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effect”, with which a cooled upper mantle domain is emplaced along an otherwise
rapid intermediate-spreading ridge (Fig. 2 ).
Despite the rapid intermediate-spreading rate, mantle melting beneath each short
first-order segment was effectively inhibited due to the “sandwiched” geometry of
the two adjacent transform faults, causing primarily tectonic extension in the sand-
wiched segment. The full-axial OCCs, large axial water depths, peridotite expo-
sures at segment mid-points, and the fertility of the peridotites from the Parece Vela
Rift suggest that little mantle melting took place beneath the cooled short first-order
segments resulting from an extreme transform fault effect caused by closely-spaced
fracture zones (Fig. 2 ).
The onset of magma-starved rifting did not appear to coincide with the change
to short spreading segment geometry within the Parece Vela Rift. Instead, spreading
in this segment geometry initially produces well-ordered spreading-normal abyssal
hills. Pronounced tectonic extension only initiated during a later phase of the second-
stage spreading. We thus infer that the transform sandwich effect on mantle melting
gradually increased in the sandwiched segments after the PVB ridge axes started
rotating counter-clockwise at ~19 Ma. We further infer that formation of OCCs
initiated when mantle upwelling beneath the sandwiched segments was lowered
below a critical threshold for melting (Fig. 2 ).
This mechanism appears to explain the tectonic and magmatic features of the
Ascension FZ in the Mid-Atlantic Ridge at ~7°S. The Mid-Atlantic Ridge in this
region is unusually magmatically robust given a typical slow-spreading rate of
3.3 cm/year full-rate (Bruguier et al. 2003 ; based on the global plate motion model
NUVEL-1A by DeMets et al. 1994 ). In spite of the slow-spreading rate, there are an
unusually shallow axial depth (~1,200 m) forming a volcanic ridge rather than an
axial valley, pseudofaults and overlapping spreading centers, and thickened crust of
10-11 km in this region (Brozena and White 1990 ; Bruguier et al. 2003 ). It has there-
fore been postulated that a mantle plume is responsible for these anomalous charac-
teristics (Brozena and White 1990 ), although Bruguier et al. ( 2003 ) suggested that a
small heterogeneity in the mantle is responsible for these. Irrespective of the origin of
the anomalies, the Mid-Atlantic Ridge near the Ascension FZ is similar to the
Reykjanes Ridge: a ridge with slow-spreading rate showing the robust magmatism
typical of a fast-spreading ridge due to the presence of excess melt (in the case of the
Reykjanes Ridge caused by the Iceland hotspot; Bell and Buck 1992 ; Searle et al.
1998 ). Despite its slow-spreading rate, the Mid-Atlantic Ridge near the Ascension FZ
is therefore similar to the magmatically robust portions of the PVB and other inter-
mediate- and fast-spreading ridges in terms of robust magmatic budgets.
The Ascension FZ offsets the Mid-Atlantic Ridge right laterally by ~200 km,
and in fact consists of two fracture zones: the North and South Ascension FZs
(Fig. 3 ). A large OCC (here named the Ascension OCC) and a possible fossil OCC
are clearly recognizable between the two fracture zones (Fig. 3 ). The Ascension
OCC is relatively large, with the dimension of ~95 km perpendicular to the axis and
~25 km along the axis. Peridotites were recovered at the ridge-transform intersec-
tion adjacent to the Ascension OCC by a German expedition (Schulz et al. 1999 )
(Fig. 3 ). The Mid-Atlantic Ridge in this region has two distinct characteristics similar
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