Tectonics of Unusual Crustal Accretion in the Parece Vela Basin - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 156

of the second-stage spreading (Fig. 2 ). This is consistent with a further shortening
of segments of the Parece Vela Rift (Figs. 1b and 2 ) and the collapse of spreading
in the system.
An important feature of the OCCs in the central PVB is that many of these are
developed along the full length of segments (Ohara et al. 2001 ) (Figs. 1b and 2 ). In
the Mid-Atlantic Ridge, OCCs normally develop at inside-corners of ridge-transform
intersections (e.g., Tucholke et al. 1998 ). Furthermore, the peridotites from the
Parece Vela Rift expose at segment mid-points and include fertile compositions
with spinel Cr# (i.e., molar ratio of Cr/(Al+Cr)) ~0.17 (Ohara et al. 2003a ; Ohara
2006 ). This is unusual, because segment mid-points are generally considered to be
the most magmatically robust part of a ridge system, with the highest degree of
mantle melting (Lin et al. 1990 ). In addition, fertile peridotites from the Parece Vela
Rift are among the least depleted in a global mantle peridotite context (Ohara et al.
2003a ; Ohara 2006 ). This is also unusual, because more depleted peridotites similar
to those occur at the Garret Fracture Zone (FZ) in the East Pacific Rise (Constantin
1999 ) are expected in a relatively fast spreading ridge.

4 Mechanisms That Account for the Unusual Characteristics

of the Parece Vela Basin

The unusual tectono-magmatic characteristics of the PVB indicate a relative
magma starvation there compared to the crust created at intermediate-spreading
ridges (Ohara et al. 2001, 2003a ). Based on the geologic interpretations of crust
with analogous features on global mid-ocean ridges, we propose three possible
mechanisms that can account for these characteristics: (1) presence of a cold and/
or refractory mantle domain, (2) declining spreading rate during the later phase of
the second-stage spreading of the PVB, and (3) a transform sandwich effect. The
first one primarily accounts for the unusual characteristics observed in the western
PVB, whereas the last two account for those in the central PVB.

4.1 Presence of a Cold and/or Refractory Mantle Domain

The Chaotic Terrain occupies a part of the well-ordered abyssal hill floor in the
western PVB. It abruptly emerged as a patchy area within the well-ordered abyssal
hill floor produced under a rapid intermediate-spreading environment when E-W
orthogonal spreading occurred, indicating that tectonic extension was occurring in
an otherwise magmatic spreading environment.
The presence of persistent transform faults and/or fracture zones is not evident
in the western PVB, and therefore the emergence of the Chaotic Terrain was not
due to a transform fault effect. The transform fault effect is thought to be a cooling
of the axial mantle as a consequence of juxtaposition of a young, hot and thin axial
lithosphere against an old, cold and thick lithosphere at a ridge segment end (Fox