Masato Joshima

The North Fiji Basin basalts and their magma sources: Part II. Sr-Nd isotopic and trace element constraints

Abstract Sr-Nd isotope and trace element data are reported for basalts from the North Fiji Basin (NFB). NFB basalts are characterized by extreme variations in isotopic ratios and trace element abundances which are related to mantle heterogeneities. Values of 87 Sr 86 Sr for basalts from the northern segments, N160° and triple junction range from 0.7029 to 0.7041, whereas 143 Nd 144 Nd values vary from 0.51281 to 0.51313. Most of the basalts from these segments are characterized by strong relative enrichments in Rb, Ba, Sr, K, Nb, Ta La, Ce and Ti that are comparable to OIB components. The central segments, N15° and N-S have 143 Nd 144 Nd ratios between 0.51298 and 0.51363 and 87 Sr 86 Sr ratios between 0.7029 and 0.7033, and are depleted in large ion lithophile, light rare-earth and high field strength elements similar to N-MORB. Covariation between trace element and isotopic ratios among NFB basalts supports a model in which melts from the NFB rift system are derived by mixing of OIB-type and depleted N-MORB mantle components. The Sr-Nd isotopic and trace element variability indicates that the NFB basalt source is heterogeneous on the scale of individual melt batches.

Physiographical and sedimentological characteristics of submarine canyons developed upon an active forearc slope: The Kushiro Submarine Canyon, northern Japan

Comprehensive geological surveys have revealed the physiographical and sedimentological characteristics of the Kushiro Submarine Canyon, one of the largest submarine canyons around Japan. The canyon indents the outer shelf along a generally straight, deeply excavated course of more than 230 km in length upon the active forearc slope of the Kuril Trench in the Northwest Pacific. The forearc slope has a convex-upward geometry that can be divided into upper and lower parts separated by an outer-arc high (3200–3500 m water depth). The upper slope consists of gently folded forearc sediments, and the lower slope is underlain by sedimentary rocks deformed by subduction-related processes. The upper reaches of the canyon (~3250 m of thalweg water depth) are developed on the upper slope, showing a weakly concave-upward longitudinal profile with a gradual down-canyon increase in relief between the thalweg and the canyon rim. Although an infill of hemipelagic mud and the absence of turbidite deposits indicates that the upper part of the upper reaches of the canyon (~900 m thalweg water depth) is inactive, the lower part of the upper reaches (900–3250 m thalweg water depth) is considered to be an active conduit to the lower reaches, as determined from voluminous turbidites recovered in sediment cores (~76-yr intervals) and rockfalls observed in the canyon bottom by deep-sea camera. A number of gullies developed upon the northern slope of the lower part of the upper reaches might well provide a frequent supply of turbidity currents, giving rise to a down-canyon increase in the frequency of flow events. The down-canyon increase in flow occurrence is related to a gradual decrease in gradient, demonstrating an inverse power-law relationship between slope and drainage area. In contrast, the lower reaches of the canyon (3250–7000 m thalweg water depth) are characterized by a gradual decrease in relief, a high gradient, and extremely low sinuosity. The limited increase in drainage area down-canyon of the confluence with the Hiroo Submarine Channel, which is the largest tributary of the main canyon, indicates that the erosional force of turbidity currents decreases down-canyon. The gradient of the lower reaches largely reflects the morphology of the forearc slope along the canyon, which has been deformed by subduction-related tectonics. The lack of an inverse power-law relationship between gradient and drainage area in the lower canyon supports the hypothesis that the topography of the lower reaches is dominated by subduction-related tectonic deformation of the substrate rather than canyon erosion. Interrelationships between canyon erosion by currents and tectonic processes along the forearc slope are important in the development of the physiography of submarine canyons upon active forearc margins.

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.

Ultrafine-scale magnetostratigraphy of marine ferromanganese crust

Hydrogenetic ferromanganese crusts are iron-manganese oxide chemical precipitates on the seafloor that grow over periods of tens of millions of years. Their secular records of chemical, mineralogical, and textural variations are archives of deep-sea environmental changes. However, environmental reconstruction requires reliable high-resolution age dating. Earlier chronological methods using radiochemical and stable isotopes provided age models for ferromanganese crusts, but have limitations on the millimeter scale. For example, the reliability of 10 Be/ 9 Be chronometry, commonly considered the most reliable technique, depends on the assumption that the production and preservation of 10 Be are constant, and requires accurate knowledge of the 10 Be half-life. To overcome these limitations, we applied an alternative chronometric technique, magnetostratigraphy, to a 50-mm-thick hydrogenetic ferromanganese crust (D96-m4) from the northwest Pacific. Submillimeter-scale magnetic stripes originating from approximately oppositely magnetized regions oriented parallel to bedding were clearly recognized on thin sections of the crust using a high-resolution magnetometry technique called scanning SQUID (superconducting quantum interference device) microscopy. By correlating the boundaries of the magnetic stripes with known geomagnetic reversals, we determined an average growth rate of 5.1 ± 0.2 mm/m.y., which is within 16% of that deduced from the 10 Be/ 9 Be method (6.0 ± 0.2 mm/m.y.). This is the finest-scale magnetostratigraphic study of a geologic sample to date. Ultrafine-scale magnetostratigraphy using SQUID microscopy is a powerful new chronological tool for estimating ages and growth rates for hydrogenetic ferromanganese crusts. It provides chronological constraints with the accuracy promised by the astronomically calibrated magnetostratigraphic time scale (1–40 k.y.).

Age of the Solomon Sea Basin from magnetic lineations

Magnetic anomalies measured in the central to western half of the Solomon Sea, when considered with other magnetic data, reveal the existence of linear patterns. Magnetic lineation anomaly models of the Cenozoic, 65 to 0 Ma, suggest that an age between 34 and 28 Ma and a half-rate spreading speed of 5.8 cm/yr for the northern flank of a former spreading center best fits our present magnetic data in the Solomon Sea Basin. Heat flow and bathymetry data support this preferred model.

Bathymetry and magnetic anomalies in the Havre Trough and southern Lau Basin: from rifting to spreading in back-arc basins

Abstract The Havre Trough and Lau Basin are active back-arc basins related to Pacific-Australian plate convergence. These back-arc basins provide insight into the evolution of rifting through various stages, including a final stage of oceanic spreading. We carried out a swath bathymetry and magnetic anomaly survey focused on the less-studied Havre Trough and southern Lau Basin. The western portion of the whole Havre Trough is deeper and more thickly sedimented whereas shallow bathymetry and volcanic edifices are dominant in the east. We suggest that crustal rifting progresses asymmetrically with respect to the trough axis. The active tectono-magmatic zone migrates trenchward during ongoing widening. Rifting along normal faults trending in the longitudinal trough strike occurs in the southern Havre Trough. Volcanism is restricted to a line along the normal faults. Circular or elongated knolls in the northern Havre Trough indicate that en echelon type segmented rifting of short strike length and successive volcanic intrusion takes place. Volcanism in this area is dominated by diapir-like magma intrusion into an extended and thinned arc crust. The southernmost tip of the spreading center of Lau Basin is defined by the deep graben, striking at N20°E, located at 24°00′S, 177°10′W.

Kinematics of active spreading in the central North Fiji Basin (Southwest Pacific)

Abstract Based on a synthesis of magnetic and bathymetric data, we re-evaluate the kinematics of the recent opening of the central part of the North Fiji Basin (NFB). The westward motion of the Pacific plate along the left-lateral North Fiji Fracture Zone (NFFZ) results in the opening of two N-S trending spreading ridges, located at 173°30′E and 176°E. Both ridges show complex features such as propagating rifts, ridge jumps and overlapping spreading centres. Their spreading rates are similar: 7.6 to 4 cm yr −1 across the western ridge, 5.5 cm yr −1 across the eastern one. While the NFFZ is purely strike-slip to the east of the eastern ridge, it becomes more complex to the west: it changes from transpressional to transtensional, then to purely transform and finally joins the western N-S ridge in a RRR-type triple junction. Our kinematic analysis shows that most of the left-lateral motion along the NFFZ is transferred to the eastern ridge at the RTF-type 176°E triple junction. It suggests that the western N-S ridge is probably connected to the north with another left-lateral transform, possibly the South Pandora “ridge”.

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.