Kyoko Okino

Modes of seafloor generation at a melt-poor ultraslow-spreading ridge

We report on extensive off-axis bathymetry, gravity, and magnetic data that provide a 26-m.y.-long record of axial tectonic and magmatic processes over a 660-km-long and melt-poor portion of the ultraslow Southwest Indian Ridge. We describe a new type of seafloor (the smooth seafloor) that forms at minimal ridge melt supply, with little or no axial volcanism. We propose possible mechanisms leading to this avolcanic or nearly avolcanic mode of spreading, in contradiction with the traditional view of mid-ocean ridges as primarily volcanic systems. We also show evidence for large-offset asymmetric normal faults and detachments at the ridge axis, with asymmetry persisting in some cases for tens of millions of years.

The Philippine Sea: New survey results reveal the structure and the history of the marginal basins

The Hydrographic Department of Japan initiated a long-term geological and geophysical survey of the Philippine Sea in 1983. We here summarize the bathymetry and magnetic anomalies of this area, focusing on the structure and the evolutionary process of the marginal basins. The Shikoku and Parece Vela Basins developed as one backarc system in the later phase of their formation; the significant difference between the two basins, however, is clearest near the extinct spreading centers. The steep Oki-Daito Escarpment, located in the northern West Philippine Basin, was mapped for the first time during this survey. The N-S trending spreading fabric north of the escarpment is in striking contrast to the NW-SE to E-W lineament in the south. Our data reveal a more complicated history of the western Philippine Sea than has been proposed in previous studies.

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.

Shikoku Basin and Its Margins

The Shikoku Basin is an inactive backarc basin located south of the southwest Japan arc in the northwestern Pacific margin. Its characteristic features are summarized on the basis of updated geophysical and geological data, including swath bathymetry, gravity, magnetics, and seismic reflection profiling records as well as results from the DSDP/ODP drilling holes. It has been proposed that the Shikoku Basin was born as a rift separating the N-S trending paleo-Kyushu-Palau Ridge at its northern end. The rifting rapidly propagated southward. It was succeeded by seafloor spreading that formed a narrow triangular trough bounded by steep scarps on both east and west margins. Magnetic and bathymetric data indicate that the spreading center has changed its trend at least twice, first at 23 Ma and then at 19 Ma. Widespread off-ridge volcanism occurred after extinction of spreading at 15 Ma. The Kinan seamount chain was formed at this stage. Most of the rocks constituting the basin are tholeiite similar to MORB, whereas some of the off-ridge magmas are alkali basalt. Igneous basement is overlain mostly by hemipelagic sediments containing dispersed detrital clays and interbedded tephra layers. The Shikoku Basin lithosphere is subducting at the Nankai Trough beneath southwest Japan in a NNW direction. Deep-focus earthquakes are not observed at depths greater than 80 km, implying that the subducted young lithosphere loses its rigidity below such depths. It seems plausible to presume that the Shikoku Basin was subducting at the Nankai Trough at 15–12 Ma while the basin floor underwent extensive volcanic intrusions. Emplacement of intermediate to felsic rocks at the outer zone and Setouchi Province of southwest Japan probably originated from this unusual circumstance.

Enigmatic extinct spreading center in the West Philippine backarc basin unveiled

The Central Basin fault in the center of the West Philippine Basin was first discovered ∼50 yr ago. It is a 1000-km-long ridge oriented northwest to southeast and is cut by north-south–trending fracture zones. Hypotheses about the origin and development of the Central Basin fault have remained unresolved until recently. Submersible observations and SeaBeam surveys show that the Central Basin fault is a segmented spreading ridge having a morphology similar to that of a slow spreading ridge, with a nontransform offset, a nodal deep, and an inside corner high. The distance from the ridge versus the depth of the sea floor, the obliqueness of sets of small trough and ridge structures, and heat-flow values both of the crestal and off-axis areas of the Central Basin fault suggest that the fault is not a simple spreading center, but rather underwent multiple spreading episodes. The texture and chemistry of basalts obtained from the ridge suggest that the lavas were formed in a backarc basin setting. These data confirm that the Central Basin fault is a slow backarc spreading center that has a more complicated evolutionary history than previously realized.

Late amagmatic extension along the central and eastern segments of the West Philippine Basin fossil spreading axis

The spreading processes within the West Philippine Basin (WPB) remain partly unknown. This study presents an analysis of the tectono-magmatic processes that happened along its spreading axis during the conclusion of the last spreading phase at 33/30 Ma. We demonstrate that the late episode of N^S opening from an E^W-trending spreading system has been followed by a late tectonic event occurring in the central and eastern parts of the basin. This event was responsible for transtensional strain accommodated along the NW^SE faults cutting through the former E^W rift valley in the central part of the basin. In its eastern part, the same event occurred at a larger extent and led to the creation of a new NW^SE axis, obliquely cutting the older E^W spreading segments and their associated spreading fabrics. At this location, several tens of kilometers of slightly oblique amagmatic extension occurred following a V60‡ direction. We propose that this late event is associated with the onset of E^W opening of the Parece-Vela Basin located along the eastern border of the WPB at 30 Ma. Extensive stresses within this basin were probably transmitted to the hot and easily deformable rift zone of the WPB. The newly-created NW^SE axis most likely propagated from east to west, being responsible for scissors opening within the WPB. NE^SW extension ceased when well-organized spreading started at 26 Ma in Parece-Vela Basin, accommodating entirely the global extensive stress pattern. ; 2002 Elsevier Science B.V. All rights reserved.

Hydrothermal activity on the ultra-slow spreading southern Knipovich Ridge

We report first evidence for hydrothermal activity from the southern Knipovich Ridge, an ultra-slow spreading ridge segment in the Norwegian-Greenland Sea. Evidence comes from optical backscatter anomalies collected during a systematic side-scan sonar survey of the ridge axis, augmented by the identification of biogeochemical tracers in the overlying water column that are diagnostic of hydrothermal plume discharge (Mn, CH4, ATP). Analysis of coregistered geologic and oceanographic data reveals that the signals we have identified are consistent with a single high-temperature hydrothermal source, located distant from any of the axial volcanic centers that define second-order segmentation along this oblique ridge system. Rather, our data indicate a hydrothermal source associated with highly tectonized seafloor that may be indicative of serpentinizing ultramafic outcrops. Consistent with this hypothesis, the hydrothermal plume signals we have detected exhibit a high methane to manganese ratio of 2–3:1. This is higher than that typical of volcanically hosted vent sites and provides further evidence that the source of the plume signals reported here is most probably a high-temperature hydrothermal field that experiences some ultramafic influence (compare to Rainbow and Logachev sites, Mid-Atlantic Ridge). While such sites have previously been invoked to be common on the SW Indian Ridge, this may be the first such site to be located along the Arctic ultra-slow spreading ridge system.

Survey maps Philippine Sea structures

Detailed structural images of two basins in the Philippine Sea and related data collected by the Japanese Continental Shelf Surveys Project are increasing our understanding of how back-arc basins evolve. The area surveyed covers the southeastern East China Sea, the northern Philippine Sea, and the westernmost Northwest Pacific Basin (Figure 1). One purpose of the survey is to gather information to help define Japans legal continental shelf in the context of the United Nations Convention on the Law of the Sea, which became effective in 1994.

High‐resolution magnetic signature of active hydrothermal systems in the back‐arc spreading region of the southern Mariana Trough

High-resolution vector magnetic measurements were performed on five hydrothermal vent fields of the back-arc spreading region of the southern Mariana Trough using Shinkai 6500, a deep-sea manned submersible. A new 3-D forward scheme was applied that exploits the surrounding bathymetry and varying altitudes of the submersible to estimate absolute crustal magnetization. The results revealed that magnetic-anomaly-derived absolute magnetizations show a reasonable correlation with natural remanent magnetizations of rock samples collected from the seafloor of the same region. The distribution of magnetic-anomaly-derived absolute magnetization suggests that all five andesite-hosted hydrothermal fields are associated with a lack of magnetization, as is generally observed at basalt-hosted hydrothermal sites. Furthermore, both the Pika and Urashima sites were found to have their own distinct low-magnetization zones, which could not be distinguished in magnetic anomaly data collected at higher altitudes by autonomous underwater vehicle due to their limited extension. The spatial extent of the resulting low magnetization is approximately 10 times wider at off-axis sites than at on-axis sites, possibly reflecting larger accumulations of nonmagnetic sulfides, stockwork zones, and/or alteration zones at the off-axis sites.

Tectonics of Unusual Crustal Accretion in the Parece Vela Basin

Despite its rapid intermediate-spreading rate, the Parece Vela Basin (PVB) shows unusual characteristics that indicate a depressed magmatic budget, such as the occurrence of numerous oceanic core complexes (OCCs) and rugged terrain, exposing abundant peridotites and gabbros. Based on the geologic interpretations of crust with analogous features on global mid-ocean ridges, we propose three possible mechanisms that can account for these unusual characteristics: (1) presence of a cold and/or refractory mantle domain, (2) declining spreading rate during the later phase of the second-stage spreading of the PVB, and (3) a transform sandwich effect. Recent numerical modeling for formation of OCC suggests that there is a minimum as well as a maximum magmatic supply necessary to produce long-lived detachment fault. In the western PVB, a cold and/or refractory mantle domain inhibited a large amount of mantle melting within an intermediate-spreading ridge, attaining the limited window of the condition of magma supply demonstrated in the numerical model in an otherwise robust magmatic environment. In the central PVB, a transform sandwich effect and/or declining spreading rate inhibited a large amount of mantle melting within an intermediate-spreading ridge, also attaining the limited window of the condition of magma supply demonstrated in the numerical model in an otherwise robust magmatic environment.