Zagros mountains salt dome: © NASA Earth Science and Remote Sensing Unit, NASA Johnson Space Center
The Zagros Mountains make up a belt of deformed crustal rocks located in south eastern Iran. They extend for over 1500km from eastern Turkey in the northwest through to the Gulf of Oman in the southeast.
The Zagros Mountains form an important part of the Alpine-Himalayan mountain chain which spans much of south west Asia and the Middle East. This orogenic belt originated from collision of the African, Arabian and Indian continental plates with the Eurasian plate and the northward subduction and subsequent closure of the Neo-Tethys Ocean. The Zagros Mountains formed as a result of convergence between the Arabian plate and the Eurasian plate in the Late Cretaceous-Early Miocene. This process is still in operation today at a rate of roughly 25mm year-1, causing the Zagros Mountains and the Iranian Plateau to increase in height each year.
The formation of the Zagros Mountain belt is the result of a complex and lengthy orogeny beginning in the Palaeozoic and carrying through to present day. Prior to the onset of subduction in the Late Cretaceous, the Neo-Tethys Ocean was still an actively spreading narrow ocean basin between the Arabian and Eurasian Plates. The Zagros Mountains then formed a passive continental margin on the north eastern flank of the Arabian plate. Thick successions of shallow marine limestones, dolomites, siltstones, shales and salts were deposited on this continental shelf from the Palaeozoic through to the Cenozoic era.
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The Zagros folded zone: © Joshua Doubek
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As the Arabian plate began to move northwards, and the Neo-Tethys Ocean was subducted beneath the Eurasian plate, some slithers of oceanic crust, known as ophiolites, were obducted on top of the continental shelf sedimentary sequence. Further convergence generated stresses in the lithosphere and induced extensive folding and thrusting in these shelf sediments. The compressional stresses generated several large-scale, north-east dipping thrust faults which now separate the Zagros Mountains into five tectonic belts which run parallel to each other. Additionally, numerous northwest-southeast trending parallel folds developed, creating spectacular high-angled anticlines (arch-shaped folds with the oldest beds at the core) and mountain peaks that rise between 3000m -3650m above sea level.
Subsequent erosion has now removed the softer rocks, such as the mudstones and siltstones leaving more resistant limestone and dolomite rocks. This differential erosion formed the linear ridges and parallel valleys of the Zagros Mountains.
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Fars domain of the Zagros fold-thrust belt: © NASA
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The continental shelf deposition and tectonic history of the Zagros Mountain sediments were conducive to the formation and trapping of petroleum, today forming a globally significant petroleum reserve. Carbon-rich shales deposited on the Arabian continental shelf were heated as successive sedimentary layers were deposited above, leading to the generation of oil and gas. As folding and thrusting initiated in the region, these hydrocarbons became trapped in the strongly folded anticlines in places where impermeable sedimentary layers overlay porous reservoir rocks. Oil fields in the Zagros mountain chain are therefore elongate and parallel to the northwest-southeast trending folds.
Further reading:
* Berra and Angiolini (2014) The evolution of the Tethys region throughout the Phanerozoic: A brief tectonic reconstruction, in Petroleum systems of the Tethyan region: AAPG Memoir 106, p. 1–27.
* Hessami et al. (2005) Active deformation within the Zagros Mountains deduced from GPS measurements, Journal of the Geological Society, 163, 143-148.