Tectonics of Unusual Crustal Accretion in the Parece Vela Basin - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 154

Deep axial
water depth

Axis

Full-axial OCC

Fertile peridotite
exposure at
segment mid-point

Pseudofault

Chaotic Terrain

First-stage
(8.8 cm/y full-rate)

Fig. 2 Schematic cartoon illustrating the bathymetric features of the Parece Vela Basin. Thick
dashed lines indicate fracture zones, whereas thin lines abyssal hills. OCCs are indicated by grey-
hatch with axis-normal lineated lines. White circles show peridotite exposures (Ohara et al.
2003b )

criterion by Macdonald et al. ( 1991 ). In the central PVB, the magnetic lineation
pattern is highly segmented and identification of the anomalies was thus impossible.
During the YK00-01 cruise of R/V Yokosuka , total geomagnetic intensity was
measured with a deep-towed proton magnetometer along a transect across the
seafloor of segment S3 in the central PVB (Fujioka et al. 2000 ). The magnetic
anomaly profile from this survey is almost symmetrical about the extinct axis of
segment S3. The overall observed anomaly pattern was reasonably explained by
spreading with a theoretical half-rate of 3.5 cm/year and cessation of spreading at 12 Ma
(Ohara et al. 2003a ). The postulated 7.0 cm/year full-rate for the second-stage of the
basin evolution is still close to the higher end of intermediate-spreading rates
(4-9 cm/year). Ohara et al. ( 2003a ) concluded on this basis that the PVB opened at
rapid intermediate-rate 8.8-7.0 cm/year full-rates. The evolution of the spreading
rate during the last few million years, as it declined to zero, is presently unknown.