Zagros fold and thrust belt
Encyclopedia
The Zagros fold and thrust belt (Zagros FTB) is a ~1800 km long zone of deformed crustal rocks, formed in the foreland of the collision
between the Arabian Plate
and the Eurasian Plate
. It is host to one of the world's largest petroleum provinces, containing about 49% of the established hydrocarbon reserves in fold and thrust belts and about 7% of all reserves globally.
mountains and the Lesser Caucasus mountains
to the north of the Iranian plateau
and along the zone formed by the Greater Caucasus mountains, the Apsheron-Balkan Sill and the Kopet Dag
mountains further north again.
accretionary wedge
to the southeast. The belt varies in width with two main salients in the Lorestan and Fars domains and two main embayments at Kirkuk and Dezful. The variation in geometry along strike is attributed to the distribution of the late Proterozoic Hormuz salt layer, with a thick and continuous salt being present beneath the salients and either missing, thin or discontinuous in the embayments. The distribution of the Hormuz salt is controlled by the extent of late Proterozoic basins. The belt is also divided into zones from northeast to southwest. Next to the Main Zagros Reverse Fault is a zone sometimes referred to as the 'High Zagros', the highest part of the Zagros Mountains reaching heights in excess of 4500 m, with the 'High Zagros Fault' forming its southwestern boundary. The next zone between the High Zagros Fault and the Main Frontal Fault (also Mountain Front Flexure) is known as the 'Simply Folded Zone', characterised by many elongate folds and very few surface faults. The zone to the southwest of the Main Frontal Fault is considered to be part of the foreland basin although active structures are observed as far out as the Zagros Frontal Fault.
levels and shallower one between the Dashtak décollement and the surface. There is also evidence of a thick-skinned element to the deformation. The lack of Hormuz salt exposed at the surface makes the nature of the basal detachment less certain than in the Fars domain, but the presence of this salt layer is inferred from the observation that the taper angle (the angle between the basal detachment and the current topographic slope) is identical to that where the salt is proven further south.
s, although it may be missing over the Fars platform, a continuation of an area of the foreland in which the salt layer is not present.
and the area of lowland occupied by the alluvial plain
of the Tigris–Euphrates river system, known as the 'Mesopotamian Basin', together represent the active foreland basin
to the Zagros FTB, caused by the loading of the leading edge of the Arabian Plate by the Zagros thrust sheets. The gulf is being progressively infilled by the southeastward prograding delta of the river system.
oceanic crust
continued along this portion of the Eurasian Plate until continental crust
of the Arabian Plate became involved in this convergent boundary
. The exact timing of the onset of the subsequent collision is uncertain although there is evidence of some deformation during the deposition of the Asmari Formation in the Oligocene
, or possibly as early as the Late Eocene
. Deformation within the Zagros FTB has continued from then until the present day, although unconformities of regional extent suggest that there have been several distinct phases of deformation. Unconformities have been recognised at the base and top of the Asmari Formation (Late Eocene and Early Miocene), at the base of the Agha Jari Formation (late mid-Miocene) at the base of the Bakhtyari Formation (latest Pliocene) and mid Pleistocene, interpreted to date these separate phases. These deformation pulses were associated with southwestward migration of the active deformation front.
There is no evidence of continued subduction of oceanic crust beneath the Eurasian Plate along this part of the plate boundary, in contrast to the neighbouring segment beneath the Makran where a dipping slab is well imaged by seismic tomography
. The ending of subduction has been linked to evidence that the Main Zagros Reverse Fault is no longer active, suggesting that further deformation is occurring by distributed deformation of the leading edge of the Arabian Plate. This is consistent with observations of recent seismicity.
with over ten billion barrels of remaining oil reserves as of 1998.
Continental collision
Continental collision is a phenomenon of the plate tectonics of Earth that occurs at convergent boundaries. Continental collision is a variation on the fundamental process of subduction, whereby the subduction zone is destroyed, mountains produced, and two continents sutured together...
between the Arabian Plate
Arabian Plate
The Arabian Plate is one of three tectonic plates which have been moving northward over millions of years and colliding with the Eurasian Plate...
and the Eurasian Plate
Eurasian Plate
The Eurasian Plate is a tectonic plate which includes most of the continent of Eurasia , with the notable exceptions of the Indian subcontinent, the Arabian subcontinent, and the area east of the Chersky Range in East Siberia...
. It is host to one of the world's largest petroleum provinces, containing about 49% of the established hydrocarbon reserves in fold and thrust belts and about 7% of all reserves globally.
Plate tectonic setting
The Zagros FTB is formed along a section of the plate boundary that is subject to oblique convergence with the Arabian Plate moving northwards with respect to the Eurasian Plate at about 3 cm per year. The degree of obliqueness reduces southwards along the Zagros, with the collision becoming near orthogonal within the Fars domain. The relative movement between the plates is only partly taken up within the Zagros, the remainder is taken up by deformation in the AlborzAlborz
Alborz , also written as Alburz, Elburz or Elborz, is a mountain range in northern Iran stretching from the borders of Azerbaijan and Armenia in the northwest to the southern end of the Caspian Sea, and ending in the east at the borders of Turkmenistan and Afghanistan...
mountains and the Lesser Caucasus mountains
Caucasus Mountains
The Caucasus Mountains is a mountain system in Eurasia between the Black Sea and the Caspian Sea in the Caucasus region .The Caucasus Mountains includes:* the Greater Caucasus Mountain Range and* the Lesser Caucasus Mountains....
to the north of the Iranian plateau
Iranian plateau
The Iranian plateau, or Iranic plateau, is a geological formation in Southwest Asia. It is the part of the Eurasian Plate wedged between the Arabian and Indian plates, situated between the Zagros mountains to the west, the Caspian Sea and the Kopet Dag to the north, the Hormuz Strait and Persian...
and along the zone formed by the Greater Caucasus mountains, the Apsheron-Balkan Sill and the Kopet Dag
Kopet Dag
The Kopet Dag, Kopet Dagh, or Koppeh Dagh , also known as the Turkmen-Khorasan Mountain Range is a mountain range on the frontier between Turkmenistan and Iran, extending about 650 km along the border, east of the Caspian Sea. The highest peak of the range in Turkmenistan is southwest of the...
mountains further north again.
Geometry
The Zagros FTB extends for about 1800 km from the Bitlis suture zone in the northwest to the boundary with the MakranMakran
The present day Makran is a semi-desert coastal strip in the south of Sindh, Balochistan, in Iran and Pakistan, along the coast of the Arabian Sea and the Gulf of Oman. The present day Makran derived its name from Maka, a satrap of Achaemenid Empire....
accretionary wedge
Accretionary wedge
An accretionary wedge or accretionary prism is formed from sediments that are accreted onto the non-subducting tectonic plate at a convergent plate boundary...
to the southeast. The belt varies in width with two main salients in the Lorestan and Fars domains and two main embayments at Kirkuk and Dezful. The variation in geometry along strike is attributed to the distribution of the late Proterozoic Hormuz salt layer, with a thick and continuous salt being present beneath the salients and either missing, thin or discontinuous in the embayments. The distribution of the Hormuz salt is controlled by the extent of late Proterozoic basins. The belt is also divided into zones from northeast to southwest. Next to the Main Zagros Reverse Fault is a zone sometimes referred to as the 'High Zagros', the highest part of the Zagros Mountains reaching heights in excess of 4500 m, with the 'High Zagros Fault' forming its southwestern boundary. The next zone between the High Zagros Fault and the Main Frontal Fault (also Mountain Front Flexure) is known as the 'Simply Folded Zone', characterised by many elongate folds and very few surface faults. The zone to the southwest of the Main Frontal Fault is considered to be part of the foreland basin although active structures are observed as far out as the Zagros Frontal Fault.
Kirkuk embayment
In northern Iraq the Zagros FTB is relatively narrow and this area is referred to as the Kirkuk embayment. This part of the fold belt lacks an effective basal Hormuz salt detachment.Lorestan domain
The Lorestan (or Lurestan) domain forms the northerly of the two main salients in the Zagros FTB. The structural style of the simply folded zone is interpreted here to be dominated by detachment folding with some degree of disharmonic folding. The disharmony occurs between a deeper layer bounded by the basal and Dashtak Formation décollementDecollement
Décollement is a gliding plane between two rock masses. In French, "décoller" means "to detach from" or "to rip off" and was first used by geologists studying the structure of the Swiss Jura Mountains, but is also known as a detachment zone. This is a structure of strata owing to deformation,...
levels and shallower one between the Dashtak décollement and the surface. There is also evidence of a thick-skinned element to the deformation. The lack of Hormuz salt exposed at the surface makes the nature of the basal detachment less certain than in the Fars domain, but the presence of this salt layer is inferred from the observation that the taper angle (the angle between the basal detachment and the current topographic slope) is identical to that where the salt is proven further south.
Dezful embayment
Between the two main salients of the Zagros FTB, the Dezful embayment developed in an area that lacked an effective basal Hormuz salt detachment, resulting in a steeper topographic slope of 2°, compared to 1° for both the Lorestan and Fars domains. During the Miocene this area became a depocentre in which locally thick Gachsaran salt was deposited. The presence of locally thick Gachsaran salt has caused disharmonic folding between the sequences above and below that layer.Fars domain
The Fars salient forms the southeastern end of the Zagros FTB. This area is underlain by a thick Hormuz salt layer that comes to the surface in several places where it extrudes from the crests of anticlines, forming salt glacierSalt glacier
A salt glacier is a flow of salt that is created when a rising diapir in a salt dome breaches the surface. Gravity causes the salt to flow like glaciers into adjacent valleys. Most of the flow occurs during the winter, when the salt is wet, as the strength of salt is critically dependent on its...
s, although it may be missing over the Fars platform, a continuation of an area of the foreland in which the salt layer is not present.
Kazerun fault system
This dextral fault system transfers some of the dextral displacement along the Main Recent Fault onto thrust faults and folds of the Fars domain as the relative motion changes from strongly oblique to near orthogonal. It also forms the effective southeastern boundary to the Dezful embayment. In detail the Kazerun fault system consist of a series of en echelon segments within an overall fan shaped zone. From the focal depth of earthquakes along this zone it is clear that these faults are developed within the underlying basement rocks.Persian Gulf
The Persian GulfPersian Gulf
The Persian Gulf, in Southwest Asia, is an extension of the Indian Ocean located between Iran and the Arabian Peninsula.The Persian Gulf was the focus of the 1980–1988 Iran-Iraq War, in which each side attacked the other's oil tankers...
and the area of lowland occupied by the alluvial plain
Alluvial plain
An alluvial plain is a relatively flat landform created by the deposition of sediment over a long period of time by one or more rivers coming from highland regions, from which alluvial soil forms...
of the Tigris–Euphrates river system, known as the 'Mesopotamian Basin', together represent the active foreland basin
Foreland basin
A foreland basin is a depression 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 lithosphere to bend, by a process known as lithospheric flexure...
to the Zagros FTB, caused by the loading of the leading edge of the Arabian Plate by the Zagros thrust sheets. The gulf is being progressively infilled by the southeastward prograding delta of the river system.
Development of the Zagros fold and thrust belt
Northeastward subduction of TethyanTethys Ocean
The Tethys Ocean was an ocean that existed between the continents of Gondwana and Laurasia during the Mesozoic era before the opening of the Indian Ocean.-Modern theory:...
oceanic crust
Oceanic crust
Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium...
continued along this portion of the Eurasian Plate until continental crust
Continental crust
The continental crust is the layer of igneous, sedimentary, and metamorphic rocks which form the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial due to more felsic, or granitic, bulk composition, which lies in...
of the Arabian Plate became involved in this convergent boundary
Convergent boundary
In plate tectonics, a convergent boundary, also known as a destructive plate boundary , is an actively deforming region where two tectonic plates or fragments of lithosphere move toward one another and collide...
. The exact timing of the onset of the subsequent collision is uncertain although there is evidence of some deformation during the deposition of the Asmari Formation in the Oligocene
Oligocene
The Oligocene is a geologic epoch of the Paleogene Period and extends from about 34 million to 23 million years before the present . As with other older geologic periods, the rock beds that define the period are well identified but the exact dates of the start and end of the period are slightly...
, or possibly as early as the Late Eocene
Eocene
The Eocene Epoch, lasting from about 56 to 34 million years ago , is a major division of the geologic timescale and the second epoch of the Paleogene Period in the Cenozoic Era. The Eocene spans the time from the end of the Palaeocene Epoch to the beginning of the Oligocene Epoch. The start of the...
. Deformation within the Zagros FTB has continued from then until the present day, although unconformities of regional extent suggest that there have been several distinct phases of deformation. Unconformities have been recognised at the base and top of the Asmari Formation (Late Eocene and Early Miocene), at the base of the Agha Jari Formation (late mid-Miocene) at the base of the Bakhtyari Formation (latest Pliocene) and mid Pleistocene, interpreted to date these separate phases. These deformation pulses were associated with southwestward migration of the active deformation front.
There is no evidence of continued subduction of oceanic crust beneath the Eurasian Plate along this part of the plate boundary, in contrast to the neighbouring segment beneath the Makran where a dipping slab is well imaged by seismic tomography
Seismic tomography
Seismic tomography is a methodology for estimating the Earth's properties. In the seismology community, seismic tomography is just a part of seismic imaging, and usually has a more specific purpose to estimate properties such as propagating velocities of compressional waves and shear waves . It...
. The ending of subduction has been linked to evidence that the Main Zagros Reverse Fault is no longer active, suggesting that further deformation is occurring by distributed deformation of the leading edge of the Arabian Plate. This is consistent with observations of recent seismicity.
Economic importance
The Zagros FTB contains 49% of the world's hydrocarbon reserves hosted in fold and thrust belts and about 7% of all reserves. The Zagros province includes many giant and supergiant oilfields, such as the Kirkuk FieldKirkuk Field
Kirkuk Field is an oilfield at Baba Gurgur near Kirkuk, Iraq. It was discovered in 1927. The Kirkuk oil field was brought into use by the Iraq Petroleum Company in 1934 and has ever since remained the basis of northern Iraqi oil production with over of proven remaining oil reserves as of 1998...
with over ten billion barrels of remaining oil reserves as of 1998.