Considering geological and structural units of Iran, three main structural units or zones could be distinguished in Iran. These units or zones are separated from each other by ophiolite–bearing sutures. Other criteria such as structural style, crustal character and age of basement consolidation, age and intensity of deformation, age and nature of magmatism, are used to subdivide these major zones into smaller elements. The three major units and their main constituents could be defined as the southern, central and northern units. The southern unit has a crystalline basement consolidated in the Precambrian time and a platform–type Paleozoic development and comprises the Zagros folded belt, southern and southwestern parts of the Zagros. This section made a part of the Arabian Plate that was located on the margin of Rodinia and Pannotia in the Neoproterozoic, and of Gondwana in the Paleozoic and Mesozoic. The central unit, which comprises the central Iran and the Alborz, is interpreted as an assemblage of fragments that were in the vicinity of the Arabian Plate and formed a marginal section of Rodinia and Pannotia in the Neoproterozoic and of Gondwana in the Cambrian, Ordovician, Silurian and Devonian. These fragments were detached from Gondwana in the Carboniferous. They were submerged, moved northwards, and were finally attached to the Eurasian section of the northern supercontinent Laurasia in the Late Triassic. These fragments or microplates were fused together and made the Iranian Plate which was rejoined by Gondwanic Afro-Arabia in the Late Cretaceous. As Afro-Arabia moved northwards towards Eurasia, the Arabian Plate ultimately collided with the Iranian Plate in the Miocene. Finally there is the northern unit which is separated from the central unit by the North Iran Suture. It is characterized by continental crust including remnants of more or less cratonized former Paleozoic oceanic crust that seems to reflect the Paleotethys. The northern unit represents a marginal strip of the Hercynian realm of Central Asia- broadly overlapped by the Alpine realm. It was deformed and largely consolidated by the Early Cimmerian folding and the Late Alpine folding. The northern unit comprises the South Caspian Depression and the Kopet Dagh Range.
These three main structural units are divided into some smaller geological and structural subdivisions which include the following zones:
This zone extends from Bandar Abbas in the south to Kermanshah in the northwest and continues through to Iraq. Zagros is in fact the northeastern edge of the Arabian plate. Some important features of Zagros include: Absence of magmatic and metamorphic events after Triassic, and low abundance of the outcrops of Paleozoic rocks. Structurally, it consists of large anticlines and small synclines and continuous marine sedimentation from the Carboniferous to the Miocene. On the whole, a sequence of Precambrian to Pliocene rocks about 8-10 kilometres in thickness has undergone folding from the Miocene to the Recent time in the Zagros Mountains. The Zagros fold and thrust belt was formed by the collision of two tectonic plates — the Iranian Plate and the Arabian Plate. This collision primarily happened during the Miocene and folded the entire rocks that had been deposited from the Carboniferous to the Miocene in the geosyncline in front of the Iranian Plate. The process of collision continues to the present and as the Arabian Plate is being pushed against the Iranian Plate, the Zagros Mountains and the Iranian Plateau are getting higher and higher. The Zagros mountain range, itself, has a totally sedimentary origin and is made primarily of limestone. In the Elevated Zagros or the Higher Zagros, the Paleozoic rocks could be found mainly in the upper and higher sections of the peaks of the Zagros Mountains along the Zagros main fault. On the both sides of this fault, there are Mesozoic rocks, a combination of Triassic and Jurassic rocks that are surrounded by Cretaceous rocks on the both sides. The Folded Zagros (the mountains south of the Elevated Zagros and almost parallel to the main Zagros fault) is formed mainly of Tertiary rocks, with the Paleogene rocks south of the Cretaceous rocks and then the Neogene rocks south of the Paleogene rocks.
This zone is located to the south-southwest of Central Iran and the northeastern edge of Zagros range. In the north and northeast, this zone is separated from Central Iran by depressions like Lake Orumiyeh, Gavkhouni and faults like Shahr-e-Babak and Abadeh, and to the south-southwest by the main thrust fault of Zagros. A striking feature of this zone is the presence of immense volumes of magmatic and metamorphic rocks of Paleozoic and Mesozoic eras. As far as the trends, and particularly the folding style is concerned, some researchers consider the Sanandaj–Sirjan Zone as being similar to Zagros; however, considerable differences exist in rock types, magmatism, metamorphism, and orogenic events. There are some similarities between Sanandaj–Sirjan and Central Iran.
Sahand–Bazman Volcanic Belt
This volcanic belt, which is usually called the Central Iranian Range, runs east and almost parallel to the Sanandaj–Sirjan Zone, and owes its existence to the widespread and intensive volcanic activity which developed on the Iranian plate from the Upper Cretaceous to Recent time. The peak of this volcanism happened in the Eocene. The Sahand-Bazman volcanic belt is supposed to have resulted from the collision of the Arabian and Central Iranian continental plate margins. It is represented by sub-alkaline volcanics that vary in composition from basaltic through andesitic to rhyolitic composition.
Located in a triangle in the middle of Iran, Central Iran is one of the most important and complicated structural zones in Iran. In this zone, rocks of all ages, from Precambrian to Quaternary, and several episodes of orogeny, metamorphism, and magmatism can be recognized. Central Iran in a broad sense, comprising the whole area between the North and South Iranian ranges. Within the Iranian plate the Central-East Iran microplate is bordered by the Great Kavir Fault in the north, by the Nain–Baft Fault in the west and southwest and by the Harirud Fault in the east. It is surrounded by the Upper Cretaceous to Lower Eocene ophiolite and ophiolitic melange. The microplate consists of different structural components; Kerman-Tabas Block, Yazd Block and Anarak-Khur Block.
Eastern Iran can be divided into two parts: Lut Block and Flysch Zone (flysch or coloured melange of Zabol–Baluch Zone). Located to the west of Zabol–Baluch Zone, Lut Block is the main body of Eastern Iran. Lut Block extends for about 900 km in a north–south direction. It is bounded in the north by Dorooneh fault and in the south by Jazmurian depression. In the east, it is separated from Flysch Zone by the Nehbandan fault, whereas the western boundary with Central Iran is Nayband fault and Shotori Mountains. The oldest units include upper Precambrian Lower Cambrian schists overlain by Permian limestone and other Paleozoic sedimentary rocks. Flysch Zone (Zabol–Baluch) is located between Lut Block to the west and Helmand (in Afghanistan) to the east. In contrast to Lut Block, the Flysch Zone is highly deformed and tectonized and consists of thick deep-sea sediments like argillaceous and silicic shales, radiolarite, and pelagic limestone and volcanic rocks such as basalt, spilitic basalt, diabase, andesite, dacite, rhyolite, and subordinate serpentinized ultramafic rocks. The basement is likely composed of an oceanic crust. Most rock units in this zone fall into three main groups: flyschoid sediments; volcanic, volcanosedimentary, and intrusive rocks; and ophiolitic series.
Southeastern Iran or Makran
Southeastern Iran or Makran zone is located to the south of Jazmurian depression. Its western boundary is Minab fault; to the south, it is restricted by the Gulf of Oman, and to the east, it extends into Pakistan. The northern part is characterized by dominance of east–west trending faults, Bashagard fault being the most important one. Along these faults lies large section of ophiolite series. The oldest rocks in this zone are the ophiolites of Late Cretaceous–Paleocene overlain by a thick sequence (about 5,000 m) of sandstone, shale, and marl. The whole sequence is deformed prior to Early Miocene. Thick sequence of Neogene rock units, in excess of 5,000 m, covers the older series.
The northeastern active fold belt of Iran, the Kopet Dagh, is formed on the Hercynian metamorphosed basement at the southwestern margin of the Turan Platform. The belt is composed of about 10 kilometres of Mesozoic and Tertiary sediments (mostly carbonates) and, like the Zagros, was folded into long linear northwest-southeast trending folds during the last phase of the Alpine Orogeny, in the Miocene and Plio-Pleistocene time. No magmatic rocks are exposed in Kopet Dagh except for those in the basement in the Aghdarband and some Triassic basic dikes. This basin was located in the northeastern Iran. From Middle Jurassic, it was covered with a vast continental shelf sea. In this period of time and due to transgression as well as rapid subsidence basin, the western part became deeper. In this basin, a thick sequence of continuous marine and continental sediments was deposited (about 10 km). No major sedimentary gap or volcanic activities during Jurassic to Miocene have ever been reported. This sedimentary complex provides suitable conditions for accumulation of hydrocarbons. Kopet Dagh sedimentary rocks were placed in their current position due to uplifting at the end of the Miocene. The Kopet Dag Range, itself, is made chiefly of Cretaceous rocks with a smaller portion of Jurassic rocks in the southeastern parts. The mountains were mainly formed in the Miocene during the Alpine orogeny. As the Tethys Sea was closed and the Arabian Plate collided with the Iranian Plate and was pushed against it, and with the clockwise rotation of the Eurasian Plate towards the Iranian Plate and their final collision, the Iranian Plate was pressed against the Turan Platform. This collision folded the entire rocks that had been deposited in this geosyncline or basin from the Jurassic to the Miocene and formed the Kopet Dag Mountains.
The Alborz mountain range forms a barrier between the south Caspian and the Iranian plateau. This range is located in northern Iran, parallel to the southern margin of Caspian Sea. Alborz is characterized by the dominance of platform-type sediments, including limestone, dolostone, and clastic rocks. Rock units from Precambrian to Quaternary have been identified, with some hiatuses and unconformities in Paleozoic and Mesozoic. Unlike its northern and southern boundaries, (Caspian Sea and Central Iran, respectively) there is not a consensus regarding the eastern and the western limits of Alborz. The Binalud Mountains in the east, although the continuation of the Alborz, bear features comparable to those of Central Iran. The Alborz mountain range is only 60–۱۳۰ km wide and consists of sedimentary series dating from Upper Devonian to Oligocene, prevalently Jurassic limestone over a granite core. Continental conditions regarding sedimentation are reflected by thick Devonian sandstones and by Jurassic shales containing coal seams. Marine conditions are reflected by Carboniferous and Permian strata that are composed mainly of limestones. In the Eastern Alborz Range, the far eastern section is formed by the Mesozoic (chiefly Triassic and Jurassic) rocks, while the western part of the Eastern Alborz Range is made primarily of the Paleozoic rocks. The Precambrian rocks can be found chiefly south of the city of Gorgan situated in the southeast of the Caspian Sea and in much smaller portions in the central and western parts of the Central Alborz Range. The central part of the Central Alborz Range is formed predominantly of the Triassic and Jurassic rocks, while the northwestern section of the range is made chiefly of Jurassic rocks. Very thick beds of the Tertiary (mostly of the Eocene) green volcanic tuffs and lavas are found mainly in the southwestern and south-central parts of the range. The far northwestern part of the Alborz that constitutes what is called the Western Alborz Range or the Talish Mountains is made mainly of the Upper Cretaceous volcano-sedimentary deposits with a strip of Paleozoic rocks and a band of Triassic and Jurassic rocks in the southern parts, both in a northwest-southeast direction. With the northward movement of Africa and the Arabian plate and with the closure of the Tethys Sea as the Arabian Plate collided with the Iranian Plate and was pushed against it, and with the clockwise movement of the Eurasian Plate towards the Iranian Plate and their final collision, the Iranian Plate was pressed from both sides. The collisions finally caused the folding of the Upper Paleozoic, Mesozoic, and Paleogene rocks, and the Cenozoic (chiefly the Eocene) volcanism to form the Alborz Mountains mainly in the Miocene. The Alpine orogeny began, therefore, with Eocene volcanism in southwestern and south-central parts of the Alborz and continued with the uplift and folding of the older sedimentary rocks in the northwestern, central and eastern parts of the range during the orogenic phases of importance that date from the Miocene and the Pliocene epochs.
There is no agreement regarding the geological setting of Azerbaijan. According to some authors, the northeastern corner could be included in Alborz and the southeastern part in Sanandaj–Sirjan. Some believe that most of Azerbaijan lies in a zone called Azerbaijan–Alborz, and as they indicate, this zone is bounded in the north by Alborz fault, in the west by Tabriz–Urumiyeh fault, and in the south by Semnan fault. According to the some authors, the northern part of Azerbaijan continues to the Caucasus Mountains in Caucasia and the Pontus Mountains in Turkey and the Southern Azerbaijan is comparable with Central Iran and Western Iran and extends to the Taurus Mountains in Turkey. The significant structural event occurring in Early Devonian was accompanied by faulting and fragmentation that led to a different sedimentary facies in Azerbaijan. This orogenic episode generated the Tabriz fault, extending in a northwest–southeast direction from Zanjan depression to the northern mountains of Tabriz (Mishu, Morou) and northwest of Azerbaijan and the Caucasus. This event divided Azerbaijan into two blocks, one block in the northeast with subsidence and sedimentation in Early Devonian and the other in the southwest which remained high until Late Carboniferous.