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., 1997; 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 10-1000 square km 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:

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:

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.
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.
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.
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

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
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