Extinct spreading center of the Parece Vela Basin - a result of deep-tow
magnetic survey
Kyoko OKINO, Chiori TAMURA, and Kensaku TAMAKI

okino@ori.u-tokyo.ac.jp, Ocean Research Institute, University of Tokyo
Hiromi FUJIMOTO
Tohoku University

ineylurve

Survey Results
TheSeaBeam bathymetryshows clear normal
spreadingfabric,however there is a
domed topographic
high just west of theFigure 1 Bathymetry ofrift. And we can also
the Parece Vela Riftrecognize the corru(Modified Ohara et al., ingation not parallel topress). Thick and Thin linethe spreading fabric on
indicate deep-tow
andthe top of the dome. Itsurface-tow
magnetic
may be similar to thesurvey lines, respectively.
mullion structure, theexposed lower crust, reported in the Mid-Atlantic Ridge
slow spreading center. We also obtained a long magnetic
Ps

Introduction
The Shikoku and Parece Vela Basins are theextinct backarc basins occupying eastern half of thePhilippine Sea. They were formed during late Oligocene
and Middle Miocene behind the proto Izu-BoninMariana arc-trench system. Two basins were developed
as one backarc system in the later stage of the spreading.
We can see the clear spreading fabric, and a sea mountchain along the axis of the Shikoku Basin and a chain ofdepressions at the Parece Vela Basin. Magnetic lineation
pattern also presents an evolutionary history of thesebasins as follows; the earlier east-west spreading from
26 Ma to 20/19 Ma, and the later northeast-southwest
spreading. The spreading ceased at around 15 Ma in theShikoku Basin. Detailed reconstruction was carried out
in the Shikoku Basin, however the precise history of theParece Vela Basin is still questionable, because of its
weak and complicated magnetic anomaly and lack of age
dating of basement rocks.
The Parece Vela Rift, the remnant spreading
center of the Parece Vela Basin, shows very deep(>6000m) rift valley, which may indicate an amagmatic
extension was dominant in the terminal phase of thebasin formation (Fig. 1). The spreading axis is highly
segmented and the off-axis basin floor is characterized
by prominent fracture zones and the perpendicularspreading fabric. A unique corrugated seafloor like themega-mullion structure in the MAR is observed near theaxis (Ohara et al., in press). In order to obtain precise
opening history and the phenomena in the terminal phaseof the backarc spreading, two surveys were conductedby Japanese research ships. Our fist survey was acrossthe Parece Vela Rift
PVR
by using surface-tow
proton magnetometerand SeaBeam (Fig. 1,
Parece Vela Riftthin line). Then,based on the resultsof this survey, wecarried out the deeptow
magnetometer
survey along twoparts of the surfacetow
surveyline(Fig.1, thick lines).

DTAbstract. The Parece Vela Rift, a north-south trending
chain of deep depressions, is the remnant spreading
center of the Parece Vela Basin in the Philippine Sea.
The rift is deeper than 6000 m and is highly segmentedby prominent fracture zones. The results of our deep-tow
magnetic survey show the intermediate-rate spreading inaverage and a possibility of asymmetric spreading at theterminal phase of the basin formation.

Deep-Tow Magnetic Anomaly (YK0001)
Southwestern Wing

Northeastern Wing
PVRPVR
200

200

100

100

00-100-100-200-200

Surface-Tow

Deep-Tow

1002005B215.2Ma

200

5AC0mullion?

2005AB5A212.4Ma

100100

5A20-100

Theoretical
half-rate=3.5cm/y-200-200

5C-1005B 5AD

5AA05A0100

5A

5AA2005AD 5B

Figure 2 (Top) Results of deep and surface-tow magnetic anomaly survey and their interpretation.
(Bottom) Theoretical magnetic anomaly pattern from at spreading rate of 3.5 cm/y (half-rate).
anomaly profile across the PVR. In the southwestern
wing, we can tentatively identify the peaks as 5D to 5A,
though we could not label these small peaks completely.
On the other hand, the northeastern wing is not so clear
partly due to the obliquity of the survey line.
In order to obtain perfect match of the small
anomaly peaks for both wings, we conducted the deeptow magnetic survey along almost the same line of thesurface-tow survey line. The towing depth is not enough,but we can obtain larger amplitude profiles with shorter
wavelength anomaly (Fig. 2). In southwestern wing, wecan distinguish 5B1 from 5B2, which could not berecognized in the surface profile. And 5 peaks of Chron
5AD to 5A2 are confirmed. 5A1 is 12.4 Ma. In the sameway, we identify Anomalies 5AD to 5A2 in thenortheastern wing. Spreading half-rate 3.5 cm/y showed
best fit. In the east, the youngest anomaly is 5A2; on theother hand, we observe a couple of peaks younger than
5A2 just west of the rift. The pattern of profile seems
different from those in other part and it maybe due to thecharacter of the magnetized body of the domed mullion
like structure.

Discussion and Conclusion
Based on surface and deep-tow magnetometermeasurements, geomagnetic polarity Chron 5D to 5A2
are identified in the central part of the Parece Vela Basin.
It corresponds to 18.2 to 12.4 Ma (Cande and Kent,1995). It is much younger than previously considered,
for the spreading stopped at 14.8 Ma in the Shikoku
Basin. And it may imply the different age of final
shutdown of the Shikoku - Parece Vela spreading system.
Spreading rate of the terminal phase after 15 Ma, isestimated as 7 cm/y, an intermediate-spreading rate. Theasymmetry of anomaly pattern near the rift valley
indicates that the southwestern wing continues extension
after 12 Ma. It supports the idea of mullion structure,
which is asymmetric extension of the rift. The mullion
structure is interpreted as an exposure of the footwallalong a large detachment fault in the slow-spreading
center (Tucholke and Lin, 1994), however themechanism of asymmetric opening at intermediate-rate
spreading center has remained to be solved.