and Gallo 1984 ; Morgan and Forsyth 1988 ). The abrupt emergence of the Chaotic
Terrain may instead require an intrinsic tectonic episode related to the presence of
a cold and/or refractory mantle domain.
The cold mantle domain may relate to the emergence of a mantle cold plume
beneath the area, and the mantle thus had not undergone significant melting during
the certain period of the first-stage spreading of the PVB. A cold mantle domain has
been proposed in the Equatorial Mid-Atlantic Ridge (Bonatti et al. 1993 ) as well as
in the Southeast Indian Ridge (at the AAD) (Christie et al. 1998 ). Numerical modeling
by Gerya and Yuen ( 2003 ) demonstrated that hydration and partial melting of the
mantle along a subducting slab can trigger Rayleigh-Taylor instabilities that evolve
into cold plumes that rise through the hot asthenospheric mantle wedge. They dem-
onstrated that these cold plumes are 300-400°C cooler than the ambient mantle.
Heterogeneous mantle containing a refractory domain could also account for the
Chaotic Terrain. Long-term preservation of refractory domains in asthenospheric
mantle is invoked by Re-Os studies on peridotites from mid-ocean ridges (Harvey
et al. 2006 ; Liu et al. 2008 ), although there are yet no Re-Os studies on the Chaotic
Terrain peridotite to test this hypothesis.
Declining Spreading Rate During a Later Phase of the
Second-Stage Spreading of the Parece Vela Basin
Another mechanism to explain the unusual characteristics of the PVB is related to
declining spreading rate during a later phase of the second-stage spreading of the
PVB. This mechanism could contribute to the central PVB OCCs.
For the second-stage spreading, a postulated 7.0 cm/year full-rate was obtained
by deep-towed proton magnetometer data (Fig. 2 ; Ohara et al. 2003a ). During the
second-stage, the full spreading rate varied from 7.0 cm/year to zero (i.e., cessation)
at 12 Ma with the ridge jump to the east and the initiation of the Mariana Trough.
However, the exact evolution of the decline in spreading rate cannot be constrained
from the currently available data. One interpretation of the deep-towed magnetom-
eter data is to assume that the PVB spread at 7.0 cm/year for the entire second-stage
and halted abruptly at 12 Ma. However, it is more plausible to suppose a decline in
spreading rate during a later phase of the second-stage. If the latter interpretation is
valid, then the tectono-magmatic characteristics of the central PVB would have
been governed by slower-spreading rate tectonics, which would imply a lower
magmatic budget and greater tectonism.
Transform Sandwich Effect
A further explanation of the unusual characteristics of the central PVB is an
extreme transform fault effect (Fox and Gallo 1984 ; Morgan and Forsyth 1988 )
beneath the Parece Vela Rift. Ohara et al. ( 2003a ) proposed a “transform sandwich