DEFINITION OF TECTONIC EVOLUTION OF THE ARABIAN SHIELD

The Arabian shield is divided into five major and numerous

smaller terranes separated by four major and many smaller

suture zones, many with ophiolites along them. The five

major 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 terrane is considered continental,

and the Ar-Rayn terrane is considered to be probably

continental. The four suture zones include the Bi’r Umq,

Yanbu, Nabitah, and Al-Amar-Idsas. These suture zones represent

the collision and suturing that took place between different

tectonic terranes in the Arabian shield. For example,

the Bi’r Umq suture zone 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

suture 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 zone

represents the 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 it can be said that the Mozambique

belt in the African craton underwent rifting in the time interval

between 1,200 million and 950 million years ago. This rifting

resulted in the formation of 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 micro-continental

plates such as the Afif micro-continental 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 old.

These rocks are of mafic or bimodal composition and are

considered products of early island arcs, particularly in the

Asir terrane. These rocks show mixing or the involvement of

rocks and fragments that 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 terrane is considered to be the oldest

island arc, formed between 900 million and 800 million years

ago. This terrane may have encountered some 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 the Bir Omq suture

zone took place between 700 million and 680 million years

ago, while the island arc magmatic activity in the Midyan terrain

continued until 600 million years ago. It appears that the

collision along the Nabitah suture zone was diachronous

along strike. The collision started in the northern part of the

Nabitah suture between the Afif and Hijaz terranes at about

680 million 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 million

years ago. 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 million to 630 million years ago). Many investigators

suggest that the Ar-Rayn terrain is part of a bigger continent

(one that extends under the eastern Phanerozoic cover and is

exposed in Oman) that collided with or into the Arabian

shield from the east and was responsible for the development

of Najd left-lateral fault system.

By 640 million years ago the five major terranes had collided

with each other forming the four mentioned suture

zones and the Arabian shield was stabilized. Since then, the

shield behaved as one lithospheric plate until the rifting of the

Red Sea. However, orogenic activity inside the Arabian shield

continued for a period of about 80 million years after collision,

during which 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 peneplained, 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 and collision along the margins

of the plate during the closure of the Tethys Sea until

rifting of the Red Sea in the Tertiary.

See also CRATONS; KUWAIT; OMAN MOUNTAINS; ZAGROS

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