Tectonics of Unusual Crustal Accretion in the Parece Vela Basin - Accretionary Prisms and Convergent Margin Tectonics in the Northwest Pacific Basin - page 160

5 Relationships Between Mantle Thermal Anomaly, Fracture

Zone Geometry, Occurrence of Oceanic Core Complexes
and Spreading Rate

Although the unusual characteristic of the western PVB is explained by assuming
a cold and/or refractory mantle domain, those of the central PVB cannot be
explained solely by a single mechanism. We thus examined the literatures to inves-
tigate the relationship between mantle thermal anomaly, fracture zone geometry,
occurrence of OCCs, and spreading rate in several regions (Table 1 ). These specific
examples are characterized by their unique tectono-magmatic signatures, including
the occurrence of cold mantle domains, multiple fracture zones, and OCCs at inter-
mediate-spreading rates.

5.1 St. Paul FZ in the Mid-Atlantic Ridge

In the Equatorial Atlantic, the Mid-Atlantic Ridge is highly segmented and forms
en-echelon geometry by the left lateral large-offset transform faults (e.g., the St. Paul
and Romanche FZs). The ridge axis reaches a maximum depth of 4-5 km at this
region, exposing abundant peridotites. Bonatti et al. ( 1993 ) and Schilling et al. ( 1995 )
suggested that the mantle beneath the Equatorial Mid-Atlantic Ridge is colder than
the other region of the Atlantic Ocean based on major element systematics. Recent
global mantle tomography model in fact demonstrated the presence of a fossil
detached cold subducted slab beneath the Equatorial Atlantic (Sichel et al. 2008 ).
The Mid-Atlantic Ridge in this region has a typical slow-spreading rate of 3.2
cm/year full-rate (calculation based on NUVEL-1A). The St. Paul FZ offsets the
ridge left laterally ~580 km. The fracture zone is made up of four transform faults
linked by three short first-order segments (Hékinian et al. 2000 ). The length of
these short first-order segments are ~15 to ~45 km. Presence of OCCs associated
with these short first-order segments was not noted by Hékinian et al. ( 2000 ),
although Sichel et al. ( 2008 ) interpreted the ~90 km long massif that host the St.
Paul Islet as a large OCC, called “Saint Peter Saint Paul Megamullion”. However,
they provided no information supporting their contention, and the presence of the
OCC is not fully assessed. On the other hand, the bathymetric map shown in
Brunelli and Seyler ( 2010 ) shows that one of the short first-order segments has a
similar morphology to the “smooth seafloor” reported by Cannat et al. ( 2006 ). The
“smooth seafloor” was first described in the 61°-67°E Southwest Indian Ridge,
occurring in the form of broad ridges with a smooth, rounded topography (Cannat
et al. 2006 ). It is considered as a non-volcanic terrain characterized by successive
fault surfaces primarily consisted of peridotites. The peridotites from these short
first-order segments are relatively fertile with the minimum spinel Cr# 0.22
(Brunelli and Seyler 2010 ).
The Mid-Atlantic Ridge near the St. Paul FZ is thus similar to the western PVB
in that a cold mantle domain is assumed as well as to the central PVB in that a