Carnegie Ridge
Encyclopedia
The Carnegie Ridge is an aseismic ridge on the Nazca Plate
that is currently being subducted
beneath the South American Plate
. The ridge is thought to be a result of the passage of the Nazca Plate over the Galapagos hotspot
. It is named for the research vessel Carnegie
, which discovered it in 1929.
. Wide-angle seismic reflection and refraction data acquired over the central and eastern part of the ridge give crustal thicknesses of 13 km and 19 km respectively for crust that has estimated ages of about 11 Ma and 20 Ma. Layer 2 thicknesses are similar to the neighbouring normal oceanic crust with thickening taking place in layer 3.
and the formation of the separate Cocos
and Nazca Plates. At about 19.5 Ma, the Galapagos Rise
spreading center
moved so that most of the hotspot magmatism affected the Nazca Plate, forming the combined Carnegie and Malpelo Ridges. At about 14.5 Ma the spreading center jumped south, such that most of the magmatism affected the Cocos Plate and caused the Malpelo Ridge to rift away from the Carnegie Ridge. This stage caused the narrowing of the Carnegie Ridge now seen between 85° W and 87° W. At about 9.5 Ma rifting between the Malpelo and Carnegie Ridges ceased. The Galapagos Rise moved north again at about 5 Ma, leaving the hotspot entirely within the Nazca Plate, which is the current situation.
(15 Ma) to about Pleistocene
(2 Ma). Although there is agreement that the ridge is currently being subducted, there is little agreement on the effect that this has had on either the subducting or over-riding plates. Some models argue that the buoyancy associated with the thickened crust of the ridge has caused the downgoing Nazca Plate to tear, leaving a relatively flat section carrying the ridge, flanked by two sections with steeper dip. The presence of a flat section is not supported by a more recent study of earthquake
hypocenter
s, which found a constant dip of about 25°–35° down to 200 km.
Nazca Plate
]The Nazca Plate, named after the Nazca region of southern Peru, is an oceanic tectonic plate in the eastern Pacific Ocean basin off the west coast of South America. The ongoing subduction along the Peru-Chile Trench of the Nazca Plate under the South American Plate is largely responsible for the...
that is currently being subducted
Subduction
In geology, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate, sinking into the Earth's mantle, as the plates converge. These 3D regions of mantle downwellings are known as "Subduction Zones"...
beneath the South American Plate
South American Plate
The South American Plate is a continental tectonic plate which includes the continent of South America and also a sizeable region of the Atlantic Ocean seabed extending eastward to the Mid-Atlantic Ridge....
. The ridge is thought to be a result of the passage of the Nazca Plate over the Galapagos hotspot
Galápagos hotspot
The Galápagos hotspot is a volcanic hotspot in the East Pacific Ocean responsible for the creation of the Galapagos Islands as well as three major aseismic ridge systems, Carnegie, Cocos and Malpelso which are on two tectonic plates. The hotspot is located near the Equator on the Nazca Plate not...
. It is named for the research vessel Carnegie
Carnegie (ship)
The Carnegie was a brigantine yacht, equipped as a research vessel, constructed almost entirely from wood and other non-magnetic materials to allow sensitive magnetic measurements to be taken for the Carnegie Institution's Department of Terrestrial Magnetism. She carried out a series of cruises...
, which discovered it in 1929.
Extent
The Carnegie Ridge is seen to extend eastwards over 1,000 km from the Galapagos islands to the Colombia-Ecuador trench and is interpreted to continue beneath northern Ecuador for about a further 700 km. The subducted extent is disputed, with some workers arguing that there is no evidence of a subducted ridge beneath Ecuador extending more than about 60 km from the trench.Structure
The Carnegie Ridge consists of thickened oceanic crustOceanic crust
Oceanic 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...
. Wide-angle seismic reflection and refraction data acquired over the central and eastern part of the ridge give crustal thicknesses of 13 km and 19 km respectively for crust that has estimated ages of about 11 Ma and 20 Ma. Layer 2 thicknesses are similar to the neighbouring normal oceanic crust with thickening taking place in layer 3.
History
The formation of the Carnegie Ridge and other aseismic ridges in this part of the Pacific started at about 20 Ma when the Galapagos hotspot formed, following the break-up of the Farallon PlateFarallon Plate
The Farallon Plate was an ancient oceanic plate, which began subducting under the west coast of the North American Plate— then located in modern Utah— as Pangaea broke apart during the Jurassic Period...
and the formation of the separate Cocos
Cocos Plate
The Cocos Plate is an oceanic tectonic plate beneath the Pacific Ocean off the west coast of Central America, named for Cocos Island, which rides upon it.-Geology:...
and Nazca Plates. At about 19.5 Ma, the Galapagos Rise
Galapagos Rise
The Galapagos Rise is a divergent boundary located between the South American coast and the triple junction of the Nazca Plate, the Cocos Plate, and the Pacific Plate. The volcanically active Galapagos Islands exist on the Galápagos hotspot above the Galapagos Rise...
spreading center
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...
moved so that most of the hotspot magmatism affected the Nazca Plate, forming the combined Carnegie and Malpelo Ridges. At about 14.5 Ma the spreading center jumped south, such that most of the magmatism affected the Cocos Plate and caused the Malpelo Ridge to rift away from the Carnegie Ridge. This stage caused the narrowing of the Carnegie Ridge now seen between 85° W and 87° W. At about 9.5 Ma rifting between the Malpelo and Carnegie Ridges ceased. The Galapagos Rise moved north again at about 5 Ma, leaving the hotspot entirely within the Nazca Plate, which is the current situation.
Subduction
The onset of subduction of the Carnegie Ridge beneath the South American Plate has been dated variously from about mid-MioceneMiocene
The Miocene is a geological epoch of the Neogene Period and extends from about . The Miocene was named by Sir Charles Lyell. Its name comes from the Greek words and and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene follows the Oligocene...
(15 Ma) to about Pleistocene
Pleistocene
The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
(2 Ma). Although there is agreement that the ridge is currently being subducted, there is little agreement on the effect that this has had on either the subducting or over-riding plates. Some models argue that the buoyancy associated with the thickened crust of the ridge has caused the downgoing Nazca Plate to tear, leaving a relatively flat section carrying the ridge, flanked by two sections with steeper dip. The presence of a flat section is not supported by a more recent study of earthquake
Earthquake
An earthquake is the result of a sudden release of energy in the Earth's crust that creates seismic waves. The seismicity, seismism or seismic activity of an area refers to the frequency, type and size of earthquakes experienced over a period of time...
hypocenter
Hypocenter
The hypocenter refers to the site of an earthquake or a nuclear explosion...
s, which found a constant dip of about 25°–35° down to 200 km.