Gibraltar Arc

Last updated April 04, 2019

The Gibraltar Arc is a geological region corresponding to an arcuate orogen surrounding the Alboran Sea, between the Iberian Peninsula and Africa. It consists of the Betic Cordillera (south Spain), and the Rif (North Morocco).^[1] The Gibraltar Arc is located at the western end of the Mediterranean Alpine belt^[2] and formed during the Neogene due to convergence of the Eurasian and African plates.^[3]

The Alboran Sea from Arabic is the westernmost portion of the Mediterranean Sea, lying between the Iberian Peninsula and the north of Africa. The Strait of Gibraltar, which lies at the west end of the Alboran Sea, connects the Mediterranean with the Atlantic Ocean.

The Iberian Peninsula, also known as Iberia, is located in the southwest corner of Europe. The peninsula is principally divided between Spain and Portugal, comprising most of their territory. It also includes Andorra, small areas of France, and the British overseas territory of Gibraltar. With an area of approximately 596,740 square kilometres (230,400 sq mi)), it is both the second largest European peninsula by area, after the Scandinavian Peninsula, and by population, after the Balkan Peninsula.

Africa is the world's second largest and second most-populous continent, being behind Asia in both categories. At about 30.3 million km² including adjacent islands, it covers 6% of Earth's total surface area and 20% of its land area. With 1.2 billion people as of 2016, it accounts for about 16% of the world's human population. The continent is surrounded by the Mediterranean Sea to the north, the Isthmus of Suez and the Red Sea to the northeast, the Indian Ocean to the southeast and the Atlantic Ocean to the west. The continent includes Madagascar and various archipelagos. It contains 54 fully recognised sovereign states (countries), nine territories and two de facto independent states with limited or no recognition. The majority of the continent and its countries are in the Northern Hemisphere, with a substantial portion and number of countries in the Southern Hemisphere.

Mulhacén is the highest mountain in continental Spain and in the Iberian Peninsula. It is part of the Sierra Nevada range in the Cordillera Penibética. It is named after Abu l-Hasan Ali, known as Muley Hacén in Spanish, the penultimate Muslim King of Granada in the 15th century who, according to legend, was buried on the summit of the mountain.

The Guadalquivir is the fifth longest river in the Iberian Peninsula and the second longest river with its entire length in Spain. The Guadalquivir river is the only great navigable river in Spain. Currently it is navigable from the Gulf of Cádiz to Seville, but in Roman times it was navigable to Córdoba.

Sebou is a river in northern Morocco. At its source in the Middle Atlas mountains it is known as the Guigou River. The river is 496 kilometers long and has an average water flow of 137 m³/s, which makes it the largest North African river by volume. It passes near the city of Fes and discharges to the Atlantic Ocean in Mehdia. Sebou is navigable for only 16 km as far as the city of Kenitra, which has the only river port in Morocco. Its most important tributaries are the Ouergha River, Baht River and Inaouen River. The river supports irrigation in Morocco's most fertile region: the Gharb.

Tectonic evolution

North–south convergence of the Eurasian and African plates occurred during the middle Oligocene to the late Miocene, followed by northwest–southeast convergence from the late Tortonian to present. The Gibraltar Arc was formed during the Neogene due to a combination of western migration of the orogenic mountain front and late orogenic extension. The present convergence rate of the plates is estimated to be approximately 4.5 to 5.0 mm/year with an azimuth of 135–120°.^[3]

The Oligocene is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present. As with other older geologic periods, the rock beds that define the epoch are well identified but the exact dates of the start and end of the epoch are slightly uncertain. The name Oligocene was coined in 1854 by the German paleontologist Heinrich Ernst Beyrich; the name comes from the Ancient Greek ὀλίγος and καινός, and refers to the sparsity of extant forms of molluscs. The Oligocene is preceded by the Eocene Epoch and is followed by the Miocene Epoch. The Oligocene is the third and final epoch of the Paleogene Period.

The Miocene is the first geological epoch of the Neogene Period and extends from about 23.03 to 5.333 million years ago (Ma). The Miocene was named by Charles Lyell; its name comes from the Greek words μείων and καινός and means "less recent" because it has 18% fewer modern sea invertebrates than the Pliocene. The Miocene is preceded by the Oligocene and is followed by the Pliocene.

The Tortonian is in the geologic timescale an age or stage of the late Miocene that spans the time between 11.608 ± 0.005 Ma and 7.246 ± 0.005 Ma. It follows the Serravallian and is followed by the Messinian.

The eastward Gibraltar Arc oceanic subduction system was active during the Early and Middle Miocene and has likely been inactive since. At this time, the Alboran Sea acted as a back-arc basin during the deposition of accretionary units.^[4] Since the Late Miocene, north–south to northwest–southeast continental convergence forced the subduction system along the arc that is oriented N20°E to N100°E. There is a lithospheric slab dipping east from the Strait of Gibraltar down to 600 km depth beneath the Alboran Sea.^[4]

Subduction A geological process at convergent tectonic plate boundaries where one plate moves under the other

Subduction is a geological process that takes place at convergent boundaries of tectonic plates where one plate moves under another and is forced to sink due to gravity into the mantle. Regions where this process occurs are known as subduction zones. Rates of subduction are typically in centimeters per year, with the average rate of convergence being approximately two to eight centimeters per year along most plate boundaries.

Back-arc basins are geologic basins, submarine features associated with island arcs and subduction zones. They are found at some convergent plate boundaries, presently concentrated in the western Pacific Ocean. Most of them result from tensional forces caused by oceanic trench rollback and the collapse of the edge of the continent. The arc crust is under extension or rifting as a result of the sinking of the subducting slab. Back-arc basins were initially a surprising result for plate tectonics theorists, who expected convergent boundaries to be zones of compression, rather than major extension. However, they are now recognized as consistent with this model in explaining how the interior of Earth loses heat.

Accretion (geology) process by which material is added to a tectonic plate or a landmass

Accretion, in geology, is a process by which material is added to a tectonic plate or a landmass. This material may be sediment, volcanic arcs, seamounts, or other igneous features.

The crustal structure of the Gibraltar Arc is characterized by an arcuate bulge parallel to the arc, with crustal thinning occurring uniformly from the margins of the mountain ranges towards the Alboran Sea. The lithospheric mantle also has an arcuate bulge below the arc with extreme mantle thinning in the Alboran Sea,^[3] which is the typical structure of a back-arc basin located on the concave side of an arcuate mountain belt.^[5]

A lithosphere is the rigid, outermost shell of a terrestrial-type planet, or natural satellite, that is defined by its rigid mechanical properties. On Earth, it is composed of the crust and the portion of the upper mantle that behaves elastically on time scales of thousands of years or greater. The outermost shell of a rocky planet, the crust, is defined on the basis of its chemistry and mineralogy.

A major left-lateral strike-slip fault zone, the Trans-Alboran Shear Zone, crosscuts the Gibraltar Arc with a NE trend from the eastern Betics to the western Rif. It was active during the Neogene, contributing to the westward advance of the Gibraltar Arc. Some of the previous fault segments are active, with left-lateral transpressive faulting and moderate to significant clockwise stress rotations. Oblique to this shear zone, there are two major right-lateral strike-slip fault systems, the Maro-Nerja and Yusuf systems. These trend NW and have transtensive deformation. The present-day stress pattern is probably the consequence of the interference between two stress sources: ongoing continent convergence and secondary stress sources from variations in crustal thickness, sedimentary accumulations causing loading, and the active strike-slip fault.^[3]

In geology, transpression is a type of strike-slip deformation that deviates from simple shear because of a simultaneous component of shortening perpendicular to the fault plane. This movement ends up resulting in oblique shear. It is generally very unlikely that a deforming body will experience "pure" shortening or "pure" strike-slip. The relative amounts of shortening and strike-slip can be expressed in the convergence angle alpha which ranges from zero to 90 degrees. During shortening, unless material is lost, transpression produces vertical thickening in the crust. Transpression that occurs on a regional scale along plate boundaries is characterized by oblique convergence. More locally, transpression occurs within restraining bends in strike-slip fault zones.

Transtension is the state in which a rock mass or area of the Earth's crust experiences both extensive and transtensive shear. As such, transtensional regions are characterised by both extensional structures and wrench structures. In general, many tectonic regimes that were previously defined as simple strike-slip shear zones are actually transtensional. It is unlikely that a deforming body will experience 'pure' extension or 'pure' strike-slip.

Geology

The Arc has two sections: the Internal and External Zones. The Internal Zone, which is located on the inner side of the arc, adjacent to the Alboran Sea, mostly consists of high-pressure, low-temperature metamorphic rocks. The External Zone, located on the outer side of the arc, is mostly made of sediments deposited on the passive margins of Africa and Iberia. These rocks became highly deformed during the westward emplacement of the hinterland of the subduction system, with some units in the External Rif having undergone medium-pressure, low-temperature metamorphism during the Oligocene. Flysch units from the Cretaceous to Early Miocene are located between the External and Internal Zones. These were folded and thrusted during the westward migration of the Internal Zones and can be interpreted as the palaeo-accretionary wedge of the subduction system that was active during the Lower and Middle Miocene.^[4]

Related Research Articles

An orogeny is an event that leads to both structural deformation and compositional differentiation of the Earth's lithosphere at convergent plate margins. An orogen or orogenic belt develops when a continental plate crumples and is pushed upwards to form one or more mountain ranges; this involves a series of geological processes collectively called orogenesis.

Oceanic trench Long and narrow depressions of the sea floor

Oceanic trenches are topographic depressions of the sea floor, relatively narrow in width, but very long. These oceanographic features are the deepest parts of the ocean floor. Oceanic trenches are a distinctive morphological feature of convergent plate boundaries, along which lithospheric plates move towards each other at rates that vary from a few millimeters to over ten centimeters per year. A trench marks the position at which the flexed, subducting slab begins to descend beneath another lithospheric slab. Trenches are generally parallel to a volcanic island arc, and about 200 km (120 mi) from a volcanic arc. Oceanic trenches typically extend 3 to 4 km below the level of the surrounding oceanic floor. The greatest ocean depth measured is in the Challenger Deep of the Mariana Trench, at a depth of 11,034 m (36,201 ft) below sea level. Oceanic lithosphere moves into trenches at a global rate of about 3 km²/yr.

Obduction was originally defined by Coleman to mean the overthrusting of oceanic lithosphere onto continental lithosphere at a convergent plate boundary where continental lithosphere is being subducted beneath oceanic lithosphere.

Convergent boundaries are areas on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other causing a process known as subduction. The subduction zone can be defined by a plane where many earthquakes occur, called the Benioff Zone. These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, earthquakes, orogenesis, destruction of lithosphere, and deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere. The geologic features related to convergent boundaries vary depending on crust types.

African Plate Tectonic plate underlying Africa west of the East African Rift

The African Plate is a major tectonic plate straddling the equator as well as the prime meridian. It includes much of the continent of Africa, as well as oceanic crust which lies between the continent and various surrounding ocean ridges. Between 60 million years ago and 10 million years ago, the Somali Plate began rifting from the African Plate along the East African Rift. Since the continent of Africa consists of crust from both the African and the Somali plates, some literature refers to the African Plate as the Nubian Plate to distinguish it from the continent as a whole.

The islands of Japan are primarily the result of several large ocean movements occurring over hundreds of millions of years from the mid-Silurian to the Pleistocene as a result of the subduction of the Philippine Sea Plate beneath the continental Amurian Plate and Okinawa Plate to the south, and subduction of the Pacific Plate under the Okhotsk Plate to the north.

The Messinian Salinity Crisis (MSC), also referred to as the Messinian Event, and in its latest stage as the Lago Mare event, was a geological event during which the Mediterranean Sea went into a cycle of partly or nearly complete desiccation throughout the latter part of the Messinian age of the Miocene epoch, from 5.96 to 5.33 Ma. It ended with the Zanclean flood, when the Atlantic reclaimed the basin.

The geology of the Iberian Peninsula consists of the study of the rock formations on the Iberian Peninsula, which includes Spain, Portugal, Andorra, and Gibraltar. The peninsula contains rocks from every geological period from Ediacaran to Holocene, and many types of rock are represented. World-class mineral deposits are also found there.

Accretionary wedge The sediments accreted onto the non-subducting tectonic plate at a convergent plate boundary

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 downgoing slab of oceanic crust, but in some cases the wedge includes the erosional products of volcanic island arcs formed on the overriding plate.

The Azores–Gibraltar Transform Fault (AGFZ), also called a fault zone and a fracture zone, is a major seismic fault in the Central Atlantic Ocean west of the Strait of Gibraltar. It is the product of the complex interaction between the African, Eurasian, and Iberian plates. The AGFZ produced the large-magnitude 1755 Lisbon and 1969 Horseshoe earthquakes and, consequently, a number of large tsunamis.

Philippine Mobile Belt Complex portion of the tectonic boundary between the Eurasian Plate and the Philippine Sea Plate, comprising most of the country of the Philippines

The Philippine Mobile Belt is a complex portion of the tectonic boundary between the Eurasian Plate and the Philippine Sea Plate, comprising most of the country of the Philippines. It includes two subduction zones, the Manila Trench to the west and the Philippine Trench to the east, as well as the Philippine Fault System. Within the Belt, a number of crustal blocks or microplates which have been shorn off the adjoining major plates are undergoing massive deformation.

The Andean orogeny is an ongoing process of orogeny that began in the Early Jurassic and is responsible for the rise of the Andes mountains. The orogeny is driven by a reactivation of a long-lived subduction system along the western margin of South America. On a continental scale the Cretaceous and Oligocene were periods of re-arrangements in the orogeny. Locally the details of the nature of the orogeny varies depending on the segment and the geological period considered.

The Hellenic arc or Aegean arc is an arcuate tectonic feature of the eastern Mediterranean Sea related to the subduction of the African Plate beneath the Aegean Sea Plate. It consists of an oceanic trench, the Hellenic Trench, on its outer side; two arcs—a non-volcanic outer arc and an inner volcanic arc, the South Aegean Volcanic Arc; and a marginal sea on its inner side.

Tectonic subsidence is the sinking of the Earth's crust on a large scale, relative to crustal-scale features or the geoid. The movement of crustal plates and accommodation spaces created by faulting create subsidence on a large scale in a variety of environments, including passive margins, aulacogens, fore-arc basins, foreland basins, intercontinental basins and pull-apart basins. Three mechanisms are common in the tectonic environments in which subsidence occurs: extension, cooling and loading.

The Carpathian Flysch Belt is an arcuate tectonic zone included in the megastructural elevation of the Carpathians on the external periphery of the mountain chain. Geomorphologically it is a portion of Outer Carpathians. Geologically it is a thin-skinned thrust belt or accretionary wedge, formed by rootles nappes consisting of so-called flysch - alternating marine deposits of claystones, shales and sandstones which were detached from their substratum and moved tens of kilometers to the north (generally). The Flysch Belt is together with Neogene volcanic complexes only tectonic zone occurring along the whole Carpathian arc.

The Kutai sedimentary basin extends from the central highlands of Borneo, across the eastern coast of the island and into the Makassar Strait. With an area of 60,000 km², and depths up to 15 km, the Kutai is the largest and deepest Tertiary age basin in Indonesia. Plate tectonic evolution in the Indonesian region of SE Asia has produced a diverse array of basins in the Cenozoic. The Kutai is an extensional basin in a general foreland setting. Its geologic evolution begins in the mid Eocene and involves phases of extension and rifting, thermal sag, and isostatic subsidence. Rapid, high volume, sedimentation related to uplift and inversion began in the Early Miocene. The different stages of Kutai basin evolution can be roughly correlated to regional and local tectonic events. It is also likely that regional climate, namely the onset of the equatorial ever wet monsoon in early Miocene, has affected the geologic evolution of Borneo and the Kutai basin through the present day. Basin fill is ongoing in the lower Kutai basin, as the modern Mahakam River delta progrades east across the continental shelf of Borneo.

The geology of Myanmar is shaped by dramatic, ongoing tectonic processes controlled by shifting tectonic components as the Indian plate slides northwards and towards southeastern Asia. Myanmar spans across parts of three tectonic plates separated by north-trending faults. To the west, a highly oblique subduction zone separates the offshore Indian plate from the Burma microplate, which underlies most of the country. In the center-east of Myanmar, a right lateral strike slip fault extends from south to north across more than 1000 km. These tectonic zones are responsible for large earthquakes in the region. The India-Eurasia plate collision which initiated in the Eocene provides the last geological pieces of Myanmar, and thus Myanmar preserves a more extensive Cenozoic geological record as compared to records of the Mesozoic and Paleozoic eras. Myanmar is physiographically divided into three regions: the Indo-Burman Range, Myanmar Central Belt and the Shan Plateau; these all display an arcuate shape bulging westwards. The varying regional tectonic settings of Myanmar not only give rise to disparate regional features, but they also foster the formation of petroleum basins and a diverse mix of mineral resources.

Divergent double subduction is a special type of subduction system where two parallel subduction zones with different directions are developed on the same oceanic plate. In conventional plate tectonics theory, an oceanic plate subducts under another plate and new oceanic crust is generated somewhere else, commonly along the other side of the same plates However, in divergent double subduction, the oceanic plate subducts on two sides. This results in the closure of ocean and arc-arc collision. This concept was first proposed and applied to the Lachlan fold belt in southern Australia. Since then, geologists have applied this model to other regions such as the Solonker Suture Zone of the Central Asian Orogenic belt, the Jiangnan Orogen, the Lhasa–Qiangtang collision zone and the Baker terrane boundary. Active examples of this system are 1) the Molucca Sea Collision Zone in Indonesia, in which the Molucca Sea plate subducts below the Eurasian plate and the Philippine Sea plate on two sides, and 2) the Adria microplate in the Central Mediterranean, subducting both on its western side and on its eastern side . Note that the term "divergent" is used to describe one oceanic plate subducting in different directions on two opposite sides. It should not be confused with use of the same term in 'divergent plate boundary' which refers to a spreading center that separates two plates moving away from each other.

The geology of Sicily records the collision of the Eurasian and the African plates during westward-dipping subduction of the African slab since late Oligocene. Major tectonic units are the Hyblean foreland, the Gela foredeep, the Apenninic-Maghrebian orogen, and the Calabrian Arc. The orogen represents a fold-thrust belt that folds Mesozoic carbonates, while a major volcanic unit is found in an eastern portion of the island. The collision of Africa and Eurasia is a retreating subduction system, such that the descending Africa is falling away from Eurasia, and Eurasia extends and fills the space as the African plate falls into the mantle, resulting in volcanic activity in Sicily and the formation of Tyrrhenian slab to the north.

References

↑ Flinch, J.F. (1994). "Tectonic evolution of the Gibraltar Arc". Doctoral Thesis, Rice University.
↑ Miller, M.S.; Allam, A.A.; Becker, T.W.; Di Leo, J.F.; Wookey, J. (2013). "Constraints on the tectonic evolution of the westernmost Mediterranean and northwestern Africa from shear wave splitting analysis". Earth and Planetary Science Letters. 375: 234–243. doi:10.1016/j.epsl.2013.05.036.
1 2 3 4 Fernández-Ibañez, F.; Soto, J.I.; Zoback, M.D.; Morales, J. (2007). "Present-day stress field in the Gibraltar Arc (western Mediterranean)". Journal of Geophysical Research: Solid Earth. 112 (B8). doi:10.1029/2006jb004683.
1 2 3 Pedrera, A.; Ruiz-Constán, A.; Galindo-Zaldívar, J.; Chalouan, A.; Sanz de Galdeano, C.; Marín-Lechado, C.; Ruano, P.; Benmakhlouf, M.; Akil, M.; López-Garrido, A.C.; Chabli, A.; Ahmamou, M.; González-Castillo, L. (2011). "Is there an active subduction beneath the Gibraltar orogenic arc? Constraints from Pliocene to present-day stress field". Journal of Geodynamics. 52 (2): 83–96. doi:10.1016/j.jog.2010.12.003.
↑ Polyak, B.G.; Fernàndez, M.; Khutorskoy, M.D.; Soto, J.I.; Basov, I.A.; Comas, M.C.; Khain, V.Y.; Alonso, B.; Agapova, G.V.; Mazurova, I.S.; Negredo, A.; Tochitsky, V.O.; de la Linde, J.; Bogdanov, N.A.; Banda, E. (1996). "Heat flow in the Alboran Sea, western Mediterranean". Tectonophysics. 263 (1): 191–218. doi:10.1016/0040-1951(95)00178-6.

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[1] Flinch, J.F. (1994). "Tectonic evolution of the Gibraltar Arc". Doctoral Thesis, Rice University.

[2] Miller, M.S.; Allam, A.A.; Becker, T.W.; Di Leo, J.F.; Wookey, J. (2013). "Constraints on the tectonic evolution of the westernmost Mediterranean and northwestern Africa from shear wave splitting analysis". Earth and Planetary Science Letters. 375: 234–243. doi:10.1016/j.epsl.2013.05.036.

[Fernandez-3] 1 2 3 4 Fernández-Ibañez, F.; Soto, J.I.; Zoback, M.D.; Morales, J. (2007). "Present-day stress field in the Gibraltar Arc (western Mediterranean)". Journal of Geophysical Research: Solid Earth. 112 (B8). doi:10.1029/2006jb004683.

[Pedrera-4] 1 2 3 Pedrera, A.; Ruiz-Constán, A.; Galindo-Zaldívar, J.; Chalouan, A.; Sanz de Galdeano, C.; Marín-Lechado, C.; Ruano, P.; Benmakhlouf, M.; Akil, M.; López-Garrido, A.C.; Chabli, A.; Ahmamou, M.; González-Castillo, L. (2011). "Is there an active subduction beneath the Gibraltar orogenic arc? Constraints from Pliocene to present-day stress field". Journal of Geodynamics. 52 (2): 83–96. doi:10.1016/j.jog.2010.12.003.

[5] Polyak, B.G.; Fernàndez, M.; Khutorskoy, M.D.; Soto, J.I.; Basov, I.A.; Comas, M.C.; Khain, V.Y.; Alonso, B.; Agapova, G.V.; Mazurova, I.S.; Negredo, A.; Tochitsky, V.O.; de la Linde, J.; Bogdanov, N.A.; Banda, E. (1996). "Heat flow in the Alboran Sea, western Mediterranean". Tectonophysics. 263 (1): 191–218. doi:10.1016/0040-1951(95)00178-6.

Gibraltar Arc

Contents

Tectonic evolution

Geology

See also

Related Research Articles

References