Kiyoyuki Kisimoto

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.

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.

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.

Crustal structure variation along the central rift/ridge axis in the North Fiji Basin: Implications from seismic reflection and refraction data

Abstract Seismic reflection and refraction surveys carried out in the accretionary region of the North Fiji Basin (NFB) have revealed several features of the crustal structure along the axial domain, some of which can be interpreted as the results similar to the phenomena of the active rifting and spreading in the mid-ocean ridges system. Although the multi-channel seismic (MCS) reflection profiles recorded along the central axial graben and ridge, where the most active hydrothermal activities were observed and medium to fast crustal spreading rate was implied, do not show clear structural evidence of the axial magma chamber (AMC) reflector, very shallow Moho (approximately 3 km below the sea floor, unusually thin crustal thickness) is deduced from the refraction data obtained from the site around station 14, where the most dominant spreading is occurring to date in the NFB. The seafloor in this region is sediment free. This is interpreted to be simply because the crust is young and the volcanic material is directly exposed at the seafloor as a result of the recent volcanism. This is also confirmed from the velocity profiles obtained by refraction experiments. Although the station distribution of the refraction experiments is not dense enough to cover all along the axial region, the along-axis lateral variation of the velocity-depth profile of the crust is implied. In general, the feature of the crustal velocity structure in the axial domain deduced from seismic reflection and refraction experiments indicates that the central spreading/rifting system of the NFB is very similar to that of mid-ocean ridge crust rather than those of island- or back-arc type or the typical mature oceanic crust, which are observed in the NFB, outside of the central domain.

High-resolution bathymetry using Alvin scanning sonar at the Southern East Pacific Rise and its implication to the formation of collapsed lava lakes

A high-resolution bathymetric map of a collapsed lava lake on the ridge crest of the superfast-spreading Southern East Pacific Rise was constructed. The data were acquired during the MOAI’98 Cruise using a pencil-beam scanning sonar that was installed on the submersible Alvin. The map covers an area of 200 × 350 m at the Oasis hydrothermal site (17°25.4′S, 113°12.3′W) where low-temperature fluids are venting from a collapsed lava lake. The collapsed lava lake is 250 m long, with a mean width of 10 m in parallel to the spreading axis on the western flank of the ridge crest. The estimated volume of lava that has drained out is about 7000 m3 erupted from a single event. This small amount of melt and frequent contacts between “young” and “younger” flows indicate that volcanism at the superfast-spreading ridge system is characterized by frequent eruptions of very small volumes of magma.

Sea Floor Mapping using the Data of Forward Looking Sonar and Side-Scan Sonar around the Hydrothermal Sites, South Mariana Trough

Processed data of the mesotech 971 forward looking sonar was successfully plotted on a map and compared with side-scan-sonar (SSS) data and color image of original forward sea floor record. The accuracy of positions by LBL acoustic system is insufficient to process the sea-floor mapping. The accuracy and density of positions controls, in other saying, decides the quality of mapped seafloor images.