Ayu Trough Research Articles

Spreading rate, opening time, and kinematic history of the Ayu Trough

Spreading rate, opening time, and kinematic history of the Ayu Trough

Meter-Scale Seafloor Geodetic Measurements Obtained from Repeated Multibeam Sidescan Surveys

We calibrate a technique to use repeated multibeam sidescan surveys in the deep ocean to recover seafloor displacements greater than a few meters. Displacement measurements from seafloor patches (3 km by 20 km) on the port and starboard side of the ship are used to estimate vertical and across-track displacement. We present displacement measurements from a survey of the Ayu Trough southwest of the Marianas Trench using a 12 kHz multibeam. Vertical and across-track displacement errors for the 12 kHz multibeam sonar are typically 0–2 m with RMS uncertainties of 0.25–0.67 m in the across-track and 0.37–0.75 m in the vertical as determined by 3-way closure tests. The uncertainty of the range-averaged sound velocity is a major error source. We estimate that variations in the sound velocity profile, as quantified using expendable bathythermographs (XBTs) during data collection, contribute up to 0.3 m RMS uncertainty in the across-track direction and 1.6 m RMS uncertainty in the vertical direction.

Geochemistry of basalt from the Ayu Trough, equatorial western Pacific

The Ayu Trough is an approximately 600-km-long divergent margin between the Philippine Sea and Caroline plates located in the western Pacific near the equator. Characterized by rugged seafloor topography and deep axial depths that average greater than 4000 m, the Ayu Trough has been considered as a slow spreading center. Here we report geochemical analyses of Ayu Trough basalts (ATB) dredged from seven sites along the axis from 0°30′ to 4°30′N. The ATB vary geochemically from N-MORB (La/SmN<1) to relatively enriched (E)-MORB (La/SmN>1.4). N-MORB were recovered from five sites of axis wall, while E-MORB were sampled from one site, an axial volcanic ridge inside the median valley at 2°45′N. At one site, transitional (T)-MORB were recovered in addition to N-MORB. Variations in incompatible element distribution patterns and Sr–Nd–Pb isotopic ratios between the N-MORB and E-MORB samples are too large to be explained solely by melting of a uniformly depleted mantle source. It appears that at least two distinct mantle components, depleted and enriched, were involved in the formation of the ATB. N-MORB isotope ratios are similar to those associated with Indian-Ocean-type mantle, whereas those of E-MORB lie on a trend towards those of EM2. Based on incompatible element and isotope ratios, we suggest that the enriched component, source of E-MORB, beneath the Ayu Trough was generated in a subduction zone by metasomatism of low-degree melt and addition of a small amount of sediment. Although N-MORB generally conform to global correlations between the axial depth and basalt chemistry, E-MORB retrieved from the axial volcanic ridge include two distinct lava suites with (Sm/Yb)N ratios of ∼1.8 and 1.3, suggesting that they were generated at different mean depths of melting, and possibly different mean degrees of partial melting. If the low-degree melt samples are younger than the higher degree of melt samples, then this finding is consistent with inferences based on geophysical observations such as at least 50–70-m-thick sediment on the axis, that spreading in the Ayu Trough may have recently slowed dramatically or even ceased, leading to a diminished melt production and construction of a localized axial volcano.

Bacillus alveayuensis sp. nov., a thermophilic bacterium isolated from deep-sea sediments of the Ayu Trough

Two thermophilic, spore-forming strains, TM1(T) and TM5, were isolated from deep-sea sediment (4000 m below sea level) of the Ayu Trough in the western Pacific Ocean. Cells of the two strains were Gram-positive, motile and rod-shaped. Their spores were ellipsoidal, subterminal to terminal and occurred in swollen sporangia. The two strains grew at temperatures up to 65 degrees C and in the pH range 6.5-9.0. The NaCl concentration for optimal growth was 3.0 % (w/v) and growth was inhibited by 5 % (w/v) NaCl. Phylogenetic analysis based on 16S rRNA gene sequences indicated that strains TM1(T) and TM5 belonged to the genus Bacillus, and that strain TM1(T) was most closely related to Bacillus aeolius DSM 15084(T) (96.7 %), Bacillus smithii DSM 4216(T) (96.1 %), Bacillus methanolicus NCIMB 13113(T) (95.8 %) and Bacillus pallidus DSM 3670(T) (95.7 %). Between the 16S rRNA gene sequences of strains TM1(T) and TM5 there were only three nucleotide differences, implying that the two strains were of the same species. The cellular fatty acid profiles of the two strains were also very similar, with iso-C(15 : 0), iso-C(16 : 0), C(16 : 0), iso-C(17 : 0) and anteiso-C(17 : 0) as the major components. The G + C content of strain TM1(T) was 38.7 %. On the basis of phenotypic and molecular data, strains TM1(T) and TM5 represent a novel species of the genus Bacillus, for which the name Bacillus alveayuensis sp. nov. is proposed. The type strain is TM1(T) (= KCTC 10634(T) = JCM 12523(T)).

Vector magnetic analysis within the southern Ayu Trough, equatorial western Pacific

We present the analysis of magnetic field data of the Ayu Trough, a divergent plate margin located in the southern Philippine Sea plate. The survey, which mapped both axial and off-axis regions of the southern Ayu Trough, recorded the total field using a proton precession magnetometer towed behind the ship and the vector magnetic field using a shipboard three-component fluxgate magnetometer together with multibeam bathymetry. The two magnetic field anomalies show good agreement, indicating that the reduction of vector field data was successful. Our inspection of E–W profiles shows that there is little correlation in magnetic anomalies between adjacent profiles and lack of symmetry with respect to the trough axis. However, a careful examination of gridded magnetic anomaly map reveals that symmetry may be found to the north of 1°N along N120E, which is roughly the direction of spreading at this region as revealed by the fine-scale seafloor fabric by our survey. Vector magnetic anomalies exhibit important differences with strike of magnetic boundaries within the inner region between Tobi and Mapia Ridges trending predominantly N–S, whereas in the outer region they are almost E–W. This finding suggests that the outer region appears to retain the magnetization and crustal structure of the old West Philippine Basin, while the inner region is inherited from the E–W seafloor spreading in the Ayu Trough. The short along-axis segmentation and complex seafloor morphology caused by slow oblique spreading may explain the lack of symmetry in the magnetic anomalies with respect to the trough axis and a wider occurrence of 3-D magnetic source crust roughly to the south of 1°N.

남부 필리핀해 Ayu Trough에서의 지형과 선상지구물리 관측: 지각 확장과 구조 고찰

The Ayu Trough, located in the southern end of the Philippine Sea, represents a divergent boundary between the Philippine Sea and the Caroline Plates. A detailed geophysical survey was carried out in the Ayu Trough by R/V Onnuri. Topographically, the Ayu Trough resembles an slow spreading ridge. The trough can be divided into three sections: the south <TEX>$(0^{\circ}-1^{\circ}30'N),\;middle\;(1^{\circ}30'-4^{\circ}N)$</TEX>, and north <TEX>$(4^{\circ}-6^{\circ}30'N)$</TEX>. The seafloor in the middle section is characterized by features asymmetric with respect to the axis. These features were probably produced by NW-SE and NNW-SSE extensions and seem to support the argument that the opening of the Ayu Trough occurred in an oblique fashion. Farther south, a long transform fault but with a short offset defines the boundary between middle and southern sections. The axial depth increases a stepwise to the south of <TEX>$1^{\circ}30'N$</TEX>. A clear difference can be seen between the southern and middle sections with the latter exhibiting much higher mantle Bouguer anomaly values in the axial region. The anomaly indicates that the axial crust perhaps experienced a much higher degree of extension in the middle than in the southern section. The analyses of magnetic field data reveal that the region beyond 100km exhibits considerable variations, whereas the magnetic anomalies within 100km from the trough axis are very much subdued. This observation suggests that the opening of the Ayu Trough involved an initial stage of rifting of existing volcanic arcs, followed by production of new seafloor.

Reflection Seismology in the Southern Ayu Trough, a Slow-spreading Divergent Boundary

A multichannel seismic survey was conducted in the southern Ayu Trough which is the only spreading boundary between the Philippine Sea and Caroline plates. The seismic system used in this study comprises of 2.46-l sleeve gun and a 12-channel streamer with a group interval of 6.25m. Migration technique was used to analyze seismic velocity, and poststack depth migration was applied to the stacked data. The sediment thickness obtained from the depth section tends to increase with distance from the spreading axis. Sedimentation rates are poorly constrainted in the study area. The apparent half-spreading rates estimated from the sediment thickness and sedimentation rate from DSDP hole on the caroline plate are 4.7mm/yr and 7.9mm/yr at <TEX>$1^{\circ}24'N\;and\;0^{\circ}42'N$</TEX>, respectively, which are fester than Previously suggested. On the basis of new oblique spreading geometry, the recalculated spreading rates are 5.4mm/yr and 9.1mm/yr at <TEX>$1^{\circ}24'N\;and\;0^{\circ}42'N$</TEX>, respectively. Seismic sections show that the topography is asymmetric across the Ayu Trough and the acoustic basement is rough. These features are consistent with the earlier suggestion that the Ayu Trough is a slow-spreading divergent boundary. A detailed examination of seismic profiles away from the axis shows that sediments can be divided into two layers which implies a possible change in the spreading rate anuor sedimentation condition during the formation of the trough.

The active period of the Ayu Trough estimated from K-Ar ages:the southeastern spreading center of Philippine Sea Plate.

In order to estimate the active period of the Ayu Trough, which lies at the boundary between the Philippine Sea Plate and the Caroline Plate, we measured K-Ar ages of submarine rocks dredged from the region. Samples recovered from the trough axis are similar to typical MORB and/or BABB in chemical and petrological features and show K-Ar ages of around 1 Ma. This suggests that the Ayu Trough might have been a spreading center at least during the last 1 million years. While, samples recovered at a site 62 km east of the axial rift valley show K-Ar ages of 20–25 Ma, and they are high-alkali andesites. If these samples indicate that an island arc existed there at 25 Ma, the activity of the Ayu Trough might have started at 25 Ma or later.

Morphological studies of the Ayu Trough, Philippine Sea – Caroline Plate Boundary

The Ayu Trough lies on the southern boundary between the Philippine Sea Plate and Caroline Plate. Although this trough may be the best place to study the evolution and kinematics of the Philippine Sea Plate, the origin and evolution of this trough are poorly understood. Our geophysical and morphological surveys in the northern part of the Ayu Trough revealed that the trough shares morphological similarities with slow‐spreading mid‐ocean ridges. The seafloor ages and an average spreading rate of the trough were inferred from the average length of the ridge segments, distribution of sediment thickness, and basement subsidence. Based on the seismic section at 3°30′N, the opening of the trough started about 25 Ma with an average half‐spreading rate 4.1 mm/yr. This spreading rate is much slower than some previous estimates. The present magma production rate and activity of the trough is estimated to be even lower than the 25 m.y. average. The seafloor depth of the trough axis and relief of the axial valley are approximately twice as deep as that of active mid‐ocean ridges. The trough axis valley is covered with sediment in contrast with the axial valleys of active mid‐ocean ridges. Sedimentation at the trough axis shows that the spreading rate has been quite slow for the last 2.5 Ma.

A28 アユトラフ火山岩の K-Ar 年代

A28 アユトラフ火山岩の K-Ar 年代

P42 アユトラフについて : 白鳳丸 KH92-1 航海地球物理地質・調査の成果

P42 アユトラフについて : 白鳳丸 KH92-1 航海地球物理地質・調査の成果

The Manokwari Trough and the western end of the New Guinea Trench

The New Guinea Trench is a sparsely surveyed seafloor depression which runs parallel to the north coast of New Guinea for a distance of some 700 km. Its western end lies about 600 km east of the Philippine Trench; the intervening region is occupied by a complex series of north trending ridges and troughs, of which the Ayu Trough and the Tobi and Mapia ridges are the most prominent. It has been suggested and is now widely accepted that the trench marks a site of subduction, but the degree of present‐day activity is disputed. The western termination has recently been defined by combined single‐channel seismic and long range sidescan sonar surveys as lying at the ridge system which culminates in Mapia Island. The trench contains as much as 1 km of flat‐lying undisturbed sediments within the area surveyed, and the southern slopes are extensively channeled, suggesting a lack of recent deformation. The same survey has also shown that for a distance of some 400 km west of the western end of the trench, the north coast of New Guinea is flanked, at a distance of only a few tens of kilometers, by a deep trough. The sonar imagery of this “Manokwari Trough” suggests recent convergence as well as transcurrent movement. Both the existence of this trough and the abrupt termination of the New Guinea Trench are consequences of the seafloor spreading which took place in the Ayu Trough after subduction had ceased at the trench.

Mineralogy and geochemistry of volcanic and plutonic rocks from the boundaries of the Caroline plate: Tectonic implications

Chemical and mineralogical data from rocks dredged at six sites along plate boundaries around the Caroline Sea define at least three separate igneous suites. Three sites were located in the Sorol Trough, along the Caroline-Pacific plate boundary, and three sites were located in the Ayu Trough along the Caroline-Philippine plate boundary. Geophysical data suggest that both features have originated through processes of crustal extension. Weathered basalts with mid-ocean ridge basalt (MORB) affinities were recovered from the base of the northern margin of the Sorol Trough. Fresh transitional-to-alkalic pillow basalts from a seamount in the central part of the trough have geochemical and Sr-isotopic characteristics similar to enriched MORB or volcanics associated with off-ridge volcanism. A suite of tholeiitic rocks consisting of high-Mg basalt, ferrobasalt and a variety of mafic to ultramafic rocks dredged from the western section of the Sorol Trough has chemical and mineralogical characteristics of other intraplate tholeiitic suites like those from Hawaii. The ferrobasalts could have been derived from a high-Mg basaltic liquid through fractional crystallization of olivine, clinopyroxene and plagioclase. The geochemical data indicate that both MORB and “hot-spot” type sources have been involved in the generation of magmas beneath the Sorol Trough. Furthermore, the highly metamorphosed and tectonized mafic to ultramafic rocks recovered with the tholeiites suggest that a significant component of shear accompanied crustal accretion along “leaky” transform faults within the Sorol Trough. The chemical and isotopic characteristics of fresh to weathered pillow-lava fragments from the axial rift and eastern margin of the Ayu Trough, suggest that a MORB-type source was tapped during crustal accretion along a central spreading centre. An assemblage of metabasalts, metagabbros and silicic plutonic rocks (plagiogranites), recovered from a scarp along the western margin of the trough, have petrologic and chemical features analogous to rocks from plutonic complexes in ophiolites and some oceanic ridges. The plagiogranites could be differentiates from a relatively hydrous MORB magma, but cannot be related to the slightly LIL-enriched basalts in the Ayu Trough by crystal fractionation. Two separate mantle sources are implicated. Metamorphic features in the plutonic rocks are inferred to be the result of hydrothermal and dynamic events associated with the inception of spreading and to subsequent tectonism which exposed them. The petrochemistry of rocks from the Ayu and Sorol troughs is consistent with the hypothesis that crustal accretion occurred along the northern and western boundaries of the Caroline plate and further suggests that a variety of sub-oceanic mantle sources have been involved in the genesis of the crust in each region.

Petrochemistry of the Sorol and Ayu Troughs: implications for crustal accretion at the northern and western boundaries of the Caroline plate

Major element and trace element analyses of samples dredged from six sites along extensional plate boundaries around the Caroline Sea in the western equatorial Pacific define three separate igneous suites. Three dredge sites were located in the Sorol Trough along the Caroline-Pacific plate boundary and three sites were located in the Ayu Trough along the Philippine-Caroline plate boundary. Within the Sorol Trough, fresh transitional-to-alkalic pillow basalts were dredged from a seamount, weathered oceanic tholeiites were sampled at the base of the northern margin, and fresh, Ti-rich ferrobasalts and ultramafics were recovered from a deep valley in the western section. Pyroxenites, high-Mg basalt and a lherzolite, chemically related to the ferrobasalts in the western section of the trough, include schistose fragments and visibly tectonized ultramafics. High-grade metamorphic features observed in these specimens suggest that a significant component of shear accompanies crustal accretion in the Sorol Trough. Fresh pillow-lava fragments from the axial rift and weathered pillows from the eastern margin of the Ayu Trough have compositions similar to Mid-Ocean Ridge basalts (MORBs) and indicate that the Ayu Trough formed through crustal accretion processes. Additionally, an intriguing assemblage of meta-basalts, meta-gabbros and silicic plutonic rocks were recovered from a shallow scarp along the western margin of the Ayu Trough. The chemical characteristics of the meta-basalts and meta-gabbros suggest that they may be related through crystal fractionation. The chemistry of the silicic plutonics, however, indicates that they may not be derived by fractionation of primary magmas with chemical compositions similar to those of the metamorphosed oceanic crustal rocks. Alternatively, the silicic plutonics may represent an assemblage of intrusive island-arc tholeiitic rocks which formed during the initial stages of magma genesis in the island arc that was rifted apart by seafloor spreading in the nascent Ayu Trough.

Is there a Caroline plate?

We examine available marine geophysical and seismological data from the Caroline Sea region and conclude that a separate Caroline plate currently exists. The Caroline plate is moving relative to the Pacific plate on its northern and eastern boundaries, the Philippine plate on its western margin, and the Indian and smaller plates along its southern side in New Guinea and the Bismarck Sea area. The southern Yap Trench, the Palau Trench, and an accreting plate boundary within the Ayu Trough manifested as an axial rift valley comprise the Caroline-Philippine plate boundary. On the basis of sediment thickness and subsidence of basement away from the rift, we estimate that the Ayu Trough started to open during the Miocene. The northern section of Pacific-Caroline plate boundary coincides with the Sorol Trough which exhibits both strike-slip and extensional characteristics. The southeastern section of this boundary occurs along the Mussau Trench where Caroline plate underthrusts the Pacific plate. The section of plate boundary between the Sorol Trough and Mussau Trench is characterized by highly unusual deformational tectonics. Convergence between the Pacific and Caroline plates is apparently accommodated here by overthrusting of small slivers of sea floor towards the northeast. The intensity of deformation appears to increase southward towards the Mussau Trench. Our calculated instantaneous angular rotation vector for the Pacific-Caroline plates predicts that convergence rates increase uniformly south along the overthrust and underthrust sections of plate boundary. The transition in tectonic style from overthrusting to underthrusting occurs between 3° and 4°N.