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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.

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.

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.

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.

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.

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.

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 (

The eastern margin of the Japan Sea has been under convergent tectonics since sometime after the Pliocene. The suture line is on the ocean-continent boundary between the Japan-Yamato Basins and the Tohoku (Northeast Japan) Arc. Incipient subduction, and obduction, of the oceanic and sub-oceanic crust are observed along the suture line with the occurrence of numerous N-S trending, fault-bounded ridges and troughs. Focal mechanism solutions of several compressional type earthquakes with magnitudes of M = 6.9 to 7.7 which have occurred along the zone are consistent with the observation of geological thrust faults. The convergent stresses in the Japan Sea may be due to India-Eurasia collision and its associated intra-plate or inter-microplate movements in East Asia. The eastward movement of the Amurian Plate causes Baikal extension along its western margin and the Japan Sea convergence along its eastern margin.