Kensaku Tamaki

Japan Sea, opening history and mechanism: A synthesis

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.

Hydrothermal activity along the southwest Indian ridge

Twenty years after the discovery of sea-floor hot springs, vast stretches of the global mid-ocean-ridge system remain unexplored for hydrothermal venting. The southwest Indian ridge is a particularly intriguing region, as it is both the slowest-spreading of the main ridges and the sole modern migration pathway between the diverse vent fauna of the Atlantic and Pacific oceans. A recent model postulates that a linear relation exists between vent frequency and spreading rate and predicts vent fields to be scarcest along the slowest-spreading ridge sections, thus impeding migration and enhancing faunal diversity. Here, however, we report evidence of hydrothermal plumes at six locations within two 200-km-long sections of the southwest Indian ridge indicating a higher frequency of venting than expected. These results suggest that fluxes of heat and chemicals from slow-spreading ridges may be greater than previously thought and that faunal migration along the southwest Indian ridge may serve as an important corridor for gene-flow between Pacific and Atlantic hydrothermal fields.

The Bonin Arc

The Bonin Arc is a well-developed island arc, associated with a deep trench, an active volcanic chain, and a back-arc basin. The Bonin Arc is approximately 1100 km long, extending north—south from latitude 35°N to 25°N within longitude 139°E and 145°E, facing the Pacific plate in front and the Shikoku Basin behind (Fig. 1). At its northern limit, the Bonin Arc forms part of a triple junction of trench—trench—transform fault or trench—trench—trench with the Tohoku and the Seinan (southwest) Japan arcs. The Mariana Arc continues the line of the Bonin Arc to the south. There is no marked morphological boundary between the Bonin and the Tohoku arcs, but one is inferred by the depression along the Sagami Trough (see Fig. 13). The southern boundary of the Bonin Arc is even less well defined than the northern margin. The Bonin Arc extends to the Mariana Arc as part of the same morphological sequence from north to south. These three arcs face the Pacific plate.

A new Mesozoic isochron chart of the northwestern Pacific Ocean: Paleomagnetic and tectonic implications

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.

Preliminary analysis of the Knipovich Ridge segmentation: influence of focused magmatism and ridge obliquity on an ultraslow spreading system

Abstract 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.

Age-depth correlation of the Philippine Sea back-arc basins and other marginal basins in the world

Abstract 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 (

Opening Tectonics of the Japan Sea

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.

TOBI sidescan sonar imagery of the very slow-spreading Southwest Indian Ridge: evidence for along-axis magma distribution

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.

Methane, manganese, and helium-3 in newly discovered hydrothermal plumes over the Central Indian Ridge, 18°–20°S

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 (<−27.5‰), suggesting possible influences of several methane input processes to the fluid chemistry.

Discovery of New Hydrothermal Activity and Chemosynthetic Fauna on the Central Indian Ridge at 18°–20°S

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.