The Azores–Gibraltar Transform Fault (AGFZ), also called a

fault zone In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust result from the action of plate tectoni ...

and a

fracture zone A fracture zone is a linear feature on the ocean floor—often hundreds, even thousands of kilometers long—resulting from the action of offset mid-ocean ridge axis segments. They are a consequence of plate tectonics. Lithospheric plates on e ...

, is a major

seismic zone In seismology, a seismic zone or seismic belt is an area of seismicity potentially sharing a common cause. It may also be a region on a map for which a common areal rate of seismicity is assumed for the purpose of calculating probabilistic ground ...

in the Eastern

Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa, Europe ...

between the

Azores ) , motto =( en, "Rather die free than subjected in peace") , anthem= ( en, "Anthem of the Azores") , image_map=Locator_map_of_Azores_in_EU.svg , map_alt=Location of the Azores within the European Union , map_caption=Location of the Azores wi ...

and the

Strait of Gibraltar The Strait of Gibraltar ( ar, مضيق جبل طارق, Maḍīq Jabal Ṭāriq; es, Estrecho de Gibraltar, Archaic: Pillars of Hercules), also known as the Straits of Gibraltar, is a narrow strait that connects the Atlantic Ocean to the Medi ...

. It is the product of the complex interaction between the African,

Eurasian Eurasia (, ) is the largest continental area on Earth, comprising all of Europe and Asia. Primarily in the Northern and Eastern Hemispheres, it spans from the British Isles and the Iberian Peninsula in the west to the Japanese archipela ...

, and Iberian plates. The AGFZ produced these large-magnitude earthquakes and, consequently, a number of large tsunamis: 1755 Lisbon, 1761 Lisbon, 1816 North Atlantic, 1941 Gloria Fault earthquake, 1969 Horseshoe and

1975 It was also declared the ''International Women's Year'' by the United Nations and the European Architectural Heritage Year by the Council of Europe. Events January * January 1 - Watergate scandal (United States): John N. Mitchell, H. R. ...

Geologic setting

Forming the Atlantic segment of the boundary between the African and Eurasian plates, the AGFZ is largely dominated by compressional forces between these converging () plates, but it is subject to a dynamic tectonic regime that also involves extension and transform faulting. The oceanic

lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of up to thousands of years ...

in the area is directly related to the

opening of the North Atlantic Ocean The opening of the North Atlantic Ocean is a geological event that has occurred over millions of years, during which the supercontinent Pangea broke up. As modern-day Europe ( Eurasian plate) and North America ( North American Plate) separated d ...

and is one of the oldest preserved on Earth. The western end of the AGFZ, the

Azores Triple Junction The Azores Triple Junction (ATJ) is a geologic triple junction where the boundaries of three tectonic plates intersect: the North American Plate, the Eurasian Plate and the African Plate. This triple junction is located along the Mid-Atlantic ...

on the

Mid-Atlantic Ridge The Mid-Atlantic Ridge is a mid-ocean ridge (a divergent 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, the ridge separates the North A ...

(MAR), is where the North American, African, and Eurasian plates meet. Spreading in the MAR is faster south of the AGFZ than north of it, which results in a transcurrent movement along the AGFZ at about . The eastern segment of the fault is complex and characterised by a series of seamounts and ridges separating the Tores and

Horseshoe A horseshoe is a fabricated product designed to protect a horse hoof from wear. Shoes are attached on the palmar surface (ground side) of the hooves, usually nailed through the insensitive hoof wall that is anatomically akin to the human ...

abyssal plains. The active compressional deformation in this segment is an extremely rare example of compression between two oceanic lithospheres.

Plate tectonics

The Atlantic Ocean is surrounded by

passive margin A passive margin is the transition between oceanic and continental lithosphere that is not an active plate margin. A passive margin forms by sedimentation above an ancient rift, now marked by transitional lithosphere. Continental rifting cre ...

s with the exception of three subduction zones: the Lesser Antilles Arc in the Caribbean, the

Scotia Arc The Scotia Arc is the island arc system forming the north, east and south border of the Scotia Sea. The northern border, the North Scotia Ridge, comprises (from west to east) Isla de los Estados at the tip of Tierra del Fuego, the Burdwood, Davis, ...

in the South Atlantic, and the Gibraltar Arc in the western

Mediterranean The Mediterranean Sea is a sea connected to the Atlantic Ocean, surrounded by the Mediterranean Basin and almost completely enclosed by land: on the north by Western and Southern Europe and Anatolia, on the south by North Africa, and on ...

. The Gibraltar Arc is propagating westward into the Atlantic over an east-dipping oceanic slab (one of the remainders of the

Tethys Ocean The Tethys Ocean ( el, Τηθύς ''Tēthús''), also called the Tethys Sea or the Neo-Tethys, was a prehistoric ocean that covered most of the Earth during much of the Mesozoic Era and early Cenozoic Era, located between the ancient continents ...

). This subduction/

back-arc basin A back-arc basin is a type of geologic basin, found at some convergent plate boundaries. Presently all back-arc basins are submarine features associated with island arcs and subduction zones, with many found in the western Pacific Ocean. Most o ...

system is developing in front of the Alboran Block (under the

Alboran Sea The Alboran Sea (from Arabic , ''al-Baḥrān'') is the westernmost portion of the Mediterranean Sea, lying between the Iberian Peninsula and the north of Africa (Spain on the north and Morocco and Algeria on the south). The Strait of Gibraltar, w ...

) at a rate faster than that of the Africa-Iberia convergence. Consequently, this area is a rare case of a passive margin slowly being transformed into an

active margin Active may refer to: Music * ''Active'' (album), a 1992 album by Casiopea * Active Records, a record label Ships * ''Active'' (ship), several commercial ships by that name * HMS ''Active'', the name of various ships of the British Roya ...

. The extension of this subduction system, known as the "allochthonous unit of the Gulf of Cadiz" (AUGC), marks the continuing propagation of the

Alpide belt The Alpide belt or Alpine-Himalayan orogenic belt,K.M. Storetvedt, K. M., ''The Tethys Sea and the Alpine-Himalayan orogenic belt; mega-elements in a new global tectonic system,'' Physics of the Earth and Planetary Interiors, Volume 62, Issues 1 ...

into the Atlantic along the AGFZ. In the context of the Wilson Cycle, this suggests that the beginning of the closure of the Atlantic is taking place in front of the three Atlantic subduction zones.

References

Notes

Sources

* * * * *

Geologic setting

Plate tectonics

See also

References

Notes

Sources

Further reading