The Nazca Ridge is a

submarine ridge 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 ...

, located on the

Nazca Plate The Nazca Plate or Nasca 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 ...

off the west coast of South America. This plate and ridge are currently subducting under the

South American Plate The South American Plate is a major tectonic plate which includes the continent of South America as well as a sizable region of the Atlantic Ocean seabed extending eastward to the African Plate, with which it forms the southern part of the Mid-A ...

at a

convergent boundary A convergent boundary (also known as a destructive boundary) is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can be defined by a ...

known as the Peru-Chile Trench at approximately per year. The Nazca Ridge began subducting obliquely to the collision margin at 11°S, approximately 11.2 Ma, and the current subduction location is 15°S. The ridge is composed of abnormally thick basaltic ocean crust, averaging 18 ±3 km thick. This crust is buoyant, resulting in

flat slab subduction Flat slab subduction is characterized by a low subduction angle (<30 degrees to horizontal) beyond the

seismogenic layer

under

Peru , image_flag = Flag of Peru.svg , image_coat = Escudo nacional del Perú.svg , other_symbol = Great Seal of the State , other_symbol_type = National seal , national_motto = "Firm and Happy f ...

. This flat slab subduction has been associated with the uplift of

Pisco Basin Pisco Basin ( es, Cuenca de Pisco) is a sedimentary basin extending over in southwestern Peru.Solís Mundaca, 2018, p.1 The basin has a thick sedimentary fill, which is about half the thickness of more northern foreland basins in Peru. The old ...

and the cessation of Andes volcanism and the uplift of the Fitzcarrald Arch on the South American continent approximately 4 Ma.

Morphology

The Nazca Ridge is approximately wide, long, and has of bathymetric relief. The gradient of the slopes is 1-2 degrees. The ridge is located at a depth of below sea level, above the carbonate compensation depth. It is blanketed in a thin covering of thick pelagic calcareous ooze. Based on

Rayleigh wave Rayleigh waves are a type of surface acoustic wave that travel along the surface of solids. They can be produced in materials in many ways, such as by a localized impact or by piezo-electric transduction, and are frequently used in non-destructive ...

analysis, the ridge has an average crustal thickness of 18 ±3 km, but could have a localized maximum thickness up to . This is abnormally thick for oceanic crust. By comparison, the underlying Nazca Plate adjacent to the ridge ranges from thick, and is comparable to the worldwide average of around thick.

Formation

Based on basalt ages, the portion of the Nazca Ridge that is currently exposed dates from 31 ± 1 Ma at the Peru-Chile trench, to 23 ± 1 Ma where the Nazca Ridge and

Easter Seamount Chain Easter,Traditional names for the feast in English are "Easter Day", as in the ''Book of Common Prayer''; "Easter Sunday", used by James Ussher''The Whole Works of the Most Rev. James Ussher, Volume 4'') and Samuel Pepys''The Diary of Samuel P ...

are adjacent. Basalt composition has also been used to show that the Nazca Ridge and Easter Seamount Chain formed from the same magma source, with the formation of the Easter Seamount Chain occurring after the Nazca Plate changed direction. Formation began along the Pacific-Farallon/Nazca spreading center, and has been attributed to hot spot volcanism. There is some debate as to where this hot spot was originally located however, with locations near

Easter Island Easter Island ( rap, Rapa Nui; es, Isla de Pascua) is an island and special territory of Chile in the southeastern Pacific Ocean, at the southeasternmost point of the Polynesian Triangle in Oceania. The island is most famous for its nearly ...

and Salas y Gomez both being proposed. The ridge is primarily composed of

mid-ocean ridge 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 ...

basalt, which erupted on the Nazca Plate when the plate was already 5-13 Ma old. Based on isotopic ratios and

rare earth element The rare-earth elements (REE), also called the rare-earth metals or (in context) rare-earth oxides or sometimes the lanthanides (yttrium and scandium are usually included as rare earths), are a set of 17 nearly-indistinguishable lustrous silve ...

composition, it is estimated that the magma was sourced at approximately 95 km depth from a 7% partial melt. The Nazca Ridge has a conjugate feature on the

Pacific Plate The Pacific Plate is an oceanic tectonic plate that lies beneath the Pacific Ocean. At , it is the largest tectonic plate. The plate first came into existence 190 million years ago, at the triple junction between the Farallon, Phoenix, and Iza ...

, the Tuamotu Plateau. Magnetic anomalies have shown that there was symmetrical spreading at the Pacific-Farallon/Nazca center, so the Tuamotu Plateau can be used as a proxy for the pre-subducted Nazca Ridge geometry.

Subduction and migration history

The Nazca Plate began subducting into the Peru-Chile trench 11.2 Ma at 11°S. Due to the oblique orientation of the ridge to the Nazca-South American plate collision zone, the ridge has migrated south along the active margin to its current location at 15°S. Based on Tuamotu Plateau mirror relationship, it is estimated that of the Nazca Ridge has already subducted. The speed of migration has slowed over time, with the ridge migrating at per year until 10.8 Ma, then slowing to per year from 10.8-4.9 Ma. The current ridge migration rate is per year. The current plate subduction rate is per year.

Continental margin interaction

The ridge is buoyant, resulting in flat slab subduction of the Nazca Plate underneath Peru. Buoyancy is related to crustal age, and the buoyancy effect can be seen in oceanic crust aged from 30-40 Ma. The Nazca Plate is dated to 45 Ma where it subducts into the Peru-Chile trench. The extreme thickness of the buoyant ridge is responsible for the flat slab subduction of the older underlying plate. Modeling has shown that this type of subduction is only concurrent with submarine ridges, and accounts for approximately 10% of convergent boundaries. The most recent estimate of the subduction angle for the Nazca Plate is 20° to a depth of at inland. At depth, approximately inland, the plate shifts to a horizontal orientation, and continues to travel horizontally for up to inland, before resuming subduction into the

asthenosphere The asthenosphere () is the mechanically weak and ductile region of the upper mantle of Earth. It lies below the lithosphere, at a depth between ~ below the surface, and extends as deep as . However, the lower boundary of the asthenosphere is not ...

Large magnitude earthquakes occur in association with the area around the Nazca Ridge subduction zone, known at the Peru megathrust. These include, but are not limited to, a magnitude 8.1 earthquake in 1942, a magnitude 8.0 earthquake in 1970, a magnitude 7.7 earthquake in 1996, a magnitude 8.4 earthquake in 2001, and a magnitude 8.0 earthquake in 2007. Earthquake records for this area of subduction go back to

1586 Events * January 18 – The 7.9 Tenshō earthquake strikes the Chubu region of Japan, triggering a tsunami and causing at least 8,000 deaths. * June 16 – The deposed and imprisoned Mary, Queen of Scots, recognizes Philip II of ...

. All of these ruptures were located either on the coast of Peru or within the Peru-Chile Trench between 9°S and 18°S, coincidental with the subducting Nazca Ridge, and include both intraplate and interplate rupturing. No large earthquakes have been located between 14°S and 15.5°S, where the bathymetric high of the ridge is subducting. Interplate earthquakes do not occur in direct conjunction with the Nazca Ridge. There has been little geomorphic affect to the Peru-Chile trench due to the ridge subduction beyond a shallowing from above the ridge location. However, this is a tectonic erosion margin. There is no

accretionary wedge An accretionary wedge or accretionary prism forms from sediments accreted onto the non- subducting tectonic plate at a convergent plate boundary. Most of the material in the accretionary wedge consists of marine sediments scraped off from the ...

forming in the trench, and what sediment is found there is from continental sources, based on fossil assemblage. The calcareous ooze blanketing Nazca Ridge is completely subducted. Crustal erosion of the

forearc basin Forearc is a plate tectonic term referring to a region between an oceanic trench, also known as a subduction zone, and the associated volcanic arc. Forearc regions are present along a convergent margins and eponymously form 'in front of' the vol ...

has resulted in the loss of of the South American Plate since 11 Ma. The

Pisco Pisco is a colorless or yellowish-to-amber colored brandy produced in winemaking regions of Peru and Chile. Made by distilling fermented grape juice into a high-proof spirit, it was developed by 16th-century Spanish settlers as an alternative ...

located above the subducting ridge has experienced uplift since the Late Pliocene or Pleistocene an uplift that is attributed to the subduction of the Nazca Ridge.

Influence in Amazon tectonics

The flat slab subduction associated with the Nazca Ridge has been linked to the cessation of volcanism in the Andes Mountains at about 4 Ma. The subduction has also been linked with the formation of the Fitzcarrald Arch, which is a , high, domed topographic feature that defines the Amazon drainage Basin. Studies indicate that the uplift of the arch also began 4 Ma. The uplift of the Fitzcarrald Arch intersects with the Andes Mountains where there is a shift from high-gradient topography to the low-gradient

Amazon Basin The Amazon basin is the part of South America drained by the Amazon River and its tributaries. The Amazon drainage basin covers an area of about , or about 35.5 percent of the South American continent. It is located in the countries of Boliv ...

. This topographic uplift effectively divides the Amazon drainage basin into three sub-basins, the

Ucayali The Ucayali River ( es, Río Ucayali, ) is the main headstream of the Amazon River. It rises about north of Lake Titicaca, in the Arequipa region of Peru and becomes the Amazon at the confluence of the Marañón close to Nauta city. The city o ...

to the northwest, the Acre to the northeast, and the Madre De Dios to the southeast. It's hypothesized that significant modifications to sedimentary, erosional, and hydrological processes have resulted from the uplift of the Fitzcarrald Arch. Evolutionary paths for freshwater fish began to diverge in the Amazon sub-basins approximately 4 Ma as well. The uplift of the Fitzcarrald Arch could also be the catalyst that lead to these differing evolutionary paths, effectively isolating fish populations from each other.

References

External links

* {{Coord, 18, S, 79, W, type:mountain, display=title Underwater ridges of the Pacific Ocean Oligocene volcanism Geology of Peru