JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. B1, 2040, doi:10.1029/2001JB001095, 2003

The Central Basin Spreading Center in the Philippine Sea:
Structure of an extinct spreading center and implications
for marginal basin formation
Kyoko Okino
Ocean Research Institute, University of Tokyo, Tokyo, Japan

Kantaro Fujioka
Japan Marine Science and Technology Center, Yokosuka, Japan
Received 14 August 2001; revised 25 June 2002; accepted 23 August 2002; published 25 January 2003.

[1] Newly acquired marine geophysical data along the Central Basin Spreading Center(CBSC), the extinct spreading axis of the West Philippine Basin (WPB), display along-axis
variations of spreading style. We have described and analyzed the tectonic spreading fabricand segmentation patterns along a 1000-km-long section of the fossil ridge between
126°000 and 133°300E. ‘‘Slow-spreading features’’ (deep rift valleys and nodal basins,
rough abyssal hills on the ridge flanks, and mantle Bouguer anomaly lows beneath segment
centers) are observed in the eastern CBSC. In contrast, ‘‘fast-spreading’’ features(overlapping spreading centers, volcanic axial ridges, and smooth abyssal hill fabric) are
identified along the western CBSC. We attribute the large morphological and geophysicalvariations along the CBSC to higher melt supply in the western region caused by highmantle temperature and/or mantle heterogeneity, which may be related to a relatively smallscale mantle plume forming the oceanic plateaus located in the WPB. Another prominent
feature of the CBSC is the development of a deep valley oblique to the spreading fabric.
Reconstructions of the plate boundary geometry through time, using abyssal hill geometry
as well as other measurements, provide evidence for a later stage of amagmatic extension(i.e., reactivation) along the CBSC after the formation of the main basin. This
stage is dominated by tectonic deformation with minor amounts of volcanism (mainly
located in eastern segments), resulting in the observed surface brittle deformation
INDEX TERMS: 3035 Marine Geology
distributed within a broad zone of ductile deformation.
and Geophysics: Midocean ridge processes; 3045 Marine Geology and Geophysics: Seafloor morphology
and bottom photography; 3010 Marine Geology and Geophysics: Gravity; 3040 Marine Geology andGeophysics: Plate tectonics (8150, 8155, 8157, 8158); 3005 Marine Geology and Geophysics: Geomagnetism(1550); KEYWORDS: marginal basin formation, seafloor morphology, extinct spreading center, Philippine Sea
Citation: Okino, K., and K. Fujioka, The Central Basin Spreading Center in the Philippine Sea: Structure of an extinct spreading
center and implications for marginal basin formation, J. Geophys. Res., 108(B1), 2040, doi:10.1029/2001JB001095, 2003.

1. Introduction[2] Geophysical and geochemical observations combinedwith model studies show that one key parameter controlling
mid-ocean ridge processes is the spreading rate [MacDonald, 1988; Parmentier and Phipps-Morgan, 1990; Lin andPhipps-Morgan, 1992]. Recent studies have pointed out that
mantle temperatures [e.g., Cochran et al., 1997] and chemical heterogeneities in the upper mantle [Niu et al., 2001] arealso critical factors governing ridge process. Ridge morphology, gravity signature, chemical variability, and segment
geometry exhibit large variations between fast-spreading andslow-spreading ridges, and the transition occurs at spreading
rates in the 50– 80 mm/yr. range [Small, 1994]. How plate
Copyright 2003 by the American Geophysical Union.
0148-0227/03/2001JB001095$09.00

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kinematics, mantle upwelling, and decompression melting
variations result in such large differences is a basic questionin understanding crustal accretion. Most of our knowledge ofaccretion processes is based on studies of active fast-spreading and slow-spreading midocean ridges, primarily the East
Pacific Rise and the northern Mid-Atlantic Ridge. Spreading
systems in marginal basins are important sites of crustal
accretion; however, back arc basins and other marginalbasins have not been studied from the perspective ofmorphotectonic variations nearly as intensely as have been
fast-spreading and slow-spreading midocean ridges. Backarc basins often show more complicated tectonic fabric,higher heat flow values, greater depths, and different basalt
chemistry. In this paper we present the morphological andgeophysical characteristics along an extinct spreading centerin a marginal basin, the Central Basin Spreading Center(CBSC), in the western Philippine Sea. The CBSC is

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considered to have ceased spreading in the middle Oligocene. Satellite gravity data suggest that the spreading system
is located near oceanic plateaus and seamount chains in its
western part and is connected to the remnant arc at its easternend. The CBSC provides an ideal setting for the investigation of the influence of anomalous magmatic processes onridge morphology, and also provides new insight into
structural, morphological, and volcanological changes thatmight occur during the death of a spreading system.
[3] We have conducted a detailed geophysical and petrologic analysis of the CBSC between 126°000 and 133°300E
with the goal of understanding the tectonics of the Philippine Sea, which is a complex of active and extinct marginalbasins and arcs. The CBSC is a long NW-SE-trending
structure located in the western Philippine Sea and isconsidered to be the Eocene-middle Oligocene spreading
center of the West Philippine Basin (WPB) [Hilde and Lee,1984]. In spite of their importance for understanding crustal
accretion processes in marginal basins in general (and theearly history of the Philippine Sea Plate in particular), nosystematic, combined morphological and geophysical surveys of back arc spreading centers have been conducted
before our cruises. We compiled data acquired during the
STEPS (Structure, Tectonics, and Environmental Studies of
the Philippine Sea) cruises from 1998 to 2000, which
covered the spreading axes, as well as up to 45 km of offaxis coverage. Our bathymetric and gravimetric analyses
show large along-axis variations in the style of spreading
features of the CBSC. These range from ‘‘fast-spreading’’features such as overlapping spreading centers in the west to‘‘slow-spreading’’ features such as rough abyssal hills,
nontransform discontinuities and segment-center mantle
Bouguer anomaly (MBA) minima in the east. We proposethat these characteristics reflect different thermal histories
and/or mantle heterogeneity beneath these distinct regions
along the CBSC. The NW-SE-trending deep valley andlineaments that are overprinted on the north-south spreading
fabric indicate later extension/deformation after the mainstage of basin formation. Our results provide constraints on
the crustal accretion processes of the CBSC and theirrelationship to the evolution of this marginal basin.

2. Overview of the Western Philippine Sea[4] The western Philippine Sea consists of two distinct
regions, the northern Daito Ridge province and the southern
WPB (Figure 1). The features of the Daito Ridge province
have been interpreted as Paleocene to Eocene remnant arcs
[e.g., Karig, 1975; Mizuno et al., 1978; Klein and Kobayashi, 1981; Tokuyama et al., 1986] and continental fragments[Nur and Ben-Avraham, 1982]. The WPB is the oldest basinin the Philippine Sea Plate and has been attributed to backarc spreading [Karig, 1975] or to trapping of normal ocean
crust in a back arc position [Uyeda and Ben-Avraham,
1972; Hilde and Lee, 1984]. Previous mapping of seafloor
fabrics and identification of magnetic lineations show that it
was formed by NE-SW spreading from the CBSC followedby north-south extension from Eocene to Oligocene [BenAvraham et al., 1972; Louden, 1976; Hilde and Lee, 1984].
Drilling results [Ozima et al., 1977; Kroenke et al., 1980]
give some constraints for the reconstruction of the WPB;
however, no widely accepted models of the early history of

the Philippine Sea Plate have emerged [Seno and Maruyama, 1984; Haston and Fuller, 1991; Hall et al., 1995].
One general consensus is that the CBSC is a remnant
spreading center; however, systematic geophysical surveys
along the axis had not been completed until our cruises[Fujioka et al., 1999].
2.1. The WPB[5] The WPB occupies the western part of the Philippine
Sea, extending from 5° to 23°N and is the largest anddeepest inactive basin on the Philippine Sea Plate. The
average depth of the basin ranges from 5500 to 6000 m,
deeper than that expected from the standard age-depth curve
but in accord with the curve derived for back arc basins[Sclater, 1972; Park et al., 1990]. The WPB is bounded on
the east side by the Kyushu-Palau Ridge and the Tobi
Ridge, on the west side by the Ryukyu (Nansei-Shoto)Taiwan-Philippine arc-trench system, and on the northernside by the Daito Ridges (Figure 1). Two midplate plateaus,
the Benham Rise and the Urdaneta Plateau, are located in
the western part of the basin.
[6] Most previous studies concluded that the WPB was
spread symmetrically from the CBSC, though interpretationand identification of magnetic anomalies differ. Proposedages for the cessation of spreading include 39 [Louden, 1976;
Mrozowski et al., 1982], 26 [Shih, 1980], and 35 Ma [Hildeand Lee, 1984]. Shih [1980] first mapped offsets of thelineation patterns in the basin, and Lewis and Hayes [1980]
pointed out that several fracture zones offset the CBSC by.
Hilde and Lee [1984] mapped more than 10 fracture zones in
the middle of the basin and concluded that north-south
spreading followed earlier NE-SW spreading, in contrast to
earlier studies that proposed simple NE-SW spreading.
[7] In the central part of the basin, the orientation of the
spreading-generated abyssal hill fabric changes systematically from NW-SE away from the CBSC, to WNW-ESE, toeast-west near the CBSC. A southerly facing, WNW-ESE
trending, steep escarpment named the Oki Daito Escarpment (ODE) is found in the northern part of the basin[Ohara et al., 1997]. The spreading fabric of the basin floorchanges abruptly against this escarpment; the trend is nearly
north-south in the north and NW-SE in the south. The eastwest/NE-SW curvilinear feature at 8° – 10°N revealed bysatellite altimetry data [Sandwell and Smith, 1997] may be
one candidate for an ODE counterpart south of the CBSC
[Okino et al., 1999].
[8] Deep sea drilling was carried out in the WPB during
Deep Sea Drilling Project (DSDP) Legs 31 and 59 [Scott andKroenke, 1980]. Sites 292 and 293 were located on twoplateaus, and Sites 290, 291, 294, 295, and 447 were locatedon the basin floor (Figure 2). At Site 291, in the southwesternpart of the basin, the basement basalt is overlain by late
Eocene (37.5 –43 Ma) sediments [Louden, 1976]. At Sites
294 and 295, in the northern WPB, the age of the basement
basalt was determined to be 49 Ma by 40Ar/39Ar dating[Ozima et al., 1977]. Sites 290 and 447 are separated by only
35 km in the eastern WPB. At Site 290, the sediment was
penetrated up to 255 m, but the basement was not contacted.
Volcanic conglomerates and breccia with Eocene-Oligocene(about 37.5 Ma) fossils were recovered; thus Karig [1975]
suggested that the age of the basal sedimentary unit represented the basement age. At Site 447, the basalt unit was

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Figure 1. Tectonic setting of the Philippine Sea Plate and surrounding areas overlaid on shaded
predicted bathymetry [Smith and Sandwell, 1997]. The arrows indicate the current relative plate motions[DeMets et al., 1990]. The box indicates the area illustrated in Figures 3, 4, 5, 6, and 7.
overlain by Middle Oligocene (29 – 34 Ma) sediment; however, a hiatus may be present at the sediment/basalt contact[Kroenke et al., 1980].
2.2. The CBSC
[9] TheCBSC is a prominent linear bathymetric feature
first described by Hess [1948] as a NW-SE-trending transcurrent fault. He named it the Central Basin Fault (CBF);
therefore some previous studies used the name CBF insteadof CBSC.
[10] Until recently, the CBSC has not been systematically
mapped and studied, although some ship bathymetry tracks
from transits crossed this area. Satellite-derived gravity andbathymetry data [Sandwell and Smith, 1997; Smith andSandwell, 1997] reveal that the CBSC consists of east-westtrending segments, providing support for the Hilde and Lee[1984] model. During two submersible dives on rift wallsof the CBSC at 130°100E, exposed talus blocks, steep
manganese-coated outcrops, and some slightly altered pil-low basalts and breccias were observed [Fujioka et al.,1997]. Age dating was carried out on the collected samples,
and the ages range between 28.1 and 14.88 Ma [Fujioka etal., 1999]. These ages are considerably younger than those
expected from previous models based on magnetic anomalies and drilling results. In 1996 the French ship R/Vl’Atalante acquired a single-swath NW-SE transect along
the CBSC [Deschamps et al., 1999]. Several rhomboidal
structures in the vicinity of the rift valley were found andwere attributed to dextral shear along the rift axis during thelast spreading phase. Deschamps et al. [1999] also proposed that the east-west spreading fabrics were crosscut
during a final, largely amagmatic, extensional phase to
produce a 130°-trending deep rift valley. Fujioka et al.
[1999] compiled the survey results of R/V Kairei in 1998
and showed that the morphological characteristics of the
CBSC around 128°300E were similar to a typical slow
spreading center. Interpretation of these previous data andof bathymetry and geophysical data presented below eluci-

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Figure 2. Track chart of the study area. Ship tracks from SeaBeam 2100 and HS 10 cruises areindicated by solid bold and dotted lines, respectively. Closed circles are the previous drilling sites in theDSDP.
date a more complex pattern and large along-axis variationsof the rift structure.

3. Data[11] Between 1998 and 2000, R/V Kairei and R/V Yokosuka conducted geological and geophysical surveys of the
CBSC. During these cruises (KR9801, KR9812, KR9910,
and YK0001), swath bathymetry, using SeaBeam 2100,
underway gravity, and magnetic data were collected as wellas dredge and piston core samples [Fujioka et al., 1998, 1999,
2000; Yamazaki et al., 1999]. The primary track lines were set
NW-SE along the general trend of the CBSC at a spacing of 7
nautical miles, resulting in almost complete coverage within
45 km of the axis, except for some local topographic highs(Figure 2). The survey area spans from 131°000E, the junctionof the CBSC and the remnant island arc of the Kyushu-Palau
Ridge, to 126°300E, where the prominent rift valley of the

CBSC cannot be followed on the satellite-derived gravity
anomaly map [Sandwell and Smith, 1997]. Additional swathbathymetry data from the R/V Yokosuka have been merged,
although these data were collected using the lower-resolution
HS-10 system. We utilized Generic Mapping Tools (GMT)[Wessel and Smith, 1998] and the MB system [Caress andChayes, 1996] for filtering, gridding, and plotting data sets.
[12] The SeaBeam 2112 is a multibeam survey systemthat generates data for and produces wide-swath contour
maps and side-scan images. The system consists of a
12-kHz projector array along the ship’s keel and a hydrophone array across the projector. SeaBeam 2112 synthesizes 2° by 2° narrow beams at an interval of 1°, and theswath width varies from 120° at intermediate depth to 90°
in deeper water. The ping interval increases with water
depth, for example about 13 s at 4000 m. The accuracy ofthe depth measurement is reported as 0.5% of the depth.
One of the important parameters for depth and position

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Figure 3. Shade relief bathymetry of the CBSC illuminated from north. Contour interval is 200 m.
Across-axis profiles along the lines labeled as ‘‘a’’ to ‘‘f’’ are shown in Figure 9.

accuracy of the bathymetric survey is the sound velocity
profile. SeaBeam 2112 system uses the velocity data not
only for calculating the depth and position of each beam
but also for the beam-forming process. The temperature of
the surface layer is most important in this regard, and thesystem measures and uses surface temperature in real time.
During our cruises, data from expendable bathythermograph (XBT) measurements were used to about 1830 m.
We removed extreme depth variations from the outer edgeof the swath. The chosen grid size is 150 m, and the swath
is masked to approximately the original swath coverage,
allowing interpolation of small gaps within 1 km. The
shaded relief map, illuminated from the north with a 100m contour interval, and the tectonic fabric extracted fromthe swath bathymetry are shown in Figures 3 and 4,
respectively.
[13] For the Fourier transform analyses described below,
a complete bathymetry grid without gaps was required. To
achieve this, a 2-min bathymetry grid predicted fromsatellite-derived gravity anomaly [Smith and Sandwell,1997] was merged with shipboard data to produce asmoothed data set with a reduced grid spacing of 1 min.
[14] The shipboard free-air gravity data from four cruises
were merged with navigation and filtered along-track using
a weighted averaging procedure to reduce short-period

noise. Drift and the Eotvos corrections were made, andthe free-air gravity anomaly was calculated using the standard gravity formula. The internal crossover errors of free-air
anomalies in each cruise are within 3– 4 mGal. A DC-shiftof less than 15 mGal was applied to minimize the crossover
errors among the four cruises. These corrected data fromfour cruises were combined and gridded in 1-min cells.
Small data gaps within the survey are was interpolated using
satellite gravity [Sandwell and Smith, 1997]. The 10-mGal
contour map made from this grid is shown in Figure 5.
[15] In order to estimate the crustal thickness of the area
we calculated the MBA [Lin et al., 1990]. The gravity
effects of the water-seafloor and the crust-mantle interfaces
were forward modeled following the method described byParker [1972] using the 1-min bathymetry grid. A density
contrast of 1800 kg/m3 was assumed at the seafloor and oneof 500 kg/m3 for the Moho. The Moho was assumed to lie
everywhere 5 km below the seafloor. The calculated gravity
effects of both interfaces were gridded and then subtracted
from the gridded free-air gravity anomaly. The resulting
MBAs are contoured at 10-mGal intervals in Figure 6. The
MBA at midocean ridges primarily reflects crustal thickness
variations [Lin et al., 1990]. Other contributors to the MBA
include the thermal effects of plate cooling with age andsubcrustal density variations related to the pattern of mantle

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Figure 4. Seafloor fabric chart. Several important tectonic components, such as axial rifts, failed rifts,
segment discontinuities, and sea knolls, are recognized. The segments are labeled as S1 – S7.
upwelling, temperature variations, and the distribution ofpartial melt [Lin and Phipps-Morgan, 1992]. The CBSC is
not an active ridge system; therefore, the contributions fromthe latter parameters can be neglected in this discussion.
[16] Shipboard total-field magnetic anomaly data (International Geomagnetic Reference Field (IGRF1995) removed)from four cruises were compiled. The crossover errors show
unusually large values, sometimes larger than 100 nT. The
large DC-shift between cruises is due to uncertainties in thetime-varying coefficients that determine the IGRF; this isespecially evident as the surveyed area is far from themagnetic observatories deciding the IGRF. We therefore
recalculated the anomalies by using DGRF1995 and
IGRF2000, obtaining a significant decrease in the shift
between cruises. Another factor contributing to decreased
accuracy is solar disturbance. The period of the survey
cruises, during the later 1990s, corresponds to a period of
increased solar activity: some magnetic storms and smallscale disturbances occurred during the survey. The closest
available magnetic observatory is Langpang (Taiwan), 800
km NW of the study area, so an accurate diurnal correction
could not be obtained. In consequence the derived gridded
data set (1 min) still includes erroneous values, and the data
quality is not high enough to stand detailed discussion or
further processing. The 25-nT contour map shown in Figure
7 therefore serves primarily as an indicator of the roughpattern of seafloor magnetic characteristics.

4. Results
4.1. On-Axis and Off-Axis Morphology[17] The compiled SeaBeam bathymetry data (Figure 3)
are shown in the form of a contoured shaded-relief map. The

large morphological variations along the CBSC reflect thecomplex tectonic history of this area. The morphology andsegmentation characteristics have important implications forthe nature of the spreading process of the CBSC because
the morphotectonic variability between segments may becontrolled by a combination of plate separation rates, asthenospheric flow patterns and temperatures, and magma supply. For most midocean ridges first-order discontinuities aredefined by transform faults where strike-slip motion isconcentrated in a narrow zone. Second-order discontinuities,
at slow spreading ridges, are defined by nontransform
discontinuities with small offsets (<30 km) and off-axis
traces, while overlapping spreading centers are observed at
fast-spreading ridges [MacDonald et al., 1988, 1991; Grindlay et al., 1991]. The segmentation pattern of the CBSC
between the Kyushu-Palau Ridge and 126°300E is characterized by one long-lived transform fault (129°340E), five
nontransform discontinuities, and one overlapping spreading
center. The segments between these discontinuities havebeen numbered in sequence from east to west (Figure 4).
In this section we point out four basic observations of the onaxis and off-axis morphology in the surveyed area. Moredetailed descriptions of each morphological feature are givenin the Appendix.
1. The discontinuities and morphological features exhibit
both ‘‘slow-spreading’’ features and ‘‘fast-spreading’’
characteristics, although the spreading rate is estimated as
36 mm/yr at the cessation of spreading, with no significant
changes along the axis [Hilde and Lee, 1984]. The threeeastern discontinuities are typical nontransform discontinuities with off-axis traces of nodal deeps; these exhibit‘‘slow-spreading’’ characteristics. In contrast, the westernmost segment exhibits an overlapping spreading center

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Figure 5. Free-air gravity anomalies contoured at 10-mGal intervals. Thick lines show the axial rifts
identified with bathymetry.

geometry typical of fast spreading ridges (Figure 8). A
major bathymetric feature in the area is a series ofabandoned rifts near 16°N that show a left-stepping enechelon alignment and form a sinuous escarpment. The
morphology and alignment of the abandoned rifts resembles
the geometry of failed rifts in the ‘‘cyclic failure’’ rift
propagation model [Wilson, 1990]. Across-axis bathymetry
profiles (Figure 9a) show that the relief of the abyssal hills
is smoother in the western segments S6 and S7, in contrast
to the rough seafloor east of 131°300E. The roughness
indicator ‘‘MAD’’ (Median Absolute Deviation [Small,1994]) also shows increasing roughness in the eastern
CBSC (Figure 9b), although the MAD values are rather
scattered. These results indicate a marked difference inmagma supply along the CBSC. Two large seamounts
located at segment S7 (Figure 8) also suggest that a higher
magma supply existed in the western segment of the CBSC.
2. The seafloor more than 45 km from the CBSC shows
relatively continuous NW-SE trending fabric, which supports the interpretation of NE-SW-directed spreading at
intermediate rates in the early stages of the formation of the
WPB [Hilde and Lee, 1984]. Between 129°450 and 131°000E
some oblique (about 135°), narrow depressions that abut the

NW-SE fabric are recognized on the swath bathymetry map(small arrows in Figure 4). These are interpreted as pseudofaults associated with eastward propagation of the ridge
segment. On the other hand, north-south-directed spreading
during the final formation stage of the WPB is confirmed byeast-west-trending abyssal hills near the axis. A slightchange of the off-axis fabric orientation from 095°, 080° andthen 090° near the axial valley, suggests that spreading
directions were highly variable or unstable during the latest
stage of basin formation.
3. Deep rift valleys that are oblique to the 080° –095°
trending off-axis abyssal hills are prominent along the entire
CBSC. These rifts cut the spreading fabric, and they areinterpreted to be the results of renewed tectonism after the
cessation of north-south-directed spreading. Segment S6 isthe best segment to understand the complicated history of the
CBSC. The axial rift valley and abyssal hills trend east-west,
and oblique lineaments and grabens trending NW-SE are
overprinted on this older spreading fabric. The oblique rifts
are observed over the entire CBSC, although their characteristics differ between segments. We consider that the obliquefeature is a result of NE-SW extension that postdates theformation of the basin floor and was constrained by the

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Figure 6. MBAs contoured at 10-mGal intervals. Thick lines show the axial rifts identified withbathymetry.
preexisting crustal architecture. The existence of thetriangular nodal basins at 16°180N in S5 and at 16°100E in
S4 support the idea that the older east-west spreading axes arelocated south of the oblique rift and that the rift developed bylinking the old segment ends.
4. The offset of axial discontinuities across the rift axis isobserved for all discontinuities east of the 129°340E fracturezone. The break in the northern rift wall at the intersection
with the off-axis trace of the discontinuity is located east of
the corresponding break in the southern rift wall. This
indicates the existence of a right-lateral sense of shear
component in the latest stage of tectonism.
4.2. Gravity Anomaly[18] Off-axis free-air anomalies away from large topographic features generally ranges from 10 to 10 mGal
(Figure 5). Large positive anomalies are associated with therift flank mountains (40 mGal) and with a large seamount(>80 mGal) at the westernmost end of the area. Negative
anomalies are measured in deep oblique rifts and deep
basins within the rift valley. Small negative anomalies arefound over the off-axis traces of the eastern axial discontinuities and over the abandoned rifts at the northwestern endof the survey area. These free-air anomalies generally reflect

topographic relief. Most of the smaller-scale features related
to bathymetric relief in the free-air anomalies are removed
in the MBA calculations (Figure 6). The large seamount at
the western end of the survey area exhibits a 120 mGal
MBA low reflecting thicker crust beneath the seamount.
Large negative MBAs are also found in the eastern end of
the area, indicating the existence of a thick root supporting
the Kyushu-Palau Ridge. The large negative anomalies
along the oblique deep rifts are nearly eliminated by the
topographic correction.
[19] Figure 6 shows the segmentation and the along-axis
variation of the subsurface structure in the CBSC. The
129°340E fracture zones and eastern nontransform discontinuity traces are associated with higher positive MBA
anomalies, probably reflecting thinned crust formed atsegment ends resulting from three-dimensional upwelling,similar to slow spreading ridges. A circular MBA low of20 mGal is observed at the midpoint of Segment S6. The
eastern segments are also associated with MBA lows;
however, the centers of the negative anomalies are not at
the midpoints of the deep rift valleys. These MBA lows of20 mGal are centered a few tens of kilometers south ofthe deep rift valleys, where the topographic fabric trends
approximately 090°. This indicates that, in terms of theOKINO AND FUJIOKA: CENTRAL BASIN SPREADING CENTER

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Figure 7. Total field magnetic anomalies contoured at 25-nT intervals. Thick lines show the axial rifts
identified with bathymetry. Thin lines and numbers are magnetic anomaly lineations identified by Hildeand Lee [1984].
density structure, the spreading segment centers are locatedto the south of the deepest rift valleys. The quasi-triangulardepression on the swath bathymetry map is found west of
the MBA minimum, and we interpret this depression as an
extinct nodal basin at the original ridge-transform intersection. Along S7, the westward propagating rift containing the overlapping spreading center (OSC), the 20
mGal MBA contour line defines an ellipse elongated along
the rift valley. The off-axis MBA values are also lower inthe western part. This may reflect thicker crust beneath thewestern segments, suggesting vigorous magmatism in thisarea. In contrast, small positive anomalies are recognizedabove the oblique rifts, especially in S2E, S4, and thewestern part of S5, suggesting amagmatic extension inthese rifts.
[20] Circular MBA lows at segment midpoints, called‘‘bull’s-eyes,’’ are typically observed in active slow
spreading centers [e.g., Kuo and Forsyth, 1988; Lin etal., 1990; Detrick et al., 1995; Thibaud et al., 1998].
These are commonly interpreted in terms of variations incrustal thickness, crustal or mantle density, and thethermal effects of upwelling at the ridge axis. The crust
is interpreted to be thicker beneath the segment midpoints

and thinner beneath the segment ends. The residual MBA
pattern, after removing thermal effects, exhibit a band of
low MBA from the segment center toward the off-axis
areas instead of ‘‘bull’s-eyes,’’ indicating the contributionof melt-rich upwelling mantle diapir beneath the ridge
axis [Lin et al., 1990]. Although the CBSC is an extinct
spreading center where the thermal effects should be
negligible, ‘‘bull’s-eyes’’ patterns are observed. Similar
gravity anomalies have been recognized in other extinct
spreading centers (Labrador Sea, CBSC, Coral Sea, andMagellan Trough) by Jonas et al. [1991]. They indicatedthat a frozen-in low-density structure is confined towithin about 10 km of either side of the extinct axis,
which is consistent with the presence of a gabbroic root
beneath the axis. On the other hand, Olser and Louden[1992, 1995] showed that the combination of thin crustand serpentinized upper mantle due to the reduction ofmelt supply during the death of the spreading system
could explain both the gravity anomaly and seismic
structure beneath the Labrador Sea. Either case is plausible for the CBSC, and distinguishing between them is
not possible with the currently available data. Regardless,circular MBA lows along the CBSC correspond to

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Figure 8. Detailed shaded relief bathymetry with 100-m contours in the western part of the CBSC.
Tectonic components, such as failed rifts and oblique lineaments, show the overlapping spreading. FR,
failed rift; IPF, inner pseudo-fault; OPF, outer pseudo-fault.

locations of spreading segment centers at the time thatthe system died, and the off-axis extent of low MBAs in
the western segments indicates thicker crust in the western region.
4.3. Magnetic Anomalies[21] The limited off-axis survey, the ridge parallel survey lines, and relatively low quality of the original data
make it difficult to identify the anomaly lineations and to
discuss the magnetization of the ridge axis. Therefore we
only point out three important pieces of information
derived from Figure 7.
[22] First, the east-west-trending magnetic lineations areoffset by fracture zones and discontinuities, indicating
north-south-directed spreading and segmentation of theridge axis in the later phases of spreading. A set of obliquelineations is observed in the area north of segment S7,
supporting the existence of the OSCs. The previous identification of magnetic anomalies [Hilde and Lee, 1984]
shown in dotted lines with numbers in Figure 7 is generally
consistent with our data east of S6; however, reexamination
may be required to understand the detailed segmentation
patterns of the western part of the CBSC.
[23] Second, the anomaly pattern shows rough symmetry
about the axis at S6 (indicated in Figure 7), and the offset
between S6 and S5 is approximately 90 km. The axis of S5,
estimated from the magnetic anomaly pattern, is coincident
with the location of the relict east-west-trending rift andwith the location of the MBA minimum. No clear anomaly
pattern is associated with the oblique rifts in S5. This
indicates that the oblique rifts were formed by postspreading amagmatic tectonism.

OKINO AND FUJIOKA: CENTRAL BASIN SPREADING CENTER

EPM

5 - 11exhibit rather flat bottom profiles within the segment,
while the profiles of the eastern segments are nearlysinusoidal. Segment midpoints are well correlated withlocal minima of MBAs. The ridge segments defined by the
MBA lows reflect the underlying crust and mantle structure well and correspond to the volcanic segmentation. The
along-axis half-segment MBA gradient (as defined by Lin
and Phipps-Morgan [1992]) for S7 is 0.26 mGal/km,
which is similar to gradients observed at the fast-spreading
East Pacific Rise (EPR) [e.g., Wang and Cochran, 1993].
The MBA low at the segment midpoint is largest at S6,
where the MBA relief exceeds 25 mGal, and the gradientis 0.58 mGal/km, which is similar to values reported from
the Mid-Atlantic Ridge [e.g., Lin and Phipps-Morgan,
1992; Detrick et al., 1995]. The eastern segments, S1and S2, have relatively high MBA gradients, similar to
those described on slow-spreading ridges. The characteristics of segments change at the 129°340E transform fault.
The average MBA is low in the western segments S6 andS7, and the average depth is a little shallower. The segment length is also longer compared with eastern segments. These results may reflect the higher melt supply inthe western CBSC.

5. Discussion

Figure 9. (top) Across-axis bathymetry profiles. The
locations of six profiles are shown in Figure 3. (bottom)
Bathymetry roughness versus longitude. Roughness isdefined as the MAD of the residual profile, which is
obtained by removing the first five modes from the original
profile.

[24] Third, the highest positive anomalies are parallel tothe oblique rifts and are observed north of segments S1and S2. These may be related to the latest volcanic
activities.
4.4. Along-Axis Variation[25] Along-axis variation in bottom topography andMBAs are shown in Figure 10 to demonstrate both segment-scale variation and long-wavelength trends along the
CBSC. The thin lines indicate the profile along the deepest
point of the rift valley, including the oblique rifts in theeastern segments. The thick gray lines indicate the 090°trending extinct spreading center identified from the swathbathymetry and the location of MBA lows. Both lines areshown in plan view in Figure 4. In general each segment
center is associated with a local topographic maximum.
Segment S2 exhibits a local minimum at the segment
midpoint that is interpreted as a higher-order discontinuity
between S2E and S2W. The western segments S6 and S7

5.1. The Origin of ‘‘Fast-Spreading’’ Featuresin the Western CBSC
[26] The westernmost part of the survey area exhibits
unique characteristics as follows: (1) smoother and slightly
shallower bathymetry, (2) overlapping spreading center
geometry, and (3) low MBA in off-axis areas. The enechelon, short, deep troughs and sets of oblique lineaments
resemble features observed near microplates or OSCs typical of fast spreading ridges. Abyssal hills created by the
westward-propagating ridge change orientation from almost
east-west to 080° near the 128°360E discontinuity, similar tothe change in orientation of abyssal hills spread fromsegments S5 and S6. The 110°-trending rift valley extends
to 127°000E and cuts the abyssal hill fabric at an obliqueangle, so the valley was formed after dueling propagation.
The MBA low is centered near the midpoint of S7; however, the 20 mGal contour is elongated, not circular. The
low MBA area is not focused along the extinct spreading
center but encompasses the off-axis seafloor west of
128°350E. This indicates thick crust beneath the westernpart of the CBSC, probably reflecting higher rates of melt
production.
[27] The western CBSC exhibits ‘‘fast-spreading’’ features, although the full spreading rate was 36 mm/yr in thenorth-south-directed spreading stage of the West Philippine
Basin [Hilde and Lee, 1984]. In contrast, the eastern CBSC
shows typical slow-spreading features such as rough abyssal
hills and nontransform discontinuities (NTDs). Although
the Euler pole and angular velocity of spreading along the
CBSC have not been established, the limited length of thesurveyed spreading center and the trend of the abyssal hills
support the idea that differences in spreading rate between
the eastern and western survey areas were small. These dataand observations support the interpretation that, althoughspreading rate is clearly an important parameter controllingridge morphology and segmentation [MacDonald, 1982;

EPM

5 - 12

OKINO AND FUJIOKA: CENTRAL BASIN SPREADING CENTER

Figure 10. (top) Along-axis variations in MBA and (bottom) depth along the CBSC. Thick lines are
profiles along the latest rift system, including oblique deep rifts in segments S4 and S5. Gray lines
illustrate the values along the rifts of north-south spreading. The locations of axial rifts are shown in
Figure 4.

Lin and Phipps-Morgan, 1992], there should be othercontrol factors deciding the spreading style.
[28] The effect of thermal structure on ridge morphology
and segmentation is clearly seen along the Southeast Indian
Ridge (SEIR), where the ridge spreads at a narrow range ofintermediate rates but exhibits the full range of slow-spreading to fast-spreading features [Cochran et al., 1997; Sempere et al., 1997; Small et al., 1999]. The western SEIR ischaracterized by a long-wavelength increase in axial depths,
OSCs, and large negative MBAs, whereas deeper axial
valleys, nontransform discontinuities, and higher MBAs
are observed toward the Australian-Antarctic Discordance(AAD). This large variation in the style of crustal accretioncan be attributed to a gradient in mantle temperature
between regions influenced by the Amsterdam and Kerguelen hot spots and the AAD [Sempere et al., 1997]. Similar
along-axis variations are also reported for ridges near hot
spots, e.g. the Azores hot spots [Detrick et al., 1995]. In
general, ridge-hot spot interaction causes robust magmatism, which forms thicker crust and resultant shallow seafloor.
[29] Niu et al. [2001] emphasize the importance of mantle
compositional control on the extent of mantle melting, crust
production, gravity anomaly, ridge morphology, and segmentation. They analyze the observed difference between
two segments of the Mid-Atlantic Ridge (MAR) between
the Oceanographer and Hayes transforms, where the mantle
potential temperature is the same and the plate spreading

rate is the same. They demonstrate that morphologicaldifferences are ultimately controlled by variations in fertile
mantle source composition, as reflected in lava chemistry.
Samples from the magmatically robust segment are moreenriched in incompatible elements, total alkalis, and H2Ocontents relative to samples from the magma-starved segment. Their results indicate that the heterogeneity of fertile
mantle source composition is important as are mantle
temperature and plate spreading rate variations.
[30] The ‘‘fast-spreading’’ morphology and large MBAlows (indicating thicker crust) in the western CBSC are
interpreted to be the result of higher rates of melt production. Although the CBSC is not presently active and thevelocity structure obtained by seismic tomography does notdirectly reflect the thermal structure during the period ofbasin formation, the shear wave velocity of the upper
mantle beneath the CBSC exhibits relatively low velocities
in the western part of the basin [Nakamura and Shibutani,1998]. This may reflect remnant high mantle temperaturesduring basin formation.
[31] On the other hand, variations in basalt geochemistryalong the CBSC may point to heterogeneity of the upper
mantle source, requiring no variation in potential temperature. As described in section 4.2, large blocky seamounts
are located near and within the rift west of 127°300E. The
westernmost seamount, centered at 126°450E, 18°350N, hasrelief greater than 4000 m and is delineated by 070°trending troughs (Figure 3). Dredge sampling operations

OKINO AND FUJIOKA: CENTRAL BASIN SPREADING CENTER

were carried out by a Russian vessel at this seamount[Shcheka et al., 1995], named the Vinogradov Seamount.
The sampled basalts are classified in the leucocratic potassium-sodium group, characterized by low abundances ofmafic elements (Cr, Ni) and anomalously high contents oflithophyle elements (P, Ba, Sr, Zr, and Rb). These characteristics match the composition of Ocean Island Basalt (OIB).
Based on the satellite altimetry map, the Vinogradov Seamount is located at the northern end of a seamount chain
extending from the Benham Rise. The Benham Rise is arelatively small but significant oceanic plateau at the westernmost edge of the WPB (Figure 2). It was probably
formed by volcanism near the CBSC and was moved
toward its present position by seafloor spreading [Hildeand Lee, 1984]. DSDP Site 292 was drilled on the southern
Benham Rise, where basement consisting of olivine tholeiite was recovered [Karig, 1975]. The basalt chemistry bears
out the OIB-like character of the Benham Rise by its muchhigher TiO2 and other high field strength element (HFSE)
composition [Hickey-Vargas, 1998]. These signatures could
reflect heterogeneous upper mantle, which could be
explained by the contamination of the upper mantle bymantle plumes. The geophysical and geochemical observations support the idea that the ‘‘fast-spreading’’ characteristics and thicker crust along the western CBSC were
influenced by hotter upper mantle and/or by heterogeneous
mantle source. And the heterogeneity of upper mantle may
be due to local contamination by a small-scale mantle
plume, which formed the plateaus in the western WPB.
Although no plumes are found in the WPB at present,
tectonic reconstruction of the Philippine Sea Plate [Hall etal., 1994] places it north of Australia at 50 Ma, above theplume-rich Indian Ocean asthenospheric domain [HickeyVargas, 1998]. The large geophysical variations along the
CBSC and the ‘‘fast-spreading’’ to ‘‘slow-spreading’’ characteristics may reflect variations in the underlying thermal/
chemical structure of the asthenosphere.
5.2. Development of the CBSC
[32] Tectonic spreading fabric typically parallels spreading center segments, and fracture zones are generally
orthogonal to spreading segments. Adjacent to the CBSC,
however, we observe multiple orientations of lineaments,
reflecting a more complicated history. Although the fullbathymetric coverage is somewhat limited, coverage along
the remnant spreading axis and up to 45 km away from the
CBSC shows three stages of seafloor spreading as follows:(1) NE-SW spreading (Figure 11a), (2) north-south spreading (Figures 11b and 11c) and (3) NE-SE rifting (Figure11d). The first and second stages are generally similar to thescenario proposed by Hilde and Lee [1984]; however, wehave documented a more detailed picture of development of
the CBSC.
[33] The seafloor generated during the first NE-SWdirected spreading stage can only be seen along somenorth-south transit lines in our map. The orientation of thelineaments varies from 120° in the north to 105° near theaxis. The abyssal hills are relatively continuous, and theirrelief is relatively low, which may indicate stable intermediate rate spreading (88 mm/yr, Hilde and Lee [1984]).
The fracture zone at 129°340E has existed since this period,
based on the satellite altimetry map, although no fractureEPM

5 - 13zone is recognized north of 18°300N in Figure 3. The
Benham Rise and the Urdaneta Plateau were formed near
the ridge at that time, which was influenced by higher
mantle temperatures beneath the western CBSC. The
spreading fabric trends 045° at the northern end of line
128°400E (small arrow in Figure 8). These resemble the setof lineaments seen in the overlapping spreading basin near
the axis, so these may be related to an older duelingpropagation. This indicates that the western part of thebasin also exhibited ‘‘fast-spreading’’ features in the earlier
stages of basin genesis. In contrast, east of the 129°340E
fracture zone, 135°-trending troughs oblique to the spreading fabric are seen on four survey lines (small arrows in
Figure 4). These troughs appear to be long pseudo-faults
related to eastward segment propagation, indicating that thesegment east of this pseudo-fault decreased in length as thebasin developed (Figure 11a). The sense of propagationshows that the temporal evolution of segmentation isaffected by the thermal regime in the underlying mantle
and asthenosphere, which is reported at Mid-Atlantic Ridge
near the Azores hot spot [Phipps Morgan and Parmentier,
1985; Thibaud et al., 1998].
[34] The second-order discontinuities between segments
S3, S4, S5, and S6 can be traced about 35 km from theaxial rift both on the swath map (Figure 4) and the satellite
altimetry data. The spreading direction changes from NESW to almost north-south at that point. This change inpole location and plate motion vectors was accommodatedby breakup of the spreading axes into multiple smaller
segments offset by discontinuities (Figure 11b), preserving
the overall NW-SE-trending thermal structure. The reliefof the east-west abyssal hills is higher than that of the
NW-SE fabric formed earlier, which may indicate that the
spreading rate decreased at the time of the change ofextension direction. A similar change of spreading style,i.e., rotation of spreading axis, higher segmentation, andspreading rate decrease, is also observed in the final stagesof other back arc basins in the Philippine Sea [Okino etal., 1999].
[35] During the north-south-directed spreading stage, aslight change of spreading direction is recognized from thegeometry of abyssal hills and fracture zones (Figure 11c). It
is most clearly seen in segment S6, where the trend ofabyssal hills rotates progressively from 095°, 080° to 090°
near the rift valley. The fracture zone at 129°350E alsochanged orientation to 173°, corresponding to the orientation of the abyssal hills and resulting in the formation of an
arcuate escarpment. The same change can be observed in
the off-axis seafloor of the eastern segments, although thefabric is disturbed by later deformation. This indicates thatthe spreading regime was unstable near the end of basinformation. The location of the MBA centers and themagnetic lineations suggest that the youngest east-westridge segments are located south of the present-day deep
rift and that the small deep depressions at the end of therelict ridges are remnant nodal basins. In the western part of
the CBSC, the dueling propagators formed an overlapping
spreading basin between them.
[36] The main part of the WPB was formed in the abovementioned first and second stages. The third stage isessentially amagmatic and takes the form of NE-SW-directed rifting. This created fault scarps, grabens, and extremelyEPM

5 - 14

OKINO AND FUJIOKA: CENTRAL BASIN SPREADING CENTER

Figure 11. Schematic figures of the evolution of the
CBSC. FR, failed rift; FZ, fracture zone; PF, pseudo faults;
BR, Benham Rise; VS, Vinogradov Seamount; UP,
Urdaneta Plateau. (a) NW-SE-spreading phase. The Benham Rise is formed near the CBSC in the western basin.
Eastern segments propagate toward east. (b) North-south
spreading phase. The ridge axis are segmented. The westernpart of the CBSC is characterized by cyclic duelingpropagation. (c) Spreading direction is unstable in the lastphase of the basin formation. (d) Cessation of volcanismand postspreading deformation forming oblique rifts.

deep valleys cutting the preexisting fabric formed by northsouth-directed seafloor spreading. It is also responsible forthe offset of nontransform discontinuities in a right-lateral
sense. Deformation is concentrated in the areas surroundedby adjacent segment ends (Figure 11d). This may indicatethat the deformable zone was constrained by the thermal
regime (and hence the mechanical properties of the lithosphere) inherited from the earlier stages of seafloor spreading. The amount of deformation is smaller in the westernsegments of the CBSC; the deep rift valleys develop in thesegments east of the 129°340E fracture zone. We suggestthat this difference was caused by the difference of subsurface structure between the eastern and western CBSC. This
stage is tectonic dominant and generally amagmatic; however, some evidence of volcanism occurs on the seafloor. In
segments S1 and S2, elongated minor ridges located within
the rift valley may be the youngest axial volcanic zones.
Figure 7 shows that segments S2 and S3 have positive
magnetic anomalies along the central rift, although the otherpart of the CBSC is occupied by negative anomalies. Thissuggests that a late magmatic phase occurred in the easternsegment.
[37] We did not infer an absolute timescale for the evolutionary history of the CBSC. It is hard to identify magnetic
lineations based only on our results because of the alongaxis survey line direction. The most widely supported
evolutionary model of the WPB [Hilde and Lee, 1984]
proposes a NE-SW-directed opening at 88 mm/yr between
56 and 45 Ma (Anomaly 22) followed by a north-southdirected opening at 36 mm/yr that ceased at 35 Ma(Anomaly 13). However, recent age dating of rock samples
from the CBSC indicates younger ages than previously
reported. Two basalt samples from the oblique rift at S5[Fujioka et al., 1999] have ages of 28.10 ± 0.16 (40Ar/39Ar),
26.1 ± 0.9 (K/Ar), and 14.88 ± 0.16 Ma (40Ar/39Ar). Some
K/Ar dates are also reported from the CBSC: 27.4 [Okino etal., 1999] and 16.2 Ma [Ohara, pers. commun.] at S1.
Though the K/Ar data may include large errors due todegassing, there is a peak around 27 Ma, which is younger
than the previous model [Hilde and Lee, 1984]. We examined two scenarios: (1) slow (35 mm/yr) north-south spreading ceased at 35 Ma, then minor amounts of volcanism
occurred along the remnant axis around 27 and 15 Ma; and(2) slow north-south spreading ceased at 27 Ma, and theminor activity occurred at 15 Ma because a large percentageof the age data indicate that volcanism occurred at this time.
We prefer the former one because the second scenario
required a completely new identification of magnetic
anomalies, and we do not have any evidence denying the
Hilde and Lee [1984] identification. The age of 27 Ma forthe volcanism in the CBSC is almost the same age as that of
the eastern Parece Vela Basin opening [Okino et al., 1998].
The cessation of spreading and/or the final extensional stage
are probably related to the opening of the back arc basin
along the Kyushu-Palau Ridge. Compiled heat flow measurement in the Philippine Sea Plate [Yamano and Kinoshita,
1995] show relatively higher values along the CBSC,
suggesting that either active tectonism or minor volcanism
has occurred since the basin ceased spreading. Magmatic
activity after basin formation at a remnant spreading axis isalso reported at the Kinan Seamount Chain in the Shikoku
Basin [Sakamoto and Kim, 1999; Ishii et al., 2000] and at

OKINO AND FUJIOKA: CENTRAL BASIN SPREADING CENTER

the Scarborough Seamount Chain in the South China Sea[Wang et al., 1985].
5.3. Implication for the Reconstruction of theWest Philippine Basin[38] This paper focuses on the tectonics of the extinct
spreading center rather than the genesis of the whole basin.
However, these new data have some implications for thereconstruction of the WPB.
[39] First, the western part of the basin is closely associated with ocean plateau formation. The Benham Rise andsome small seamounts were formed near the CBSC, where
the basalt chemistry shows OIB-like character. The western
CBSC exhibits prominent OSC geometry. This apparent‘‘fast-spreading’’ morphology is attributed to the higher
magma supply related to elevated mantle temperatures ora heterogeneous mantle source. The northwestern part of thebasin was mapped by the Japan Hydrographic Department[Okino et al., 1999], and there is evidence for failed rifts andoblique sets of lineaments [Yoshida et al., 2000]. These datasuggest that the western part of the basin has had a higher
magma supply for a long time. The segment propagation in
the WPB was perhaps influenced by its rate.
[40] Second, the age of cessation of WPB formation isstill equivocal. There are several viable ages given for thehalt of volcanism in the WPB, 35 [magnetic anomaly, Hildeand Lee, 1984], 27, and 15 Ma [40Ar/39Ar dating, Fujioka etal., 1999]. These various ages raise several possibilities forthe rifting in the basin. If the 35 Ma age is correct, then thelater volcanism is related to reactivation after a hiatus.
Another possibility is that the central part of the basin isyounger than has been previously considered. The dating
results and the rifts on the topographic high at the intersection with the Kyushu-Palau Ridge provide the evidence
for activity of the CBSC even in the stage of back arc
opening of the eastern Philippine Sea. The ages reported
along the CBSC have not yet been compiled and arescattered in a relatively large range, partly due to inaccurate
K/Ar dating. However, the age data have two reliable peaks
at 26 –27 and 15 Ma, so the volcanism continued after basinformation. Oblique rifts were also formed after basin formation, although the timing is not known. These postspreading activities may reflect a general reorganization ofplate motions in the region.
[41] Third, the off-axis morphology shows a more complicated history of the basin than that proposed by Hilde andLee [1984]. Recently a large escarpment (the Oki-Daito
Escarpment, Figure 2) was reported in the northern part of
the WPB, north of which the spreading fabric indicatesnorth-south-directed opening [Ohara et al., 1998; Okino etal., 1999]. Pseudo-faults indicating propagating ridges arealso recognized in our data set. Although there has been nosystematic survey in the southern Philippine Sea until now,a clear linear structure crossing the basin can be mapped on
the satellite-derived gravity map (Figure 1), which may be a
candidate for the counter part of the Oki-Daito Escarpment.
Older side-scan data [Andrews, 1980] and single-swath data
by R/V l’Atalante [Deschamps, 1999] show large variationsof the spreading fabric on the basin floor. In order toreconstruct the detailed history of the basin, recompilationof the magnetic anomaly, recent age dating, and the fragments of known tectonic fabric data should be combined.

EPM

5 - 15

6. Conclusions[42] We have studied the morphological and geophysicalcharacteristics of the extinct CBSC between 126°000 and
133°300E. Our study supports the following conclusions:
1. Along the CBSC the ridge segments and discontinuities exhibit a morphological range from typical slowspreading features in the eastern segments to fast spreading
features in the westernmost segment. The spreading rate isalmost the same in this geographically limited area;
therefore, this variety is related to the rate of magma supply
relative to spreading rate.
2. The western part of the CBSC is characterized byoverlapping spreading centers, extensive MBA lows, andsmoother off-axis bathymetry. These fast-spreading features
are in striking contrast with the eastern CBSC, showing atypical slow-spreading character. We consider that theobserved differences are caused by variations in crustal
thickness along the CBSC resulting from higher melt supply
in the western CBSC.
3. Relics of cyclic rift failure caused by dueling propagation that are found in the northwestern part of the basin
indicate that higher melt supply may have affected thewestern WPB throughout basin formation. The proposed
increased melt supply may reflect higher mantle temperatures
suggested by previous seismic topography results [Nakamura and Shibutani, 1998] or reflect heterogeneity in upper
mantle geochemical composition due to mantle plumecontamination, suggested by OIB-like signatures of basalt
samples [Hickey-Vargas, 1998].
4. Based on the orientation of abyssal hills, the latest
stage of the WPB opening is characterized by north-southdirected spreading from a highly segmented ridge. The
prominent deep rift and lineaments obliquely cut theseabyssal hills, indicating that an amagmatic rifting stage
occurred after the main basin formed. The oblique features
are observed only between ridge-transform intersection
zones, suggesting that inherited mechanical and/or thermal
characteristics may have controlled the expression of
postspreading deformation. Circular MBA lows are centered at the east-west-trending extinct ridge axis, not at theoblique deep rift. This indicates that the anomalous density
structure beneath the spreading center is preserved after the
cessation of spreading.
5. New age data from the CBSC axis range from 27 to 15
Ma. These dates indicate that reactivation or deformation
along the CBSC continued long after the basin formed andmay be related to the opening of the Parece Vela Basin.
[43] Our results indicate that higher magma supply,
perhaps related to a small mantle plume, is a key factor
controlling the morphological and geophysical variations
along the CBSC. This elevated magma supply affected not
only the spreading style but also the evolutionary history of
the basin.

Appendix A: Detailed Description ofMorphological Features
A1. On-Axis and Off-Axis Morphology[44] We define seven segments and intervening discontinuities mainly based on swath bathymetry data along the
CBSC (Figures 3 and 4); however, we used supplementaryEPM

5 - 16

OKINO AND FUJIOKA: CENTRAL BASIN SPREADING CENTER

Table 1. Segments and Discontinuities of the CBSC Between 126°000 and 133°300E
Name

Length 1a, km

S1
S2
S2E
S2W
S3
S4
S5
S6
S7
S8

61
100
49
51
57
40
109
107
90
(series of failed rifts
in our survey area)

Length 2a, km

Maximum Rift
Width, km

Average
Depth, m

Rift
Height, m

123

19
19

5900
6500

1500
1100 – 1500

25
23
26
20
20

6400
6800
7700
6000
6200

1400, 109°
1300, 123°
1600 – 2800
900 – 1500
1300

44
46
83
107

Trend, deg

Neo Volcanic
Zone (NVZ)

115°
112°
114°
110°
No
No
138°
092°
116°

yes
yes
yes
?
no
no
no

Off-Axis Trace
Name

Location

S1 – S2
S2E – S2W
S2 – S3
S3 – S4
S4 – S5
S5 – S6
S6 – S7
S7 – S8132°210E NTD
131°550E
131°300E NTD
130°520E NTD
130°17.50E
129°340E ETF & FZb
129°300E short TFc

TypeOffsetOur Map

not clear
no
yes
yes

ambiguousvery short
yes
yes
yes
yes
yes
yes

NTD
yes
yes
OSC

SatelliteyesaLength is measured along the current deepest rift, and Length 2 is the distance between discontinuities at both ends.
b
FZ, fracture zone.
cTF, transform fault.

satellite-derived gravity anomaly data [Sandwell andSmith, 1997] for detecting the off-axis traces of discontinuities. The locations of discontinuities, segment length,
rift valley relief and rift wall elevation, and other parameters are summarized in Table 1. All segments are rightlaterally offset by the discontinuities and preserve thegeneral NW-SE trend inherited from the earlier NE-SWdirected spreading stage. Segment length, defined here as
the distance between neighboring discontinuities, ranges
from 44 to 123 km. The longest segment, S2, can bedivided into two third-order segments, S2W and S2E.
These have approximately the same length as the second-order segments, but the offset between them has nooff-axis trace.
[45] The off-axis seafloor is primarily characterized bylinear abyssal hills indicating continuous seafloor spreading from the CBSC. The general trend of the abyssal hills
gradually varies from 110° to 080°, except for the area
west of 128°300E, where oblique lineaments formed by the
OSC are found. The orientation of the off-axis fabricchanges from 095° far from the axis, to 080° and 090°
near the axial rift. This change can be most clearly seen in
S6 but is a common feature along other segments. The
129°340E fracture zone changes orientation by 10° (from003° to 353°) north of the rift valley. The location of thischange is approximately coincident with the disappearanceof the nontransform discontinuity traces, suggesting thatridge segmentation and reorganization compensated for thechange of spreading direction.
[46] The deep rifts oblique to the off-axis fabrics areobserved over the entire CBSC, but detailed characteristics differ between segments. In Segment S1, where theeast-west-trending rift is wider than in the other segments,
two 115°-trending, en echelon rifts are developed within
the east-west-trending rift. Small ridges about 200 m highlocated in the oblique rifts may reflect weak volcanism

during rifting. The oblique rift trends about 110° in S2and S3, where the rift shoulder mountains are characterized by east-west lineaments but are interrupted by theoblique rifts at the axial zone. Two 200-m-high ridges arelocated symmetrically within the oblique rift at S2W,
which suggests that they are the remnants of a neovolcanic zone split by extensional tectonics. The very
deep and highly oblique rifts deform the east-west-spreading fabric in S4 and S5. The axis of symmetry in theolder east-west-spreading fabric is located south of theoblique rift.
A2. The Western CBSC Overlapping
Spreading Center[47] Just west of the 128°360E discontinuity the seafloor
fabric is similar to the fabric observed in segment S6, where
the orientation of abyssal hills varies from east-west to 080°
near the axis. The 140°E-trending central rift becomes
indistinct between 127° and 127°300E. A major bathymetricfeature in the area is a series of abandoned rifts near 16°N
that show a left-stepping en echelon alignment, indicating
successive rift failures. The seafloor fabric between thesefailed rifts and the 140°E-trending rift valley exhibits avariation from almost east-west to a 030° trend. The area
also has a shallow average depth when compared with otheroff-axis areas. The abandoned rifts seem to have beencreated by successive rift failures with a short repeat cycle.
The western (northern) ridge segment, which is not numbered in this paper, was abandoned while the eastern segment (S7) propagated westward.
[48] The length and width of the overlap region are 100
km, so that the length to width ratio is close to 1:1. The
OSC is typical of those seen at fast spreading centers, e.g.,
the East Pacific Rise, the Pacific-Cocos, and the PacificAntarctic plate boundaries. Large-scale dueling propagation
forms trapped microplates similar to the Easter microplate

OKINO AND FUJIOKA: CENTRAL BASIN SPREADING CENTER[Handschumacher et al., 1981; Hey et al., 1985; Naar andHey, 1991] or the Juan Fernandez microplate [Francheteau
et al., 1987; Larson et al., 1992; Martinez et al., 1997].
Small-scale OSCs are observed at second-order discontinuities along fast-spreading centers.
[49] At 126°450E, 18°300N, a large seamount higher than3000 m is located south of the diminishing rift valley. The
surrounding area is characterized by 075°-trending lineaments, and the seamount itself is bounded by faults with thesame orientation. This may reflect some structural control of
volcanic construction. The horizontal scale and volume of
the mountain are estimated to be 90  30 km and 3000 km3,
respectively, although our survey does not cover the entire
mountain. To the east a smaller but similarly structurally
controlled seamount whose length and height are 15 km and2000 m is located at the center of the rift valley. Thisseamount shows a marked contrast to the small axial
volcanic ridges observed in S1 and S2. These observations
suggest that a higher magma supply existed in the westernsegment of the CBSC.
[50] Small seamounts with relief <600 m are closely
spaced on the southern flanks of segments S6 and S7(indicated by a small dot in Figure 4). The typical size ofthese seamounts is a 5 – 7 km diameter. Similar size seamounts are also located in the off-axis areas of the easternsegments, but they are more widely distributed. The seamount cluster coincides with a MBA low, suggesting
thicker crust and presumably more abundant magmatism
than in eastern areas.

[51] Acknowledgments. We thank the captain and crew of R/V
Kairei for their efficient work during the STEPS cruises. We also thankT. Hilde, E. Honza, T. Yamazaki, S. Lallemand, A. Deschamps, and all the
onboard scientists for data acquisition and useful discussion. Constructive
reviews of the manuscript by K. Tamaki and D. Curewitz are greatly
appreciated. Y. Ohara, O. Ishizuka, and H. Sato gave us valuable information and comments from the viewpoint of petrology. Thanks also to F.
Martinez, an anonymous reviewer, and the Associate Editor for their helpfulcomments.

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K. Fujioka, Japan Marine Science and Technology Center, Yokosuka,
Japan.
K. Okino, Ocean Research Institute, University of Tokyo, 1-15-1
Minamidai, Nakano, Tokyo 164-8639, Japan. (okino@ori.u-tokyo.ac.jp)