Easter Microplate

Easter Plate is located to the west of Easter Island off the west coast of South America in the middle of the Pacific Ocean, bordering the Nazca plate to the east and the Pacific plate to the west.[1] It was discovered from looking at earthquake distributions that were offset from the previously perceived Nazca-Pacific Divergent boundary.[2] This young plate is 5.25 million years old and is considered a microplate because it is small with an area of approximately 160,000 km2.[3] Seafloor spreading along the Easter microplate's borders have some of the highest global rates, ranging from 50 to 140 mm/yr.[4]

Structure and tectonics (present)

From the 1970s to 1990s, multiple efforts were made to collect data on the area, including several magnetic and gravitational anomaly surveys. These surveys show that Easter plate is uniquely shallow, bordered by spreading centers and transform boundaries, with a triple junction located at the southern and northern tip.[5]

Along the eastern border, there are several spreading centers south of 27°S and 3 northward propagating rifts to the north of 27°S. The axis further north is a graben reaching a depth of approximately 6000 m.[1] Northward propagation of the eastern rifts is continuous at a speed of 150 mm/yr.[4] The spreading ridge between 26°S and 27°S has a spreading rate of 120 mm/yr, but is asymmetrical on Nazca plate side. Bathymetry data shows the depth is 2100 m near 26°30'S and progressively gets deeper to the north, reaching depths of 3300 m in an axial valley.[4] There is approximately a 25 km gap at the northern end of the east rift with no rift connecting the northern boundary to the eastern boundary.[4]

Easter Microplate Boundaries
Easter microplate (from GeoMapApp) with rough plate boundaries.[6] Four spreading sections make up the eastern border. The west section comes from the north side down in a southwest manner, until it breaks off and becomes the southwest section. Triple junctions on north and south ends are not well defined.

[1] The northern border has wide ridges, greater than 1 km tall, linked side-by-side with the steeper slopes to the south. The southern trough area sits deeper than the areas to the north. The very eastern end of the northern border has pure strike-slip motion,[1] while the western end is marked by the Northern Pacific-Nazca-Easter triple junction.[4] This triple junction is a stable rift-fracture-fracture zone with anomalous earthquakes occurring to the northeast portion, indicating a possible second spreading axis.[4] The rest of the northern boundary to the east and west of the triple junction are colinear transform boundaries. A trough, approximately 3700 m deep, borders the north along this transform boundary to the east connecting to a 5300 m deep hole, called the "Pito Deep" because of its close proximity to the Pito Seamount, at the northeastern limit.[4]

The western border is divided into two parts. The west section has 2 spreading segments running north to south with spreading rates that approximately range from 120–140 mm/yr. These segments are connected by sinistrally slipping transform faults around 14°15'S .[4] A relay basin runs north to south along the southernmost segment as a result of past counter-clockwise rotation.[1] The southwest consists of one slower spreading center (50–90 mm/yr) that runs northwest to southeast until joining the southern transform boundary.[4]

Like the western end of the northerner border, the southern end also has an inferred rift-rift-fracture triple junction, but no data has been gathered yet to verify its existence.[4] A single transform fault runs west to east and is home to the most rugged and shallow terrain with high seismic activity.[4]

Evolution

In 1995, routine magnetic, gravity, and echosounder data, supplemented with data from GLORIA (a long-range side scan sonar), German Sea Beam, SeaMARC II, and data from the World Data Center in Boulder, CO were all utilized to construct a two-stage model for the evolution of the Easter microplate.[1]

Stage 1: 5.25 to 2.25 million years ago

Approximately 5.25 million years ago, the boundary between the Pacific and Nazca plates was not connected and did not completely separate the two plates. The Easter microplate began to grow to the north-south throughout this period. The eastern rift, having not yet connected to the western rift, began to propagate northward by pseudofaults that appear to the west and east of the rift and continued until approximately 2.25 million years ago when the tip reached 23°S. While this was occurring, the west rift was propagating southward, north of the east rift, breaking into segments connected by transform faults that trend towards the southwest. The entire microplate continued a counter-clockwise rotation rate of 15° every million years throughout the entire history of the Easter microplate.[1]

Stage 2: 2.25 million years ago to present

The Easter microplate grew at a slower rate in the east-west dimension during this period, as it stopped growing north-south due to the cessation of east rift propagation. The east rift did continue angular spreading while keeping the same growth rate, but did not propagate any further northward. The west rift continued adjusting with more segmenting until the southwest rift began to open and propagate to the east. The southwest rift continued propagation until the present day southern triple junction was created.[1]

Future predictions

Though other evolution models have argued that the microplate was created approximately 4.5 million years ago,[6] there is currently only one hypothesis for future evolution of the Easter microplate. It is believed that due to the slowing spreading rates at the southwest rift and the northern end of the east rift, the southwest and west rift will cease spreading activity and completely transfer the microplate from the Nazca to the Pacific plate. This has been the case for other areas where extensive rift propagation studies have been conducted.[7]

Dynamics

Driving forces

Divergence of the Nazca and Pacific plates generate a pulling force acting on the Easter microplate, causing its rotation. Two types of driving forces are believed to act on the Nazca-Pacific plate divergence: shear and tension. Shear driving forces occur along the north and south boundaries, which explain failures due to compression in the northern end of the plate. Tension driving forces occur at the east and west rifts. Because of the fast spreading rates along these boundaries, the Easter microplate has a thin lithosphere. The normal tensional forces applied across the east and west rifts is enough to drive the microplate's rotation. Due to the slowing trend of these spread rates along these rifts to the north, it is believed the lithosphere gets thicker near the north and the shear forces are believed to contribute to the overall driving force.[8]

Resisting forces

Mantle basal drag accounts for 20% of the forces applied to the Easter microplate. Mantle basal drag force is calculated using the equation: , where is the mantle drag force per unit area, is the proportionality constant, and is absolute velocity of microplate using a fixed hotspot as the reference frame. The value for represents a quantification of the total resisting force that the ductile asthenosphere applies to the brittle lithosphere floating on top.

The other 80% of the resisting forces come from the rotation of the Easter microplate. As the microplate is rotating, normal resistances are applied to the microplate at the north and south ends where there are no rifts to help microplate adjustment. Both tension and compression contribute to the resistance, but compressional forces along the ends of the rifts have more of an impact. These compressional forces are what create the elevated regions that surround the "Pito Deep".[8]

References

  1. ^ a b c d e f g h Rusby, Ruth; Searle, Roger (July 1995). "A History of Easter Microplate, 5.25 Ma to present". Journal of Geophysical Research. 100 (B7): 12617–12640. Bibcode:1995JGR...10012617R. doi:10.1029/94JB02779.
  2. ^ Handschumacher, D. W. (1981). "Structure and evolution of the Easter Plate". Nazca Plate : Crustal Formation and Andean Convergence : A Volume Dedicated to George P. Woollard: 63.
  3. ^ Alden, Andrew (Feb 28, 2017). "Here Are the Sizes of Tectonic or Lithospheric Plates". Thought Co.
  4. ^ a b c d e f g h i j k Hey, R.; Naar, David (September 26, 1985). "Microplate Tectonics Along a Superfast Seafloor Spreading System near Easter Island". Nature. 317 (6035): 320–325. Bibcode:1985Natur.317..320H. doi:10.1038/317320a0.
  5. ^ Anderson, Roger; Forsyth, Donald; Molnar, Peter; Mammerickx, Jacqueline (December 1974). "Fault plane solutions of earthquakes on the Nazca plate boundaries and the Easter plate". Earth and Planetary Science Letters. 24 (2): 188–202. Bibcode:1974E&PSL..24..188A. doi:10.1016/0012-821X(74)90096-X.
  6. ^ a b Naar, David; Hey, R. (May 10, 1991). "Tectonic Evolution of the Easter Microplate". Journal of Geophysical Research. 96 (B5): 7961–7993. Bibcode:1991JGR....96.7961N. doi:10.1029/90JB02398.
  7. ^ Engeln, Joseph; Stein, Seth (May 1984). "Tectonics of the Easter plate". Earth and Planetary Science Letters. 68 (2): 259–270. Bibcode:1984E&PSL..68..259E. doi:10.1016/0012-821X(84)90158-4.
  8. ^ a b Neves, M. C.; Searle, R. C.; Bott, M. H. P. (October 2002). "Easter Microplate Dynamics". Journal of Geophysical Research. 108. doi:10.1029/2001JB000908. hdl:10400.1/11125.
Crough Seamount

Crough Seamount (named after the geologist Thomas Crough) is a seamount in the Pacific Ocean, within the exclusive economic zone of Pitcairn. It rises to a depth of 650 metres (2,130 ft) and is paired with a taller but overall smaller seamount to the east. This seamount has a flat top and probably formed an island in the past. It is about 7-8 million years old, although a large earthquake recorded at its position in 1955 may indicate a recent eruption.

The seamount appears to be part of a long geological lineament with the neighbouring Henderson and Ducie islands, as well as the southern Tuamotus and Line Islands. Such a lineament may have been generated by a hotspot; the nearby Easter hotspot is a candidate hotspot.

Easter Island

Easter Island (Rapa Nui: Rapa Nui, Spanish: Isla de Pascua) is a Chilean island in the southeastern Pacific Ocean, at the southeasternmost point of the Polynesian Triangle in Oceania. Easter Island is most famous for its nearly 1,000 extant monumental statues, called moai, created by the early Rapa Nui people. In 1995, UNESCO named Easter Island a World Heritage Site, with much of the island protected within Rapa Nui National Park.

It is believed that Easter Island's Polynesian inhabitants arrived on Easter Island sometime near 1200 AD. They created a thriving and industrious culture, as evidenced by the island's numerous enormous stone moai and other artifacts. However, land clearing for cultivation and the introduction of the Polynesian rat led to gradual deforestation. By the time of European arrival in 1722, the island's population was estimated to be 2,000–3,000. European diseases, Peruvian slave raiding expeditions in the 1860s, and emigration to other islands, e.g. Tahiti, further depleted the population, reducing it to a low of 111 native inhabitants in 1877.Chile annexed Easter Island in 1888. In 1966, the Rapa Nui were granted Chilean citizenship. In 2007 the island gained the constitutional status of "special territory." Administratively, it belongs to the Valparaíso Region, constituting a single commune of the Province Isla de Pascua. The 2017 Chilean census registered 7,750 people on the island, of whom 3,512 (45%) considered themselves Rapa Nui.Easter Island is one of the most remote inhabited islands in the world. The nearest inhabited land (around 50 residents in 2013) is Pitcairn Island, 2,075 kilometres (1,289 mi) away; the nearest town with a population over 500 is Rikitea, on the island of Mangareva, 2,606 km (1,619 mi) away; the nearest continental point lies in central Chile, 3,512 kilometres (2,182 mi) away.

Easter Island is considered part of Insular Chile.

Hydrothermal vent

A hydrothermal vent is a fissure on the seafloor from which geothermally heated water issues. Hydrothermal vents are commonly found near volcanically active places, areas where tectonic plates are moving apart at spreading centers, ocean basins, and hotspots. Hydrothermal deposits are rocks and mineral ore deposits formed by the action of hydrothermal vents.

Hydrothermal vents exist because the earth is both geologically active and has large amounts of water on its surface and within its crust. Under the sea, hydrothermal vents may form features called black smokers or white smokers. Relative to the majority of the deep sea, the areas around submarine hydrothermal vents are biologically more productive, often hosting complex communities fueled by the chemicals dissolved in the vent fluids. Chemosynthetic bacteria and archaea form the base of the food chain, supporting diverse organisms, including giant tube worms, clams, limpets and shrimp. Active hydrothermal vents are thought to exist on Jupiter's moon Europa, and Saturn's moon Enceladus, and it is speculated that ancient hydrothermal vents once existed on Mars.

Kiwa hirsuta

Kiwa hirsuta is a crustacean discovered in 2005 in the South Pacific Ocean. This decapod, which is approximately 15 cm (5.9 in) long, is notable for the quantity of silky blond setae (resembling fur) covering its pereiopods (thoracic legs, including claws). Its discoverers dubbed it the "yeti lobster" or "yeti crab".

List of tectonic plates

This is a list of tectonic plates on the Earth's surface. Tectonic plates are pieces of Earth's crust and uppermost mantle, together referred to as the lithosphere. The plates are around 100 km (62 mi) thick and consist of two principal types of material: oceanic crust (also called sima from silicon and magnesium) and continental crust (sial from silicon and aluminium). The composition of the two types of crust differs markedly, with mafic basaltic rocks dominating oceanic crust, while continental crust consists principally of lower-density felsic granitic rocks.

Martin Bott

Martin Harold Phillips Bott (12 July 1926 – 20 October 2018) was a British geologist and Professor in the Department of Earth Sciences at the University of Durham, England.

Mid-Pacific Mountains

The Mid-Pacific Mountains (MPM) is a large oceanic plateau located in the central North Pacific Ocean or south of the Hawaiian–Emperor seamount chain. Of volcanic origin and Mesozoic in age, it is located on the oldest part of the Pacific Plate and rises up to 2 km (1.2 mi) (Darwin Rise) above the surrounding ocean floor and is covered with several layers of thick sedimentary sequences that differ from those of other plateaux in the North Pacific. About 50 seamounts are distributed over the MPM. Some of the highest points in the range are above sea level which include Wake Island and Marcus Island.

The ocean floor of the MPM dates back to the Jurassic-Cretaceous, some of the oldest oceanic crust on Earth.The MPM is a range of guyots with a lava composition similar to those found in Iceland and the Galapagos Islands, and they probably formed similarly at or near a rift system.

In the Cretaceous, they formed large tropical islands located closer to the Equator that began to sink in the late Mesozoic.The MPM formed in the Early Cretaceous (at c. 110 Ma) over a hotspot that uplifted the ocean floor of the still young Pacific Plate. Reefs developed on the subsiding islands and renewed volcanism in the Late Cretaceous helped maintain some of eastern islands but inevitably the guyots sank to their present depth.

It has been proposed that the MPM has crossed over several hotspots, and the MPM guyots are indeed older on the western MPM than the eastern part, but the guyots do not form chains that can be traced to any known hotspots. The MPM, nevertheless, must have originated over the South Pacific Superswell. Among the guyots in the Mid-Pacific Mountains are Allison Guyot, Horizon Guyot, Resolution Guyot and Darwin Guyot.The western half of the Easter hotspot chain, a lineament that includes the Line Islands and Tuamotu archipelago, begins near the eastern part of the MPM. The formation of the MPM thus probably occurred at the Pacific-Farallon Ridge and the Easter hotspot, or where the Easter Microplate is now located.

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 Andean orogeny. The Nazca Plate is bounded on the west by the Pacific Plate and to the south by the Antarctic Plate through the East Pacific Rise and the Chile Rise respectively. The movement of the Nazca Plate over several hotspots has created some volcanic islands as well as east-west running seamount chains that subduct under South America. Nazca is a relatively young plate both in terms of the age of its rocks and its existence as an independent plate having been formed from the break-up of the Farallon Plate about 23 million years ago. The oldest rocks of the plate are about 50 million years old.

Pito Seamount

Pito Seamount is a seamount in the Pacific Ocean. It rises to a depth of 2,250 metres (7,380 ft) and features hydrothermal activity in the form of black smokers, which were discovered in 1993.

Rano Rahi seamounts

Rano Rahi (Rapa Nui: "Many volcanoes") is a field of seamounts in the Pacific Ocean. These seamounts in part form a series of ridges on the Pacific Plate pointing away from the neighbouring East Pacific Rise and which were volcanically active until about 230,000 years ago, and possibly even more recently.

The origin of these seamounts is unclear. Proposals include the effect of tectonic forces on the Pacific Plate and the presence of mantle flow towards the East Pacific Rise.

Superswell

A superswell is a large area of anomalously high topography and shallow ocean regions. These areas of anomalous topography are byproducts of large upwelling of mantle material from the core–mantle boundary, referred to as superplumes. Two present day superswells have been identified: the African superswell and the South Pacific superswell. In addition to these, the Darwin Rise in the south central Pacific Ocean is thought to be a paleosuperswell, showing evidence of being uplifted compared to surrounding ancient ocean topography.

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