Hajar Mountains
The Hajar 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 inland from the Gulf of Oman.
Geology
Orography and tectonic setting
The Hajar Mountains extend for 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 wide, with Jabal Shams being the highest peak at in the central region of the mountains.Currently, the Arabian Plate is moving north relative to the Eurasian Plate at 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.
Formation
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.
Lithology
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 that were transported from the northeast to the southwest horizontally for more than. This was a major tectonic event during the late Cretaceous. This process is called obduction, where Permian to middle Cretaceous continental slope-rise sedimentary rocks and late Cretaceous oceanic crust were thrust 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.Structures
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 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.The two culminations are separated by the Semail Gap. This is a prominent linear structure, trending NNE—SSW. However, it is still debated as to what this structure is. Different geologists claim that it is a left-lateral strike-slip fault, a normal fault, a lateral ramp, a monocline due to a blind thrust, or a fault with multiple phases of deformation.
There is some debate over whether the topography of the Hajar Mountains is due to thin or thick-skinned tectonics. Recent Bouguer gravity and magnetotelluric geophysical data suggest deep basement faults that have been activated as thrust faults during the collisional event that obducted the Semail Ophiolite. This has resulted in a much shallower depth to basement in the Hajar Mountains and a deeper depth to basement in the foreland basin to the west. This has implications on economic geology such as the mining for precious minerals and the extraction of oil and gas.
Modern topography
The late Cretaceous obduction event created the proto-Hajar Mountains. However, this topography subsided and shallow marine sedimentation covered the region, beginning in the Paleocene. Paleocene to Eocene sedimentary rocks are found at above sea level within the Hajar, and are folded. This indicates that the present day topography formed after the late Eocene. The exact timing is debated, and various interpretations indicate the topography formed anywhere between the late Eocene through to the Miocene.The driving forces that formed the Hajar is also debated. Many geologists relate the Zagros Collision as the reason for the uplift forming the mountains, as currently the Musandam Peninsula is uplifting due to this collision. However, Jabal Shams, the highest peak of the central mountains is over away from this zone. In addition, there is no major seismicity within the central mountains, indicating that the mountains are not currently deforming, even though the Zagros collision is. This indicates that the uplift that created the present day topography occurred in the past, possibly before the initiation of the Zagros collision, by a mechanism that is not fully understood.
Geoconservation
Oman's geological record is valuable to geologists. It contains the most complete ophiolite on Earth, of which it is most well known for among geologists. The ophiolite sequence has pillow basalt, as well as exposures of the fossil crust-mantle boundary. Generally, ophiolites are obducted prior to continental collision, which highly deforms the structure of the original oceanic crust. However, because continental collision has not occurred in the Hajar, the Semail ophiolite is still intact. Oman also has one of the best exposed mega-sheath folds ever discovered, the Wadi Mayh sheath fold. Additionally, the relatively small outcrop of eclogite is important. Eclogite is rare on the Earths surface, as it is a rock that forms at high pressures deep within the crust or mantle. Geologists can learn about what is occurring in the Earths interior and tectonic processes from these rocks. There are also various fossil localities in Oman. There is concern in the geological community that with the development of infrastructure these rocks that contain a great deal of information will be excavated and destroyed.Geography
Central Hajar
The central section of the Hajar is the highest and wildest terrain in the country. Jabal Shams is the highest of the range, followed by Jebel Akhdar. The latter and the smaller Jebel Nakhl range are bounded on the east by the low Samail Valley.Eastern Hajar
East of Samail are the Eastern Hajar, which run east to the port city of Sur, almost at the easternmost point of Oman.Western Hajar
The mountains to the west of Sama'il Valley, particularly those in Musandam Peninsula and the UAE, are known as the Western Hajar, also known as the "Oman proper". Since Jabal Akhdar and mountains in its vicinity are west of the valley, they may be regarded as Western Hajar.Outlier(s)
In the region of Tawam, which includes the adjacent settlements of Al-Buraimi and Al Ain, on the border of Oman and the UAE Emirate of Abu Dhabi, lies the outlier of Jebel Hafeet, which measures in height. Due to its proximity to the main range, it may be treated as one of the Hajar Mountains, sensu lato. This mountain has ridges which stretch northwards to the city of Al Ain.''Ru'us al-Jibal''
The northernmost mountains of the Hajar range are found on the Musandam Peninsula. For this reason, the phrase Ru'us al-Jibal is applied to them, or the peninsula itself. Despite being physically part of the western Hajar, they differ in geology and hydrology to the rest of the range. The highest point in the UAE is located at Jebel Jais near Ras Al Khaimah, which measures from sea level, but since the summit is on the Omani side, Jabal ar Rahrah, measuring over, has the highest peak in the UAE.''Shumayliyyah''
The mountains bordering the Shamailiyyah coast on the Gulf of Oman, forming parts of the northern UAE Emirates of Sharjah, Ras Al-Khaimah and Fujairah, may also be called the Shumayliyyah. In this region is Jebel Al-Ḥeben.Flora and fauna
The mountains are rich in plant life compared to most of Arabia, including a number of endemic species. The vegetation changes with altitude, the mountains are covered with shrubland at lower elevations, growing richer and then becoming woodland, including wild olive and fig trees between, and then higher still there are junipers. Fruit trees such as pomegranate and apricot are grown in the cooler valleys and in places there are rocky outcrops with little vegetation. The flora shows similarities with mountain areas of nearby Iran, as well as with areas along the Red Sea in the Horn of Africa. For example, the tree Ceratonia oreothauma is found here and also in Somalia.A number of birds are found in the mountains including Egyptian and lappet-faced vultures. Mammals include mountain gazelles and the Arabian tahr. Other endemic species include a number of geckos and lizards: Asaccus montanus, Asaccus platyrhynchus and a subspecies of Wadi Kharrar rock gecko are found only in Oman while Musandam leaf-toed gecko, Gallagher's leaf-toed gecko, Oman rock gecko, Jayakar lizard and Oman blue-tailed lizard are found only in the Hajar. The endangered Arabian leopard had been recorded here, particularly in the area of Khasab in northern part of the Musandam.
Like the Ru'us al-Jibal, the area of Jebel Hafeet is noted for hosting rare flora and fauna. For example, in February 2019, an Arabian caracal was sighted here, and in March, a Blanford's fox, which has also been reported in the mountains of Ras Al-Khaimah. In September 2024 it was reported that the rare white-edged rock brown butterflies were spotted in the area.