Oceanic core complex

An oceanic core complex (OCC), or megamullion, is a seabed geologic 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 1500 m in height.

History, distribution and exploration

The first oceanic core complexes described were identified in the Atlantic Ocean (Cann et al., 1998; Tucholke et al., 1998). 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 (Fujimoto et al., 1999; O'Hara et al., 2001). Examples include large expanses of ocean floor and therefore of the oceanic lithosphere, particularly along the Mid-Atlantic Ridge (Smith et al., 2006; Escartin et al., 2008) and the Southwest Indian Ridge (Cannat et al., 2006). 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 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 low-angle detachment faults. The core complex builds on the uplifted side of the fault where most of the gabbroic (or crustal) material is stripped away to expose mantle peridotite. They comprise peridotites ultramafic rocks of 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.

Examples

Saint Peter Saint Paul Megamullion, Equatorial Atlantic Ocean after Motoki et al. (2007)

Some 50 oceanic core complexes have been identified, including:

• Godzilla Mullion, part of the Parece Vela Rift in the Western Pacific Ocean between Japan and the Philippines was discovered in 2001. It is about 150 km long, about as large as the State of Delaware, and is the largest Ocean Core Complex in the world.
• The St Peter Saint Paul complex lies in the equatorial Atlantic Ocean. It is 90 km long and 4000 m high. 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.

• To investigate the formation of detachment faults
• To investigate the development of oceanic core complexes:

In 2005 Deborah Smith from the Woods Hole Oceanographic Institute discovered a series of complexes in the North Atlantic, 1,500 miles (2,400 km) from Bermuda.^[1] 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 their development.

• 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 TAG hydrothermal field on the Atlantis Massif.

• 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 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

• Metamorphic core complex

References

1. ^ Deborah K. Smith, et. al., Widespread active detachment faulting and core complex formation near 13° N on the Mid-Atlantic Ridge, Nature 442, 440-443 (27 July 2006) | doi:10.1038/nature04950 abstract

• Cannat, M., et. al., 2006, Modes of seafloor generation at a melt-poor ultraslow-spreading ridge: Geology, v. 34 pp. 605-608
• Escartín, J., Smith, D.K., Cann, J., Schouten, H., Langmuir, C.H., and Escrig, S., 2008, Central role of detchment faults in accretion of slow-spreading oceanic lithosphere: Nature, v. 455, p. 790-794, doi: 0.1038/nature07333
• Fujimoto et al. (1999) InterRidge News 8(1) 22-4.
• MacLeod, C. J., et al., Life cycle of oceanic core complexes, Earth and Planetary Science Letters, Volume 287, 15 October 2009, Pages 333-344 doi:10.1016/j.epsl.2009.08.016 abstract
• Ohara, Y. (2001) Marine Geophysical Researches 22(1), 47-61.
• Smith, D.K., Cann, J.R., and Escartin, J., 2006, Widespread active detachment faulting and core complex formation near 13 degrees N on the Mid-Atlantic Ridge: Nature, v. 442 pp. 440-443 doi: 10.1038/nature04950
• Tucholke et al. (1998) Megamullions and mullion structure defin-ing oceanic metamorphic core complexes on the mid-Atlantic ridge, Journal of Geophysical Research, v. 103 pp. 9857-9866 doi: 10.1029/98JB00167