The Gibraltar Arc is a geological region corresponding to an arcuate
orogen
An orogenic belt, or orogen, is a zone of Earth's crust affected by orogeny. An orogenic belt develops when a continental plate crumples and is uplifted to form one or more mountain ranges; this involves a series of geological processes collect ...
surrounding the
Alboran Sea, between the
Iberian Peninsula
The Iberian Peninsula (),
**
* Aragonese and Occitan: ''Peninsula Iberica''
**
**
* french: Péninsule Ibérique
* mwl, Península Eibérica
* eu, Iberiar penintsula also known as Iberia, is a peninsula in southwestern Europe, defi ...
and
Africa
Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...
. It consists of the
Betic Cordillera (south Spain), and the
Rif
The Rif or Riff (, ), also called Rif Mountains, is a geographic region in northern Morocco. This mountainous and fertile area is bordered by Cape Spartel and Tangier to the west, by Berkane and the Moulouya River to the east, by the Mediterrane ...
(North Morocco). The Gibraltar Arc is located at the western end of the Mediterranean
Alpine belt and formed during the Neogene due to convergence of the Eurasian and African plates.
Maximum altitudes of the region are reached at the
Mulhacén
Mulhacén (), with an elevation of , is the highest mountain in peninsular Spain and in all of 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 Hacé ...
peak at the Cordillera Betica. Precipitation is collected mainly by the
Guadalquivir
The Guadalquivir (, also , , ) is the fifth-longest river in the Iberian Peninsula and the second-longest river with its entire length in Spain. The Guadalquivir is the only major navigable river in Spain. Currently it is navigable from the Gulf ...
(Betics) and
Sebou (Rif) rivers, which have delivered most sedimentary infill of the homonym sedimentary
foreland basin
A foreland basin is a structural basin that develops adjacent and parallel to a mountain belt. Foreland basins form because the immense mass created by crustal thickening associated with the evolution of a mountain belt causes the lithospher ...
s..
Tectonic evolution
North–south convergence of the Eurasian and African plates occurred during the middle
Oligocene
The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but the ...
to the late
Miocene
The Miocene ( ) is the first geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and means "less recent" ...
, followed by northwest–southeast convergence from the late
Tortonian
The Tortonian is in the geologic time scale an age or stage of the late Miocene that spans the time between 11.608 ± 0.005 Ma and 7.246 ± 0.005 Ma (million years ago). It follows the Serravallian and is followed by the Messinian.
The Tortonia ...
to present. The Gibraltar Arc was formed during the
Neogene
The Neogene ( ), informally Upper Tertiary or Late Tertiary, is a geologic period and system that spans 20.45 million years from the end of the Paleogene Period million years ago ( Mya) to the beginning of the present Quaternary Period Mya. ...
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°.
The eastward Gibraltar Arc oceanic
subduction
Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, ...
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
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 of ...
during the deposition of
accretionary units.
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.
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
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 o ...
mantle also has an arcuate bulge below the arc with extreme mantle thinning in the Alboran Sea,
which is the typical structure of a back-arc basin located on the concave side of an arcuate mountain belt.
A major left-lateral
strike-slip
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 tectonic ...
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.
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
Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...
. The External Zone, located on the outer side of the arc, is mostly made of sediments deposited on the
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 crea ...
s 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
Flysch () is a sequence of sedimentary rock layers that progress from deep-water and turbidity flow deposits to shallow-water shales and sandstones. It is deposited when a deep basin forms rapidly on the continental side of a mountain building ...
units from the
Cretaceous to Early Miocene are located between the External and Internal Zones. These were
folded and
thrust
Thrust is a reaction force described quantitatively by Newton's third law. When a system expels or accelerates mass in one direction, the accelerated mass will cause a force of equal magnitude but opposite direction to be applied to that syst ...
during the westward migration of the Internal Zones and can be interpreted as the palaeo-
accretionary wedge
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 ...
of the subduction system that was active during the Lower and Middle Miocene.
See also
*
Messinian salinity crisis
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 partial or nearly complete desiccation (dry ...
References
{{Regions of the world
Geology of Europe
Geology of Gibraltar
Geology of Spain
Geology of Africa
Geology of Morocco