Megamullion
Encyclopedia
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.
(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.
. These zones have low upper mantle
temperatures and long transform faults develop. Rift valley
s do not develop along the expansion axes of slow spreading boundaries. Expansion takes place along low-angle detachment fault
s. The core complex builds on the uplifted side of the fault where most of the gabbro
ic (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.
Some 50 oceanic core complexes have been identified, including:
. This expedition was the first to associate the complex structures with detachment faults. Research includes:
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
feature that forms a long ridge perpendicular to a mid-ocean ridge
Mid-ocean ridge
A mid-ocean ridge is a general term for an underwater mountain system that consists of various mountain ranges , typically having a valley known as a rift running along its spine, formed by plate tectonics. This type of oceanic ridge is characteristic of what is known as an oceanic spreading...
. 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 RidgeMid-Atlantic Ridge
The Mid-Atlantic Ridge is a mid-ocean ridge, a divergent tectonic plate boundary located along the floor of the Atlantic Ocean, and part of the longest mountain range in the world. It separates the Eurasian Plate and North American Plate in the North Atlantic, and the African Plate from the South...
(Smith et al., 2006; Escartin et al., 2008) and the Southwest Indian Ridge
Southwest Indian Ridge
The Southwest Indian Ridge is a divergent tectonic plate boundary located along the floor of the southwest Indian Ocean. It separates the African Plate to the north from the Antarctic Plate to the south....
(Cannat et al., 2006). Some of these structures have been drilled and sampled, showing that the footwall can be composed of both mafic
Mafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
plutonic and ultramafic rocks (gabbro
Gabbro
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
and peridotite
Peridotite
A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...
primarily, in addition to diabase
Diabase
Diabase or dolerite is a mafic, holocrystalline, subvolcanic rock equivalent to volcanic basalt or plutonic gabbro. In North American usage, the term diabase refers to the fresh rock, whilst elsewhere the term dolerite is used for the fresh rock and diabase refers to altered material...
), 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 magmaMagma
Magma is a mixture of molten rock, volatiles and solids that is found beneath the surface of the Earth, and is expected to exist on other terrestrial planets. Besides molten rock, magma may also contain suspended crystals and dissolved gas and sometimes also gas bubbles. Magma often collects in...
. These zones have low upper mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
temperatures and long transform faults develop. Rift valley
Rift valley
A rift valley is a linear-shaped lowland between highlands or mountain ranges created by the action of a geologic rift or fault. This action is manifest as crustal extension, a spreading apart of the surface which is subsequently further deepened by the forces of erosion...
s do not develop along the expansion axes of slow spreading boundaries. Expansion takes place along low-angle detachment fault
Detachment fault
Detachment faulting is associated with large-scale extensional tectonics. Detachment faults often have very large displacements and juxtapose unmetamorphosed hanging walls against medium to high-grade metamorphic footwalls that are called metamorphic core complexes...
s. The core complex builds on the uplifted side of the fault where most of the gabbro
Gabbro
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
ic (or crustal) material is stripped away to expose mantle peridotite
Peridotite
A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...
. They comprise peridotites ultramafic rocks of mantle
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
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
- Godzilla Mullion, part of the Parece Vela Rift in the Western Pacific OceanPacific OceanThe Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
between JapanJapanJapan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
and the PhilippinesPhilippinesThe Philippines , officially known as the Republic of the Philippines , is a country in Southeast Asia in the western Pacific Ocean. To its north across the Luzon Strait lies Taiwan. West across the South China Sea sits Vietnam...
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 OceanAtlantic OceanThe Atlantic Ocean is the second-largest of the world's oceanic divisions. With a total area of about , it covers approximately 20% of the Earth's surface and about 26% of its water surface area...
. It is 90 km long and 4000 m high. The apex forms the Saint Peter and Paul RocksSaint Peter and Paul RocksThe Saint Peter and Saint Paul Archipelago is a group of 15 small islets and rocks in the central equatorial Atlantic Ocean. It lies in the Intertropical Convergence Zone, a region of severe storms...
. 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 MassifAtlantis Massif
The Atlantis Massif is a prominent undersea massif, a dome-shaped region approximately 10 mi. across and rising about 14,000 ft. from the sea floor, in the North Atlantic Ocean. It is located at approximately 30°8'N latitude 42°8'W longitude; just east of the intersection of the Mid-Atlantic...
. This expedition was the first to associate the complex structures with detachment faults. Research includes:
- To investigate the structure of the mantleMantle (geology)The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
:
- 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 faultDetachment faultDetachment faulting is associated with large-scale extensional tectonics. Detachment faults often have very large displacements and juxtapose unmetamorphosed hanging walls against medium to high-grade metamorphic footwalls that are called metamorphic core complexes...
s - To investigate the development of oceanic core complexes:
- To investigate the formation of detachment fault
- In 2005 Deborah Smith from the Woods Hole Oceanographic Institute discovered a series of complexes in the North Atlantic, 1500 miles (2,414 km) from BermudaBermudaBermuda is a British overseas territory in the North Atlantic Ocean. Located off the east coast of the United States, its nearest landmass is Cape Hatteras, North Carolina, about to the west-northwest. It is about south of Halifax, Nova Scotia, Canada, and northeast of Miami, Florida...
. 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 anomaliesMagnetic anomalyIn geophysics, a magnetic anomaly is a local variation in the Earth's magnetic field resulting from variations in the chemistry or magnetism of the rocks. Mapping of variation over an area is valuable in detecting structures obscured by overlying material...
arose in the upper, extrusive layer of the oceanic crustOceanic crustOceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium...
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.