry of Research financed ship, ROV and AUV time (Graviluck’06, MOMAR’08, Bathyluck’09, MOMARSAT cruises in 2010-2015

Interseismic strain build-up on the submarine North Anatolian Fault offshore Istanbul

Using offshore geodetic observations, we show that a segment of the North Anatolian Fault in the central Sea of Marmara is locked and therefore accumulating strain. The strain accumulation along this fault segment was previously extrapolated from onshore observations or inferred from the absence of seismicity, but both methods could not distinguish between fully locked or fully creeping fault behavior. A network of acoustic transponders measured crustal deformation with mm-precision on the seafloor for 2.5 years and did not detect any significant fault displacement. Absence of deformation together with sparse seismicity monitored by ocean bottom seismometers indicates complete fault locking to at least 3 km depth and presumably into the crystalline basement. The slip-deficit of at least 4m since the last known rupture in 1766 is equivalent to an earthquake of magnitude 7.1 to 7.4 in the Sea of Marmara offshore metropolitan Istanbul

A Multi-Observation Least-Squares Inversion for GNSS-Acoustic Seafloor Positioning

Monitoring deformation on the seafloor is a major challenge for modern geodesy and a key to better understanding tectonic processes and assess related hazards. The extension of the geodetic networks offshore can be achieved by combining satellite positioning (GNSS) of a surface platform with acoustic ranging to seafloor transponders. This approach is called GNSS-Acoustic (GNSS-A). The scope of this work is to provide a tool to identify and quantify key points in the error budget of such experiment. For this purpose, we present a least-squares inversion method to determine the absolute position of a seafloor transponder array. Assuming the surface platform is accurately positioned by GNSS, the main observables are the two-way travel time in water between the transponders on the seafloor and the surface platform acoustic head. To better constrain transponder positions, we also consider the baseline lengths and the relative depth-differences between different pairs of them. We illustrate the usefulness of our forward modeling approach and least-square inversion by simulating different experimental protocols (i.e., platform trajectories, with or without information on the distance and depth between transponders). We find that the overall accuracy of a GNSS-A experiment is significantly improved with additional information about the relative depths of the instruments. Baseline lengths also improve the accuracy, but only when combined with depth differences. The codes in Python3 used in this article are freely available online

The sources of deep ocean infragravity waves observed in the North Atlantic Ocean

International audienceInfragravity waves are long-period (25-250 s) ocean surface gravity waves generated in coastal zones through wave-wave interactions or oscillation of the breaking point. Most of the infragravity wave energy is trapped or dissipated near coastlines, but a small percentage escapes into the open oceans. The source of deep ocean infragravity waves is debated, specifically whether they come mostly from regions with strong source waves or from sites with particular morphologies/orientations. We correlate measurements of infragravity waves in the deep North Atlantic Ocean with infragravity wave generation parameters throughout the Atlantic Ocean to find the dominant sources of deep ocean infragravity wave energy in the North Atlantic Ocean. The deep ocean infragravity wave data are from a 5 year deployment of absolute pressure gauges west of the Azores islands (37°N, 35°W) and shorter data sets from seafloor tsunami gauges (DART buoys). Two main sources are identified: one off of the west coast of southern Europe and northern Africa (25°N-40°N) in northern hemisphere winter and the other off the west coast of equatorial Africa (the Gulf of Guinea) in southern hemisphere winter. These regions have relatively weak source waves and weak infragravity wave propagation paths to the main measurement site, indicating that that the site morphology/orientation dominates the creation of deep ocean infragravity waves. Both regions have also been identified as potential sources of global seismological noise, suggesting that the same mechanisms may be behind the generation of deep ocean infragravity waves and global seismological noise in the frequency band from 0.001 to 0.04 Hz. © 2015. American Geophysical Union. All Rights Reserved

Oceanographic Signatures and Pressure Monitoring of Seafloor Vertical Deformation in Near-coastal, Shallow Water Areas: A Case Study from Santorini Caldera

International audienceBottom pressure, tilt, and seawater physical properties were monitored for a year using two instruments within the immerged Santorini caldera (Greece). Piggybacked on the CALDERA2012 cruise, this geodetic experiment was designed to monitor evolution of the 2011–2012 Santorini unrest. Conducted during a quiescent period, it allowed us to study oceanographic and atmospheric signal in our data series. We observe periodic oceanographic signals associated with tides and seiches that are likely linked to both the caldera and Cretan Basin geometries. In winter, the caldera witnesses sudden cooling events that tilt an instrument towards the Southeast, indicating cold water influx likely originating from a passage into the caldera between Thirasia island and the northern end of Thera island to the northwest. We did not obtain evidence of long-term vertical seafloor deformation from the pressure signal, although it may be masked by instrumental drift. However, tilt data suggest a local seafloor tilt event »1/year after the end of the unrest period, which could be consistent with inflation under or near Nea Kameni. Seafloor geodetic data recorded at the bottom of the Santorini caldera illustrate that the oceanographic signature is an important part of the signal, which needs to be considered for monitoring volcanic or geological seafloor deformation in shallow water and/or nearshore areas

Shoreline changes in a rising sea level context: the example of Grande Glorieuse, Eparses Islands, Western Indian Ocean

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Three Approaches to Interseismic Slip Rates on the Main Marmara Fault and Their Tensorial Correlations with the Kostrov-Based Strain Rates

The interseismic slip distribution in the Marmara fault system represents both observational and modelling challenges. The observational challenge is obvious: the faults are under water and to understand their interseismic behavior (creeping versus locked) requires expensive and logistically difficult underwater geodetic measurements, alongside those on land. Up to now, two such underwater studies have been conducted and they suggest that the segment to the south of Istanbul zone (so-called Central segment) is locked while some creep is probably going on along the neighboring segment to the west. Given these two important findings, the slip distribution problem is still non-trivial due to the fact that our experiments so far demonstrate that the block-based slip inversions and those that only consider a single fault (with the same geometry as one of the boundaries of the blocks) give significantly different results. In this study we approach the problem using three methodologies: block models with spatially non-varying strains within individual blocks, a boundary element approach and a continuum kinematic approach. Although the block model does not give spatially varying strains, the inversion results from the block model can be used as an input to model strain field in the vicinity of the fault. We construct a formulation to correlate the results from these with the strain rates obtained using focal mechanism summations

Activité magmatique, tectonique et hydrothermale actuelle sur la Dorsale Est Pacifique entre17°-et 19°S(campagne NAUDUR)

The submersible-study of four segments of the East Pacific Rise between 17 degrees and 18 degrees 40'S, where the spreading rate is greater than 150 mm/a, allows us to confirm the variability of the accretion processes. The segment between 17 degrees and 17 degrees 30'S shows an intense present-day magmatic activity. The one between 18 degrees 10' and 18 degrees 22'S shows a mainly tectonic activity. The segment between 18 degrees 22' and 18 degrees 37'S is an intermediate stage with an axial graben in which fresh lavas were emplaced by an extremely young eruption. On all the segments the hydrothermal activity exists ranging from shimmering water up to black smokers, It is associated with different stages of animal colonization.L'étude par le submersible Nautile de quatre segments de la Dorsale Est Pacifique entre 17° et 18°40'S où le taux d'ouverture est de plus de 150 mm/an, permet de confirmer la variabilité des processus de l'accrétion dans le temps et dans l'espace. Le segment compris entre 17° et 17°30'S montre une intense activité magmatique actuelle ou sub-actuelle, celui compris entre 18°10' et 18°22'S montre une activité essentiellement tectonique et celui compris entre 18°22' et 18°37'S une combinaison des deux, le graben d'extension étant partiellement rempli de laves émises lors d'une éruption extrêmement récente. Partout existe une activité hydrothermale, allant de l'émission de fluides moirés à des fumeurs noirs, associée à divers stades de colonisation animale