Arabian shield

The Arabian shield comprises the core of
the Arabian Peninsula, a landmass of near trapezoidal shape
bounded by three water bodies. The Red Sea bounds it from
the west, the Arabian Sea and the Gulf of Aden from the
south, and the Arabian Gulf and Gulf of Oman on the east.
The Arabian Peninsula can be classified into two major geological
provinces, including the Precambrian Arabian shield
and the Phanerozoic cover.
The Precambrian shield is located along the western and
central parts of the peninsula. It narrows in the north and the
south but widens in the central part of the peninsula. The
shield lies between latitudes 12° and 30° north and between
longitudes 34° and 47° east. The Arabian shield is considered
as part of the Arabian-Nubian shield that was formed in the
upper Proterozoic Era and stabilized in the Late Proterozoic
around 600 million years ago. The shield has since subsided
and been covered by thick deposits of Phanerozoic continental
shelf sediments along the margins of the Tethys Ocean.
Later in the Tertiary the Arabian-Nubian shield was rifted
into two fragments by the Red Sea rift system.
Phanerozoic cover overlies the eastern side of the Arabian
shield unconformably and dips gently toward the east.
Parts of the Phanerozoic cover are found overlying parts of
the Precambrian shield, such as the Quaternary lava flows of
Harrat Rahat in the middle and northern parts of the shield
as well as some sandstones, including the Saq, Siq, and
Wajeed sandstones in different parts of the shield.
History of Tectonic Models
The Arabian shield includes an assemblage of Middle to Late
Proterozoic rocks exposed in the western and central parts of
the Arabian Peninsula and overlapped to the north, east, and
south by Phanerozoic sedimentary cover rocks. Several parts
of the shield are covered by Tertiary and Quaternary lava
flows that were extruded concurrently with rifting of the Red
Sea. Rocks of the Arabian shield may be divided into assemblages
of Middle to Late Proterozoic stratotectonic units, volcano-
sedimentary, and associated mafic to intermediate
intrusive rocks. These rocks are divided into two major categories,
the layered rocks and the intrusive rocks. Researchers
variously interpret these assemblages as a result of volcanism
and magmatism in ensialic basins or above subduction zones.
More recent workers suggested that many of these assemblages
belong to late Proterozoic volcanic-arc systems that
comprise distinct tectonic units or terranes, recognized following
definitions established in the North America cordillera.
Efforts in suggesting models for the evolution of the Arabian
shield started in the 1960s. Early workers suggested that
the Arabian shield experienced three major orogenies in the
Late Proterozoic Era. They also delineated four classes of plutonic
rocks that evolved in chemistry from calc-alkaline to
peralkaline through time. In the 1970s a great deal of
research emerged concerning models of the tectonic evolution
of the Arabian shield. Two major models emerged from this
work, including mobilistic plate-tectonic models, and a nonmobilistic
basement-tectonic model.
The main tenet of the plate-tectonic model is that the evolution
of the Arabian shield started and took place in an
oceanic environment, with the formation of island arcs over
subduction zones in a huge oceanic basin. On the contrary, the
basement-tectonic model considers that the evolution of the
Arabian shield started by the rifting of an older craton or continent
to form intraoceanic basins that became the sites of
island arc systems. In both models, late stages of the formation
of the Arabian-Nubian shield are marked by the sweeping
together and collision of the island arcs systems, obduction of
the ophiolites, and cratonization of the entire orogen, forming
one craton attached to the African craton. Most subsequent
investigators in the 1970s supported one of these two models
and tried to gather evidence to support that model.
As more investigations, mapping, and research were carried
out in the 1980s and 1990s, a third model invoking
microplates and terrane accretion was suggested. This model
suggests the existence of an early to mid-Proterozoic
(2,000–1,630-million-year-old) craton that was extended, rifted,
then dispersed causing the development of basement fragments
that were incorporated as allochthonous microplates
into younger tectonostratigraphic units. The tectonostratigraphic
units included volcanic complexes, ophiolite complexes,
and marginal-basin and fore-arc stratotectonic units that
accumulated in the intraoceanic to continental-marginal environments
that resulted from rifting of the preexisting craton.
These rocks, including the older continental fragments, constituted
five large and five small tectonostratigraphic terranes
that were accreted and swept together between 770 million
and 620 million years ago to form a neo-craton on which
younger volcano-sedimentary and sedimentary rocks were
deposited. Most models developed in the period since the
early 1990s represent varieties of these three main classical
models, along with a greater appreciation of the role that the
formation of the supercontinent of Gondwana played in the
formation of the Arabian-Nubian shield.
Geology of the Arabian Shield
Peter Johnson and coworkers have synthesized the geology of
the Arabian shield and proposed a general classification of
the geology of the Arabian shield that attempts to integrate
and resolve the differences between the previous classifications.
According to this classification, the layered rocks of the
Arabian shield are divided into three main units separated by
periods of regional tectonic activity (orogenies). This gives an
overall view that the shield was created through three tectonic
cycles. These tectonic cycles include early, middle, and late
Upper Proterozoic tectonic cycles.
The early Upper Proterozoic tectonic cycle covers the
time period older than 800 million years ago and includes the
oldest rock groups that formed before and up to the Aqiq
orogeny in the south and up to the Tuluhah orogeny in the
north. In this general classification, the Aqiq and Tuluhah
orogenies are considered as part of one regional tectonic
event or orogeny that is given a combined name of the Aqiq-
Tuluhah orogeny.
The middle Upper Proterozoic tectonic cycle is considered
to have taken place in the period between 800 and 700
million years ago. It includes the Yafikh orogeny in the south
and the Ragbah orogeny in the north. These two orogenies
were combined together into one regional orogeny named the
Yafikh-Ragbah orogeny.
The late Upper Proterozoic tectonic cycle took place in
the period between 700 and 650 million years ago. It includes
the Bishah orogeny in the south and the Rimmah orogeny in
the north. These two orogenies are combined together into
one regional orogeny named the Bishah-Rimmah orogeny.
Classification of Rock Units
The layered rocks in the Arabian shield are classified into
three major rock units, each of them belonging to one of the
three tectonic cycles mentioned above. These major layered
rock units are the lower, middle, and upper layered rock units.
The lower layered rock unit covers those rock groups
that formed in the early upper Proterozoic tectonic cycle
(older than 800 million years ago) and includes rocks with
continental affinity. The volcanic rocks that belong to this
unit are characterized by basaltic tholeiite compositions and
by the domination of basaltic rocks that are older than 800
million years. The rock groups of this unit are located mostly
in the southwestern and eastern parts of the shield.
The rock groups of the lower layered unit include rocks
that were formed in an island arc environment and that are
characterized by basic tholeiitic volcanic rocks (Baish and
Bahah Groups) and calc-alkaline rocks (Jeddah Group).
These rocks overlie in some places highly metamorphosed
rocks of continental origin (Sabia Formation and Hali
schists) that are considered to have been brought into the system
either from a nearby craton such as the African craton,
or from microplates that were rifted from the African plate
such as the Afif microplate.
The middle layered rock unit includes the layered rock
groups that formed during the middle upper Proterozoic tectonic
cycle in the period between 800 and 700 million years
ago. The volcanic rocks are predominately intermediate
igneous rocks characterized by a calc-alkaline nature. These
rocks are found in many parts of the shield, with a greater
concentration in the north and northwest, and scattered outcrops
in the southern and central parts of the shield.
The upper layered rock unit includes layered rock groups
that formed in the late upper Proterozoic tectonic cycle in the
period between 700 and 560 million years ago and are predominately
calc-alkaline, alkaline intermediate, and acidic
rocks. These rock groups are found in the northeastern, central,
and eastern parts of the shield.
Intrusive Rocks
The intrusive rocks that cut the Arabian shield are divided into
three main groups. These groups are called (from the older to
the younger) pre-orogenic, syn-orogenic, and post-orogenic.
The pre-orogenic intrusions are those intrusions that cut
through the lower layered rocks unit only and not the other
layered rock units. It is considered older than the middle layered
rock unit but younger than the lower layered rock unit.
These intrusions are characterized by their calcic to calcalkaline
composition. They are dominated by gabbro, diorite,
quartz-diorite, trondhjemite, 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 1,000
and 700 million years old. Geochemical signatures including
strontium isotope ratios show that these intrusions were
derived from magma that came from the upper mantle.
The syn-orogenic intrusions are those intrusions that cut
the lower and the layered rock units as well as the pre-orogenic
intrusions, but that do not cut or intrude the upper layered
rocks unit. These intrusions are considered older than
the upper layered rocks unit and younger than the pre-orogenic
intrusions, and they are assigned ages between 700 and
620 million years old. Their chemical composition is closer to
the granitic calc-alkaline to alkaline field than the preorogenic
intrusions. These intrusions include granodiorite,
adamalite, monzonite, granite, and alkali granite with lesser
amount 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 of the pre-orogenic intrusions and indicates
that these intrusions were derived from a magma that
was generated in the lower crust.
Post-orogenic intrusions are intrusions that cut through
the three upper Proterozoic layered rocks units as well as the
pre- and syn-orogenic intrusions. These are assigned ages
between 620 and 550 million years old. They form circular,
elliptical, and ring-like bodies that range in chemical composition
from alkaline to peralkaline. These intrusions are made
mainly of alkaline and 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 post-orogenic magmatic suite is bimodal in silica content.
These intrusions are found 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 post-orogenic intrusions
ranges between 0.704 and 0.7211, indicating that these
intrusions were derived from a magma that was 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
belts strike east to northeast. These ophiolite belts include:
1. The Amar-Idsas ophiolite belt
2. Jabal Humayyan–Jabal Sabhah ophiolite belt
3. The Bijadiah-Halaban ophiolite belt
4. Hulayfah-Hamdah “Nabitah” ophiolite belt
5. Bi’r Umq–Jabal Thurwah ophiolite belt
6. Jabal Wasq–Jabal Ess ophiolite belt
These rocks were among other mafic and ultramafic
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 noticeable 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 million
years, 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 that was formed during and after
the existence of the Najd fault system is the Ji’balah 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 Ji’balah Group formed
between 600 and 570 million years ago. The Ji’balah Group
is composed of coarse-grained clastic rocks and volcanic
rocks in the lower parts, by stromatolitic and cherty limestone
and argillites in the middle parts, and by fine-grained
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 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
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.
20 Arabian shield
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