Hajar Mountains

The Hajar Mountains (Arabic: جِبَال ٱلْحَجَر, romanized: Jibāl al-Ḥajar, The Rocky Mountains or The Stone Mountains) are one of the highest mountain ranges in the Arabian Peninsula, shared between northern Oman and eastern United Arab Emirates. Also known as "Oman Mountains", they separate the low coastal plain of Oman from the high desert plateau, and lie 50–100 km (31–62 miles) inland from the Gulf of Oman.

The Hajar Mountains extend for 700 kilometres (430 miles) through the UAE and Oman. They are located on the north-east corner of the Arabian Plate, reaching from the Musandam Peninsula through to the east coast of Oman. The range is about 100 km (62 mi) wide, with Jabal Shams being the highest peak at 3,009 m (9,872 ft) in the central region of the mountains.

Currently, the Arabian Plate is moving north relative to the Eurasian Plate at 2–3 cm (0.79–1.18 in) per year. Continental collision is occurring at the Zagros fold and thrust belt west of the Musandam Peninsula. This collisional plate boundary transitions into a subduction zone, towards the east. Here, oceanic crust of the Arabian Plate is subducted northwards beneath Eurasia, called the Makran subduction zone.

The Hajar Mountains are the product of polyphase mountain building. Uplift and deformation of the Arabian passive margin began during the late cretaceous as the African-Arabian Plate began to subduct under the South Tethyan Oceanic Plate imitated at an intra oceanic subduction zone. This initiation may have been the result of plate rotation due to the breakup of Gondwana.

Similar to the modern convergence of the Australian passive margin under Eurasian oceanic crust, Arabian passive margin sediments became highly deformed and shortened forming an imbricated thrust belt. This also coincided with the emplacement of the Semail Ophiolite. By the early Maastrichtian, deformation ceased, and stable continental shelf conditions resumed.

A second episode of deformation began during the Eocene Epoch around 45-40 Ma. This episode saw the reactivation of cretaceous thrust faults and the development of long and short wavelength folding of Paleocene marine sediments that infilled previous foredeep accommodation. Low temperature thermodchronometry of apatite grains has given ages to this deformation and subsequent exhumation of the mountain belt. Exhumation occurred in two states, first between 45 and 40 Ma and again from 20 to 15 Ma. This later unroofing could also be related to tectonic uplift in the nearby Zagros Mountains of Iran.

The geology of the Hajar can be grouped into four major tectonostratigraphic groups. Group one are the pre-Permian basement rocks, a sedimentary sequence of clastics, carbonates and evaporites. Group two are a middle Permian to Late Cretaceous sequence of continental shelf carbonates, which were deposited unconformably above the basement. Group three are a series of nappes (allochthonous rocks) that were transported from the northeast to the southwest horizontally for more than 300 km (190 mi). This was a major tectonic event during the late Cretaceous. This process is called obduction, where Permian to middle Cretaceous continental slope-rise (shallow to deep marine) sedimentary rocks and late Cretaceous oceanic crust (Semail ophiolite) were thrust (obducted) above the rocks from groups one and two. Lastly, group four are late Cretaceous to Miocene shallow marine and terrestrial sedimentary rocks that were deposited on top of all three previous groups.

The high topography is around two major culminations: Jabal Akhdar and Saih Hatat, which are large scale anticlines. The Saih Hatat culmination contains eclogite in the northeast at As Sifah. These rocks were subducted to about 80 km (50 mi) depth into the mantle, and then exhumed back to the surface. This exhumation event created possibly the largest megasheath fold on Earth, the Wadi Mayh megasheath fold. The common view is that these eclogites were originally basic volcanic rocks within the leading edge of the continental crust of the Arabian Plate. This leading edge was then subducted by a NE-dipping subduction zone. However, some geologists have interpreted that these eclogites were subducted through a SW-dipping subduction zone.