The Carnegie Ridge is an aseismic ridge on the Nazca Plate that is being

subducted Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

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.

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 crust Oceanic crust is the uppermost layer of the oceanic portion of the tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike complex, and the lower oceanic crust, composed of troctolite, gabbro and ultramafic cumu ...

. 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 Plate The Farallon Plate was an ancient oceanic plate. It formed one of the three main plates of Panthalassa, alongside the Phoenix Plate and Izanagi Plate, which were connected by a triple junction. The Farallon Plate began subducting under the west ...

and the formation of the separate Cocos and Nazca Plates. At about 19.5 Ma, the

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 ...

spreading center A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a diver ...

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-

Miocene The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recen ...

(15 Ma) to about

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

(2 Ma). Although there is agreement that the ridge is 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 An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...

hypocenter In seismology, a hypocenter or hypocentre () is the point of origin of an earthquake or a subsurface nuclear explosion. A synonym is the focus of an earthquake. Earthquakes An earthquake's hypocenter is the position where the strain energy ...

s, which found a constant dip of about 25°–35° down to 200 km.

References

{{Reflist Underwater ridges of the Pacific Ocean Tectonic plates