Geoscience Reference
In-Depth Information
3.2
Oceanic Core Complexes in the Parece Vela Basin
The PVB experienced two major episodes of OCC formation during the basin's
spreading history. The first was related to formation of the Chaotic Terrain, and the
second was related to the OCCs in the axis of the central PVB just before the ridge
went extinct.
The Chaotic Terrain in the western PVB consists of a series of rugged, isolated
and elevated domes capped by corrugated axis-normal lineations, and associated
deep basins (maximum depth ~6,200 m) (Figs.
1b
and
2
). This terrain is similar to
other areas where OCCs are abundant, like the off-axis regions of the 61°-67°E
Southwest Indian Ridge (Cannat et al.
2006
) and the 13°N segment of the Mid-
Atlantic Ridge (Smith et al.
2008
). The axis-normal length of each dome varies from
~3.5 to ~15 km, comparable to the length observed in the OCCs in the Mid-Atlantic
Ridge (Ohara et al.
2007
). A high mantle Bouguer anomaly (~30 mgal) indicates the
presence of relatively thin crust beneath the area (Okino et al.
1998
; Ohara et al.
2001
). The morphology and gravity signatures of these isolated domes are very similar
to global OCCs. Dredging on these domes yielded peridotites and gabbros (Ohara
et al.
2003b
), also sharing a common characteristic with the global OCCs.
The presence of the Chaotic Terrain indicates the dominance of tectonic over
magmatic extension during the time from Chron 6C to 6A (corresponding to
24-21 Ma) of the first-stage spreading of the PVB. This is unusual, since a higher
magmatic budget is generally expected for an intermediate-spreading ridge. Among
intermediate-spreading ridges, the presence of off-axis OCCs similar to those in the
Chaotic Terrain is known in the Australian-Antarctic Discordance (AAD) on the
Southeast Indian Ridge (~7.2 cm/year full-rate; Okino et al.
2004
).
In the central PVB, Godzilla Megamullion is developed at segment S1 of the
Parece Vela Rift (Figs.
1b
and
2
). It is overall a relatively flat to domal elongated
massif with distinct corrugated surface. In detail, it consists of several individual
domes. The older edge of the massif is marked sharply by a linear abyssal hill.
Well-ordered abyssal hills parallel the strike of the spreading axis further off-axis.
The younger edge of the massif consists of an axis-parallel irregular edifice, termi-
nating the corrugations on the massif. Godzilla Megamullion has slightly elevated
mantle Bouguer anomalies and yields peridotites and gabbros, also supporting the
contention that it is an analogue to global OCCs. Godzilla Megamullion is the largest
OCC found to date (Ohara et al.
2001
); it extends ~125 km normal to the axis and
~55 km along the axis.
Other segments of the Parece Vela Rift also host rift mountain massifs (Figs.
1b
and
2
). Although these massifs have no prominent corrugated surfaces, the domal
shape and smooth surface of many of these are distinct from the surrounding
NW-SE trending well-ordered abyssal hills. The massifs at segments S2 and S3 also
yield peridotites (Ohara et al.
2003a, b
), supporting the contention that these are
analogues to global OCCs. The second-stage spreading of the PVB had an interme-
diate-spreading rate (postulated 7.0 cm/year full-rate), initially having produced
well-ordered abyssal hills. Pronounced tectonic extensions initiated in a later phase
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