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Fluid chemistry and microbial community patterns in chimney habitats were investigated in two hydrothermal fields located at the Central Indian Ridge. Endmember hydrothermal fluid of the Solitaire field, located ~3 km away from the spreading center, was characterized by moderately high temperature (307°C), Cl depletion (489 mm), mildly acidic pH (≥4.40), and low metal concentrations (Fe ≤ 105 μm and Mn = 78 μm). Chloride depletion indicates that the subseafloor source fluid had undergone phase separation at temperatures higher than ~390°C while the metal depletion was likely attributable to fluid alteration occurring at a venting temperature of around 307°C. These different temperature conditions suggested from fluid chemistry might be associated with an off-spreading center location of the field that allows subseafloor fluid cooling prior to seafloor discharge. The microbial community in the chimney habitat seemed comparable to previously known patterns in typical basalt-hosted hydrothermal systems. Endmember hydrothermal fluid of the Dodo field, standing on center of the spreading axis, was characterized by high H2 concentration of 2.7 mm. The H2 enrichment was likely attributable to fresh basalt–fluid interaction, as suggested by the nondeformed sheet lava flow expansion around the vents. Thermodynamic calculation of the reducing pyrite–pyrrhotite–magnetite (PPM) redox buffer indeed reproduced the H2 enrichment. The quantitative cultivation test revealed that the microbial community associated with the hydrothermal fluid hosted abundant populations of (hyper)thermophilic hydrogenotrophic chemolithoautotrophs such as methanogens. The function of subseafloor hydrogenotrophic methanogenic populations dwelling around the H2-enriched hydrothermal fluid flows was also inferred from the 13C- and D-depleted signature of CH4 in the collected fluids. It was observed that the hydrothermal activity of the Dodo field had ceased until 2013.

Bathymetry, gravity and deep-tow sonar image data are used to define the segmentation of a 400 km long portion of the ultraslow-spreading Knipovich Ridge in the Norwegian–Greenland Sea, Northeast Atlantic Ocean. Discrete volcanic centers marked by large volcanic constructions and accompanying short wavelength mantle Bouguer anomaly (MBA) lows generally resemble those of the Gakkel Ridge and the easternmost Southwest Indian Ridge. These magmatically robust segment centers are regularly spaced about 85–100 km apart along the ridge, and are characterized by accumulated hummocky terrain, high relief, off-axis seamount chains and significant MBA lows. We suggest that these eruptive centers correspond to areas of enhanced magma flux, and that their spacing reflects the geometry of underlying mantle upwelling cells. The large-scale thermal structure of the mantle primarily controls discrete and focused magmatism, and the relatively wide spacing of these segments may reflect cool mantle beneath the ridge. Segment centers along the southern Knipovich Ridge are characterized by lower relief and smaller MBA anomalies than along the northern section of the ridge. This suggests that ridge obliquity is a secondary control on ridge construction on the Knipovich Ridge, as the obliquity changes from 35° to 49° from north to south, respectively, while spreading rate and axial depth remain approximately constant. The increased obliquity may contribute to decreased effective spreading rates, lower upwelling magma velocity and melt formation, and limited horizontal dike propagation near the surface. We also identify small, magmatically weaker segments with low relief, little or no MBA anomaly, and no off-axis expression. We suggest that these segments are either fed by lateral melt migration from adjacent magmatically stronger segments or represent smaller, discrete mantle upwelling centers with short-lived melt supply.

New deep tow sidescan sonar data from the Southwest Indian Ridge reveal complex volcanic/tectonic interrelationships in the axial zone of this ultra-slow spreading ridge. While some constructional volcanic features resemble examples documented at the slow-spreading Mid-Atlantic Ridge, such as axial volcanic ridges, hummocky and smooth lava flows, their distribution and dimensions differ markedly. The largest axial volcanic ridges occur at segment centres, but fresh-looking volcanic constructions also occur at segment ends and in the deep basins marking the non-transform discontinuities. The orientations of the dominant fault population and main volcanic ridges are controlled by tectonic processes such as orthogonal extension in the sections of the ridge perpendicular to the spreading direction and transtensional extension in the obliquely spreading sections of the ridge. Minor faults and small volcanic ridges striking parallel to the axis in the oblique part of the ridge are not controlled by these extensional regimes. This observation suggests that the ridge axis acts as a zone of weakness and that magmatic processes, with associated fractures opening in response to magma pressure, may control local emplacements of axial volcanic ridges at obliquely spreading ridges. This non-systematic pattern of ridge characteristics suggests an along-axis variation between focused and distributed magmatic supply, a model which is supported by our interpretation of low-amplitude mantle Bouguer anomalies calculated for the area. We propose that a change of the axial segmentation pattern, from two segments to the present-day three segments, may have introduced additional instability into the crustal accretion process.

The Ayu Trough lies on the southern boundary between the Philippine Sea Plate and Caroline Plate. Although this trough may be the best place to study the evolution and kinematics of the Philippine Sea Plate, the origin and evolution of this trough are poorly understood. Our geophysical and morphological surveys in the northern part of the Ayu Trough revealed that the trough shares morphological similarities with slow-spreading lid-ocean ridges. The seafloor ages and an average spreading rate of the trough were inferred from the avcrage length of the ridge segments, distribution of sediment thickness, and basement subsidence. Based on the seismic section at 3°30'N, the opening of the trough started about 25 Ma with an average half-spreading rate 4.1 mm/yr. This spreading rate is much slower than some previous estimates. The present magma production rate and activity of the trough is estimated to be even lower than the 25 m.y. average. The seafloor depth of the trough axis and relief of the axial valley are approximately twice as deep as that of active lid-ocean ridges. The trough axis valley is covered with sediment in contrast with the axial valleys of active lid-ocean ridges. Sedimentation at the trough axis shows that the spreading rate has been quite slow for the last 2.5 Ma.

We wanted to have some observational method to evaluate hydrothermal discharging behavior as it was. DIDSON is an acoustic lens-based sonar. Institute of Industrial Science, University of Tokyo (IIS) has understood DIDSON's superior performance and tried to find new method for utilization of it. A next challenge is to develop an observation method based on DIDSON for hydrothermal discharging from seafloor vent. In October 2009, we conducted seafloor reconnaissance using a manned deep-sea submersible (DSV) Shinkai 6500 in Central Indian Ridge 18–20deg. S, where hydrothermal plume signatures were previously perceived. DIDSON was equipped on the top of Shinkai 6500 in this cruise. DIDSON could obtain acoustic images of seafloor hydrothermal flows, even though we could not fully utilize the performance of DIDSON due to low-quality wire connection between DIDSON and Shinkai6500. Contrasting density inside the acoustic image of the hydrothermal flow could be distinguished. DIDSON showed its possibility of observation tool that can delineate spatial and temporal change of internal structure of the hydrothermal flows.

SUMMARY Late Jurassic to Early Cretaceous (Mesozoic) magnetic anomaly lineations (the Phoenix, Magellan, Mid-Pacific Mountains, Hawaiian and Japanese lineation sets) with fracture zones in the west-central Pacific Ocean were identified more comprehensively than in any previous studies. We fixed 2100 positions of identified magnetic anomalies based on magnetic data of 283 cruise tracks. Two remarkable fracture zones, the Phoenix and Central Pacific Fracture Zones, were mapped and newly named. Our newly identified lineations from MlON to MO around the Mid-Pacific Mountains, which belong to the Hawaiian lineation set, illustrated that the sea-floor south of the Mid-Pacific Mountains has the same age as that of the north (132-118 Ma). Our analysis of skewness parameters revealed that the older part of the Phoenix set (M17-M29) has skewness different from that of the younger part (Ml-M14), implying an effect of magnetic overprints by the Cretaceous volcanism. It was confirmed that the spreading rate of the Mesozoic Pacific spreading system was the fastest in the world in the Mesozoic. A drastic change in spreading rates occurred simultaneously at the period between chrons M21 and M20 (149.5-148.5 Ma) in all the Mesozoic Pacific spreading systems. The event appears to be synchronous with events in other oceans such as the Mesozoic Atlantic and Indian Oceans.

Two-dimensional magnetic modeling was performed to investigate the magnetic structure beneath the southern Okinawa Trough. The E-W elongating magnetic anomalies accompanied with the geologic segment of the Yaeyama Graben are explained by a simple model assuming of the seismic layers of 3.6-6.4km/s with a uniform normal magnetization of 2.5A/m as the magnetic layer. The upper surface of the magnetic layer coincides with an acoustic basement on multi-channel seismic reflection profile. The E-W elongating anomalies are caused by the V-shaped subsided pre-rifting layers. The high magnetic intensity of 2.5A/m given to the model suggests that the pre-rifting layers contain a large amount of magnetic material. Our investigating results do not support an existence of magnetic lineations of seafloor spreading origin in the southern Okinawa Trough.

International audience; Near-bottom magnetic measurements on board submersible Nautile were carried out on the Mid-Atlantic Ridge 21 degrees 40'N segment, and deep-sea geomagnetic vector anomalies along 19 dive tracks were obtained by applying the processing method for shipboard three-component magnetometer data. A forward modeling technique using short-wavelength components of the anomalies arising from local topography and vertical motion of the submersible was designed to estimate the absolute magnetization intensity of the seafloor. In the vicinity of the spreading axis a considerable number of magnetization estimations are reliably confirmed by the high correlation between observed and modeled anomalies, whereas less reliable estimations are obtained off-axis, probably because the sediment buries the basement topography. The natural remanent magnetization (NRM) measured on basalt samples collected during these dives is compared with the magnetization from anomalies. Though both results give a similar range of magnetization intensity, no correlation is confirmed between them, possibly because the magnetization from anomalies represents laterally averaged seafloor magnetization, whereas the NRM has variations at the scale of individual pillow or lava pile. Equivalent magnetization inverted from the sea-surface magnetic anomalies shows axial magnetization increases significantly from the segment center to the segment ends. However, the results of eight dives conducted near the spreading axis at different locations along the segment show much less variation in magnetization intensity along the axis. We ascribe the high equivalent magnetization at segment ends to preferential serpentinization of peridotite near the segment ends and the associated formation of magnetite. The results of three across-axis transects composed of 15 dives running in the spreading direction can be consistently interpreted as recording geomagnetic paleointensity variations during the Brunhes epoch. Although magnetization lows are generally correspondent to periods of low paleointensity, they show deeper drop than predicted from the paleointensity variation.

We report first evidence for hydrothermal activity from the southern Knipovich Ridge, an ultra-slow spreading ridge segment in the Norwegian-Greenland Sea. Evidence comes from optical backscatter anomalies collected during a systematic side-scan sonar survey of the ridge axis, augmented by the identification of biogeochemical tracers in the overlying water column that are diagnostic of hydrothermal plume discharge (Mn, CH4, ATP). Analysis of coregistered geologic and oceanographic data reveals that the signals we have identified are consistent with a single high-temperature hydrothermal source, located distant from any of the axial volcanic centers that define second-order segmentation along this oblique ridge system. Rather, our data indicate a hydrothermal source associated with highly tectonized seafloor that may be indicative of serpentinizing ultramafic outcrops. Consistent with this hypothesis, the hydrothermal plume signals we have detected exhibit a high methane to manganese ratio of 2–3:1. This is higher than that typical of volcanically hosted vent sites and provides further evidence that the source of the plume signals reported here is most probably a high-temperature hydrothermal field that experiences some ultramafic influence (compare to Rainbow and Logachev sites, Mid-Atlantic Ridge). While such sites have previously been invoked to be common on the SW Indian Ridge, this may be the first such site to be located along the Arctic ultra-slow spreading ridge system.

[1] The analysis of the Towed Ocean Bottom Instrument (TOBI) side scan sonar images along the Southwest Indian Ridge between 63°40′E and 65°40′E reveals strong focusing of magmatic activity and long amagmatic accretionary ridge segments. Fresh-looking volcanic terrains are observed at distinct locations along the axis separated by highly tectonized and sedimented terrains of an along-axis extent as much as 82 km. The largest tectonized section corresponds to a dramatically thin crust area with moderate magnetization anomalies. We suggest that seafloor spreading is mainly amagmatic in this tectonized section of the Southwest Indian Ridge with upper mantle rocks exposed at the seafloor. Amagmatic accretionary ridge segments of such dimensions are quite distinct from what is observed at the Mid-Atlantic Ridge but are also recognized at the Gakkel Ridge and may thus be characteristic of ultra-slow spreading ridges. The correlation between the distribution of fresh-looking volcanic terrains, the occurrence of shallow areas crowned by axial volcanic ridges, and high magnetization values suggests a shallow segmentation of the ridge mainly related to variation in the thickness and/or the intrinsic magnetization of the basaltic source layer. By contrast, strong along-axis variations of the gravity-derived crustal thickness are shrunken in length relative to this shallow segmentation of the ridge and occur only beneath the elevated segments. Adjacent to these elevated segments, small bathymetric swells with fresh-looking volcanic constructions do not correspond to thicker crust areas. This suggests a highly focused melt supply beneath the elevated segments which may feed volcanic constructions up to 60 km from the center of these segments by shallow lateral melt migration in the crust, probably through large dikes. Neither the ultra-slow spreading rate nor the ridge obliquity explains the variation of the magmatic vigor along the ridge. Mantle source heterogeneities together with lower mantle temperatures beneath the easternmost Southwest Indian Ridge could partly control its segmentation.

The examination of the crustal structure and ODP deep-sea drilling results introduced the following opening model of the Japan Sea. The opening of the Japan Sea was initiated by the extension and thinning of the proto-Japan island arc, which was situated on the margin of the Eurasian continent at ca. 30 Ma. During the extension process a large strike-slip fault was generated at the eastern margin of the proto-Japan Sea. The strike-slip movement cut through the entire lithosphere and caused it to split, triggering development of a seafloor spreading system at ca. 28 Ma. The seafloor spreading system propagated to the WSW direction into the thinned crustal zone and formed the eastern part of the Japan Basin that is presently underlain by oceanic crust. In the meantime, the southwestern part of the Japan Sea suffered continuing extension and thinning of the arc crust and formed basins and rises. The basins are composed of thinned lower arc crust overlain by volcanic layers, and the rises are composed of fragmented upper arc crusts. The propagation ceased at ca. 18 Ma, leaving the contrasting topography of the present northern and the southern Japan Sea. The extension and thinning of arc or continental crusts and subsequent development of a propagating spreading system, which is initiated at a basin margin strike-slip zone, are common and fundamental processes of most backarc basins.

Laurent Jolivet est Professeur à l'Université d'Orléans au 1er Septembre 2009; International audience; The respective tectonic effects of back arc spreading and continental collision in Asia are considered either as two independent processes or as closely interrelated. Extrusion tectonics assumes that the opening of the South China Sea and the left-lateral motion along the Red River fault are geometrically linked in a pull-apart manner. This model is not accepted by several workers because the structural link between the two processes is not clearly demonstrated. In the case of the Japan Sea, we can show without ambiguity that back arc opening was controlled by large intracontinental strike-slip faults which can be easily understood as effects of the India-Asia collision far from the indenter. The Japan Sea opened in the early Miocene in a broad pull-apart zone between two major dextral strike-slip shear zones. The first one extends from north Sakhalin to central Japan along 2000 km, it has accommodated about 400 km of finite displacement. Deformation along it varies from dextral transpression in the north to dextral transtension in the south. The second is between Korea and SW Japan and has accommodated a smaller displacement of about 200 km. The extensional domain in between lies in the back arc region of Japan. Distributed stretching of the arc crust resulted in the formation of most of the Japan Sea, while localized oceanic spreading at the southern termination of the eastern transpressional shear zone shaped the Japan Basin. The first oceanic crust was formed in a small triangle based on the eastern shear zone, and spreading propagated westward inside the pull-apart region. Timing of oceanic crust formation, of formation of the dextral shear zones and of block rotation in between, as well as the internal structure of the basins and the geometry of deformation along the master shear zones are used to reconstruct the opening history. This evolution is discussed by comparison to other manifestations of the arc and back arc activity, such as the history of sedimentation and volcanism. The paper then suggests that the collision of India can have tectonic consequences as far north as Japan and Sakhalin and describes the geometrical relation of back arc opening there and diffuse extrusion.

The progress of the study of the sea-floor spreading has mutual relationship with that of the ophiolite. The scientists to study ophiolites should fully understand the present progress of the study of the sea-floor spreading, and vice versa. We obtained the concept of the sea-floor spreading in 1960's in terms of diverging plate boundaries and started to observe geological process at the spreading axis in 1970's. Swarth mapping of topography by multi narrow beam sonar and long range sidescan sonar, manned submersible observation, multichannel seismic reflection survey of the spreading axis, and detailed geochemical study of sampled rocks dramatically improved our understanding of the geological process along the spreading axis in early 1980's. The trend of the study of the sea-floor spreading is shifting toward the global understanding of its dynamic process in late 1980's.

We have identified Late Jurassic to Early Cretaceous magnetic anomaly lineations (M0 to M35 of the Japanese and Hawaiian lineation sets) and fracture zones in the northwestern Pacific more comprehensively than previous investigators. We fixed 3500 positions of magnetic anomalies identified from magnetic data collected along cruise tracks as well as 151 positions of fracture zones from bathymetric and seismic profiles. The resultant isochron map revealed the evolution of the triple junction of the Pacific, Izanagi, and Farallon plates and the intimate relationship between the triple junction and the origin of the Shatsky Rise. The triple junction stagnated on a hotspot (the Shatsky hotspot) from chron M21 (149.5 Ma) to chron M4 (126 Ma). The Shatsky Rise is a trace of the hotspot on the Pacific plate. A sudden appearance of the Shatsky hotspot between chrons M21 and M20 caused a regional reorganization of the Pacific-Izanagi-Farallon plate boundaries.

A WNW trending lineated topographic depression (Magellan Trough) and associated magnetic anomaly lineations (Magellan lineation set) were revealed in the basin northeast of the Magellan Rise. We identified symmetric magnetic anomaly lineations from M9 (121 m.y.) to M11 (126 m.y.) of Early Cretaceous age with a symmetric axis at the Magellan Trough and concluded that the Magellan Trough is a remanent spreading center which ceased its opening at the latter half of M9 time. The Magellan lineation set fans drastically to the southeast, and it represents an asymmetric spreading system. The Magellan spreading system has two separate rotation poles for the northern flank lineations and the southern flank ones, and both poles lie on the extension of the Magellan Trough. The amplitudes of the magnetic anomalies decrease toward the poles, possibly owing to the contamination of the magnetized blocks as a function of decreasing spreading rate. The Magellan spreading system shows unusually rapid rotation rates of 4.2°/m.y. for the northern flank and 3.1°/m.y. for the southern flank, which together with the proximity of the rotation poles suggest unstableness of the spreading system. This unstable spreading may have shortened the life of the Magellan spreading system. Such an unstable spreading system could have been generated in order to balance changes in movements of the Phoenix, Farallon, and Pacific plates. The spreading center may have jumped to the northeast at M9 time as part of its evolution as the Pacific-Farallon ridge. Thus the Magellan Trough may be a fossil Pacific-Farallon ridge and the northern flank of the Magellan set a piece of captured Farallon plate.