Vol. 12(1), 2003

27

International Research: Back Arc Basins

Preliminary report of Kairei KR03-01 cruise: amagmatic tectonics andlithospheric composition of the Parece Vela Basin
Y. Ohara1, K. Okino2, J. E. Snow3 and KR03-01 Shipboard Scientific Party
1

Hydrographic and Oceanographic Department of Japan, Tokyo 104-0045, Japan
Ocean Research Institute, University of Tokyo, Tokyo 164-8639, Japan
3Max-Planck Institut für Chemie, D-55020 Mainz, Germany
2Introduction
Recent mapping studies by ourgroup (Kasuga and Ohara, 1997;Okino et al., 1998; 1999; Ohara et al.,2001) revealed that the Parece VelaBasin (PVB) in the Philippine Sea(Fig. 1) has the geodynamic characteristics expected for amagmatic extension, thus providing a rare opportunity to study the architecture andcomposition of backarc basins. The
study of backarc spreading systems
has a strong bearing on two important aspects of the Earth’s evolution,
as it relates to both subduction zone
and mid-ocean ridge dynamics.
Study of backarc basins thus has
strong impact on both the InterRidge
and MARGINS communities.
The KR03-01 cruise aboard theJapan Marine Science and Technology Center’s R/V Kairei was thus
scheduled to better understand thelithospheric architecture and composition of this unique backarc basin (Ohara et al., 2002). In this article, we report the preliminary resultsof the cruise.
Geodynamic background
The Philippine Sea occupies alarge part of the western Pacific andis composed of three large basins
separated by the Kyushu-Palau andWest Mariana ridges (both are remnant arcs; Fig. 1). Situated east of
the Kyushu-Palau Ridge, theShikoku Basin and the PVB are extinct backarc basins. The central PVB
is characterized by a N-S trending
chain of right-stepping en-echelon
depressions (the Parece Vela Rift;
Mrozowski and Hayes, 1979) bordered by escarpments extending ~N20°E from the depressions into thesurrounding basin floor (Fig. 2). The
escarpments and depressions (maximum depth ~ 7500 m) are fossil fracture zones and extinct segmentedspreading axes (first-order segments), respectively (Kasuga andOhara, 1997). Each segment is labeled as S1-S7 from south to north(Ohara et al., 2001). The PVB has a
two-stage spreading history (Kas-uga and Ohara, 1997; Okino et al.,
1998; 1999), initial E-W rifting andspreading with spreading axes trending N-S began at ~ 29 Ma (spreadingrate: 8.8 cm/y full-rate) (Okino et al.,
1998; 1999). The second stage involved counter-clockwise rotation ofspreading axes from N-S to NW-SE at~ 19 Ma (spreading rate: 7.0 cm/yfull-rate) (Ohara et al., submitted).

40 N

30 N

20 N

10 N

0120 E

130 E

140 E

150 E

Figure 1. Satellite altimetry map showing the tectonic feature of the Philippine
Sea. Dotted box indicates the location of Fig. 2.

28InterRidge NewsInternational Research: Back Arc Basins: Ohara et al., cont...

Spreading ceased at ~ 12 Ma (Oharaet al., submitted).
Ohara et al. (2001) mapped distinct rough topographic features in
the PVB, suggesting amagmatic extension in the basin, in spite of thebasin’s relatively fast-spreading rate
(8.8-7.0 cm/y full-rate). The mostdistinct topographic feature is a setof megamullions in the Parece Vela
Rift (PVR). Recently discovered“megamullions” found along slowspreading ridges have been inter-preted as exhumed footwalls of lowangle normal faults, characterized
by distinct corrugations normal tothe spreading axis (Cann et al., 1997;Blackman et al., 1998; Mitchell etal., 1998; Tucholke et al., 1998). Oneof the PVR megamullions is the largest seafloor megamullion known.
Ohara et al. (2001) named it the Giant
Megamullion, as it is ~ 10 times larger in area than the Mid-Atlantic Ridge(MAR) megamullions. The other distinct topographic feature is a rug-

21 N

20 N

19 N

18 N

17 N

16 N

15 N
135 E

136 E
-8000

137 E-6000

138 E
-4000
De pth (m)139 E-2000

140 E
0Figure 2. Shaded bathymetry of the Parece Vela Basin showing the cruise tracksof R/V Kairei during the KR03-01 cruise. Bathymetric data are from this cruise
as well as previous cruises (Ohara et al., 2001). The short, first-ordersegments are labeled S1-S7 from south to north. Asterisks indicate dredge
hauls containing peridotites, circles indicate dredge hauls containing mostly
basalts. A square indicates an empty dredge haul. Our mapping confirmed
the presence of several megamullions in the chaotic terrain, yielding mantle
peridotite from a single dredge haul. The Godzilla Mullion yields peridotites
from the entire length of over 125 km (4 dredges). Some dredge hauls from theGodzilla Mullion contain plagio-granite. A voluminous axial blocky ridge inthe rift valley of S6 yields peridotites, indicating this structure is possibly aremnant inside-corner high massif. “Crab-leg ridges” are clearly seen in thesouthern off-axis of S6.

ged “chaotic terrain” in the off-axis
region of the western PVB. This terrain consists of isolated and elevated blocks (maximum relief is ~ 1500m), capped by corrugated axis-normal lineations, and associated deeps(maximum depth ~ 6000 m). The chaotic terrain has a mantle Bouguer
anomaly distinctly higher than thesurrounding ocean basin, about 30mgal, indicating a thinner crust beneath the area (Okino et al., 1998).
The morphology and gravity signature of these individual blocks arealso similar to MAR megamullions.
Similar off-axis rugged topography
has been documented at the “high”
intermediate-spreading AustralianAntarctic Discordance (7.4 cm/y fullrate) and reflects a long-term magma
deficiency associated with a mantle
cold spot (Christie et al., 1998).
Before the KR03-01 cruise, mantle peridotites were sampled in only
two previous dredge hauls in thePVR; no bottom samples were obtained from the chaotic terrain. However, the limited sample suites revealed fundamental characteristicsof the backarc basin lithosphericcomposition (Ohara et al., in press).
The most notable characteristic of
PVR peridotites is the existence offertile peridotite (spinel Cr # (= Cr/Cr+Al ratio) ~ 0.17), accompanied bymelt-impregnated peridotite withmore depleted composition. The existence of fertile peridotite indicatesthat PVR peridotite experienced only
minor melting (4 % near-fractional
melting of a MORB-type mantle),
most likely due to inhibited mantle
melting caused by closely-spaced
fracture zones (Ohara et al., submitted). The extreme water depth of thePVR (maximum depth ~ 7500 m) also
supports cold magma. The estimated low degree of melting of the PVB
upper mantle is, to a first approximation, consistent with bathymetricfeatures suggesting amagmatic extension.
Preliminary results
The Kairei left Yokosuka on January 6, 2003 and returned to Yokosu-Vol. 12(1), 2003

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International Research: Back Arc Basins: Ohara et al., cont...

ka on January 25, 2003. The expedition to the southern Philippine Sea
was incredibly successful, resulting
in 18 dredge hauls and extensivegeophysical mapping (bathymetryand magnetic surveys; gravity data
were not recorded) during the 20days cruise, mainly focused on thechaotic terrain and the ridge-transform intersections (RTI) of segments
S1, S2, S6 and S7. Some of the remarkable results can be summarised
as follows (Fig. 2):1) Extensive swath mapping oblique tothe chaotic terrain confirmed that itis actually composed of several
individual megamullions. Dredging
on one of the several megamullions
recovered mantle peridotites andgabbros, documenting an
extremely thin crust with extensive
tectonic dismemberment.
2) Four dredge hauls on the Giant
Megamullion revealed it to becomposed nearly entirely of mantle
peridotite along its entire length ofover 125 km. This suggests a
peridotite exposure of some ~ 7000
km2, confirming this structure is
the largest exposure of amagmaticor nearly amagmatic ocean crust in
the world. We thus renamed it“Godzilla Mullion”, since it
deserves to have the name of theworld-famous Japanese monster.
3) Although oceanic plagio-granite
has been very rarely reported (e.g.,Engel and Fisher, 1975), we
recoveredplagio-granites
associated with mantle peridotites
from the Godzilla Mullion surface,suggesting small volumes of melt
were highly fractionated in thismagma-starved
spreadingenvironment.
4) We mapped the northern segments(S6 and S7), which were previously
poorly mapped . The rift valley of
S6 and S7 are both oblique to thefracture zones, indicating obliqueextension was dominant during thefinal phase of the basin evolution.
A minor elongated ridge and a
voluminous blocky ridge (~ 1600 mrelief) are located in the rift valleyof S6. Dredging the former

recovered a large volume of basalt,
whereas the latter yielded a large
volume of peridotite. We interpretthe former is a “normal”
neovolcanic ridge in S6, whereas
the latter may be a remnant insidecorner high massif.
5) The southern off-axis region of S6 ischaracterised by prominent curved
abyssal hills that look like “crablegs”. We interpret these as extreme
expressions of “hooked ridge”, welldescribed along mid-ocean ridge
RTI. Compared to normal hookedridges, the PVR crab-leg ridges areanomalous due to their abruptchanges in orientation along axis
and in the length of the “hooks”.
We interpret that these observations reveal a dramatic magmatic undersupply more typical of an ultraslow-spreading ridge than of afast- or intermediate-spreading ridgein a backarc basin. The results imply
that the morphologic and petrologic
characteristics of ridges we normally assume to be a direct function ofspreading rate are in fact solely afunction of magmatic supply.
Acknowledgements
We are grateful to captain Osamu Yukawa and crews of R/V Kairei for their professional work during the cruise. KR03-01 Shipboard
Scientific Party includes TeruakiIshii, Osamu Ishizuka, Hiroshi Sato,
Hiroyuki Yamashita, Yi Bing Li, Matthias Willbold, Yohei Shimizu, Maik
Biegler, Yutaka Matsu’ura, and Yusuke Sato.
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