Hajar Mountains

The Hajar Mountains extend for 700 kilometres (430 miles) through the UAE and Oman.[5] 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.[6][7]

Currently, the Arabian Plate is moving north relative to the Eurasian Plate at 2–3 cm (0.79–1.18 in) per year.[8][9] Continental collision is occurring at the Zagros fold and thrust belt west of the Musandam Peninsula.[10][11] 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.[12]

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.[13][14][15][16][17][18][19]

The high topography is around two major culminations: Jabal Akhdar and Saih Hatat, which are large scale anticlines.[20] The Saih Hatat culmination contains eclogite in the northeast at As Sifah.[21] These rocks were subducted to about 80 km (50 mi) depth into the mantle, and then exhumed back to the surface.[18] This exhumation event created possibly the largest megasheath fold on Earth, the Wadi Mayh megasheath fold.[22] 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.[23][24] However, some geologists have interpreted that these eclogites were subducted through a SW-dipping subduction zone.[25]

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 (sinistral) strike-slip fault,[26] a normal fault,[20] a lateral ramp,[27] a monocline due to a blind thrust,[14] or a fault with multiple phases of deformation.[28]

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.[17][29] Paleocene to Eocene sedimentary rocks are found at 2,200 m (7,200 ft) above sea level within the Hajar,[27] 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.[20][29][27][30][31]

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,[17][29][32][33] as currently the Musandam Peninsula (northwest corner of the mountain range) is uplifting due to this collision. However, Jabal Shams, the highest peak of the central mountains is over 300 km (190 mi) away from this zone. In addition, there is no major seismicity within the central mountains,[34] indicating that the mountains are not currently deforming, even though the Zagros collision is.[27] 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.

Pillow basalts at Wadi Jizz, which is part of the Semail Ophiolite sequence. These were named the Geotimes Pillow Lavas after a photo of them was published on the cover of the Geotimes magazine in 1975.

Oman's geological record is extremely valuable to geologists, and needs to be preserved.[35] It contains the most complete ophiolite on Earth, of which it is most famous for among geologists. The ophiolite sequence has spectacular pillow basalt (Geotimes pillow lava), as well as exposures of the fossil crust-mantle boundary (moho). 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.[22] 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 that need to be protected. 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.[35]

Jabal Shams, which has the highest peak in Oman

The central section of the Hajar is the highest and wildest terrain in the country. Jabal Shams is the highest of the range,[36] followed by Jebel Akhdar. The latter[37] and the smaller Jebel Nakhl range are bounded on the east by the low Samail Valley (which leads northeast to Muscat).[38]

East of Samail are the Eastern Hajar (Arabic: ٱلْحَجَر ٱلشَّرْقِي, romanized: Al-Ḥajar Ash-Sharqī), which run east (much closer to the coast) to the port city of Sur,[39] almost at the easternmost point of Oman.

Outside Al-Hoota near Nizwa, Oman

The mountains to the west of Sama'il Valley, particularly those in Musandam Peninsula and the UAE,[40] are known as the Western Hajar (Arabic: ٱلْحَجَر ٱلْغَرْبِي, romanized: Al-Ḥajar Al-Gharbī),[41] 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.[1][40]

In the region of Tawam,[42] 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 1,100–1,400 m (3,600–4,600 ft) in height.[43][44][45] Due to its proximity to the main range,[43] it may be treated as one of the Hajar Mountains, sensu lato.[46] This mountain has ridges which stretch northwards to the city of Al Ain.[47][48][49][50]

Ru'us al-Jibal in the Musandam Governorate of Oman, north of the UAE city and emirate of Ras Al Khaimah

The northernmost mountains of the Hajar range are found on the Musandam Peninsula. For this reason, the phrase Ru'us al-Jibal ("Heads of the Mountains") 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.[1][40] The highest point in the UAE is located at Jebel Jais near Ras Al Khaimah, which measures 1,911 m (6,270 ft) from sea level,[51][52] but since the summit is on the Omani side, Jabal ar Rahrah, measuring over 1,691 m (1.051 mi), has the highest peak in the UAE.[53]

Mountains in the UAE Emirate of Fujairah

The mountains bordering the Shamailiyyah (شَمَيْلِيَّة) coast on the Gulf of Oman, forming parts of the northern UAE Emirates of Sharjah, Ras Al-Khaimah and Fujairah,[40] may also be called the Shumayliyyah (شُمَيْلِيَّة).[54][55] In this region is Jebel Al-Ḥeben (جَبَل ٱلْحبن; 25°7′33″N 56°9′33″E).[56][57]