Tectonics of Unusual Crustal Accretion in the Parece Vela Basin - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 153

to OCC formation (Buck et al. 2005 ; Ildefonse et al. 2007 ; Tucholke et al. 2008 ;
MacLeod et al. 2009 ). These studies suggest that there is a minimum as well as a
maximum magmatic supply necessary to produce long-lived detachment fault.
Below this minimum, detachment faults do not form or persist for long (Tucholke
et al. 2008 ).

3 Crustal Accretion in the Parece Vela Basin

3.1 Morphology and Spreading of the Parece Vela Basin

The PVB is divided into two parts. The western part, west of about Chron 6A (at
138°E), is dominated by N-S trending well-developed abyssal hills that were pro-
duced by E-W orthogonal seafloor spreading (Fig. 1b ). Some pseudofaults are
identified in the western PVB (Fig. 1b ), indicating ridge propagation and jump
there (Okino et al. 1998 ). The “Chaotic Terrain (Ohara et al. 2001, 2007 )” is a
patchy area within the western PVB, consisting of a series of small OCCs bounded
by an otherwise well-ordered abyssal hill floor (Fig. 1b ).
The central PVB (east of 138°E) has a stair step morphology of deep rifts (the
Parece Vela Rift), as the spreading direction rotated to NE-SW (Fig. 1b ). Some
pseudofaults are also identified just east of 138°E (Fig. 1b ). The geometry of the
Parece Vela Rift is composed of a series of short length (~20 to ~55 km) first-order
segments (labeled as S1-S7 from south to north) aligned en-echelon with closely-
spaced fracture zones (Ohara et al. 2001 ). Each segment becomes shorter and the
transform faults closer together as this rotation occurred (Fig. 1b ). The Parece Vela
Rift is anomalously deep, with an average axial depth of ~6,500 m. The maximum
depth (~7,500 m) occurs in the extinct axis of segment S3, corresponding to ~6,200
m zero-age depth after correction of 12 Ma (see below for the age determination)
subsidence for a backarc basin setting (Park et al. 1990 ). The world's largest OCC,
Godzilla Megamullion, is developed at segment S1 (Ohara et al. 2001 ). Other seg-
ments, at least S2 and S3, also host smaller OCCs (Figs. 1b and 2 ).
The spreading history of the PVB consisted of two stages (Fig. 2 ). The first-
stage was E-W rifting and spreading with spreading axes trending N-S, whereas the
second-stage involved counter-clockwise rotation of spreading axes from N-S to
NW-SE (Okino et al. 1998 ).
The magnetic lineation pattern of the PVB is very weak, mostly because spreading
occurred when the basin was near the magnetic equator (Okino et al. 1998 ). In order
to obtain better magnetic information, Okino et al. ( 1998 ) conducted three-dimen-
sional inversion of the magnetic data and successfully identified N-S trending mag-
netic anomalies 7 to 6A for the western PVB (Fig. 1b ). A spreading rate of 4.4 cm/
year half-rate was thus reasonably estimated for the first-stage of the basin evolution
during the period of 26-21 Ma (Okino et al. 1998 ). The postulated 8.8 cm/year full-
rate is at the highest end of intermediate-spreading rate (4-9 cm/year) based on the