Fifteen-Twenty Fracture Zone

The Fifteen-Twenty or 15°20' Fracture Zone (FTFZ), also known as the Cabo Verde Fracture Zone, is a fracture zone located on the Mid-Atlantic Ridge (MAR) in the central Atlantic Ocean between 14–16°N. It is the current location of the migrating triple junction marking the boundaries between the North American, South American, and Nubian plates.[1][2] The FTFZ is roughly parallel to the North and South America—Africa spreading direction and has a broad axial valley produced over the last ten million years by the northward-migrating triple junction.[1][2] Offsetting the MAR by some 175 km, the FTFZ is located on one of the slowest portions of the MAR where the full spreading rate is 25 km/Myr.[3]

Geological setting

North and south of the FTFZ the axis of the MAR is near-perpendicular to the spreading direction and the spreading rate is 2.6 mm/yr. The axial valley south of the FTFZ is composed of short axial volcanic ridges separated by 8–18 km-long en echelon deeps, while north of the FTFZ the axial ridges are much longer and more linear.[4]

North and south of the FTFZ the ocean floor is relatively smooth with long abyssal hills, probably detachment faults, aligned near-parallel to the ridge axis. In contrast, close to the FTFZ the terrain is more rugged and adorned with short, oblique fault scarps. Associated with the transition between these two types of terrains (at about 15°50'N and 14°30'N respectively) are V-shaped, south-propagating structures. These transitional structures disappear away from the ridge. Within the rugged terrain serpentinized peridotite and gabbro are capped with a thin layer of extrusive basalt. In the smooth areas the lithosphere is more magmatic in composition.[4]

The FTFZ is flanked by two negative gravity anomalies associated with the accretion of igneous crust. The anomaly south of the FTFZ is twice as large as the northern one. There are also geochemical variations across the fracture zone. On the southern side basalts are enriched MORB (mid-ocean ridge basalt) but on the northern side basalts change from enriched to depleted away from the FTFZ. Peridotites collected from south of the FTFZ have an uncommon composition ascribed to a H2O-rich or hot mantle source.[3]

Megamullions

Corrugated surfaces known as megamullions or oceanic core complexes measure 25 km along-axis and 10–15 km across. When found along other mid-ocean ridges such structures occur at the inside corners of ridge discontinuities, but at the FTFZ they occur on both sides of the ridge away from any non-transform discontinuities.[4] These structures and ultramafic rocks outcropping on either side of the MAR (in contrast to other parts of the ridge) indicate considerably reduced magma supply near the FTFZ. Paradoxically, geochemical analyses of basalts near the FTFZ instead suggest an enriched mantle source and the presence of a mantle hotspot.[2]

Two models can explain these contradictions. A westward ridge jump could relocate an older megamullion on the original western flank to the opposite flank after which a new megamullion start to form on the new western flank. Near the FTFZ this would place the older megamullion in an outside corner while the younger develop in an inside corner. Alternatively, an eastward ridge jump or migration could turn a west-dipping detachment fault into an east-dipping fault, which would also result in an older abandoned and a younger active megamullion. Which is the case is currently not known.[5]

Superimposed on the larger corrugated surfaces are two systems of smaller scale corrugations: one on a 1–3 km-scale, roughly 200 m high, and another finer about 100–500 m wide. The latter occur up to 1 km from the ridge and is covered by elevated ridges running parallel to the spreading direction, about 10 m wide, hundreds of metres long, and 10 m tall. These in turn are covered with cm-scale striations running in the same direction.[6]

Triple junction

The North American–South American–African triple junction is associated with the initial opening of the Atlantic Ocean and has had a complex tectonic history. It probably migrated from near 10°N to its current location near the FTFZ between 72.5 and 35.5 Ma. Both its location and that of the North America–South-American–Caribbean triple junction are debated, however.[7] The initiation and evolution of triple junctions is often associated with mantle plumes, but, if this is the case near the FTFZ, the limited supply of magma suggest an embryonic plume or a local, anomalous mantle composition. The relative movement between the North American and South American plates is very small, but the resulting deformation could possibly explain both off-axis seismicity and the odd mantle composition.[8]

References

Notes

  1. ^ a b Fujiwara et al. 2003, Bathymetry and Geological Features, p. 4
  2. ^ a b c Fujiwara et al. 2003, Introduction, pp. 2–3
  3. ^ a b Godard et al. 2008, Geological setting, p. 414
  4. ^ a b c Escartín & Cannat 1999, Geological setting, seafloor morphology and ultramafic outcrop distribution, pp. 415–417
  5. ^ Fujiwara et al. 2003, Development of Megamullions on Ridge Flanks, pp. 20–26
  6. ^ MacLeod et al. 2002, Morphology of the Striated Surface at 15°45'N, pp. 879–880
  7. ^ Smith et al. 2008, Study area, pp. 2–3
  8. ^ Smith et al. 2008, Equatorial Atlantic, p. 20

Sources

Coordinates: 15°19′12″N 45°52′16″W / 15.320°N 45.871°W

Atlantic Ocean

The Atlantic Ocean is the second largest of the world's oceans, with an area of about 106,460,000 square kilometers (41,100,000 square miles). It covers approximately 20 percent of the Earth's surface and about 29 percent of its water surface area. It separates the "Old World" from the "New World".

The Atlantic Ocean occupies an elongated, S-shaped basin extending longitudinally between Europe and Africa to the east, and the Americas to the west. As one component of the interconnected global ocean, it is connected in the north to the Arctic Ocean, to the Pacific Ocean in the southwest, the Indian Ocean in the southeast, and the Southern Ocean in the south (other definitions describe the Atlantic as extending southward to Antarctica). The Equatorial Counter Current subdivides it into the North Atlantic Ocean and the South Atlantic Ocean at about 8°N.Scientific explorations of the Atlantic include the Challenger expedition, the German Meteor expedition, Columbia University's Lamont-Doherty Earth Observatory and the United States Navy Hydrographic Office.

List of fracture zones

Fracture zones are common features in the geology of oceanic basins. Globally most fault zones are located on divergent plate boundaries on oceanic crust. This means that they are located around mid-ocean ridges and trend perpendicular to them. The term fracture zone is used almost exclusively for features on oceanic crust; similar structures on continental crust are instead termed transform or strike slip faults, a denomination active fracture zones also can have. Some fracture zones have been created by mid-ocean ridge segments that have been subducted and may not longer exist.

Mid-Atlantic Ridge

The Mid-Atlantic Ridge (MAR) is a mid-ocean ridge, a divergent tectonic plate or constructive plate boundary located along the floor of the Atlantic Ocean, and part of the longest mountain range in the world. In the North Atlantic it separates the Eurasian and North American plates, and in the South Atlantic it separates the African and South American plates. The ridge extends from a junction with the Gakkel Ridge (Mid-Arctic Ridge) northeast of Greenland southward to the Bouvet Triple Junction in the South Atlantic. Although the Mid-Atlantic Ridge is mostly an underwater feature, portions of it have enough elevation to extend above sea level. The section of the ridge that includes Iceland is known as the Reykjanes Ridge. The ridge has an average spreading rate of about 2.5 centimetres (0.98 in) per year.

North American Plate

The North American Plate is a tectonic plate covering most of North America, Greenland, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the Azores. It extends eastward to the Mid-Atlantic Ridge and westward to the Chersky Range in eastern Siberia. The plate includes both continental and oceanic crust. The interior of the main continental landmass includes an extensive granitic core called a craton. Along most of the edges of this craton are fragments of crustal material called terranes, accreted to the craton by tectonic actions over a long span of time. It is thought that much of North America west of the Rocky Mountains is composed of such terranes.

Outline of oceanography

The following outline is provided as an overview of and introduction to Oceanography.

Outline of plate tectonics

This is a list of articles related to plate tectonics and tectonic plates.

Researcher Ridge

Researcher Ridge is an underwater ridge in the Atlantic Ocean. It appears to be a chain of seamounts named Gollum Seamount, Vayda Seamount, Bilbo Seamount, Gandalf Seamount, The Shire Seamount, Pippin Seamount, Merry Seamount, Molodezhnaya Seamount, Frodo Seamount, Sam Seamount and Mount Doom Seamount that were probably formed by a hotspot.

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-Atlantic Ridge.

The easterly edge is a divergent boundary with the African Plate; the southerly edge is a complex boundary with the Antarctic Plate, the Scotia Plate, and the Sandwich Plate; the westerly edge is a convergent boundary with the subducting Nazca Plate; and the northerly edge is a boundary with the Caribbean Plate and the oceanic crust of the North American Plate. At the Chile Triple Junction, near the west coast of the Taitao–Tres Montes Peninsula, an oceanic ridge known as the Chile Rise is actively subducting under the South American Plate.

Geological research suggests that the South American Plate is moving westward away from the Mid-Atlantic Ridge: "Parts of the plate boundaries consisting of alternations of relatively short transform fault and spreading ridge segments are represented by a boundary following the general trend." As a result, the eastward-moving and more dense Nazca Plate is subducting under the western edge of the South American Plate, along the continent's Pacific coast, at a rate of 77 mm (3.0 in) per year. The collision of these two plates is responsible for lifting the massive Andes Mountains and for creating the numerous volcanoes which are strewn throughout them.

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