Search

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.

Although the climate in the Arabian Sea is controlled primarily by the southwest monsoon, the southwest monsoon has little influence in the Gulf of Aden. To examine the different responses to monsoons between the Gulf of Aden and areas outside the gulf, a comprehensive data set of bulk organic and inorganic geochemistry, sea surface temperature, and δ15N of pheopigments was obtained from deep-sea sediment cores recovered from inner and outer regions of the gulf. The results indicated that during the past 220 kyr, the influence of the southwest monsoon was stronger in the outer region of the gulf than in the inner region, which implies that the southwest monsoon trajectory has not changed substantially during that time period. Furthermore, influxes of O2-depleted water from the Southern Ocean and the lateral advection of upwelled seawater also had limited influence in the inner region. In contrast, concentrations of lithogenic materials transported by the southwest monsoon were similar in the two regions. δ15N of pheopigments indicated that during the last glacial maximum, the southwest monsoon was weaker and the northeast monsoon was stronger than at present. A stronger southwest monsoon during interglacials enhanced primary productivity and may have caused anoxic conditions to develop in the Arabian Sea, as indicated by redox proxies in the outer region. Anoxic conditions also formed during MIS 3, but no increase in the primary productivity is indicated; therefore, another mechanism, such as an influx of O2-depleted water from the Southern Ocean, may have been the cause.

We conducted water column surveys to search for hydrothermal plumes over the spreading axes in the Gulf of Aden between 45°35′E and 52°42′E. We measured light transmission and chemical tracers Mn, Fe, CH 4 and δ 13 C of CH 4 in seawater taken using a CTD-Carrousel multi-sampling system at 12 locations including a control station in the Arabian Sea. We recognized three types of hydrothermal plumes at depths of ~650 to ~900 m (shallow plumes), ~1000 to ~1200 m (intermediate plumes), and >1500 m (deep plumes). The shallow plumes were apparently originated from newly discovered twin seamounts (12°03–06′N and 45°35–41′E) at the westernmost survey area, where two-dimensional distributions of light transmission and Mn were mapped by tow-yo observations of the CTD-sampling system with an in situ auto-analyzer GAMOS. The maximum concentrations of Mn, Fe, and CH 4 of 46 nM, 251 nM, and 15 nM, respectively, were observed for collected seawater within the shallow plumes. The intermediate plumes were characterized by anomalies of light transmission, Mn, Fe, and δ 13 C of CH 4 , but by little CH 4 anomalies, suggesting that CH 4 had been consumed down to the background level during the aging of the plumes. Anomalies of δ 3 He already reported by the World Ocean Circulation Experiment (WOCE) program exhibited a hydrothermal plume-like peak at ~2000 m depth in the Gulf of Aden, which seems to coincide with the deep plumes observed in this study. The endmember δ 13 C–CH 4 values for the shallow and the deep plumes were estimated to be in a range between −10‰ and −15‰, demonstrating that the sources of CH 4 are not biogenic but magmatic as similarly observed at sediment-starved mid-oceanic ridges.

We analyzed long-chain (C25–C36) n-alkanes and pollen grains in sediments from the Gulf of Aden covering the last 212 kyr to reconstruct the surrounding terrestrial environment, a critical region for the dispersal of Homo sapiens. Substantial increases in the flux of n-alkanes during 200–185, 120–95, and 70–50 ka were interpreted to indicate enhanced vegetation biomass in the Arabian Peninsula and the northern part of the Horn of Africa or increase in lithogenic material inputs. Periods of enhanced n-alkane flux occurred during or immediately after pluvial episodes, indicating that the increased precipitation may have induced substantially enhanced vegetation biomass, creating favorable conditions for Homo sapiens. Additionally, vegetation may have increased due to moderate precipitation unrecorded by speleothems or in accordance with the lowering of sea level, indicating that the dispersal might have been possible even after the shift to an arid environment indicated by the speleothems.

To ascertain factors controlling melt production along a typical distal, 'hotspot-interacting' mid-ocean ridge, we investigated the extent and distribution of both plume-related and plume-unrelated basalt from the central Indian ridge (CIR) between 15°S and 20°S. Comprehensive geochemical data of fresh-quenched volcanic glasses and basalts were used. Variation of Sr, Nd, and Pb isotopic compositions and Nb/Zr, Ba/Nb, and Ba/La content were interpreted by mixing of three melt end members: the Indian depleted MORB mantle derived melt; radiogenic and enriched melt derived from source mantle for Rodrigues Ridge and the intermediate series of Mauritius Island (RE2, radiogenic enriched component 2); and radiogenic but depleted melt derived from source mantle for Gasitao Ridge (RD, radiogenic depleted component). On the basis of quantitative mantle melting and melt mixing model, results show that sources for RE2 and RD are geochemically distinct from those of the Reunion plume (RE1, radiogenic enriched melt component 1). Moreover, the geochemical variation of MORB of 15°S to 20°S is unrelated to contamination of the upper mantle by the Reunion plume. These results suggest strongly that plume-unrelated heterogeneity is widespread throughout the upper mantle. The chemical characteristics of RE2 are remarkably pronounced in basalt from the central portion of ridge segment 16 around 18°S, suggesting substantial magma production. The influence of RE2 decreases along with decreasing magma production to the north, and is only slightly identifiable in basalt from the northern part of segment 18. Although the influence of RE2 decreases somewhat to the south, basalts with extreme RE2 signature were produced in the center of segment 15 around 19°S, where magma production is high. In contrast to RE2, the geochemical signature of RD in basalt is geographically limited to two localities: the south end of segment 18 and the center of segment 15. However, these observations reveal that both RE2 and RD contribute strongly to magma production on segment 15. Results show that melting of ancient recycled plate materials with a low melting point regulates voluminous magma production along the CIR.

The Knipovich Ridge (73°30′–78°40′N) is an extreme end-member of the mid-ocean ridge spreading system, both in terms of spreading rate ( Fault population characteristics conform to observational and experimental analyses of oblique rifting and spreading systems. For the ridge as a whole, faults in the axial region are short, straight, isolated (not linked into complex fault zones), and exhibit length-scaling relationships characteristic of young and active fault systems. Faults are generally oblique to both the ridge axis and the spreading direction, and orientations vary systematically with angle between ridge and spreading direction. Along-axis analysis reveals the influence of axial segmentation on fault population characteristics. Segment centers are dominated by faults perpendicular and sub-perpendicular to plate motion with longer characteristic length and generally lower fracture density. Conversely, segment ends are dominated by faults striking oblique to plate motion with shorter characteristic length and generally higher fracture density. We infer that faulting in segment centers is strongly influenced by the mechanical effects of dike intrusion perpendicular to plate motion, while faulting in segment ends is controlled by the mechanics of oblique rifting, non-transform discontinuities, and/or accommodation zones. The contrasts between these distinct structural and mechanical settings along the ridge axis are accentuated by the high obliquity and ultra-slow spreading rate of this spreading system.

The Myojin Knoll is a submarine volcano that has a classically beautiful conical-shaped silicic caldera whose surface is covered by pumice. To determine the tectonic structure inside the caldera wall and beneath the caldera floor of this pumicious submarine volcano, we carried out a structural interpretation study using newly collected deep-penetrating multichannel seismic (MCS) reflection data. We also conducted a detailed velocity analysis of the MCS data, which facilitated the interpretation study. The results demonstrate that approximately 90% of the caldera wall is composed of pumiceous volcanic breccia. This finding supports those of previous researchers who, based on seafloor observations, single-channel seismic reflection, and gravity and geomagnetic data, concluded the Myojin Knoll is a knoll having a pumiceous caldera wall underlain by a pre-caldera rhyolitic stratovolcano edifice. We also determined a down-warping reflector approximately 800 m beneath the caldera floor. A seismic unit immediately above the reflector has a higher P-wave velocity than the pumice units and shows a chaotic seismic reflection pattern. We interpreted the reflector to be the bottom of a possible shallow magma chamber where the magma would undergo repeated expansion and contraction as a result of recurrent eruption activities.

Petrographic and geochemical analyses of basaltic rocks dredged from the first segment of the Southwest Indian Ridge near the Rodriguez Triple Junction have been completed in order to investigate water-rock interaction processes during mid-ocean ridge (MOR) hydrothermal alteration in the Indian Ocean. In the study area, we have successfully recovered a serial section of upper oceanic crust exposed along a steep rift valley wall which was uplifted and emplaced along a low angle normal fault. On the basis of microscopic observation, dredged samples are classified into three types: fresh lavas, low-temperature altered rocks, and high-temperature altered rocks. The fresh lavas have essentially the same chemical composition as typical N-MORB, although LILE and Nb are slightly enriched and depleted, respectively. Low temperature alteration brought about the enrichment of K2O, Rb, and U due to the presence of K-rich celadonite and U-adsorption onto Fe-oxyhydroxide and clay minerals. On the other hand, chloritization, albitization, and addition of base metals by high temperature hydrothermal alteration result in enrichments of MnO, MgO, Na2O, Cu, and Zn and depletions of CaO, K2O, Cr, Co, Ni, Rb, Sr, and Ba. In addition, U-enrichment is also observable in the high temperature altered rocks probably due to the decrease of uranite solubility in the reducing high-temperature hydrothermal solution. These petrological and geochemical features are comparable to those of the volcanic zone to transition zone rocks in the DSDP/ODP Hole 504B, indicating that our samples were recovered from the upper ∼1000 m section of the oceanic crust. Only the alteration minerals related to off-axis alteration are absent in our samples dredged from near the spreading axis. The similarity of alteration between our samples from the Indian Ocean and the Hole 504B rocks from the Pacific Ocean suggests that MOR hydrothermal systems are probably similar across all world oceans.

Helium concentration and isotopic ratio in the ocean are influenced by four sources: the mantle, atmosphere, crust and tritium decay. In addition, the concentration of dissolved gases can be altered by air injection and ice-related mechanisms such as brine rejection, sea-ice melting and glacial melting, especially in the high-latitude seas. The Knipovich Ridge, a northern continuation of the Mid-Atlantic Ridge (74–79°N, 6–8°E), is potentially affected by all the sources and mechanisms mentioned above. This suggests that the ridge should be a good place to investigate these mechanisms. During the Knipovich 2000 expedition, water samples were collected for helium and neon analysis along the axis of the ridge. Although Mn and CH 4 results suggest the existence of hydrothermal activity in the ridge, we could not detect a significant increase of helium isotopic ratio ( 3 He/ 4 He) at corresponding water depths. Instead, the bottom waters in the northern section of the ridge were supersaturated by up to 40% for 4 He. The helium isotopic ratio of 1.3×10 −6 indicates the dominance of atmospheric helium. Using He and Ne saturation anomalies, we further explored the mechanisms affecting the distribution of helium and neon in the ridge. Around 5% supersaturation of He and Ne at most deep waters is explained by air injection and brine rejection. However, the large excess in the northern ridge was attributed to the input of 2.5% glacial meltwater, possibly originating from the glacier-covered islands, Svalbard.

We report major and trace element compositions for 45 fresh basalts from 13 dredge sites between 45.5° E and 49° E along the Gulf of Aden obtained during the Aden New Century cruise (KH-005 : RN Hakuho-maru, ORI, University of Tokyo). The basalts from Aden New Century (ANC) Seamount at around 45.5° E, discovered by bathymetric mapping during the cruise, have the highest Nb/Zr (0.22-0.27) and La/SmN (3.1-3.7) and show the strongest OIB signature among those in the Gulf of Aden. The longitudinal variation of Nb/Zr and La/SmN of the dredged basalts shows two enrichment peaks, i.e., first “keen” enrichment peak with OIB signature near ANC Seamount (45.5°E) and second “broad” enrichment peak with E-MORB signature along the ridge axis from 46° E to 48° E, and at 49° E, N-MORB is dominant. In spite of variable Nb/Zr and La/SmN, the estimated pressure of magma segregation and degree of partial melting from the dredged basalts, using reported results of experimental melting on spinel lherzolite, are relatively constant and almost similar to those of N- and T-MORBs in Pacific, Atlantic, and Indian Oceans. This suggests that the longitudinal variation of the dredged basalts reflects mantle compositions equilibrated with their primitive magmas. Nb/Zr, Nb/Y, and La/SmN systematics for the dredged basalts suggest binary magma mixing of the NMORB source with the OIB source, composed of FOZO (or C) mantle component with the incorporation of minor, if any, HIMU components.Recent studies (George et al., 1998; Orihashi et al., 1998) have suggested that two mantle plumes might be impinged beneath the Afar province in Eocene to Oligocene (45-30 Ma) and Miocene (19-15 Ma) times. Considering the above, two enrichment peaks of the mantle in the Gulf of Aden must be produced by two mantle plumes impinged in the Afar province and spread sideways sequentially, with the front of spreading first plume head corresponding to the broad enrichment peak. This broad peak is formed because the plume head emplaced beneath the lithosphere, producing a Large Igneous Province (30-26Ma), was progressively defused into and diluted by the upper mantle component (N-MORB source) over time.

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.

High-resolution seismic stratigraphy of the Yamato Basin, Japan Sea, was successfully established using core-log-seismic data integration. The construction of synthetic seismograms by the combination of physical properties and well-log data from the Ocean Drilling Program (ODP) Site 797 was the key to accomplishing the high-resolution seismic stratigraphy. To achieve resolution comparable with well-log data and core lithology, single channel seismic reflection data taken from ODP underway geophysics were reprocessed, and then carefully compared with synthetic seismogram, core and well log profiles to identify seismic units. Ten seismic stratigraphic units were identified at the site, and seismic stratigraphic interpretation was successfully extended from the site to the nearby area along the Yamato Basin margin. The opal-A/opal-CT (biogenic silica/metastable diagenetic silica) boundary has different appearances at places from strong to weak, and mostly discontinuous. One of the significant results achieved from this study is clear distinction of the opal-A/CT boundary from a very strong reflector, which appears at 22 m below the opal-A/CT boundary. Through well-log and physical properties characterization of the different units, resistivity was found to be the best indicator of diatom content and with gamma-ray it also is an indicator of chert layers in the opal-CT zone. Velocity is not greatly effected by diatom ooze in the opal-A zone, however, it shows strong peaks and has an indirect relationship with gamma-ray in the opal-CT zone. Finally, successful correlation of Gamma-ray Attenuation Porosity Evaluator density and resistivity peaks with strong seismic reflectors from upper and lower stratified layers may provide new information on the late Neogene paleoceanography of the Japan Sea in high-resolution scale.

During the Late Mesozoic and Cenozoic, extension was widespread in Eastern China and adjacent areas. The first rifting stage spanned in the Late Jurassic–Early Cretaceous times and covered an area of more than 2 million km2 of NE Asia from the Lake Baikal to the Sikhot-Alin in EW direction and from the Mongol–Okhotsk fold belt to North China in NS direction. This rifting was characterized by intracontinental rifts, volcanic eruptions and transform extension along large-scale strike–slip faults. Based on the magmatic activity, filling sequence of basins, tectonic framework and subsidence analysis of basins, the evolution of this area can be divided into three main developmental phases. The first phase, calc-alkaline volcanics erupted intensely along NNE-trending faults, forming Daxing'anling volcanic belt, NE China. The second phase, Basin and Range type fault basin system bearing coal and oil developed in NE Asia. During the third phase, which was marked by the change from synrifting to thermal subsidence, very thick postrift deposits developed in the Songliao basin (the largest oil basin in NE China). Following uplift and denudation, caused by compressional tectonism in the near end of Cretaceous, a Paleogene rifting stage produced widespread continental rift systems and continental margin basins in Eastern China. These rifted basins were usually filled with several kilometers of alluvial and lacustrine deposits and contain a large amount of fossil fuel resources. Integrated research in most of these rifting basins has shown that the basins are characterized by rapid subsidence, relative high paleo-geothermal history and thinned crust. It is now accepted that the formation of most of these basins was related to a lithospheric extensional regime or dextral transtensional regime. During Neogene time, early Tertiary basins in Eastern China entered a postrifting phase, forming regional downwarping. Basin fills formed in a thermal subsidence period onlapped the fault basin margins and were deposited in a broad downwarped lacustrine depression. At the same time, within plate rifting of the Lake Baikal and Shanxi graben climaxed and spreading of the Japan Sea and South China Sea occurred. Quaternary rifting was marked by basalt eruption and accelerated subsidence in the area of Tertiary rifting. The Okinawa Trough is an active rift involving back-arc extension. Continental rifting and marginal sea opening were clearly developed in various kind of tectonic settings. Three rifting styles, intracontinental rifting within fold belt, intracontinental rifting within craton and continental marginal rifting and spreading, are distinguished on the basis of nature of the basin basement, tectonic location of rifting and relations to large strike–slip faults. Changes of convergence rates of India–Eurasia and Pacific–Eurasia may have caused NW–SE-trending extensional stress field dominating the rifting. Asthenospheric upwelling may have well assisted the rifting process. In this paper, a combination model of interactions between plates and deep process of lithosphere has been proposed to explain the rifting process in East China and adjacent areas. The research on the Late Mesozoic and Cenozoic extensional tectonics of East China and adjacent areas is important because of its utility as an indicator of the dynamic setting and deformational mechanisms involved in stretching Lithosphere. The research also benefits the exploration and development of mineral and energy resources in this area.

Western Pacific basins are characterized by three remarkable attributes: (1) complex kinematic histories linked to global-scale plate interactions; (2) DUPAL-like contaminated mantle; and (3) rapid post-Mesozoic rollback of the confining arc-trench systems. The coincidence of slab steepening, extreme arc curvature, and vigorous basin opening associated with the Mariana convergent margin suggests that rollback continues in response to an east-directed mantle ‘wind’. Against a backdrop of conflicting kinematic and genetic interpretations we explore the notion that eastward asthenospheric flow driven by diachronous Tethyan closure caused stretching of eastern Eurasia and concomitant opening of western Pacific basins. Marking the eastern boundary of the latter, the Izu-Bonin-Mariana forearc may be regarded as a litho-tectonic ‘high-tide mark’ comprising igneous and metamorphic products from successive episodes (since ca. 45 Ma.) of arc sundering and backarc basin opening. The forearc also forms an isotopic boundary separating contaminated western Pacific mantle from the N-MORB Pacific Ocean reservoir. While the isotopic composition of western Pacific mantle resembles that feeding Indian Ocean hotspot and spreading systems, its spatial–temporal variation and the presence of subduction barriers to the south appear to preclude northward flow of Indian Ocean mantle and require an endogenous origin for sub-Eurasian contaminated mantle. It is concluded that the extrusion of Tethyan asthenosphere, contaminated by sub-Asian cratonic lithosphere, was a major cause of western Pacific arc rollback and basin opening. The model is consistent with paleomagnetic and geologic evidence supporting independent kinematic histories for constituent parts of the Philippine Sea and Sunda plates although interpretation of these is speculative. Compounded by effects of the Australia–Indonesia collision, late-Tethyan mantle extrusion appears to have produced the largest DUPAL domain in the northern hemisphere.

A dense community of benthic animals was discovered by the Japanese ROV 'Kaiko' in the hadal zone near the bottom of the Japan Trench, 7326 m deep. The community was dominated by a new species of thyasirid bivalve Maorithyas hadalis. This community appears to be sustained by chemosynthesis (nutrients being derived from reduced compounds within the sediment) for reasons including: a high concentration of sulfur contained in the thyasirid gills; the existence of numerous bacteria-like particles in the gill tissues; a sulfide smell from soft body parts and from collected sediment; the anoxic nature and high sulfide content of the sediments suggested by the dark gray to black color; the occurrence above geologic faults. Thyasirids have also been collected from other chemosynthesis-based communities around Japan, including the Japan Trench, Sagami Bay and the Nankai accretionary prism. Until now, the deepest chemosynthesis-based community known occurred on the Sanriku Escarpment of the Japan Trench, 6437 m deep. This site was dominated by the vesicomyid clam Calyptogena phaseoliformis. The discovery of a chemosynthesis-based community dominated by thyasirid clams from even deeper waters suggests a wider variety of chemosynthesis-based communities exists throughout deep-sea trenches. Additionally, the thyasirid species discussed here differs from other thyasirids by having circular symbionts which appear to exist intracellularly, and by living exposed above the sediment surface.

SUMMARY Elongated fault escarpments on the outer slopes of the western Kuril and Japan trenches have been investigated through detailed swath bathymetric mapping. Numerous horsts and grabens formed by these escarpments were identified. Distinct N70°E linear alignment of the escarpments, parallel to the magnetic anomaly lineations, was revealed on the outer slope of the western Kuril Trench. In the Japan Trench north of 39°00aeN, most of the escarpments are parallel to the trench axis and oblique to the magnetic lineations. A zig-zag pattern of faulting exists south of 39°00aeN. Each topographic profile was decomposed by computer analysis into two curves representing (1) the smoothed long-wavelength slope of the subducting ocean-crust surface and (2) the short-wavelength (

The paleomagnetism of 10 seamounts from the Joban Seamount Chain (northwestern Pacific) were studied using a method that calculates mean magnetization parameters by an inversion of magnetic anomaly and edifice bathymetry. Of the 10 seamounts, eight gave results consistent with other paleomagnetic studies of Pacific seamounts. Joban seamounts appear to have formed at two different mean paleolatitudes, contrary to what would be expected for a single hotspot origin. Furthermore, six of the consistent poles plot along the 129 to 82 Ma portion of the Pacific plate apparent polar wander path (APWP), implying the seamounts formed mainly during the mid- to Late Cretaceous. Two other poles, from Iwaki and Hitachi seamounts, are located northwest of the older end of the established Pacific APWP, possibly indicating Early Cretaceous ages. Because Iwaki and Hitachi seamounts are located in the middle of the chain, age does not progress along the chain, arguing against a single-hotspot origin. Perhaps the chain formed by recurrent volcanism along a line of weakness or by another mechanism. Iwaki and Hitachi seamounts display smaller northward drift compared to the others, consistent with the Pacific plate drifting southward from Late Jurassic or Early Cretaceous to mid-Cretaceous time.

Three-dimensional analysis of gravity and bathymetry data is used to investigate the density structure of the Rodrigues Triple Junction and the first segment of the Southeast Indian Ridge south of the triple junction. The distribution of mantle Bouguer and residual gravity anomalies suggests that focused upwelling of mantle is occurring along the nearly co-linear Central Indian and Southeast Indian Ridge limbs of the triple junction. In contrast, the mantle Bouguer anomaly over the Southwest Indian limb of the triple junction shows little variation despite nearly 4 km of topographic relief within this segment of the Southwest Indian Ridge. The absence of a significant mantle Bouguer anomaly over the Southwest Indian Ridge near the triple junction suggests that the rift valley observed on this segment is not completely compensated by thinning oceanic crust; only ∼ 1 km of crustal thinning within the axial valley is expected, based on our gravity analysis. Further, no clear long-wavelength thermal signal associated with lithospheric growth can be associated with the Southwest Indian Ridge in this region. These observations imply that focused upwelling is not occurring on this segment of the Southwest Indian Ridge and require a primarily dynamic compensation mechanism for the extreme axial topography. Our analysis indicates that the Rodrigues Triple Junction system is dominated by mantle upwelling associated with the Southeast and Central Indian Ridges.

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.

[1] We present a magnetic inversion method in the space domain using Akaike's Bayesian information criterion (ABIC). The horizontal variation of magnetization intensity is represented by a linear combination of bicubic B spline functions, and the problem is set to determine the expansion coefficients. A prior constraint on the roughness of the magnetization variation is incorporated in order to suppress the numerical instability. The ABIC give us the optimal weight of the prior constraint relative to the requirement of fitting the observed data, which is statistically determined from the quality and quantity of the data based on the entropy maximization principle. We applied this method to actual deep-sea magnetic data collected by using an autonomous underwater vehicle and successfully obtained a magnetization distribution that adequately accounts for the observation. The solution does not suffer from the inevitable smoothing due to high-cut filtering or an error caused by reducing the data onto a flat surface as sometimes happens in current inversion methods. Our method is especially useful in handling data collected along a surface of extreme topography over a relatively small area.

Indian Ocean hydrothermal vents are believed to represent a novel biogeographic province, and are host to many novel genera and families of animals, potentially indigenous to Indian Ocean hydrothermal systems. In particular, since its discovery in 2001, much attention has been paid to a so-called ‘scaly-foot’ gastropod because of its unique iron-sulfide-coated dermal sclerites and the chemosynthetic symbioses in its various tissues. Despite increasing interest in the faunal assemblages at Indian Ocean hydrothermal vents, only two hydrothermal vent fields have been investigated in the Indian Ocean. Here we report two newly discovered hydrothermal vent fields, the Dodo and Solitaire fields, which are located in the Central Indian Ridge (CIR) segments 16 and 15, respectively. Chemosynthetic faunal communities at the Dodo field are emaciated in size and composition. In contrast, at the Solitaire field, we observed faunal communities that potentially contained almost all genera found at CIR hydrothermal environments to date, and even identified previously unreported taxa. Moreover, a new morphotype of ‘scaly-foot’ gastropod has been found at the Solitaire field. The newly discovered ‘scaly-foot’ gastropod has similar morphological and anatomical features to the previously reported type that inhabits the Kairei field, and both types of ‘scaly-foot’ gastropods genetically belong to the same species according to analyses of their COI gene and nuclear SSU rRNA gene sequences. However, the new morphotype completely lacks an iron-sulfide coating on the sclerites, which had been believed to be a novel feature restricted to ‘scaly-foot’ gastropods. Our new findings at the two newly discovered hydrothermal vent sites provide important insights into the biodiversity and biogeography of vent-endemic ecosystems in the Indian Ocean.

A new compact deep-towed proton magnetometer system was developed and tested in 11 cruises around the Japanese Islands. The pressure case towed by a steel wire rope on the winch carries a portable proton magnetometer, a depth meter and their power supplies. The magnetometer sensor is connected to the pressure case with an armored cable 15 m long. The system automatically measures the total geomagnetic field intensity and the towing depth. It also stores the observed data on the internal digital memories within the instruments. This self-contained system enables us to make deep-towed observations up to a depth of 6000 m without an expensive armored towing cable. Furthermore, the simple system facilitated the operation and the production, and it reduced the costs. In our experiments, we successfully collected a maximum of 13 hours data and recorded a maximum towing depth of 3667 m. Some observed deep-towed data showed detailed information about magnetic anomalies, which could not be detected by surface-towed magnetometers. We will here explain the system with the results of two experiments and show that it is very useful for deep-towed magnetic observations.

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.

We conducted a detailed bathymetric and magnetic total force survey of five seamounts in the Joban seamount chain during cruise KH90-1 of the R/V HAKUHO MARU in 1990. The Joban seamount chain is situated to the east of the southern Japan Trench, trends along N53°E (oblique to magnetic anomaly lineations whose trend is N68°) and extends over a distance of about 394 km. We applied three-dimensional magnetic inversion to each seamount. The three northern seamounts show relatively complex magnetization structures, whereas the two southern seamounts show simple magnetization structures. The latter yielded reliable results with VGPs (Virtual geomagnetic pole) at 55.5°N/348.9°E and 52.1°N/352.4°E. We estimated the ages of the two southern seamounts to be ca. 80 to 90 Ma by comparing their VGPs with known Pacific plate apparent polar wander path. We also estimated the ages for two guyot seamounts in the northern group to be ca. 110 to 130 Ma by backtracking their crests along crustal age-depth curve to sea level. The Joban seamount chain apparently shows presence of at least two groups of inclinations. This suggests that the origin of this seamount chain is not the passage of the Pacific plate over a single stationary hotspot.

The magnetization intensity of oceanic crust in the Northeast Pacific, Atlantic, and Southeast Indian oceans was calculated from marine magnetic anomaly data to investigate long-term variations in magnetization intensity with crustal age. The magnetization intensity was derived by two-dimensional inversion of the upward-continued marine magnetic anomalies. The principal magnetization intensity variations versus age are summarized as follows: (1) In all the oceans, a high magnetization intensity exists at the ridge crest, followed by a sharp decrease within 8 My, and a minimum is reached at 8–20 Ma; (2) In the Northeast Pacific and Atlantic oceans, the magnetization intensity gradually increases up to at least 70 Ma. This variation pattern is consistent with the previous results of NRM measurements of drilled rocks and inversion of marine magnetic anomalies in the South Atlantic.

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.

A new Mesozoic isochron chart of the northwestern Pacific Ocean has been completed by utilizing all the currently available data from 357 cruises. Our chart provides the first whole view of the Mesozoic magnetic anomaly lineations in the northwestern Pacific Ocean and yields essential information for understanding the problems of the Mesozoic magnetostratigraphic time scale and the evolutional history of the Pacific plate. The Mesozoic magnetostratigraphic time scale must be recalibrated using not only the Hawaiian lineation set but also other Mesozoic lineation sets. The lack of lineation M0 in the lineation sets other than the Hawaiian lineation set indicates that a reorganization of the ridges except the Pacific-Farallon ridge occurred between chrons M1 and M0 (nearly 120 Ma). The eruption rate of the Shatsky hotspot seems to be comparable to that of the Reunion hotspot at their initial stage. The growth of the Pacific plate began in a small triangular region at about 190 Ma.

We have compiled 3.5 million data of marine magnetic anomalies in four areas in the Northeast Pacific, Atlantic, and Southeast Indian Oceans and tried to produce an expansive database of marine magnetic anomalies. The boundaries of the study areas are 20°-60°N and 170°-105°W for the Northeast Pacific, 20°-60°N and 70°-5°W for the North Atlantic, 50°-25°S and 45°W-10°E for the South Atlantic, and 60°-30°S and 125°-145°E for the Southeast Indian Ocean. Marine magnetic anomalies were obtained from geomagnetic total intensity data using IGRF/DGRF. All the data of marine magnetic anomalies in those areas were reduced to a 5-minute regular grid using weighted average interpolation. The gridded data in the North-east Pacific and the North Atlantic provide quite remarkable resolution of magnetic anomaly lineations, fracture zones, and seamounts. Results in the South Atlantic and the Southeast Indian Ocean show a limit of the gridding procedure due to low data density. We can, however, see characteristic patterns of marine magnetic anomalies corresponding to major tectonic features in parts of the areas where the data density is comparatively high. This attempt to make a database of marine magnetic anomalies has succeeded at least in the Northeast Pacific and North Atlantic Oceans, because magnetic anomaly maps confirm that characteristic features of marine magnetic anomalies in these areas are well represented by the gridded data. We can easily compare the magnetic database with other database such as the database of topography (ETOPO5). As more cruise data are accumulated in areas which have lack of magnetic data, this magnetic database will be more useful, especially to analyze a global marine magnetic anomaly.

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.

Japan Sea Legs 127 and 128 of Ocean Drilling Program were conducted in the summer of 1989. The drilling results at 6 sites during the cruises gained our understanding on the history of the Japan Sea. Based on the new chrono-stratigraphic informations, opening of the Japan Sea started at least before 18 to 20Ma, which is much older than the age of 15Ma estimated from paleomagnetic evidence by OTOFUJI and MATSUDA (1983).Lithostratigraphy of the sediments above the acoustic basement was established, which was proved to be correlatable throughout the Japan Sea. The lithostratigraphy is also correlatable with that in oilfield areas in the Tohoku region. The change in lithology through time is probably influenced by global changes in sea level as well as climate.Seismic stratigraphy is correlated with lithostratigraphy at 3 sites drilled during Leg 127. The result suggests remarkable coincidence of opal-A/CT diagenetic boundary with the Bottom Simulating Reflector [BSR].These results will have profound influences on stratigraphic studies in the Japan Sea and its environs.

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 have investigated newly discovered submarine hydrothermal plumes over the Central Indian Ridge, 18°–20°S. Onshore chemical analyses of methane, its carbon isotope, manganese, and helium-3 in seawater samples obtained from the plumes revealed their detailed geochemical characteristics. One of the newly discovered hydrothermal plumes located over the western wall of the axial valley at Segment 15B (19°33′S), called the Roger Plateau, showed constant CH4/Mn and CH4/3He ratios throughout the plume. The CH4/3He ratio (4 × 106) and δ13C (−17.5‰) are consistent with those of basalt-hosted sediment-free hydrothermal systems, although the CH4/Mn ratio (∼1) is moderately higher. These features are thought to indicate a metal-depleted fluid chemistry. The other hydrothermal field is located within the axial valley at the northern part of the smooth lava plain at Segment 16 (18°20′S), called the Dodo Great Lava Plain, where several plumes were detected. The CH4/Mn and CH4/3He ratios showed large variation through the plumes while δ13C values were almost constant. Geochemical characteristics of venting fluid estimated from those of the plumes were apparently high CH4/Mn (>6) and CH4/3He (>60 × 106) ratios and low δ13C values (

Statistical analysis of underway bathymetric profiles of the Philippine Sea back-arc basins; the Mariana Trough, the Shikoku Basin, the Parece Vela Basin and the West Philippine Basin reveals an age-depth correlation in the basins. The basement depths of the Philippine Sea range from 3200 to 6000 m with their ages from 0 to 60 Ma. The age-depth curve of the whole Philippine Sea can be represented by the following equation: Depth (m) = 3222 + 366√Age (Ma) The equation shows that the basement depth of the Philippine Sea is about 800 m deeper than that of the major ocean floors of the same age. The coefficient of 366 in this equation is close to that for major oceans, 350. This suggests that the cooling processes of the lithospheres in the Philippine Sea and major oceans are similar to each other, whereas the structure and composition of the deeper layers of the Philippine Sea seem to be different from those of major oceans. Our analysis of ETOP digital bathymetric data, however, indicates that this relationship does not hold for some other marginal basins. Marginal basins other than back-arc basins generally follow the age-depth curve of major oceans. Young back-arc basins (

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.

The autonomous underwater vehicle "r2D4", constructed in 2003, is a cruising type AUV weighing 1,600 kgf with a length of 4.4 m. It is fitted with a side scanning sonar, an interferometry sonar, a CTDO, and an in-site chemical analyzer "GAMOS" for measurement of manganese concentration. The AUV "r2D4" has been deployed off Sado-ga-shima Island along a fault line in the Sea of Japan (2003), over Kuroshima Knoll (2003), North West Rota 1 Underwater Volcano (2004) where it took video images of plumes, and Myojin-sho Underwater Crater (2005), and demonstrated its high performance in autonomy and reliability in hostile environments. During NH06-4 Leg 3 cruise in December 2006, we deployed "r2D4" in the rift valley of Segments 15 and 16 of the Central Indian Mid-Ocean Ridge System. The first area "r2D4" surveyed is "Roger Plateau (tentative name)" at 19 34S, 65 51E. The AUV "r2D4" performed three separate dives and found some anomalies in the concentration of manganese, but we could not identify the location of the origin of these anomalies. The second area is a narrow gorge in the middle of segment 16. The AUV"r2D4" descended to the east peak of the rift valley, with a depth of 2,400 m, near the center of the gorge. After calibration of its magnetometer, the AUV jumped off the cliff to the bottom of 2,700 m deep gorge. The AUV "r2D4" cruised over the bottom of the gorge for about 6 hours at 3.3 knots and took side scan images of the bottom surface covering 25 km2. The images show that there is a flat wide lava plain, which is 26 km in length and has an average width of 2.7 km, i.e. 70 km2 in area. At the north part of the plain, there is a small rock mound. When "r2D4" passed over the rocks, it found significant anomalies in the concentration of manganese and turbidity. Based on the results of the following Tow-yo survey, we came up with a conclusion that there is hydro-thermal activity at this location. It is concluded that based on a carefully-designed navigation plan the cruising type AUV can be a powerful tool for survey of mid-ocean ridge systems with complicated bottom topography.

In August 2005, we succeeded in operating AUV "r2D4", which was constructed in July 2003 as one of fruits of R-Two project, into Myojin-sho underwater Caldera in full autonomous mode. The caldera is located about 800 km in the south of Tokyo, and 8 km in diameter. There is an active underwater volcano Myojin-sho at the Northeastern part of outer rim of caldera, which erupted repeatedly in 1952. The Kuroshio current is usually running over the Caldera, so that current speed around it is sometimes more than 3 knots. It can be said that the diving of human occupied vehicle and remotely operated vehicle into Myojin-sho Caldera is very dangerous due to such hostile environment. The interferometry SONAR captured a clear image of the central cone in the middle of crater, and the in-situ chemical analyzer "GAMOS" detected high concentration of manganese ion which indicates hydro-thermal activity in the crater.

[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 Magellan trough is a WNW-trending linear depression with flanking ridges that extends more than 300 km in the Central Pacific Basin. Depth of the trough axis exceeds 6600 m. Compilations and analyses of bathymetric, seismic, and geologic data of the trough and its adjacent areas reveal three discriminate acoustic layers: a transparent layer (Unit I), a semi-opaque layer (Unit II), and an opaque layer (acoustic basement) in descending order. In the southern part of the study area (south of the trough), calcareous turbidites (Unit I) whose ages range probably from the Oligocene to the middle Eocene, were derived from the Magellan Rise. This turbidite layer tends to become thinner towards the trough. The southern flanking ridge of the trough, therefore, may have been a barrier to the deposition of the turbidites. The thickness of the transparent layer in Unit I varies from place to place, particularly in the area north of the trough, whereas the thickness of Unit II is rather constant. Some 3.5- kHz profiles show the truncation and abrupt thinning of the transparent layer (Unit I). A hiatus between the late Pliocene and the late Miocene exists in Unit I. These geological findings suggest that the deposition of Unit I has been affected by bottom water currents such as the recent Antarctic bottom water current, which plays a significant role in the deposition of sediment. Major features of the sea floor, namely the surface topography of the acoustic basement and the pattern of faults within the basement around the trough, show trends concordant with those of the trough. The concordant features trend WNW-ESE and converge to the northwest. A detailed bathymetric chart of the study area has been constructed showing left and right lateral offsets of the trough together with the flanking ridges. These bathymetric features of the trough area are concordant with the fan-shaped magnetic anomaly lineations. The bathymetric and magnetic lineations are evidence of sea-floor spreading at the axis of the trough in the Early Cretaceous (M9 to M11). These morphotectonic observations are good indicators of fundamental processes of sea-floor spreading that produced the Magellan system.