Geology of Iran

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.

Some important features of Zagros include: Absence of magmatic and metamorphic events after Triassic, and low abundance of the outcrops of Paleozoic rocks.

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.

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.

The oldest units include upper Precambrian Lower Cambrian schists overlain by Permian limestone and other Paleozoic sedimentary rocks.

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 northeastern active fold belt of Iran, the Kopeh Dagh, is formed on the Hercynian metamorphosed basement at the southwestern margin of 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 Kopeh Dagh Mountains.

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 Alborz mountain range is only 60–130 km wide and consists of sedimentary series dating from Upper Devonian to Oligocene, prevalently Jurassic limestone over a granite core.

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 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 significant structural event occurring in Early Devonian was accompanied by faulting and fragmentation that led to a different sedimentary facies in Azerbaijan.

An important post-Pan-African magmatism is documented by the widespread Doran Granite, which cuts the Upper Precambrian rocks and is covered by Lower Cambrian sediments.

The oldest rocks in Iran belong to the Kushk Series consisting of clastic sediments, acidic volcanic, tuff, and carbonates (mainly dolomite).

Middle Cambrian is characterized by uplift and regression; however, a renewed progression at this time led to the deposition of Mila and Kuhbonan Formations, consisting of limestone, dolomite, and shale, over older units.

Transition from Middle to Upper Triassic coincides with Early Cimmerian orogenic episode, which led to the segmentation of the sedimentary basin into three sub-basins: Zagros in the south and southwest, Alborz in the north, and Central Iran.

In North and Central Iran, the Upper Triassic and Lower–Middle Jurassic sediments have a detrital nature, consisting mainly of shale and sandstone with thicknesses varying from a few meters to more than 3,000 metres (9,800 ft).

In Late Cretaceous, tectonic movements related to the Laramide orogeny affects most parts of Iran, leading to uplift, folding, and faulting.

The Oligocene and Miocene stages are characterized by rapid subsidence, deposition, and facies changes in both marine and continental sedimentary basins (e.g., Mahneshan and Halab south of Zanjan).

The Mesozoic magmatic rocks are associated with Cimmerian and Laramide orogenic events that caused continental and oceanic rifting, followed by closures and collisions in vast areas of Iran (e.g., Sanandaj–Sirjan).

Data from various structural zones indicate that the volcanic and plutonic activities started in the Late Cretaceous, peaked in the Eocene, and continued, with short stops, into the Quaternary.

Ultramafic and mafic rocks of Upper Paleozoic occur as metamorphosed as well as non-metamorphosed bodies in some areas like Fariman, Shanderman, and Asalem.

The Neo-Tethys opening and rifting of Iran from the northern margin of Gondwana has probably occurred sometime between the latest Viséan to early Late Carboniferous (Bashkirian).

Many diverse features are associated with this phase, including metamorphism, magmatism, folding, faulting, creation of new basins, and facies change.

The compressional phase, happening in the Late Triassic, finally led to the closure of the Paleotethys (e.g., southeast to southwest of Caspian Sea).

This event is represented by folding, facies changes in sedimentary environments, angular unconformity, magmatism, and metamorphism (e.g., Alborz, Sanandaj–Sirjan, and Central Iran).

The compressional regime, that was associated with significant intrusive magmatic activities, led to the closure of the oceanic basins and Neothetyan rifts.

In some areas, slices of the oceanic crust have obducted onto the continental margins producing what could be called ophiolite assemblages or coloured melanges (e.g., mostly seen suture zone between Sanandaj–Sirjan and Zagros, and alongside Nehbandan fault in the east of Iran).