Journal of the Geological Society, London, Vol. 162, 2005, pp. 871–888. Printed in Great Britain.

871

Tertiary–Quaternary faulting and uplift in the northern Oman Hajar Mountains

TIMOTHY KUSKY 1, CORDULA ROBINSON 2 & FAROUK EL-BAZ 2

1Department of Earth and Atmospheric Sciences, St. Louis University, St. Louis, MO 63103, USA

(e-mail: kusky@eas.slu.edu)2Center for Remote Sensing, Boston University, Boston, MA 02215, USA

Abstract: Field mapping and remote sensing investigations reveal two new major fault sets cutting through

Tertiary rocks, Quaternary terraces and a several-hundred-year-old irrigation canal system in the Hajar

Mountains of northern Oman. They extend for tens of kilometres, forming fracture intensification zones

several hundred metres wide. WNW- to NW-oriented faults run parallel to the mountain fronts in the plains

adjoining the central Hajar range then obliquely crosscut the mountains in the north. Motion along these faults

explains how Quaternary marine terraces became elevated 190 m above sea level. A second fault set strikes

north to NNE. The associated juvenile topography suggests that they also accommodate recent uplift,

subsidiary to the WNW-striking faults, with minor strike-slip and differential movement between various

segments of the Hajar Mountains. Both fault systems, and the amount of Quaternary uplift (between 100 and

500 m), are similar to those in other active and ancient forebulge environments. Using the fracture patterns

observed, it is proposed here that the Hajar range lies on the active forebulge of a collision zone between the

NE margins of the Arabian plate, the Zagros fold belt and the Makran accretionary prism, which resulted in

the recent uplift.

Keywords: Oman, Hajar Mountains, satellite images, plate collision, uplift.

Combined use of satellite imagery and targeted high-resolution

field mapping is able to yield structural, lithological and other

data over large regions that have the potential to yield clues to

the tectonic setting and evolution of entire structural provinces.

This paper identifies and describes newly recognized fault sets

that may have accommodated Tertiary–Quaternary uplift in the

Hajar Mountains of northern Oman and the United Arab

Emirates (UAE). A new structural map is presented that is

compiled in a geographical information system (GIS) database

using Landsat Thematic Mapper (TM), high-resolution SPOT

images and Shuttle Imaging Radar (SIR-C) data as base maps.

The maps also depict regional systematic relationships between

several late fault sets that were previously unknown. Regional

and detailed structural field investigations were conducted in

areas throughout the Hajar Mountains to confirm satellite image

interpretations and to assess the age of the mapped faults.

Some of the newly identified faults have mapable offsets of

Tertiary units. They are characterized in the field as zones

several metres to several kilometres wide where fault breccia

separates less-deformed lozenges of rock and minor faults with

striated surfaces. Some are very young features, associated with

recent downcutting of drainage, faulting of cemented Quatern-

ary wadi gravels, and in one case, even cutting a falaj (a local

name for a man-made irrigation canal system used to tap water

seeps from fracture zones). Other examples do not noticeably

offset geological map units; therefore, they probably have

relatively minor displacements. Owing to the regional dimension

of these structures they are called fracture intensification zones

in this paper.

The newly recognized fault and fracture zones were not

mapped by earlier geologists in the field, possibly owing to a

covering by younger deposits. The unique perspective offered

through the analysis of satellite images, and the ground-

penetrating ability of orbital radar, coupled with regional field

investigations, however, has allowed the identification of geomor-

phological features related to these structures; thus, the discovery

of the fracture intensification zones.

Structural setting

The Hajar (meaning solid rock) Mountains in northern Oman

and the United Arab Emirates (UAE) are located on the NE

margin of the Arabian plate (Figs. 1 and 2). This plate is

bounded to the south and SW 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, respectively. On the north the plate

is marked by a complex continent–continent to continent–ocean

collision boundary along the Zagros and Makran fold and thrust

belts (Fig. 1).

The mountains are made up of several major structural units

ranging in age from Precambrian to Miocene (Fig. 2). These

include a pre-Permian basement, the Hajar unit, the Hawasina

nappes, the Samail ophiolite and metamorphic sole, and the post-

nappe structural units. Of particular interest are the Quaternary

fluvial and marine terraces that are preserved along the flanks of

the Hajar Mountains. These can be divided into an older lower

cemented terrace and an upper, younger and uncemented terrace

group (e.g. Bechennec et al. 1992). They grade both northward

and southward into coalesced alluvial fans forming the bajada

that flanks the margins of the mountains (Fig. 3). The northern

alluvial plains grade into a narrow coastal plain (the Batinah

Plain) along the Gulf of Oman.

Rocks of the Hajar supergroup preserve a history of Permian

to Cretaceous subsidence of the Arabian Platform on the margin

of the Neo-Tethys Ocean (Glennie et al. 1974; Bechennec et al.

1992). Formations that now make up the Hawasina nappes have

biostratigraphic ages of 260–95 Ma, and are interpreted to have

been deposited on the continental slope and in abyssal environ-

ments of the Neo-Tethys Ocean (Glennie et al. 1974; Bechennec

et al. 1992). By about 100 Ma, spreading in the Neo-Tethys

generated the oceanic crust of the Samail 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 to 89 Ma (Montigney et al.

1988; Nicolas 1989; Nicolas et al. 1996; Hacker & Liou 1999;

Searle & Cox 1999).

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 (Lanphere 1981; Nicolas 1989)

as indicated by greenschist-facies metamorphism in the meta-

morphic sole and by deformation of the Arabian margin

sediments.

Initial uplift of the dome-shaped basement cored antiforms of

Jabal Akhdar and Saih Hatat (Fig. 2) may have been initiated

during the late collision stages of the ophiolite with the Arabian

passive margin, and may have been localized by pre-existing

basement horst and graben structures (Le Metour et al. 1995).

The location and geometry of these massive uplifts is probably

controlled by basement ramps (Bernoulli & Weissert 1987).

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 (Glennie et al. 1974;

Mann et al. 1990). Mount et al. (1998) have suggested that the

uplift of the domes began in the Oligocene, resulting from the

propagation of a fault beneath the southern limbs of the folds.

Al-Lazki et al. (2002) presented evidence that the uplift of the

domes includes a complex history, involving several different

events. Some uplift of the domes continues today, whereas much

of the Batinah coastal plain is subsiding.

In most of the Hajar Mountains, the Hawasina nappes

Fig. 1. Tectonic map of northern Oman and

surrounding regions, showing the

relationship between thrust load and the

flexural bulge that is responsible for active

uplift of the northern Oman Mountains.

Modified after Boote et al. (1990).

T. KUSKY ET AL .872

structurally overlie the Hajar supergroup, and form a belt of

north- or NE-dipping thrust slices. However, on the southern

margins of Jabal Akhdar, Saih Hatat, and other domes (Fig. 2),

the Hawasina units form south-dipping thrust slices (Fig. 2).

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 (c. 10 km scale) allochthonous limestone blocks known as

the ‘Oman Exotics’ are also incorporated into melange zones

within the Hawasina nappes. These form light coloured, erosion-

ally resistant cuestas including Jabal Kawr and several smaller

mountains south of Al Hamra (Fig. 3).

South and SW 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 (Fig. 2; Boote et al. 1990). The Hamrat

Duru rocks include radiolarian cherts, micritic limestones,

turbiditic sandstones, shales and calcarenite, all complexly folded

and thrust faulted in a 30 km wide fold–thrust belt (Fig. 2).

A belt of regional anticlinal uplifts brings up carbonates of the

Hajar supergroup south of the main Hajar range, as exposed at

Jabal Salakh (Figs 2 and 3). 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 (Al-

Lazki et al. 2002). The folds have been attributed to flower

structures developed over deep strike-slip faults (Boote et al.

1990). 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 (Figs 2 and 3).

The Hajar Mountains locally reach over 3 km in height at

Jabal Shams (Fig. 4). They display many juvenile topographi-

cal features, such as straight mountain fronts and deep, steep-

walled canyons that probably reflect active tectonism resulting

in mountain uplift. The present height and ruggedness of the

Hajar mountainous area is a product of Cretaceous ophiolite

obduction, Tertiary extension, and rejuvenated uplift and

erosion, which was initiated at the end of the Oligocene

(Mann et al. 1990) and continues to this day. The Sayq

Plateau in Jabal Akhdar ranges from 2 to 3 km in elevation.

Jabal Shams, on the margin of the Sayq Plateau, is the

highest point in Arabia at 3075 m (Fig. 4). The mountains

Fig. 2. Simplified geological map of

northern Oman that shows the Hajar

Mountains and major structures. They are

composed of several major structural units,

ranging in age from Precambrian to

Miocene. These include a pre-Permian

basement, the Hajar Unit, the Hawasina

nappes, the Samail ophiolite and

metamorphic sole, and the post-nappe

structural units.

NEOTECTONICS OF THE HAJAR MOUNTAINS 873

decrease in height northward, reaching 2 km in the Musandam

Peninsula, where the mountain slopes drop directly into the

sea (Fig. 4).

Data and methods

Landsat TM and SIR-C radar data are used to produce a new structural

map of the Hajar Mountains (Fig. 5) assisted by field observations and

SPOT data analysis. Eight Landsat scenes are used in this study; the path

and row numbers are: 157–045, 157–044, 158–045, 158–044, 159–045,

159–044, 159–043 and 160–042. Six L-band and 6 C-band SIR-C

scenes are used; the processing run numbers are: 13144–13147 (April

1994 images) and 42794–42797, 46407-10 (October 1994 images). One

high-resolution SPOT image is used with a path and row number of

169–304.

Landsat TM data have a resolution of 28.5 m2. They reflect the

lithological and mineralogical variations of exposed rocks and soils by

separating the solar reflected energy into six spectral bands. Thermal

emitted energy is also collected in a separate band. TM bands 7 (2.08–

2.35 �m), 4 (0.76–0.90 �m) and 2 (0.50–0.60 �m) are used in this

analysis because they have low correlation and produce high-contrast

images suitable for geological interpretation. In this study, band 2 is

helpful for rock discrimination, band 4 for land–water contrasts, and

band 7 for discrimination of mineral and rock types (Jensen 2000, p.

194). The ability of Landsat data to aid in the identification of rock types

distinguishes them from radar data, which discern different lithological

units because of different surface roughnesses (e.g. Robinson 1998;

Inzana et al. 2003).

SIR-C data are processed using CEOS-READER (Committee on Earth

Observation Satellites) software (obtainable from the Jet Propulsion

Laboratory, Pasadena, CA). The resulting images have a resolution of

25 m. Images for all bands (L and C) and polarizations were generated.

The application of histogram equalization stretches and edge filters

(either simple 3 3 3 Laplacian filters or modified subtraction filters) is

particularly useful in the enhancement of SIR-C data (e.g. Robinson et

al. 1999).

Recognition of tectonic features in radar images is dependent on

viewing geometry and incidence angle. If these circumstances are

favourable, then radar images can be used to identify regionally extensive

Fig. 3. Landsat TM 742 band image

mosaic of the Hajar Mountains and the

surrounding landscape. Major

geomorphological units and place names

mentioned in the text are labelled. The 742

band combination is commonly used for

geological analysis. Placing band 7 in the

Red Gun emphasizes rock types.

Furthermore, little correlation exists

between these bands, which produces a

high-contrast image suitable for

interpretation.

T. KUSKY ET AL .874

linear features that are otherwise not clearly depicted in TM data (e.g.

Sabins et al. 1980; Robinson 1998; Robinson & Kusky 1998; Kusky &

Ramadan 2002). This is because the surface albedo in these images varies

much more than in optical imagery (as they respond to lithological, not

mineralogical, characteristics). This means that if the fault scarps are

oriented perpendicular to the radar look direction they appear brighter than

those parallel to the look direction, because the radar wave is backscattered

directly to the receiving antenna in this geometric configuration.

Principal structures are mapped in the field and using Landsat images

(Fig. 3), supplemented by SIR-C image interpretation, where the latter

are available. On-screen digitizing is applied to the satellite images to

generate digital maps of structural features in vector format. The

procedure uses geometrically corrected images as a template, which

further allows the geolocation of the interpreted structural features. The

spatial databases are created using ‘ESRI’s Arc/Info’ GIS software.

Principal youthful structural features are visible as distinct lines cutting

rocks that are Miocene or younger in age. Gently dipping thrusts and

shear zones, such as the basal thrust and metamorphic sole of the

ophiolite, are not included in the new fault and fracture maps. However,

these faults are mapped on the geological maps of the country

(Directorate General of Minerals 1993), and are consulted while mapping

the fault and fracture zones in the field and from the Landsat images.

There is a general agreement between the geological maps and the new

structural interpretation shown in Figure 5. Our new maps, however,

show additional faults and fractures that are not included on the

geological maps. These are discussed below.

Integrated image analysis and structural field mapping

The WNW–NW-striking Tertiary–Quaternary fault set includes

numerous faults that parallel the mountain front in the central

and eastern Hajar range, and cut the mountains obliquely in the

northern Hajar region (Fig. 5). They comprise a major array of

linked faults developed with c. 50 km spacing. The most notable

of the north- to NNE-striking fault set run along the Samail Gap

(Fig. 5). They too form a major group of faults characterized by

c. 50 km spacing. Seven localities with examples of these faults

sets, and the associated Tertiary–Quaternary movement, are

described below.

1. Maradi Fault (WNW striking)

The Maradi Fault (centred on 228159N, 588E) is one of the major

WNW–NW-striking faults and is located in the Suneinah fore-

Fig. 4. Digital elevation model (DEM)

generated from 1:100 000 scale

topographical maps lent by the Ministry of

Water Resources, Oman. Significant

topographical features are labelled. The

DEM shows that the highest elevations are

found in Jabal Akhdar.

NEOTECTONICS OF THE HAJAR MOUNTAINS 875

land basin to the south of the Hajar Mountains (Figs 1, 2, and 5).

It is a major, 60 km long, NW-striking fault with 10–20 m of

north-side-down, normal-sense displacement. It merges with a

foreland fold belt to the NW, and disappears beneath the

Quaternary alluvial plain to the SE (Fig. 5). It may re-emerge as

the Saiwan–Nafun Fault in the Huqf area of central Oman (Fig.

2), or it may bend to become continuous with the north- to NW-

trending Oman line to the NW (Fig. 1).

The fault is associated with a number of west-striking minor

faults that intersect the Maradi Fault in a systematic fashion

suggesting that they are pinnate fractures, formed by a compo-

nent of sinistral shear (Fig. 5). It may also have older dextral or

dip-slip displacements (Hanna & Nolan 1989) that are not

directly observable from the satellite images (Fig. 6). The fault

forms a NE-facing escarpment defined by the lower Hadhramaut

group shelf facies (Fiqa Formation), with a reduced elevation of

about 20 m on the NE side of the fault. The fault juxtaposes

Eocene rocks on the SW (Hadhramaut group) with Campanian–

Maastrichtian rocks on the NE (Barzaman Formation of the Fars

group) indicating that it has a component of vertical displace-

ment (Hanna & Nolan 1989).

Radar (SIR-C) data and field studies indicate that the Maradi

Fault has had Tertiary, sinistral strike-slip displacements, in

addition to the observable vertical displacement. SIR-C images

illustrate its continuity (Fig. 6), compared with Landsat TM

images, where identification is more difficult owing to extensive

cover with a heavy clay signature. The SIR-C image shown in

Figure 6 is collected in descending (left-looking) orbit at

incidence angles between 368 and 43.78. The relatively high

angles enhance surface roughness effects and minimize slope

effects, the necessary geometric configuration for imaging this

regionally extensive fault. Subsurface imaging also occurs at

these localities, because of the arid and gently undulating terrain

that is covered only by fine-grained material (e.g. Robinson

1998).

Two features in the SIR-C images indicate that the fault has a

component of sinistral motion, as well as its documented vertical

displacements. First, a large, pre-Miocene, unnamed drainage

Fig. 5. Structural map of the Hajar

Mountains compiled within a geographical

information system (GIS) database using

Landsat Thematic Mapper (TM) images

and Shuttle Imaging Radar (SIR-C) data to

extract fault lines. The map illustrates new

fault sets that may have accommodated the

Tertiary–Quaternary uplift of the Hajar

Mountains. Numbers 1–7 index the

localities of seven areas with evidence for

Tertiary–Quaternary faulting.

T. KUSKY ET AL .876

Fig. 6. L band SIR-C image overlain on a

Landsat TM image of the area. The arrows

point to the leftward displacement of an

unnamed drainage channel. The Upper Fars

Group Tertiary palaeo-surface across the

fault is labelled. This indicates that the

Maradi Fault experienced sinistral motion

since the Tertiary. In the SIR-C image,

drainage trends approximately NE

compared with north–south in the TM

image.

Fig. 7. Photograph of the Maradi Fault scarp (c. 10 m high). It shows

Neogene gravels that are faulted (the label points to such an example)

and Quaternary gravels that are not. Thus, movement along the fault

must have been confined to the Tertiary.

NEOTECTONICS OF THE HAJAR MOUNTAINS 877

channel is displaced in a left-lateral direction (Fig. 6). This river

bed is more defined on the L band image than the C band image

(Robinson & Kusky 1998), indicating that it exists below 50 cm

and up to 2 m in the subsurface (Schaber et al. 1997). Further

evidence of subsurface imaging at these locations comes from

the radar drainage pattern that is shifted slightly from the surface

drainage seen in the TM image at southerly locations (Fig. 6). In

the SIR-C image, drainage trends approximately NE–SW,

whereas the surface trend of drainage visible in the TM is

dominantly north–south (Fig. 6).

The second piece of evidence supporting sinistral strike-slip

movement on the fault comes from the Tertiary ferruginized and

karstified palaeosurface of the Upper Fars group that is displaced

to the left in the SIR-C image (Fig. 6), again supporting sinistral

strike-slip movement of the fault. Thus, sinistral strike-slip

displacements must have taken place along the Maradi Fault

during the Tertiary period. This age determination is also

supported by field observations showing that Neogene semi-

consolidated non-marine gravels are cut by the Maradi Fault, but

unconsolidated Quaternary gravels are not cut by fault-related

structures (Fig. 7).

2. Quriyat and Tiwi region (WNW and NNE striking)

Clear evidence for young uplift of the Hajar Mountains comes

from the Quriyat and Tiwi areas (Fig. 8). Both major sets of

young faults are identified here. The regional picture shows that

the WNW-striking set comprises a major group or array of linked

faults occurring with c. 50 km spacing. The NNE-striking set

also shows c. 50 km spacing between fault arrays, and is

intersected by a third, NE-striking fault set that shows less well-

developed 50–100 km spacing between arrays. All of these faults

cut the Tertiary limestones and several cut Quaternary marine

terraces.

Fig. 8. Structural map of the Quriyat–Tiwi

region. It shows that both the WNW-

trending and the NNE-trending faults form

a major array of linked structures, with

about 50 km spacing. All these faults cut

Tertiary limestones and several cut

Quaternary marine terraces, providing clear

evidence for young uplift of the Hajar

Mountains here.

T. KUSKY ET AL .878

Figure 9 shows a view of Pliocene–Quaternary terraces in the

Tiwi area. The terraces occur at five main levels oriented WNW,

parallel to the coast. These include terraces at 10–20 m, 30–

50 m, 110 m, 140–150 m and 180–200 m above sea level. The

terrace deposits consist of a few metres of calcarenite matrix

conglomerate with Pecten and Ostrea mollusc shells lying on old

marine platforms, with reworked Eocene foraminiferal micro-

fauna (Wyns et al. 1992). The observation that these terraces are

elevated well above the highest Pliocene–Quaternary eustatic

sea-level highs (þ80 m at 4–5 Ma) shows that the region has

experienced considerable Pliocene–Quaternary uplift that ex-

ceeds 100 m. This uplift is attributed to differential motion on

the WNW- and NNE-striking faults that cut the mountain front.

Near the coast, just north of Tiwi, a NNE-striking zone of

fracture intensification strikes into the Gulf of Oman. On the

map (Fig. 8), most of the fractures strike north–south, showing

characteristics of very steep to vertical dips. However, NNE-

striking fractures are the most abundant in the field, many of

which have visible open karst features developed along them.

Analysis of Landsat TM thermal band (band 6) data suggests that

ground water is flowing through this fault and seeping out in

large quantities offshore this location (Fielding et al. 2001). Two

ages of faulting can be discerned at this fault. First, a pre-

Tertiary fault system is well exposed near Hawyl Al Quwasim

village (Fig. 8), where karst pinnacles and collapse features

developed along the fault. These are filled with conglomerates

made of the Hajar limestone, which is Tertiary in age. These

conglomerates are cut by a second generation of faults, followed

by karst dissolution along them, which led to the collapse of the

Tertiary limestones into the newly formed sinkholes and graben.

Further evidence for very recent faulting is found in Wadi

Dayqah in the eastern Hajar range. Here a WNW-trending fault

cuts a falaj in a down-to-north normal sense (Fig. 10; Kusky et

al. 1998). The cement-lined channel of the falaj shows evidence

of having been repaired and then fractured again. There is no

quantitative knowledge of the age of this falaj, but local villagers

suggest it may be between 300 and 1000 years old. They also

speak of a time several generations ago when the ground shook,

destroying buildings in the village. It may be speculated that

motion on one of the WNW-striking faults may have caused this

historical earthquake.

Faults of the Dayqah fracture intensification zone were mapped

across the entire area for more than 50 km (Fig. 8). In the field

they appear to consist of at least three parallel zones, each a few

metres wide, separated by tens to hundreds of metres, and

characterized by a denser fracture network than in surrounding

areas. Thus, they may represent a principal fracture intensifica-

tion zone, which accommodated the Tertiary–Quaternary uplift

of the Hajar Mountains.

3. Wadi Taww–Wadi Al Ajal area (WNW striking)

The Wadi Taww–Wadi Al Ajal area is located in a complex belt

of Hawasina nappes in thrust contact against the northern margin

of Jabal Nakhl (Fig. 11). This area is located along the possible

westward extension of the listwaenite line, a late, heavily

Fig. 9. Photograph of the Pliocene–

Quaternary terraces in the Tiwi area.

Terrace surface in the foreground is 30–

50 m above sea level, whereas the

background shows several levels of terraces

up to several hundred metres above sea

level, eroded in the Cretaceous limestone of

Jabal Bani Jabir. These terraces have been

elevated by over 100 m above the Pliocene–

Quaternary eustatic sea level, indicating

that the region experienced considerable

uplift during that time period.

Fig. 10. Photograph of a faulted falaj thought to be between 1000 and

300 years old. The fault trends NW and as it crosscuts this youthful

structure it confirms that many of the NW faults mapped in Figure 5 may

be youthful structures. For scale, the black arrow in the bottom points to

a geologist.

NEOTECTONICS OF THE HAJAR MOUNTAINS 879

mineralized fault that strikes WNW across the north end of Jabal

Nakhl.

At Wadi Taww, Hawasina nappes and interthrust ophiolitic

slices are overlain by several levels of Quaternary gravel terraces,

with the lower terrace deposits being cemented and the upper

terrace deposits being only partly cemented. A detailed map

(Fig. 12) was made of the lower terrace that directly overlies the

Hawasina Group, and a reconnaissance survey of other units in

the area was also carried out to make structural measurements of

faults cutting the different units. The detailed map clearly shows

the complex deformation in the Hawasina nappes, but more

relevant to this study is the observation that two sets of faults

(WNW and NNE) cut both the Cretaceous Hawasina sedimentary

rocks and the Quaternary gravels; therefore, these faults are

demonstrably of Quaternary age.

4. Bahla–Bisyah region (NW and NNE striking)

The area around Bisyah is structurally complex in terms of

faulting. It is situated at the extrapolated junction of two major

late fracture zones, one striking NW and the other NNE (Fig.

13). The regional fracture intensification zone is not exposed in

Bisyah because of the extensive alluvial cover of Wadi Sayfam

and its tributaries. Two complementary approaches were taken in

the field to better understand the NW- and NNE-striking fracture

zones. First, field analysis of outcrops in rocks of the Hamrat

Duru group exposed near Bisyah and in the mountains to the

north and NW was aimed at locating and characterizing any

minor structures that may be associated with the main faults.

Second, time domain electromagnetic (TDEM) geophysical

surveys were carried out, and seismic reflection profiles were

obtained from the petroleum industry, both of which cross the

alluvial channels and plains (Fig. 13). These geophysical techni-

ques are directed at finding deep channels and buried fault

traces.

With respect to the first approach, Figure 13 shows the

orientation of fractures in the ophiolite (Fig. 13, plot a) and in

the Hawasina units (Fig. 13, plot b) in the Bani Shukayl area of

Bisyah. Fractures in both units have a wide range of orientations,

but show two concentrations, the strongest of which relates to a

NW-striking set and the other to a NNE-striking set (Fig. 13).

These faults are generally open structures, although calcite veins

Fig. 11. Landsat TM image showing the

location of the Wadi Taww–Wadi Al Ajal

area, on the northern margin of Jabal

Nakhl. This area is located along the

listwaenite line, a late, heavily mineralized

fault that strikes WNW.

T. KUSKY ET AL .880

seal some. Many faults have karst features developed along them,

with strings of caves visible along the faults. Some large caverns

on Jabal Kawr, for example, are developed at the intersections of

conjugate faults.

Another outcrop demonstrates the presence of the north–NNE

and WNW sets of faults, 7 km north of Bisyah (Fig. 13; plot c).

Pink and white limestone breccia of the Triassic–Cretaceous

Aqil Formation (Umar group of the Hawasina nappes) is exposed

here, where the intersection of two major faults can be examined.

Both major faults are more than 5 m wide and are characterized

by highly porous fault breccia, and many minor faults splaying

off the major structures. The minor faults have a wide range of

orientations, the most abundant of which strike NE (Fig. 13,

plot c).

In the Bahla region, a major north–south lineament is mapped

from the satellite imagery. In the field, structural manifestations

of this fault occur in outcrops on both sides of the north–south-

striking fault zone. The measurement of c. 80 fractures from this

location reveals two statistical concentrations into a north-

striking, east-dipping set and a WNW steeply north-dipping set

(Fig. 13, plot a). Farther SE at location c (Fig. 13), the north-

striking fault set is still present, but fractures of this set are

overwhelmed in number by both NE-striking, NW- and SE-

dipping conjugate faults, and NW-striking, NE- and SW-dipping

conjugate faults (Fig. 13, plot c).

West of Bahla, more closely spaced and wider strands of a

NNE-striking fault system are present, suggesting closer proxi-

mity to a major fault (Fig. 13, plot d). The faults in the outcrop

are similar in character to those at outcrop a, but also include

some serpentinized ductile shear zones that splay into brittle

fractures, which form pinnate joints of a dextral system. On the

east side of Bahla, an outcrop of cumulate gabbro was examined

(Fig. 13, location b), and this is cut by faults of both the NNE

and the WNW sets.

Thus, structural data from the Bahla area suggest that a major

NNE-striking dextral fault passes through the town and that this

fault is associated with NE- and WNW-striking minor faults.

Figure 14 shows an interpretation of seismic profiles across

Wadi Sayfam. These are important in that they show prominent

faults extending to several kilometres depth, and the surface

traces of these faults coincide with lineaments mapped from the

TM images. This confirms that the NW- and NE-striking fracture

intensification zones exist at depth and that they reach several

hundred kilometres in length (Fig. 1).

5. Al Hamra area (NNE and NW striking)

Several areas on the southern flank of Jabal Akhdar preserve

evidence of young faulting along both the north–NNE trend and

the WNW–NW trend, as described below. For example, Al

Hamra area features prominent trellis and parallel drainage that

suggests association with fractures (Fig. 15). The SPOT image of

the area SE of Misfah, near Al Hamra, shows a small swarm of

NW-striking faults intersecting the main north–south fault.

In the field it is clear that a NE-facing escarpment south of

Misfah is formed by erosion of one of the NW-striking faults that

continues into Wadi Ma’qal (Fig. 15). Many small caves are

developed along this part of the fault system. At this location a,

exposures of Cretaceous (Albian–Cenomanian) limestone of the

Natih Formation are cut by NW- and NE-striking faults and

joints (Fig. 15, plot a). It is observed that fractures of the NW-

striking set are spaced at c. 50–100 cm, whereas those of the

NE-striking set are spaced approximately every metre. Both sets

are generally open, apparently young fractures, with apertures (at

the surface) of several millimetres. At location b near Misfah

village, the NE and NW faults and fractures are present, as well

as an additional set of north-striking, vertical fractures that

control the Misfah gorge orientation (Fig. 15, plot b).

6. Ghul area (north–south striking)

A north–south-striking fault with Quaternary displacements cuts

through the Wadi Ghul–Wadi Nakhr region, along the southern

flank of the central Hajar range (Fig. 16). It forms the western

boundary to the ‘Grand Canyon’ of Oman, which offers more

than 1000 m of vertical relief (Fig. 4). The juvenile topography

of the Grand Canyon–Wadi Ghul area suggests that it formed by

relatively recent uplift, and that erosion has not yet had a chance

to develop a more mature landscape. Furthermore, fieldwork in

the southern part of the area, SW of the modern Ghul village

(Fig. 16), shows that a cemented Quaternary terrace in the wadi

bed is preserved c. 10 m below a less cemented terrace. The

older terrace is remarkable in that it is cut by a number of faults

(Fig. 16, plot a), including several subparallel strands of a north–

south-trending fault that is parallel to the major north–south

fault mapped as the western margin of Wadi Nakhr and the

Grand Canyon. The fault (Fig. 17) cuts some cobbles in the

Fig. 12. A detailed map of the lower Quaternary gravel terrace found

upstream in Wadi Taww. The map shows that the WNW and NNE faults

cut Cretaceous Hawasina sedimentary rocks and Quaternary gravels.

NEOTECTONICS OF THE HAJAR MOUNTAINS 881

conglomerate and curves around others, showing that it formed

at very shallow levels.

7. Faulted Quaternary terrace in Al Ghail, UAE (WNWstriking)

Field investigations were also carried out in the neighbouring

regions of the UAE where the analysis of Landsat images

suggests that some young faults may cut Quaternary fluvial

terraces (Fig. 18). An area upstream from Al Ghail on the

western slopes of the Hajar Mountains was investigated where

the Landsat TM image shows a prominent NW-striking fault.

Several areas with Quaternary terraces (estimated to be about

100 ka old) are exposed, providing an opportunity to investigate

the age relationship between various sets of faults and terraces.

A low cemented fluvial terrace at this location shows several

Fig. 13. Landsat TM image of the Bahla–

Bisyah area with transects of seismic

reflection profiles (dotted lines). Bisyah is

located at the intersection of a major NW-

striking and north–south-striking fracture

intensification zones. Labels denote places

where detailed structural measurements

were made in the field. Stereonets from the

Bahla area (above the image) show

orientation of fractures at locations a, b, c

and d near Bahla. Stereonets below the

image show orientation of fractures (a and

c) and beds (b) near Bisyah.

T. KUSKY ET AL .882

tectonic fractures, some of which cut clasts in the gravel. A large

Quaternary fault that cuts both the cemented and the overlying

uncemented terraces is exposed 10 km up the wadi from Al

Ghail. The fault is one of the WNW sets that cut the mountain

range in many places and consists of a zone about half a metre

thick of brecciated gravel exposed as fault gouge. The fault cuts

through the lower 15 m thick terrace, and continues into the

overlying wadi terrace (Fig. 19).

A model for the neotectonics of the Hajar Mountains,and tectonic controls on uplift, subsidence and theformation of the late fault sets

The Hajar Mountains reside on the foreland margin of the active

collision zone between the passive Arabian margin and the

Zagros–Makran fold belts (Fig. 1). This collision involves the

shallow subduction of continental crust of the Arabian Shield

Fig. 14. Interpretations of the seismic

profiles taken across Wadi Sayfam. Profile

‘Bisyah’ is the southeasternmost profile;

line ‘North of Bisyah’ is the

northwesternmost profile. These show

prominent faults that extend to several

kilometres depth. The surface traces of

these faults (indicated by vertical arrows on

lines of section) correspond to the WNW

and north–south fracture intensification

zones mapped from the Landsat TM image.

NEOTECTONICS OF THE HAJAR MOUNTAINS 883

beneath the Zagros Mountains in the west, with shallow waters

of the Persian (Arabian) Gulf in the active foredeep (Glennie et

al. 1990; Mann et al. 1990). East of the Dibba Line on the

Arabian margin, and the Zendan Fault in Iran, the collision

includes stretched continental crust grading into sea floor in the

Gulf of Oman (White & Klitgord 1976), which is being

subducted beneath the Makran accretionary wedge (Fig. 1).

The tectonic history of the northern Oman margin is complex,

including the development of a Permian–Cretaceous carbonate

platform on the southern margin of the Tethys, a remnant of

which is preserved in the Gulf of Oman (Mann et al. 1990). Late

Cretaceous deformation associated with the collision of the north

Oman margin with a probable forearc environment terminated

passive margin sedimentation, emplaced the Samail ophiolite

(Glennie et al. 1973, 1974; Pearce et al. 1981), and initiated

foredeep sedimentation in the Aruma foreland basin on the

eastern edge of the Arabian plate (Robertson 1987). Emplace-

ment of the Samail ophiolite was thus associated with closure of

the Tethys, although the ophiolite is probably forearc lithosphere

and not true Tethyan oceanic lithosphere. Ophiolite emplacement

strongly deformed and imbricated the shelf sediments of the

Hajar Group, oceanic and continental slope sediments of the

Hawasina and Sumeini Groups, and formed metamorphic rocks

of the dynamothermal aureole. The large basement-cored anti-

formal domes of Jabal Akhdar, Nakhl and Saih Hatat are

probably related to movement of the entire thrust package over

deep basement thrust ramps, deep duplexing, or blind thrust

faults propagating under the domes in the Late Cretaceous (e.g.

Hanna 1986; Coffield 1990; Al-Lazki et al. 2002). This deforma-

tion ended by Late Campanian (Glennie et al. 1974). Latest

Cretaceous and Early Tertiary rocks record shallow marine,

quiescent conditions.

Convergence between the north Oman margin and the

Zagros–Makran appears to have been re-established by the

Eocene (Coleman 1981; Mann et al. 1990). Contractional

structures on Musandam have been related to Arabian–Zagros

convergence, whereas Tertiary structures in the central Oman

Mountains have been variously ascribed to gravity tectonics

(Glennie et al. 1974) or down-to-the-basin normal faults along

the Batinah coastal plain (Mann et al. 1990). Mann et al. (1990)

and Nolan et al. (1990) showed how Maastrichtian–Oligo-

Miocene structures have controlled the distribution of several of

Fig. 15. SPOT image (169–304) of Al

Hamra, on the southern flanks of Jabal

Akhdar. It shows a swarm of NW-striking

faults that intersect the NNE-trending fault.

The stereonets further confirm the

prevalence of the NW and NE faults at

locations a and b. Both fault sets are

generally open, with apertures of several

millimetres at the surface.

T. KUSKY ET AL .884

the Tertiary stratigraphic units, and that the Saih Hatat dome

remained high during this period.

The contact between the Tertiary sedimentary sequence and

the Samail ophiolite on the Batinah coastal plain is in many

places a WNW-striking normal fault, downthrown toward the

Gulf of Oman (Mann et al. 1990). Listric faults with roll-over

anticlines are present, as are horsts and grabens, accommodation

structures and antithetic normal faults (Mann et al. 1990). The

Late Tertiary structures have a consistent down-to-the-north or

-NE sense on the Batinah Plain, and appear to be continuous

with extensional structures in the Gulf of Oman (White & Ross

1979). Although Mann et al. (1990) recognized the Tertiary

extensional province on the Batinah plain, they were not able to

provide any clear tectonic cause for the extension, suggesting

that perhaps the structural culminations of Saih Hatat and Nakhl

were collapsing, or that gravity tectonics initiated extension. We

recognize that this extension has continued through the Quatern-

ary, and present a new tectonic mechanism that explains the

regional pattern of faulting.

The driving forces for Late Tertiary–Quaternary uplift in

the northern Oman Mountains are most probably related to the

contemporaneous collision between the NE margin of the Arabian

plate and the Zagros fold belt and the Makran accretionary prism.

The axis of recent uplift in the Hajar Mountains lies about 150 km

from the active thrust front of the Makran prism, suggesting that

the NW and WNW faults may be outer-trench slope normal faults

(e.g. Bradley & Kusky 1986) that accommodate uplift on the

flexural bulge of this collision. The NNE-striking faults are

interpreted as tear faults separating blocks that have slightly

different rotational histories. The inferred strike-slip motions of

some of the WNW-striking faults are enigmatic. The geometry of

the Makran–Arabia collision implies that these faults may have a

component of dextral shear, as suggested by Hanna & Nolan

(1989). However, new observations presented here suggest that

the Maradi fault may also have experienced sinistral motions. Al-

Lazki et al. (2002) presented deep seismic data that help resolve

the conflicting kinematics inferred for the Maradi Fault. They

showed that the Maradi Fault (Fig. 5) is a deeply penetrating thrust

fault, which displaces the top of the Hajar Supergroup 900 m up

on the NE side of the fault. The fault breaks into many splays near

the surface, forming a flower structure, some strands of which

show normal displacements and some of which show thrust and

strike-slip displacements.

Geomorphological features of the Musandam Peninsula reveal

that an east–west-striking flexure passes through the centre of

the peninsula. The flexure marks a drainage divide, where wadis

Fig. 16. SPOT image (169–304) of the

Wadi Ghul–Wadi Nakhr region, near the

Grand Canyon region of Jabal Akhdar,

which is cut by the north–south-striking

fault. The stereonet shows the presence of

the north–south fault along with the WNW

structures at location c.

NEOTECTONICS OF THE HAJAR MOUNTAINS 885

to the north of the line flow to the north and those to the south of

it flow to the south (see Fig. 20). The mountains are higher in

the south, and become progressively lower and more subdued to

the north. Valleys to the north of the drainage divide are drowned

and flooded by the sea, whereas to the south the valleys are still

alluvial.

We interpret these features to mark the position of the flexural

bulge on the Musandam Peninsula. The area north of the

drainage divide is dipping to the north and subsiding under the

weight of the Zagros fold belt; hence, the drowning of the valleys

and the gently north-sloping maximum elevation surface. The

area south of the divide is on top of the flexural bulge, or just

moving up to the maximum height as the bulge migrates south-

ward. Numerous raised beaches have been recognized in this area

(Ricateau & Richie 1980).

The flexural bulge axis is located considerably further south in

the western and eastern Hajar Mountains, and we suggest that it

is offset across the Dibba Zone (Figs 1 and 2), which apparently

separates blocks of different types of lithosphere on the west and

east. The southward displacement of the bulge axis also corre-

sponds to the southward displacement of the thrust front (and

load) from the Zagros to the Makran across the Zendan Fault

(Fig. 1). The flexural lithospheric thickness and rigidity of the

Arabian lithosphere must increase toward the SE, as the bulge

axis appears to broaden and move further from the thrust front in

this direction.

Conclusions

The present work integrates remote sensing, field mapping and

GIS techniques to reveal evidence that two major sets of late

Fig. 17. Photograph of a cemented Quaternary terrace in the Wadi Ghul

area. The older terrace is cut by several strands of a north–south-trending

fault, indicating that faulting in this area continued into the Quaternary.

Ancient house of Old Ghul village in the centre of the photograph gives

scale.

Fig. 18. Landsat TM image of Al Ghail in the UAE. It shows a large

NW fault, of the WNW trend, crosscutting the area. This fault appears to

cut several Quaternary fluvial terraces as indicated in the TM image.

This was confirmed in the field (Fig. 20), supporting Quaternary

movement along the fault at this location.

Fig. 19. Photograph of a cemented Quaternary deposit, which is cut by a

fault of the WNW set, overlying uncemented wadi terraces. The fault

cuts through the lower 15 m thick terrace, and continues into the

overlying wadi terrace.

T. KUSKY ET AL .886

faults and fracture intensification domains are responsible for

Tertiary–Quaternary uplift of the northern Oman Mountains.

Evidence for the young age of uplift and faulting includes

juvenile topography, faulted Quaternary marine terraces and a

fractured falaj. The young faults occur as a WNW-striking set

parallel to the Maradi Fault with a major group or array of linked

faults occurring with c. 50 km spacing. A second set of dextral

faults striking NNE shows less well-developed 50–100 km

spacing between arrays. It is suggested that the two late fault

systems described here may have accommodated Tertiary–

Quaternary uplift of the northern Oman Mountains, and can

account for the previously unexplained juvenile topography.

A new tectonic mechanism is proposed to explain the regional

pattern of faulting and the Tertiary–Quaternary extension. The

driving forces for Late Tertiary–Quaternary uplift in the northern

Oman Mountains are most probably related to the contempora-

neous collision between the NE margin of the Arabian plate, the

Zagros fold belt and the Makran accretionary prism, suggesting

that the axis of recent uplift in the Hajar Mountains lies about

150 km from the active thrust front of the Makran prism. This

implies that the NW and WNW faults may be outer-trench slope

normal faults (e.g. Bradley & Kusky 1986) that accommodate

uplift on the flexural bulge of this collision. This is further

supported by their similarity to those faults in active and ancient

forebulge environments, including the amount of Quaternary

uplift, estimated between 100 and 500 m. The NNE-striking

faults are interpreted as tear faults separating individual blocks

that have slightly different rotational histories.

The axis of uplift is offset to the north across the Dibba line

(striking NE) to the Musandam Peninsula (Figs 2 and 20),

suggesting that it marks a fundamental lithospheric break. The

Dibba line is close to parallel to the Zendan Fault separating the

Makran and Zagros fold belts in Iran, suggesting that this break

has manifestations in both the upper and lower plates. The offset

of the axis of uplift could be related to different lithospheric

thicknesses or flexural rigidities of blocks on either side on the

lower plate, or could be related to differences in the load

geometry on the upper plate.

Implications for active tectonics and seismic hazards

Data presented here show that the Hajar Mountains of northern

Oman and the UAE have undergone very recent to active

deformation and uplift, and therefore the region has more

serious seismic risks than previously appreciated. Global posi-

tioning system and seismic monitoring stations have recently

Fig. 20. Drainage map of the Musandam

Peninsula, showing the drainage divide with

wadis flowing north and south on either

side. This divide is thought to indicate a

flexural bulge that resulted from a collision

between the NE margin of the Arabian

plate and the Zagros fold belt and the

Makran accretionary prism in Late Tertiary

to Quaternary time. The mountains are

higher to the south of the bulge axis and

progressively become lower and more

subdued in the north (Fig. 4).

NEOTECTONICS OF THE HAJAR MOUNTAINS 887

been set up in Oman under the Earthquake Monitoring Project,

based at Sultan Qaboos University in Muscat. This effort is

critical to further assess active neotectonics along the study

area. Seismic monitoring coupled with further field and geophy-

sical studies aimed at documenting Tertiary and Quaternary

structures will lead to a greater understanding of the tectonics

of the region, and a better ability to assess earthquake hazards

and risk zones.

This research was performed as part of a 3 year groundwater research

project, funded by the Ministry of Water Resources, Muscat, Sultanate of

Oman. The authors are grateful to the graduate assistants at the Center

for Remote Sensing, Boston University, who helped digitize the maps

and maintain the GIS database. Assistance in the field was provided by

A. Al-Malki and P. Considine. We thank an anonymous reviewer and

Brian Windley for comments that helped to improve the manuscript.

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Received 24 September 2004; revised typescript accepted 17 January 2005.

Scientific editing by Ian Alsop

T. KUSKY ET AL .888