| Nazca plate | |
|---|---|
 | |
| Type | Minor |
| Approximate area | 15,600,000 km2[1] |
| Movement1 | north-east |
| Speed1 | 40-53 mm/year |
| Features | Pacific Ocean |
| 1Relative to theAfrican plate | |
TheNazca plate orNasca plate,[2] named after theNazca region of southernPeru, is an oceanictectonic plate in the easternPacific Ocean basin off the west coast ofSouth America. The ongoingsubduction, along thePeru–Chile Trench, of the Nazca plate under theSouth American plate is largely responsible for theAndeanorogeny. The Nazca plate is bounded on the west by thePacific plate and to the south by theAntarctic plate through theEast Pacific Rise and theChile Rise, respectively. The movement of the Nazca plate over severalhotspots has created somevolcanic islands as well as east–west runningseamount chains that subduct underSouth America. Nazca is a relatively young plate in terms of the age of its rocks and its existence as an independent plate, having been formed from the breakup of theFarallon plate about 23 million years ago. The oldest rocks of the plate are about 50 million years old.[3]
Atriple junction, theChile triple junction,[4] occurs on the seafloor of thePacific Ocean offTaitao andTres Montes Peninsula at the southern coast ofChile. Here, threetectonic plates meet: the Nazca plate, theSouth American plate, and theAntarctic plate.
The eastern margin is aconvergent boundarysubduction zone under the South American plate and theAndes Mountains, forming thePeru–Chile Trench. The southern side is adivergent boundary with the Antarctic plate, theChile Rise, whereseafloor spreading permitsmagma to rise. The western side is a divergent boundary with thePacific plate, forming theEast Pacific Rise. The northern side is a divergent boundary with theCocos plate, theCocos–Nazca spreading centre.
The subduction of the Nazca plate under southern Chile has a history of producing massiveearthquakes, including the largest ever recorded on earth, themoment magnitude 9.51960 Valdivia earthquake.
A second triple junction occurs at the northwest corner of the plate where the Nazca, Cocos, and Pacific plates all join off the coast ofColombia. Yet another triple junction occurs at the southwest corner at the intersection of the Nazca, Pacific, and Antarctic plates off the coast of southernChile. At each of these triple junctions ananomalous microplate exists, theGalapagos microplate at the northern junction and theJuan Fernandez microplate at the southern junction. TheEaster Island microplate is a third microplate that is located just north of the Juan Fernandez Microplate and lies just west ofEaster Island.
TheCarnegie Ridge is a 1,350-kilometre-long (840 mi) and up to 300-kilometre-wide (190 mi) feature on the ocean floor of the northern Nazca plate that includes theGalápagos archipelago at its western end. It is being subducted under South America with the rest of the Nazca plate.
The absolute motion of the Nazca plate has been calibrated at 3.7 cm/year (1.5 in/year) east motion (88°), one of the fastest absolute motions of any tectonic plate. The subducting Nazca plate, which exhibits unusualflat slab subduction, is tearing as well as deforming as it is subducted (Barzangi and Isacks). The subduction has formed and continues to form thevolcanicAndes Mountain Range. Deformation of the Nazca plate even affects thegeography ofBolivia, far to the east (Tinker et al.). The1994 Bolivia earthquake occurred on the Nazca plate; this had a magnitude of 8.2, which at that time was the strongest instrumentally recorded earthquake occurring deeper than 300 km (190 mi).
Aside from theJuan Fernández Islands, this area has very few other islands that are affected by the earthquakes resulting from complicated movements at these junctions.
The precursor of the Nazca plate,Juan de Fuca plate, and theCocos plate was theFarallon plate, which split in the lateOligocene, about 22.8Mya, a date arrived at by interpretingmagnetic anomalies. Subduction under the South American continent began about 140 Mya, although the formation of the high parts of the Central Andes and the Bolivianorocline did not occur until 45 Mya. It has been suggested that the mountains were forced up by the subduction of the older and heavier parts of the plate, which sank more quickly into themantle.[5]
