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A megamullion, or oceanic core complex (OCC), is a seabed geologic feature that forms a long ridge perpendicular to a mid-ocean ridge. It contains smooth domes up to 10 to 20 miles long 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 1500m in height.

Contents

Examples

Some 50 oceanic core complexes have been identified, including:

1. The Parece Vela Megamullion (also known as the "Godzilla Megamullion"), in the northwest Pacific Ocean, between Japan and Philippines was discovered in 2001. It is 150 km long.

2. The St Peter Saint Paul Megamullion (also known as the "Brachiosaurus Megamullion") lies in the equatorial Atlantic Ocean. It is 90km long and 4000 m high. The apex forms the Saint Peter and Paul Rocks. This is the only known example where abyssal mantle rises above sea level.

Saint Peter Saint Paul Megamullion, Equatorial Atlantic Ocean, so-called "Brachiosaurus Megamullion", after Motoki et al. (2007)

Formation

Megamullion structures form at slow spreading oceanic plate boundaries which have a limited supply of upwelling magma. These zones have low upper mantle temperatures and long transform faults develop. Rift valleys do not develop along the expansion axes of slow spreading boundaries. Expansion takes place along a low-angle “detachment fault”. The megamullion builds on the upper block of the fault where most of the gabroic (or crustal) material is stripped away to expose abyssal mantle. Isotopic dating indicates that megamullion rocks are extremely old, older that the 4.6 billion year age of the earth's crust. They comprise peridotites ultramafic rocks of abyssal mantle and to a lesser extent gabbroic rocks from the earth's crust. The megamullion is essentially exposed mantle material with the crust stripped away.

Each detachment fault has three notable features: a breakaway zone where the fault began, an exposed fault surface that rides over the megamullion dome and a termination, which is usually marked by a valley and adjacent ridge. Magma supply is critical. Detachment faults are believed to need just the right amount of magma to develop: not too little and not too much so rocks don’t crack when stretched to form faults, but just enough to induce sliding on the fault.

Research

Scientific interest in megamullions has dramatically inceased following an expedition in 1996 which mapped the Atlantis Massif. This expedition was the first to associate the megamullion structure with the detachment fault. The study of megamullions is now an active research area. Research is moving forward in a number of areas:

1. To investigate the structure of the mantle.

The mullion provides a cross section of mantle material which Could only otherwise be found by drilling deep into the mantle. The deep drilling that is required to penetrate 6-7kn through the crust is beyond current technical and financial constraints. Selective sample drilling into mullion structures are already underway.

2. To investigate the formation of detachment faults

3. To investigate the development of mullions.

In 2005 Deborah Smith from the Woods Hole Oceanographic Institution discovered a series of mullion complexes in the North Atlantic, 1,500 miles from Bermuda. These structures are at various stages in their evolution—from bumps that indicated the emergence of a core complex to the faded grooves of long-exhumed core complexes that had been eroded away over millions of years. Such features will enable scientists to see active detachment faults in operation and understand: how they start, how they grow, how they mature, and how they die.

4. 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 mineral deposits. 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 TAIG hydrothermal field on the Atlantis Massif. It has a mineral deposit the size of the Coliseum in Rome.

5. To investigate marine magnetic anomalies.

The conventional view that marine magnetic anomalies arose in the upper, extrusive layer of the oceanic crust requires a rethink because perfectly normal magnetic anomalies arise at megamullions, where the crust has been stripped away. This suggests that the lower part of the ocean crust contains a substantial magnetic signature.

References

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