Tectonics of Unusual Crustal Accretion in the Parece Vela Basin - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 162

transform sandwich effect may also be working there. The transform sandwich
effect in this cold and slow-spreading ridge may be manifesting a similar tectono-
magmatic environment to an ultraslow-spreading ridge.

5.2 Australian-Antarctic Discordance in the Southeast

Indian Ridge

The Australian-Antarctic Discordance (AAD) is a part of the Southeast Indian
Ridge. The full-spreading rate is ~7.2 cm/year (Okino et al. 2004 ; based on
NUVEL-1A). The AAD is characterized by unusually deep seafloor compared with
the standard age-depth curve (Hayes 1988 ; Marks et al. 1990 ) and by its rugged
chaotic seafloor morphology (Christie et al. 1998 ). A cold mantle domain has been
proposed beneath the AAD (Christie et al. 1998 ), being attributed to the presence of
a subducted slab (Pacific Plate) beneath the Gondwana Plate (Gurnis et al. 1998 ).
Within the AAD, the spreading segments are unusually deep for a rapid interme-
diate-spreading rate ridge with mean water depth ~4,800 m (Okino et al. 2004 ).
OCCs in the AAD appears to develop along the entire second-order segment, which
is ~40 km (Okino et al. 2004 ).
Peridotites were recovered from the AAD (Christie et al. 1998 ), although no
detailed petrological data on these peridotites are published. However, the AAD
lavas have higher Na and lower Fe for a given MgO content, than the lavas from the
east of AAD (i.e., Pacific-type MORB), indicating low degree of mantle melting
and low mean pressure of melting beneath the AAD (Klein et al. 1991 ).
The Southeast Indian Ridge near the AAD is thus similar to the western PVB in
that a cold mantle domain is inferred to occur in an otherwise robust magmatic
budget environment.

5.3 Valdivia FZ in the Chile Ridge

The Chile Ridge extends from the Juan Fernandez Mircoplate to the Chile margin
triple junction. The ridge is offset by 18 transform faults with fossil fracture zones,
including two complex fracture zone systems. One of these, the Valdivia FZ system,
consists of six parallel fracture zones, separated by first order ridge segments ranging
from ~22 to ~27 km (Tebbens et al. 1997 ). The Valdivia FZ system evolved from a
single long left-lateral offset transform fault connecting the Chile Ridge to the
Pacific-Antarctic-Nazca triple junction at ~17 Ma when a 10° counterclockwise
rotation occurred (Tebbens et al. 1997 ).
The spreading half-rate of the Chile Ridge varied from ~6.1 cm/year (at ~23 Ma)
to ~3.1 cm/year (at ~6 Ma to present) (Tebbens et al. 1997 ). Since the spreading
was generally symmetric (Tebbens et al. 1997 ), we employ the doubled values as
the postulated full-rates for the Chile Ridge (i.e., a fast-spreading rate of ~12.3 cm/
year full-rate to an intermediate-spreading rate of ~6.2 cm/year) in this paper