Manabu Tanahashi

Oblique and near collision subduction, Sagami and Suruga Troughs —preliminary results of the French-Japanese 1984 Kaiko cruise, Leg 2

Abstract Leg 2 of the French-Japanese 1984 Kaiko cruise has surveyed the Suruga and the Sagami Troughs, which lie on both sides of the northwestward moving and colliding Izu-Bonin Ridge, the northernmost part of the Philippine Sea plate. The transition from the Nankai Trough to the Suruga Trough is characterized by northward decrease in width of the accretionary prism, in good agreement with the increasing obliquity between the through axis and the direction of the convergence, as the strike of the convergent boundary changes from ENE-NNE to south-north. South of the area, the southern margin of the Zenisu Ridge shows contractional deformations. This supports the interpretation made by the team of Leg 1 who studied the western extension of the area we studied, that it is an intra-oceanic thrusting of the ridge over the Shikoku Basin. In the Sagami Trough, where the relative motion is highly oblique to the plate boundary, active subduction is mostly confined in the east-west trending portions of the trough located south of the Boso Peninsula and along the lower Boso Canyon, near the TTT triple junction. In between, the present motion is mainly right-lateral along the northwest trending Boso escarpment. However, an inactive but recent (Pliocene to lower Pleistocene) accretionary prism exists south of the Boso escarpment, which suggests that the relative motion was more northerly than at present before about 1 Ma ago.

Local variability of manganese nodule facies on small abyssal hills of the Central Pacific Basin

Abstract Local variability of manganese nodule facies and associated sediments were studied around abyssal hills ( 60 × 60 km 2 ) located in the equatorial zone of the Central Pacific Basin. Small-scale variations in nodule occurrence, mineralogy, chemistry, and internal structure are discussed in relation to seismic records and sediment lithology. The slow sedimentation rate from the late Tertiary to the Quaternary appear to have promoted formation of nodules around the abyssal hills, although the Plio-Pleistocene hiatus at depths below sediment surface suggests no clear evidence of increased growth of nodules during the period. In the area of rapid and continuous sedimentation away from the hill areas, nodules are rare probably because of insufficient lifting force for nodules. Mineralogical and chemical studies reveal that the two principal nodule morphologies (smooth and rough surfaces) result from preferential deposition of hydrogenous or diagenetic manganese minerals in relation to early diagenesis of surface siliceous sediments. Regional variability of nodule-surface morphology and composition suggests a very local change in sedimentary conditions in space, and internal mineral variation suggests changes in sedimentary conditions with time. The geological factors controlling the small-scale variability of nodule facies seem to be similar to those controlling regional-scale variations.

Active Spreading and Hydrothermalism in North Fiji Basin (SW Pacific). Results of Japanese French Cruise Kaiyo 87

The aim of the Japanese-French Kaiyo 87 cruise was the study of the spreading axis in the North Fiji Basin (SW Pacific). A Seabeam and geophysical survey allowed us to define the detailed structure of the active NS spreading axis between 16° and 22° S and its relationships with the left lateral motion of the North Fiji Fracture Zone. Between 21° S and 18°10′ S, the spreading axis trends NS. From 18°10 S to 16°40 S the orientation of the spreading axis changes from NS to 015°. North of 16°40′ S the spreading axis trends 160°. These two 015° and 160° branches converge with the left lateral North Fiji fracture zone around 16°40′ S to define an RRFZ triple junction. Water sampling, dredging and photo TV deep towing give new information concerning the hydrothermal activity along the spreading axis. The discovery of hydrothermal deposits associated with living communities confirms this activity.

Trench triple junction off Central Japan—preliminary results of French-Japanese 1984 Kaiko cruise, Leg 2

Abstract Leg 2 of the French-Japanese 1984 Kaiko cruise has surveyed the trench triple junction off central Japan, where the Japan, Izu-Bonin and Sagami Trenches intersect. The Izu-Bonin Trench is deeper than the Japan Trench and filled by a thick turbiditic series. Its anomalous depth is explained by the westward retreat of the edge of the northwestward moving Philippine Sea plate. On the contrary to what happens in the Japan Trench, horst and graben structures of the Pacific plate obliquely enters the Izu-Bonin Trench, suggesting that the actual boundary between these two trenches is located to the north of the triple junction. The inner wall of the Izu-Bonin Trench is characterized in the triple junction area by a series of slope basins whose occurrence is related to the dynamics of this area. The northernmost basin is overthrust by the edge of the fore-arc area of the Northeast Japan plate. The plate boundary is hardly discernible further east, which makes it impossible to locate precisely the triple junction itself. These features suggest that large intra-plate deformation occurs there due to the interaction of the plates involved in the triple junction and the weak mechanical strength of the wedge-shaped margin of the overriding plates.

Geological structure of the central spreading system, North Fiji Basin

Abstract SeaBeam mapping, single-channel and multichannel seismic profiling, sonobuoy-OBSH refraction measurements and geomagnetic anomaly observations obtained during the cruises KAIYO87 and 88 in the North Fiji Basin reveal the detailed structure and history of jumps of young spreading systems. Currently active spreading systems north-northwest and south-southwest of a triple junction at about 17°S, 174°E, where active hydrothermal activity was discovered during KAIYO88, jumped to their present positions possibly in the late Quaternary. The spreading ridge south-southwest of the triple junction, which is oriented N15°–20°E, has a 0.7–3.5 km wide axial graben and a 15–35 km wide trapezoidal morphological profile. It cuts into a wide N40°–60°E oriented graben whose side walls themselves sharply cut previously formed structures, including a high seamount which is now separated into three highs.

In situ geological and geochemical study of an active hydrothermal site on the North Fiji Basin ridge

Abstract In June 1989 a diving cruise by the submersibleNautile was carried out on the North Fiji Basin spreading ridge. The objective of the cruise was a geological and geochemical study of the active spreading axis and associated hydrothermal processes. This operation was the third cruise of the French-Japanese Starmer project following the two cruises of the R.V.Kaiyo in 1987 and 1988. Six dives along the spreading axis between 16°58′S and 17°00S show that the axial graben consists of alternating N15—trending horsts and grabens. Extinct hydrothermal sites have been observed all along the graben. These consist of fossil chimneys, oxide staining and dead shells. At 16°59′S an active chimney has been discovered and called the “White Lady” because of its almost exclusively anhydrite composition. This chimney expels a peculiar water characterized by low chlorinity and a 285°C maximum temperature. In the northern extremity of the N15 axis a wide fossil hydrothermal site has been explored and sampled. It is located in an area cut by N15, N140 and N60-trending faults and fissures.

800-km-long N-S spreading system of the North Fiji Basin

Abstract The 800-km-long N-S spreading system in the North Fiji Basin consists of six contiguous fan-shaped rift segments each 100–200 km long with various structural styles, such as structural overprinting, triple junction, rift-propagation, voluminous magmatism, and transform or strike-slip fault influence. Deformation of the plate boundary system within the young, hot, weak lithosphere occurred under local stress conditions resulting in a unique rift segmentation distinct from that of midoceanic ridges. Spreading across a series of short and variably oriented segments produces series of fanning spreading centers whose rotation pole is located at the end of each segment. The surrounding seafloor basement has adjusted to this setting with non-rigid deformation or with fragmentation. Furthermore, the changing tectonic framework caused by the arc rotation probably makes it difficult to keep the same stress condition for a long period. Small-scale fan-shaped sea-floor spreading is short-lived and results in a rapidly evolving plate boundary geometry. This suggests a more complicated tectonic style for marginal basins than mid-ocean ridge spreading centers.

Propagating rifts in the North Fiji Basin (southwest Pacific)

The accretion system in the North Fiji marginal basin currently consists of four major, differently oriented axes. Multibeam bathymetry, recently acquired over the central north-south axis between 18 ° and 21 °S, reveals a peculiar tectonic framework. Fanned patterns of the sea-floor topography at the northern and southern tips of this segment are the general shape of a rugby ball. These data, and the interpretation of magnetic anomalies, indicate that this central sector of the ridge has lengthened northward at the expense of the axis aligned N15 °E. Both spreading and propagation rates are about 70-80 mm/yr. A southward propagation is also envisaged on the basis of the similar morphotectonic framework observed at the southern tip of the north-south ridge.

The western Fiji Transform Fault and its role in the dismemberment of the Fiji Platform

Abstract The Fiji Transform Fault (FTF) is a sinistral transform fault trending nearly E-W from the central North Fiji Basin triple junction at 17°S, 174°E to the northern Lau Basin and is characterized by very high bathymetric relief and dispersed seismicity. SeaMARC II and GLORIA sidescan imagery reveal the structural fabric along the fault trace. From the triple junction to 176°E, the fault location is uncertain but lies within an area having northeast and southeast trending structures. It is believed that a right step in the fault trace has produced a component of compression. From 176°E to 179°E, the fault is clearly delineated next to the Fiji Platform and within two left step offsets are short spreading segments. It is hypothesized that periodic propagation of these short segments into the Fiji Platform has accompanied changes in the location of the fault and in so doing has broken off parts of the Fiji Platform.

Study yields surprises about seafloor spreading in back‐arc basins

A study of the seafloor in the North Fiji Basin is leading to a better understanding of the geometry and evolution of spreading in back-arc basins. Two of the most important features of the area, the South Pandora Ridge and the Tripartite Ridge, belong to a presently active spreading system that began forming at least 7.2 m.y.a. The ridges spread at an average rate of about 1.6 cm/yr. By studying the segments of the ridges in greater detail, researchers found that each was either a “tectonic type,” with a more or less complex deep axial valley, or a “volcanic type,” with a high, rifted volcanic massif occupying the axial domain.