DEFINITION OF OMAN MOUNTAINS

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 post-nappe structural units.

The Hajar Mountains are up to 1.8 miles (3 km) high, displaying

many juvenile topographic features such as straight

mountain fronts and deep steep-walled canyons that may

reflect active tectonism causing uplift of these mountains. The

present height and ruggedness of the Hajar mountainous area

is a product of Cretaceous ophiolite obduction, Tertiary extension,

and rejuvenated uplift and erosion that 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 basement is 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

later is divided into the Late Proterozoic/Cambrian Huqf

Group, and the Ordovician Haima Group. The Huqf Group

is mainly composed of diamictites, siltstone, graywacke,

dolostone, and intercalated mafic volcanics. The Ordovician

Haima Group consists of a series of sandstones, siltstones,

quartzites, skolithos-bearing 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, Kahmmah, and Waisa Groups

of mainly carbonate lithologies, overlain by the Muti Formation

in the eastern Hajar, and the equivalent Ruus al Jibal,

Elphinstone, Musandam and Thamama Groups on the

Musandam peninsula.

The Hawasina nappes consist of a series of Late Permian/

Cretaceous sedimentary and volcanic rocks deposited in the

Hawasina basin, between the Arabian continental margin and

the open Neo-Tethys Ocean. The Hawasina nappes include the

Hamrat Duru, Al Aridh, Kawr, 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.

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 gabbros, 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 metamorphism 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.

Post-nappe 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

Campanian-Maastrictian marly limestone and polymict breccia

(Thaqab Formation). The Tertiary Hadhramaut 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.

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 that

we have been able to use 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 demonstrably

cut through the Quaternary terraces, providing some of the

best evidence for the young age of some of the faults described

here. 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 Red Sea. On the east and west its border is

marked by transcurrent 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 continentoceanic

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 260–95 Ma, interpreted to have been deposited on

the continental slope and in abyssal environments of the Neo-

Tethys Ocean. By about 100 Ma, 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 metamorphic

sole and by deformation of the Arabian margin sediments.

Initial uplift of the dome-shaped basement cored

antiforms of Jabal 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. However, on

the southern margins of Jabal Akhdar, Saih Hatat, and other

domes, the Hawasina 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 Hawasina nappes. Several

very large (~10 km scale) allochthonous limestone blocks

known as the “Oman Exotics” are also incorporated into

mélange 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 radiolarian cherts, micritic limestones, turbiditic

sandstones, shales, and calcarenite, all complexly folded and

thrust faulted in a 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, 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.

See also CONVERGENT PLATE MARGIN PROCESSES;

OPHIOLITES.

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