18InterRidge NewsInternational Ridge-Crest Research: Back Arc Basins

Parece Vela Rift and Central Basin Fault revisited - STEPS-IV(Structure, Tectonics and Evolution of the Philippine Sea) Cruise summary report
K. Fujioka1, T. Kanamatsu1, Y. Ohara2 , H. Fujimoto3, K. Okino3, C. Tamura3, S. E. Lallemand4, A. Deschamps -Boldrini4, J. A. Barretto5, N. Togashi6, H. Yamanobe6, A. So 6
Japan Marine Science and Technology Center, Yokosuka, Kanagawa 237-0061, Japan
Hydrographic Department of Japan, Tsukiji, Chuo, Tokyo 104-0045, JAPAN3
Ocean Research Institute, University of Tokyo, University of Tokyo, Tokyo 164-8639, Japan
4University Montpellier II, Montpellier cedex 5, France
5
National Institute of Geological Sciences, University of The Philippines, Quezon City, 110, Philippines
6Marine Works Japan Ltd, Yokohama, Kanagawa 236-0031, Japan
12Introduction
An enigmatic large linear structure in the Philippine Sea, the Central Basin Fault (CBF) was first foundand named by Hess (1948). It occupies the central part of the West
Philippine Basin (WPB), one of several backarc basins in the WesternPacific. The CBF is a WNW-ESEtrending 1,000 km-long linear structure with a notable rift valley like aslow spreading ridge.
We mapped areas around theCBF during the KR98-01, KR98-12,
and KR99-10 cruises. Our results
made it clear that the CBF was a slow
spreading center in a backarc basin.
French R/V LAtalante cruises (DAVAPUS from Davao to Pusan in 1994
and KAONOUM from Kaoshiung toNoumea in 1996) had two long single
transect lines across the West Philippine Basin by using SIMRAD EM12-Dual multibeam swath mapping
system and major structures of theCBF were mapped (Deschamps etal., 1999). We also took sedimentand rock samples and performed heatflow measurements at several pointsof the CBF and West Philippine Basin during the KR98-01 and KR99-10
cruises. Two Shinkai 6500 dives wereperformed in 1996. These dives collected photographs, videos and ba-salt samples from both walls of theCBF axial valley. Based on chemistry, the basalts were found to havea backarc basin affinity (Fujioka etal., 1999). However, despite theseadvances, numerous unsolved questions regarding the geology andgeophysics of the West Philippine
Basin still remain.
East of the West Philippine Basin lies the Parece Vela Basin which

exhibits two trends of magnetic lineations. The analyses of these magnetic lineations made it clear that theParece Vela Basin had two episodesof spreading history; the 1st was an
EW fast spreading event, having
several propagations to the north,followed by a NE-SW trending, slow
spreading stage accompanied byamagmatic spreading (Ohara et al.,
1996, 1997; Kasuga and Ohara, 1997;

25N

20N

15N

km
010N
130E

135E

140E

500

145E

Figure 1. Map of the survey areas showing the ships tracks
during the STEPS-IV cruise.

Vol. 9(1), 200019International Ridge-Crest Research: Back Arc Basins: Fujioka et al., continued...

17˚ 00'-6000-5000-4000-6000-400
0-400016˚ 30'-5000

000-5-5000-5000-5000

16˚ 00'00000-5-5000-60

15˚ 30'-50km

00

Okino et al., 1998, 1999). Recently
Ohara and others (in preparation)
recognized three megamullion structures along the Parece Vela Rift andof these they noted the existence ahuge megamullion-like structure
called Giant Core Complex (GCC) inthe area surrounded by 15°30N,16°30N, 138°30E, 139°30E. Thisstructure has notable transform-parallel mullion-like structure.
In the light of these prior discoveries, we set out to further investigate the origins and evolution of theWest Philippine Basin. This latest
cruise focused on data collection
from three different areas, the Parece
Vela Rift, GCC and CBF, using theR/V Yokosuka, JAMSTEC. The scientific plan consisted of surface shipobservations of gravity, bathymetry and magnetics (both proton andthree-component magnetometer)and deep-towed proton magnetometer apparatus. The cruise, YK00-01
STEPS-IV, Structure, Tectonics
and Evolution of the Philippine SeaIV, started at Guam pier on 26th January and ended at Guam pier again
on 14 February. The cruise was conducted as a JAMSTEC project, under the umbrella of SEAS (Science ofSouth East Asian Seas) and the InterRidge program. Scientists fromFrance, the Philippines as well asJapanese universities and research
institutions assembled onboard theYokosuka in order to address theabove stated scientific objectives.
The major objectives of thiscruise were two-fold: 1) to investigate the precise magnetic reversal
pattern of an extinct spreading ridgeby the deep-towed proton magnetometer and 2) investigate the evolution of the Parece Vela Basin andthe West Philippine Basin by elaborated swath mapping. The areas studied included segments of the Parece

015˚ 00'
138˚ 30'139˚ 00'50
139˚ 30'140˚ 00'

Figure 2a. Bathymetric map of the Giant Core Complex (GCC) in the Parece
Vela Basin.

Vela Rift, the megamullion GCC, theeastern part of the CBF axis, as well
as several transit lines from Parece
Vela Basin to CBF across the Kyushu Palau Remnant Arc.
Results
During the YK00-01 STEPS-IV
cruise, the following results wereobtained for the Parece Vela Basinand the CBF. Figure 1 shows theship tracks of the STEPS-IV cruise.

1. Bathymetry and morphology
We conducted swath surveys
using the SeaBeam 2112 system. The
survey covered 930 miles of track inbox surveys around the GCC and730 miles in the eastern Central Basin Fault (E-CBF).
Morphology of the GCC
Previous results conducted by
the Hydrographic Department ofJapan allowed us to recognize atleast three megamullion-like structures near three segments of the20InterRidge NewsInternational Ridge-Crest Research: Back Arc Basins: Fujioka et al., continued...

-2000

16N
-3000

-4000

-5000

15N

-6000

-7000

-8000
km
014N

131E

132E

133E

50100

134E

Figure 2b. Bathymetric map of the Eastern Central Basin Fault (E-CBF).
Parece Vela Rift from 16° - 19°N, and139° - 140°E including the IPOD
Trough named by the Russian scientists. This was the first finding ofthe megamullion structure from thebackarc basins in the world (Oharaet al., in prep.). During this cruise we
mapped one of the three mullions
called the Giant Core Complex. TheGCC occupies the southwestern halfof the extinct spreading segment ataround 16°N and the surface of theGCC is characterized by corrugations running parallel to the fracturezones. (Fig. 2a). We recognized three
types of structures, prominent fracture zone-parallel mullion structures
orientated NE-SW, as well as clear
Break Away and Termination structures. The size of GCC is 120 km longand 57 km wide and covers an area of7125 km2, which is ten times greaterthan those reported from the Atlantic Ridge (Tucholke et al., 1997).
This is the largest megamullion everreported, therefore this should becalled Gigamullion. Serpentinizedperidotites and gabbros were recovered from the IPOD Trough and theGCC (Shcheka, et al., 1995; Ohara etal., 1996; Fujioka et al., 1998). The
peridotite composition varied fromfertile lherzolite to depleted harzburgite. Many peridotite samples had
evidence of mantle-melt interaction(Ohara et al., in prep.). Proton magnetometer survey across the GCC
was also conducted across segment
3 and the result was compared withother segments.
Morphology of the E-CBF

We conducted three EW and two
NS lines at the E-CBF, close to thejunction with the Kyushu-PalauRidge. We used two more lines, one
transit line tracing a previous JAMSTEC cruise and a French cruise line
obliquely crossing the CBF for themorphologic analysis. We made asmall box at the E-CBF (Fig. 2b). The
spreading fabrics trend at 290° was
almost parallel to the axial valley.
The boundary of the CBF and theKyushu-Palau Remnant Arc wasidentified as a negative gravity anomaly along the 134°E line based on theSatellite Gravity Anomaly map(Sandwell and Smith, 1997) but thespreading fabrics of the CBF continue as far as 133°40E. The axial valleyof the CBF terminated at 133°40Egetting shallower to the east from

Vol. 9(1), 2000

21

International Ridge-Crest Research: Back Arc Basins: Fujioka et al., continued...

133°10E. The rough spreading fabrics of the seafloor on both sides of
the E-CBF, in comparison with western and central parts of the West
Philippine Basin, can be interpreted
either by a slower spreading rate ora smaller magmatic supply whencompared with those of the slow
spreading, Mid-Atlantic Ridge andfast spreading, East Pacific Rise.
Transit lines
We had two long transit lines
from the Parece Vela Rift to the ECBF across a chaotic terrain of thewest half of the Parece Vela Basin.
This terrain consisted of isolated
and elevated dome-like blocks (maximum relief was up to 1500m), cappedby axis-normal lineations and associated deeps. Ohara et al. (in prep.)interpreted these isolated dome-likeblocks to be analogues of megamullion structures.

was 1.7-2.4 knot, (average, 2.2 knot)
with respect to the ground, and thetowing depth was 2100-3000 m below sea surface. The chosen line
was across one segment of theParece Vela Rift covering both NE
and SW sides of the extinct spreading centers. High resolution magnetic intensity peaks were identified
along the line and we correlated small
peaks with theoretical magnetic reversal pattern to get a more precise
evolutionary history of the laterstage of the Parece Vela Basin for-mation (Fig. 3). Online data transmission system worked successfully during the survey.
Summary
A geophysical swath survey anda Deep-Towed Proton Magnetometer survey were carried out using theR/V Yokosuka, JAMSTEC in the vicinity of the Parece Vela Rift and theCBF during a 20 day cruise in JanFeb 2000. A huge megamullion
called Giant Core Complex was fully
mapped, including its corrugated

2. Geophysics
We conducted geophysical surveys on the gravity and magnetic
properties together with swath mapping by using a gravity meter, aproton magnetometer, the shipboard
three-component magnetometer anda DTPM system. An additional threecomponent magnetometer, which
was recently developed by scientists at Chiba University was also
used parallel to the shipboard threecomponent magnetometer. Gravity
and magnetic data will be analysedin the near future to elucidate thecrust and upper mantle structure, as
well as the magnetic lineations ofthe West Philippine Basin and theParece Vela Basin.
3. Deep Tow Proton Magnetometer
We conducted surveys along atransect (195 nautical miles, NE toSW) by using DTPM under the following conditions. The ships speed

Figure 3. Deep Tow Proton Magnetometer (DTPM), a comparison with thesurface proton magnetometer. A topographic cross section along the NESW survey line across one segment of the Parece Vela Basin.

22InterRidge NewsInternational Ridge-Crest Research: Back Arc Basins: Fujioka et al., continued...

surface structure, as well as termination and break away points. A deeptowed proton magnetometer survey
was performed across one segmentof the Parece Vela Basin to obtain
fairly good magnetic signals for abetter understanding of the spreading history. The easternmost segment of the CBF was also mappednear its junction with the Kyushu
Palau Ridge. Two N-S lines wereacquired across the extinct spreading center, perpendicular to thespreading fabrics for age determination from magnetic anomalies. These
new data contribute to our knowledge on the last spreading phases ofboth the Parece Vela Basin and theWest Philippine Basin.
Acknowledgments
The scientific party of STEPS-IV
cruise expresses its thanks to all theofficers and crew of the Yokosuka
who assured the smooth and safe
operations of geophysical swath
mapping and Deep-Towed Proton
Magnetometer survey in the Philippine Sea even under rough sea condition. We thank K. Suyehiro, K.
Tsujii and all the JAMSTEC personnels for their vigorous support andhelp in the preparation of the cruise.

Thanks are also due to K. Tamaki ofORI, University of Tokyo and to N.
Seama of Chiba University.
References
Deschamps, A., Lallemand, S.,
Dominguez, S., The last spreading episode of the West Philippine Basin revisited, Geophysical Research Letters, vol. 26, 14,2073-2076, 1999.
Fujioka, K., Fujiwara, T., Kanamatsu, T., Okino, K., Hilde, T.W.,
Honza, E., Ishii, T., Ikehara, K.,
Matsuoka, H., Hisada, K., Ohara,Y., Tsuchihashi, M., Haraguchi,
S. , Kato, K., Kawamura, K., Kaneko, G., Takasugi, H., Takahashi,M., Kodera, T., and Motoo, T.,
Morphology of Mariana Trenchand and Central Basin Fault- preliminary results of KR9801
cruise-. JAMSTEC J. Deep Sea
Res., 14, 163-192, 1998.
Fujioka, K., Okino, K., Kanamatsu,T., Ohara, Y., Ishizuka, O.,
Haraguchi, S., and Ishii, T., An
enigmatic extinct spreading center in the West Philippine backarc
basin unveiled, Geology, v.27
1135-1138,1999.
Hess, H. H., Major structural features of the western north Pacif-ic, and interpretation of H. O.
5989 bathymetric chart, Korea toNew Guinea. Bull. Geol. Soc. Am.,59, 417-446, 1948.
Kasuga, S. and Ohara, Y., 1997. A
new model of back-arc spreading in the Parece Vela Basin,
northwest Pacific margin. The
Island Arc, 6, 316-326.
Ohara Y., Kasuga S. and Ishii T.,
Peridotites from the Parece Vela
Rift in the Philippine Sea: Upper
mantle material exposed in an
extinct back-arc basin. Proceedings of the Japan Academy , 72,
Ser. B, 118-123, 1996.
Ohara, Y., Kasuga, S., Okino, K. andKato, Y., Survey maps Philippine
Sea structures. EOS, Transactions, American GeophysicalUnion., 78, Number 48, 555, 1997.
Okino, K., Kasuga, S. & Ohara, Y., A
new scenario of the Parece Vela
Basin Genesis. Mar. Geophys.
Res.20, 21-40, 1998.
Okino, K., Ohara, Y., Kasuga, S. andKato, Y., The Philippine Sea: Newsurvey results reveal the structure and the history of the marginal basin, Geophysical Research Letters, vol.26, 15, 22872290, 1999.

2000 Western Pacific Geophysics Meeting
SEAS and InterRidge Joint Business MeetingthEvening of 28 June , 2000
Convernors: Masataka Kinoshita (SEAS) and Hiromi Fujimoto (IR Back-Arc Basin WG)
Place: Meeting room in the WPGM conference building in Tokyo
SEAS (Science of East Asia Seas) and InterRidge Back-arc Basin Working group will exchange the information of recent activities of offshore research of the western Pacific by the Asian countries. We also discuss
about the possible coordination of future researches in the western Pacific.