Intrusive rocks

Intrusive rocks that cut the Arabian shield are divided into three main groups, called (from oldest to youngest) Pre-orogenic, syn-orogenic, and Post-orogenic.

The preorogenic intrusions cut through the lower-layered rocks unit only and not the other layered rock units. They are considered older than the middle-layered rock unit but younger than the lower-layered rock unit. These intrusions are characterized by their calcic to calc-alkaline composition. They are dominated by gabbro, diorite, quartz-diorite, trond-hjemite, and tonalite. These intrusions are found in the southern, southeastern, and western parts of the shield and coincide with the areas of the lower-layered rocks unit. These intrusions are assigned ages between 700 and 1,000 million years. Geochemical signatures including strontium isotope ratios show that these intrusions were derived from magma that came from the upper mantle.

The synorogenic intrusions cut the lower and the layered rock units, as well as the preorogenic intrusions but do not cut or intrude the upper-layered rock units. These intrusions are considered older than the upper-layered rocks unit and younger than the preorogenic intrusions, as well as the lower- and the middle-layered rocks units, and they are assigned ages between 620 and 700 million years. Their chemical composition is closer to the granitic calc-alka-line to alkaline field than the preorogenic intrusions. These intrusions include granodiorite, adamalite, monzonite, granite, and alkali granite, with lesser amounts of gabbro and diorite in comparison with the preorogenic intrusions. The general form of these intrusions is batholithic bodies that cover wide areas. They are found mostly in the eastern, northern, and northeastern parts of the Arabian shield. The initial strontium ratio of these intrusions is higher than that

Bouclier Arabo Nubien
Map showing the main tectonic terranes of the Arabian shield

of the preorogenic intrusions and indicates that these intrusions were derived from a magma generated in the lower crust.

Postorogenic intrusions cut through the three upper Proterozoic-layered rocks units as well as the pre- and synorogenic intrusions. These are assigned ages between 620 and 550 million years. They form circular, elliptical, and ringlike bodies that range in chemical composition from alkaline to peralkaline. These intrusions include peralkaline granites such as riebeckite granite, alkaline syenite, pink granite, biotite granite, monzogranite, and perthite-biotite granite.

Ringlike bodies and masses of gabbro are also common, and the postorogenic magmatic suite is bimodal in silica content. These intrusions are scattered in the Arabian shield, but they are more concentrated in the eastern, northern, and central parts of the shield. The initial strontium ratio of the postorogenic intrusions ranges between 0.704 and 0.7211, indicating that these intrusions were derived from a magma generated in the lower crust.

OPHIOLITE BELTS

Mafic and ultramafic rocks that comply with the definition of the ophiolite sequence are grouped into six major ophiolitic belts. Four of these belts strike north, while the other two strike east to northeast. These ophiolite belts include

• Amar-Idsas ophiolite belt

• Jabal Humayyan-Jabal Sabhah ophiolite belt

• Bijadiah-Halaban ophiolite belt

• Hulayfah-Hamdah "Nabitah" ophiolite belt

• Bir Umq-Jabal Thurwah ophiolite belt

• Jabal Wasq-Jabal Ess ophiolite belt

These rocks were among other mafic and ultra-mafic rocks considered as parts of ophiolite sequences, but later only these six belts were considered to comply with the definition of ophiolite sequences. However, the sheeted dike complex of the typical ophiolite sequence is not clear or absent in some of these belts, suggesting that the dikes may have been obscured by metamorphism, regional deformation, and alteration. These belts are considered to represent suture zones where convergence between plates or island arc systems took place, and are considered as the boundaries between different tectonic terranes in the shield.

NAJD FAULT SYSTEM

One of the most striking structural features of the Arabian shield is the existence of a fault system in a zone 185 miles (300 km) wide with a length of nearly 750 miles (1,200 km), extending from the southeastern to the northwestern parts of the shield. This system was generated just after the end of the Hijaz tectonic cycle, and it was active from 630 to 530 Ma, making it the last major event of the Precambrian in the Arabian shield. These faults are left-lateral strike-slip faults with a 150-mile (250 km) cumulative displacement on all faults in the system.

The main rock group formed during and after the existence of the Najd fault system is the Jibalah Group. This group formed in the grabens that were formed by the Najd fault system and are the youngest rock group of the Precambrian Arabian shield. The Jibalah Group formed between 600 and 570 Ma ago. The Jibalah Group is composed of coarse-grained clastic rocks and volcanic rocks in the lower parts, stromatolitic and cherty limestone and argillites in the middle parts, and finegrained clastic rocks in the upper parts. These rocks were probably deposited in pull-apart basins that developed in extensional bends along the Najd fault system.

TECTONIC EvOLUTION OF THE ARABIAN SHIELD

The Arabian shield is divided into five major ter-ranes and tectonostratigraphic units separated by four major suture zones, many with ophiolites along them. The five tectonic terranes include the Asir, Al-Hijaz, Midyan, Afif, and Ar-Rayn. The first three terranes are interpreted as interoceanic island arc terranes, while the Afif terrain is considered continental, and the Ar-Rayn terrain is considered to be probably continental. The four suture zones include the Bir Omq, Yanbu, Nabitah, and Al-Amar-Idsas belts. These suture zones represent the collision and suturing that took place between different tectonic terranes in the Arabian shield. For example, the Bir omq belt represents the collision and suturing between two island arc terranes of Al-Hijaz and Asir, while the Yanbu suture zone represents the collision zone between the Midyan and Al-Hijaz island arc terranes. The Nabitah zone represents collision and suturing between a continental microplate (Afif) in the east and island arc terranes (Asir and Al-Hijaz) in the west; Al-Amar Idsas suture represents a collision and suturing zone between two continental microplates, Afif and Ar-Rayn.

Five main stages are recognized in the evolution of the Arabian shield, including rifting of the African craton (1,200-950 million years ago), formation of island arcs over oceanic crust (950-715 million years ago), formation of the Arabian shield craton from the convergence and collision of microplates with adjacent continents (715-640 million years ago), continental magmatic activity and tectonic deformation (640-550 million years ago), and epicontinental subsidence (550 million years ago).

Information about the rifting stage (1,200-950 million years ago) is limited, but the Mozambique belt in the African craton underwent rifting in the interval between 1,200 and 950 million years ago. This rifting formed an oceanic basin along the present northeastern side of the African craton. This was a part of the Mozambique ocean that separated the facing margins of East and West Gondwana. Alternatively there may have been more than one ocean basin, separated by rifted microcontinental plates such as the Afif microcontinental plate.

The island arc formation stage (950-715 million years ago) is characterized by the formation of oceanic island arcs in the oceanic basins formed in the first stage. The stratigraphic records of volcanic and sedimentary rocks in the Asir, Al-Hijaz, and some parts of the Midyan terranes present rocks with ages between 900 and 800 million years. These rocks are of mafic or bimodal composition, and are considered products of early island arcs, particularly in the Asir terrain. These rocks show mixing or the involvement of rocks and fragments formed in the previous stage of rifting of the African craton.

The formation of island arc systems did not take place at the same time, but rather different arc systems evolved at different times. The Hijaz terrain is considered the oldest island arc, formed between 900 and 800 million years ago. This terrane may have encountered continental fragments now represented by the Khamis Mushayt Gneiss and Hali Schist, which are considered parts of, or derived from, the old continental crust from the previous stage of rifting.

Later on in this stage (760-715 million years ago) three island arc systems apparently formed simultaneously. These are the Hijaz, Tarib, and Taif island arc systems. These island arc systems evolved and formed three crustal plates, including the Asir, Hijaz, and Midyan plates. Later in this stage the Amar Andean arc formed between the Afif plate and Ar-Rayn plate, and it is considered part of the Ar-Rayn plate. Oceanic crustal plateaus may have been involved in the formation of the oceanic crustal plates in this stage.

In the collision stage (715-640 million years ago) the five major terranes that formed in the previous stages were swept together and collisions took place along the four suture zones mentioned above. The collision along these suture zones did not take place at the same time. For example, the collision along the Hijaz and Taif arcs occurred around 715 million years ago, and the collision along Bir Omq suture zone took place between 700 and 680 million years ago, while the island arc magmatic activity in the Midyan terrain continued until 600 million years ago. The collision along Nabitah suture zone was diachronous along strike. The collision started in the northern part of the Nabitah suture between Afif and the Hijaz terranes at about 680 to 670 million years ago, and at the same time the southern part of the suture zone was still experiencing subduction. Further collision along the Nabitah suture zone shut off the arc in the south, and the Afif terrain collided with the Asir terrain. As a result, the eastern Afif plate and the western island arc plates of the Hijaz and Asir were completely sutured along the Nabitah orogenic belt by 640 Ma. In this stage three major magmatic arcs developed, and later on in this stage they were shut off by further collision. These arcs include the Furaih magmatic arc that developed on the northern part of the Nabitah suture zone and on the southeastern part of the Hijaz plate, the Sodah arc that developed on the eastern part of the Afif plate, and an Andean-type arc on the eastern part of the Asir plate.

The Ar-Rayn collisional orogeny along the Amar suture was between the two continental plates of Afif and Ar-Rayn, and took longer than any other collisions in the shield (from 700 to 630 million years ago). Many investigators suggest that the Ar-Rayn terrain is part of a bigger continent, which extends under the eastern Phanerozoic cover and is exposed in oman. This terrane may have collided with or into the Arabian shield from the east and was responsible for the development of the Najd left-lateral fault system.

By 640 million years ago the five major ter-ranes had collided with each other, forming the four mentioned suture zones, and the Arabian shield was stabilized. Since then, the shield behaved as one litho-spheric plate until the rifting of the Red Sea. Oro-genic activity inside the Arabian shield, however, continued for a period of about 80 million years after collision, during which time the Najd fault system developed as the last tectonic event in the Arabian shield in the late Proterozoic Era.

After development of the Najd fault system, tectonic activity in the Arabian shield ended, and the Arabian-Nubian shield subsided and was pene-plained, as evidenced by the existence of epicontinental Cambro-Ordovician sandstone covering many parts of the shield in the north and the south. The stratigraphic records of the Phanerozoic cover show that the Arabian shield has been tectonically stable with the exception of ophiolite obduction in Oman and collision along the margins of the plate during the closure of the Tethys Sea until rifting of the Red Sea in the Tertiary.

PHANEROZOIC COVER OF THE ARABIAN SHIELD: OMAN MOUNTAINS

The northern and eastern parts of the Arabian Peninsula are composed of a series of sandstone, limestone, siltstone, evaporates, and rare volcanic rocks deposited in the Paleozoic, Mesozoic, and Cenozoic. These rocks are known as the Arabian platform, the youngest rocks of which consist of unconsolidated sands, silts, gravels, and sabkha deposits such as those that cover much of Kuwait, the Gulf States, and upper layers on the Arabian platform.

The Phanerozoic rocks of the Arabian platform dip to the east very gently, and gradually increase in thickness from a few feet where they overlie the Precambrian Arabian shield in the east, to more than six miles (10 km) in thickness in Oman, eastern Saudi Arabia, and beneath Kuwait. Since some of the thickest sections of the Arabian platform are known from Oman, these rocks are described in detail using the exposures in the northern Oman (Hajar) Mountains as examples.

The Oman, or Hajar, Mountains in northern Oman and the United Arab Emirates are located on the northeastern margin of the Arabian plate,

60-120 miles (100-200 km) from the active deformation front in the Gulf of Oman between Arabia and the Makran accretionary wedge of Asia. They are made up of five major structural units ranging in age from Precambrian to Miocene. These include the pre-Permian basement, Hajar Unit, Hawasina nappes, Semail ophiolite and metamorphic sole, and postnappe structural units.

The Hajar Mountains reach up to 1.8 miles (three km) high, displaying many juvenile topographic features such as straight mountain fronts and deep, steep-walled canyons that may reflect active tecto-nism causing uplift of these mountains. The present height and ruggedness of the Hajar mountainous area is a product of Cretaceous ophiolite obduc-tion, Tertiary extension, and rejuvenated uplift and erosion that was initiated at the end of the oligocene and continues to the present. The Sayq Plateau southwest of Muscat is 1.2-1.8 miles (2-3 km) in elevation. Jabal Shams on the margin of the Sayq Plateau is the highest point in Arabia, rising more than 1.8 miles (3 km) in the central Hajar Mountains. The heights decrease gradually northward, reaching 1.2 miles (2 km) on the Musandam peninsula. There the mountain slopes drop directly into the sea.

Pre-Permian rocks are exposed mainly in the Jabal Akhdar, Saih Hatat, and Jabal J'Alain areas. The oldest structural unit includes a Late Proterozoic basement gneiss correlative with the Arabian-Nubian shield, overlain by a Late Proterozoic/Ordovician volcano-sedimentary sequence. The latter is divided into the Late Proterozoic/Cambrian Huqf Group and the Ordovician Haima Group. The Huqf Group is composed mainly of diamictites, siltstone, gray-wacke, dolostone, and intercalated mafic volcanics. The Ordovician Haima Group consists of a series of sandstones, siltstones, quartzites, skolithos-bear-ing sandstones, and shales, interpreted as subtidal to intertidal deposits.

The Hajar Unit represents the main part of the Permian/Cretaceous Arabian platform sequence that formed on the southern margin of the Neo-Tethys Ocean. These carbonates form most of the rugged peaks of Jabal Akhdar, form a rim around the southwestern parts of Saih Hatat, and continue in several thrust sheets in the Western Hajar region. They are well exposed on the Musandam peninsula. The Hajar Unit contains the Akhdar, Sahtan, Kahm-mah, and Waisa Groups of mainly carbonate litholo-gies, overlain by the Muti Formation in the eastern Hajar, and the equivalent Ruus al Jibal, Elphinstone, Musandam, and Thamama Groups on the Musan-dam peninsula.

The Hawasina nappes consist of a series of Late Permian/Cretaceous sedimentary and volcanic rocks deposited in the Hawasina basin, between the Ara bian continental margin and the open Neo-Tethys Ocean. The Hawasina nappes include the Hamrat Duru, Al Aridh, Kawr, and Umar Groups. Chaotic deposits of the Baid Formation are interpreted as a foundered carbonate platform. The Hamrat Duru Group includes radiolarian chert, gabbro, basaltic and andesitic pillow lava, carbonate breccia, shale, limestone, and sandstone turbidites. The Al Aridh Group contains an assemblage of basaltic andesite, hyaloclastite and pillow lavas, micrites, pelagic carbonates, carbonate breccias, chert, and turbidites. This is overlain by the Kawr Group, which includes basalts, andesites, and shallow marine carbonates. The Umar Group contains basaltic and andesitic pillow lavas, cherts, carbonate breccias, and micrites.

SEMAIL OPHIOLITE

The Semail nappe forms the largest ophiolitic sheet in the world, and it is divided into numerous blocks in the northern Oman Mountains. The Semail ophiolite contains a complete classic ophiolite stratigraphy, although parts of it are unusual in that it contains two magmatic sequences, including upper and lower units. The upper magmatic unit grades downward from radiolarian cherts and umber of the Suhaylah Formation, to basaltic and andesitic pillow lavas locally intruded by trondhjemites, through a sheeted diabase dike unit, and into massive and layered gab-bros, and finally into cumulate gabbro, wehrlite, dunite, and clinopyroxenite. This upper magmatic sequence grades down from basaltic pillow lavas, into a sheeted dike complex, through isotropic then layered gabbros, then into cumulate gabbro and dunite. The Mohorovicic discontinuity is well exposed throughout the northern Oman Mountains, separating the crustal and the mantle sequences. The mantle sequence consists of tectonized harzburgite, dunite, and lherzolite, cut by pyroxenite dikes, and local chromite pods.

The metamorphic sole, or dynamothermal aureole, of the Semail ophiolite formed through meta-morphism of rocks immediately under the basal thrust, heated and deformed during emplacement of the hot allochthonous sheets. In most places it consists of two units including a lower metasedimentary horizon, and an upper unit of banded amphibolites. The metamorphic grade increases upward through the unit to upper amphibolite facies near the contact with the Semail nappe.

Postnappe units consist of Late Cretaceous and Tertiary rocks. The Cretaceous Aruma Group consists of a lower unit of Turonian-Santonian polymict conglomerate, sandstone and shale of the Qahlah Formation, and an upper unit of Campan-ian-Maastrictian marly limestone and polymict breccia (Thaqab Formation). The Tertiary Hadhramaut

The Gulf

Recent deposits

Maastrichtian-Tertiary sediments Semail ophiolite

Hawasina and Haybi allochonous units and Batinah complex Sumeini Group

Hajar Supergroup (Middle Permian to Cenomanian carbonates) I Late Precambrian-Ordovician sediments

Jebel Ja'alan schists and gneisses (Precambrian, 850 Ma) Major anticlines High angle faults

The Gulf

Recent deposits

Maastrichtian-Tertiary sediments Semail ophiolite

Hawasina and Haybi allochonous units and Batinah complex Sumeini Group

Hajar Supergroup (Middle Permian to Cenomanian carbonates) I Late Precambrian-Ordovician sediments

Jebel Ja'alan schists and gneisses (Precambrian, 850 Ma) Major anticlines High angle faults

Muscat

The Map Labrador

Gulf of Oman

Ras al Hadd

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Muscat

Gulf of Oman

Interior Oman fy^ and Arabia fyr

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Geologic map of Oman. The world's largest ophiolite, the semail, is shown in green colors.

Group comprises Paleocene to Eocene limestones, marly limestone, dolostone, conglomerate, and sandstones that outcrop along the southern edge of the Batinah coastal plain at the border with the northeast flank of the Hajar Mountains.

QUATERNARY GEOLOGY OF NORTHERN ARABIA

several levels of Quaternary fluvial terraces are preserved along the flanks of the hajar mountains in oman. These can be divided in most places into an older lower-cemented terrace and an upper younger-uncemented terrace group. The lower-cemented terrace is one of the youngest geological units and has been used as a time marker to place constraints on the ages of structures. The terraces are younger than and unconformably overlie most faults and folds, but in several places faults and fracture intensification zones cut through the Quaternary terraces, providing some of the best evidence for the young age of some of the faults along the northeastern edge of the Arabian plate. These terraces grade both northward and southward into coalesced alluvial fans, forming bajada flanking the margins of the mountains. The northern alluvial plains grade into a narrow coastal plain along the Gulf of oman.

The oman (hajar) mountains are situated at the northeastern margin of the Arabian plate. This plate is bounded to the south and southwest by the active spreading axes of the Gulf of Aden and the Red Sea. On the east and west its border is marked by trans-current fault zones of the Owen Fracture Zone and the Dead Sea Transform. The northern margin of the plate is marked by a complex continent-continent to continent-oceanic collision boundary along the Zagros and Makran fold and thrust belts.

Rocks of the Hajar Supergroup preserve a history of Permian through Cretaceous subsidence of the Arabian platform on the margin of the Neo-Tethys Ocean. Formations that now comprise the Hawasina nappes have biostratigraphic ages of 26095 Ma, interpreted to have been deposited on the continental slope and in abyssal environments of the Neo-Tethys Ocean. By about 100 Ma ago, spreading in the Neo-Tethys generated the oceanic crust of the Semail ophiolite, which was detached in the oceanic realm and thrust over adjacent oceanic crust soon after its formation. Metamorphic ages for the initiation of thrusting range from 105 Ma to 89 Ma. The ophiolitic nappes moved toward the Arabian margin, forming the high-grade metamorphic sole during transport, and progressively scraping off layers of the Hawasina sediments and incorporating them as thrust nappes to the base of the ophiolite. The ophiolite reached the Arabian continental margin and was thrust over it before 85-75 Ma, as indicated by greenschist facies metamorphism in the metamor-phic sole and by deformation of the Arabian margin sediments. Initial uplift of the dome-shaped basement cored antiforms of Jabel Akhdar and Saih Hatat may have been initiated during the late stages of the collision of the ophiolite with the Arabian passive margin, and may have been localized by preexisting basement horst and graben structures. The location and geometry of these massive uplifts is probably controlled by basement ramps. Uplift of these domes was pronounced during the Oligocene/Miocene, as shown by tilting of Late Cretaceous/Tertiary formations on the flanks of the domes. Uplift of the domes may have begun in the Oligocene, resulting from the propagation of a fault beneath the southern limbs of the folds. The uplift of the domes includes a complex history, involving several different events. Some uplift of the domes continues at present, whereas much of the Batinah coastal plain is subsiding.

In most of the Hajar Mountains, the Hawasina nappes structurally overlie the Hajar Supergroup, and form a belt of north or northeastward dipping thrust slices. On the southern margins of Jabal Akhdar, Saih Hatat, and other domes, however, the Hawa-sina form south-dipping thrust slices. Major valleys typically occupy the contact between the Hajar Supergroup and the Hawasina nappes, because of the many, easily erodable shale units within the Hawa-sina nappes. Several very large [~6 mile (10 km)

scale] allochthonous limestone blocks known as the "Oman Exotics" are also incorporated into melange zones within the Hawasina nappes. These form light-colored, erosionally resistant cuestas, including Jabal Kawr and several smaller mountains south of Al Hamra.

South and southwest of the belt of ophiolite blocks, sediments of the Hamrat Duru Group are complexly folded and faulted in a regional foreland-fold-thrust belt and then grade into the Suneinah foreland basin. The Hamrat Duru rocks include radi-olarian cherts, micritic limestones, turbiditic sandstones, shales, and calcarenite, all complexly folded and thrust faulted in an 18.5-mile (30-km)-wide fold/ thrust belt.

A belt of regional anticlinal uplifts brings up carbonates of the Hajar Supergroup in the central part of the basin, as exposed at Jabal Salakh. These elongate anticlinal domes have gentle to moderate dips on their flanks, and are cut by several thrust faults that may be linked to a deeper system. This could be a blind thrust, or the folds could be flower structures developed over deep strike-slip faults. South of the Jabal Salakh fold belt, the surface is generally flat and covered by Miocene/Pliocene conglomerates of the Barzaman Formation, and cut by an extensive network of Quaternary channels of the active alluvial plain.

Tertiary/Quaternary uplift of the northern Oman Mountains may account for the juvenile topography of the area. One of the best pieces of evidence for young uplift of the Northern Oman Mountains comes from a series of uplifted Quaternary marine terraces, best exposed in the Tiwi area 31-62 miles (50-100 km) southeast of Muscat. The uplift is related to the contemporaneous collision between the northeastern margin of the Arabian plate and the Zagros fold belt and the Makran accretionary prism. The Hajar Mountains lie on the active forebulge of this collision, and the fault systems are similar to those found in other active and ancient forebulge environments. The amount of Quaternary uplift, estimated between 300 and 1,600 feet (100-500 m), is also similar to uplift in other forebulge environments developed on continental margins. This Quaternary uplift is superimposed on an older, Cretaceous/Tertiary (Oligocene) topography.

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