Tectonics of Unusual Crustal Accretion in the Parece Vela Basin - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 163

(Table 1 ). Martinez et al. ( 1998 ) noted that the surveyed spreading segments form
axial valleys. They also reported OCCs from one area within the Valdivia FZ system
(Martinez et al. 1998 ).
Peridotites were recovered from the Valdivia FZ system, although no detailed
petrological data on these peridotites are published (F. Martinez, personal commu-
nication, 2009). However, the basalts from the Chile Ridge show higher Na and
lower Ca for a given MgO content than the basalts from the East Pacific Rise, indi-
cating low degree of mantle melting and low mantle temperature (Bach et al. 1996 ).
Bach et al. ( 1996 ) noted that the mantle beneath the Chile Ridge is characterized
by relatively low temperature due to the result of cooling associated with the
numerous transform offsets, in fact pointing out a transform sandwich effect.
The Chile Ridge near the Valdivia FZ is thus similar to the central PVB in that
these distinct characteristics of relative magma starvation are observed in an other-
wise robust magmatic budget environment, suggesting a transform sandwich effect
is working there.

6 Tectono-Magmatic Characteristics of Intra-Transform

Spreading Centers in Fast-Spreading Ridges

Transform faults along fast-spreading ridges are often segmented by intra-transform
spreading centers (ITSCs). The geometry of ITSCs is similar to that of the four
specific examples where transform sandwich effect may be working (i.e., the cen-
tral PVB, and the Ascension, St. Paul and Valdivia FZs). However, the notable
characteristic of ITSCs is that magmatic process is dominant there (Hékinian et al.
1992 ; Perfit et al. 1996 ). Here we examine two examples from the East Pacific Rise
to clarify the tectono-magmatic difference between these two similar features.
The Siqueiros Transform Fault is a left lateral transform fault located on the East
Pacific Rise between 8°15¢N and 8°30¢N. The spreading full-rate of this part of the
East Pacific Rise is ~10.7 cm/year (Pockalny et al. 1997 ; based on NUVEL-1A).
Fornari et al. ( 1989 ) identified that the transform domain is ~20 km wide and seg-
mented with four ITSCs. Length of each ITSC is ~7 to ~9 km (Pockalny et al.
1997 ). On the other hand, the Garrett Transform Fault is a right lateral transform
fault located on the East Pacific Rise at ~13°28¢S. The spreading full-rate of this
part of the East Pacific Rise is ~14.5 cm/year (Naar and Hey 1989 ). The transform
domain is ~24 km wide and consists of three ITSCs of ~10 km in length (Hékinian
et al. 1995 ). Magmatic process is dominant along the ITSCs in both the Siqueiros
and Garrett Transform Faults (Hékinian et al. 1992 ; Perfit et al. 1996 ); occurrence
of OCCs is not reported so far from the ITSCs in the East Pacific Rise, although
exposure of depleted peridotite is known from the eastern part of the Garrett
Transform Fault (Hékinian et al. 1995 ; Constantin 1999 ).
It might be expected that the lavas from ITSCs would exhibit chemical charac-
teristics indicative of classic transform fault effect, since the lithosphere within the
transform domain would be older and colder than the lithosphere along the adjacent