An oceanic core complex, or megamullion, is a seabed geology feature that forms a long ridge perpendicular to a mid-ocean ridge. It contains smooth domes that are lined with transverse ridges like a corrugated roof. They can vary in size from 10 to 150 km in length, 5 to 15 km in width, and 500 to 4000 m in height. Their counterparts on land are metamorphic core complexes, which form in areas of continental crustal extension or stretching.
History, distribution and exploration
The first oceanic core complexes described were identified in the Atlantic Ocean.
[; ] Since then numerous such structures have been identified primarily in oceanic
lithosphere formed at intermediate, slow- and ultra-slow spreading
mid-ocean ridges, as well as
back-arc basins.
[; ] Examples include 10-1000 square km expanses of ocean floor and therefore of the oceanic lithosphere, particularly along the Mid-Atlantic Ridge
and the Southwest Indian Ridge.
Some of these structures have been drilled and sampled, showing that the
footwall can be composed of both
mafic plutonic and
ultramafic rocks (
gabbro and
peridotite primarily, in addition to
diabase), and a thin
shear zone that includes hydrous
phyllosilicates. Oceanic core complexes are often associated with active hydrothermal fields.
Formation
Oceanic core complex structures form at slow-spreading diverging
oceanic
plate boundaries with only a limited supply of upwelling
magma. These zones have low upper mantle temperatures and long
transform faults develop.
do not develop along the expansion axes of slow-spreading boundaries. Expansion takes place along low-angle
. The core complex builds on the uplifted side of the fault, where most of the
(or crustal) material is stripped away to
exhumed mantle at the seabed. They consist of
,
of the mantle and to a lesser extent gabbroic rocks from the Earth's crust.
Each detachment fault has three notable features: a breakaway zone where the fault began, an exposed fault surface that rides over the dome, and a termination, which is usually marked by a valley and adjacent ridge.
However, the formation process through detachment faults hypothesis has its limitations, such as the scarce seismic evidence that low-angle normal faulting exists,
Examples
Some 50 oceanic core complexes have been identified, including:
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Godzilla Megamullion, part of the Parece Vela Rift in the Western Pacific Ocean between Japan and the Philippines was discovered in 2001. It is about 155 km long by 55 km across, and is the largest known ocean core complex in the world.
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The Saint Peter and Saint Paul Archipelago and complex lies in the equatorial Atlantic Ocean. It is 90 km long and 4000 m tall. The apex forms the Saint Peter and Paul Rocks. This is one of the few known examples where sea floor mantle rocks are exposed above sea level.
Research
Scientific interest in core complexes has dramatically increased following an expedition in 1996 which mapped the
Atlantis Massif. This expedition was the first to associate the complex structures with detachment faults. Research includes:
-
To investigate the structure of the mantle:
-
:The complexes provide cross sections of mantle material which could only otherwise be found by drilling deep into the mantle. The deep drilling that is required to penetrate 6-7 km through the crust is beyond current technical and financial constraints. Selective sample drilling into the complex structures are already underway.
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To investigate the formation of
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To investigate the development of oceanic core complexes:
-
:In 2005 scientists from the Woods Hole Oceanographic Institute discovered a series of complexes in the North Atlantic, from Bermuda.
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To study mineralisation and the release of minerals from the mantle:
-
:A steeply sloping detachment fault which penetrates deeply can be a conduit for hot mineral-rich hydrothermal fluids to circulate towards the surface and build . These deposits can grow massive because detachment faults persist for hundreds of thousands of years. The Woods Hole Institution is studying one such site, called the TAG hydrothermal field on the Mid-Atlantic Ridge.
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To investigate marine magnetic anomalies:
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:The conventional view that marine Magnetic anomaly arose in the upper, extrusive layer of the oceanic crust requires a rethink because perfectly normal magnetic anomalies arise at core complexes, where the crust has been stripped away. This suggests that the lower part of the ocean crust contains a substantial magnetic signature.
See also
Notes
Sources
