ABSTRACT

The Jabal Qumayrah area of the northern Oman Mountains records the evolution and subsequent destruction of a Mesozoic passive continental margin in the Oman segment of the Neo-Tethys Ocean, followed by the re-establishment of a passive margin, punctuated by phases of Tertiary compression. Almost uniquely along the Oman Mountains, it also contains intrusions of salt.

Detachment of oceanic sediments and volcanics during the early phases of NE-directed subduction beneath the nascent Semail Ophiolite created an in-sequence stack of imbricated thrust units comprising distal trench units (Haybi Complex), and deep-ocean and continental rise sediments derived from the Mesozoic Oman margin (the Hawasina Complex). These were emplaced onto the depressed margin beneath and ahead of the ophiolite during its obduction in the Cenomanian– Coniacian. The Mesozoic continental slope sediments of the Sumeini Group had already been largely over-ridden by the more distal thrust sheets when the Hawasina sole thrust propagated into those sediments. This detached a Sumeini Group thrust sheet, which was transported westward for at least 7 km, carrying with it the overlying Hawasina thrust stack. Structurally lower parts of the Hawasina thrust stack (Hamrat Duru Group) also extended ahead of the Sumeini Group thrust sheet, but they were not restacked with it, indicating motion continued along this part of the Hawasina sole thrust. Further footwall collapse detached at least one more imbricate within the Sumeini Group and the combined thrust stack was then folded along a N-S axis, possibly above a frontal ramp. This was associated with complex out-of-sequence forward and back-thrusting at the lower structural levels. A right-lateral scissors fault developed at right angles to the direction of nappe transport, associated with normal faulting down-to-south. Late-stage culmination within the nappe pile created an asymmetrical west-facing dome, around which the structurally overlying Hawasina thrust sheets are folded.

Passive margin sedimentation was re-established in the Campanian–Maastrichtian following subsidence of the locally emergent nappe pile and was dominated by carbonate sedimentation with little clastic input from the ophiolite or Hawasina sediments. Stable sedimentation persisted until Oligocene–Miocene compression, synchronous with the Zagros compressional event in Iran, resulted in west-facing folding along the western side of the northern Oman Mountains and their subsequent uplift.

The Jabal Qumayrah massif preserves a salt intrusion composed of gypsum and anhydrite, the top of which is now exposed in the centre of the culmination. The origin of the salt remains unclear and investigations continue. Possible sources include the extension of the major regional salt basins found in the foreland, in particular those at the Ediacaran/Cambrian boundary (Ara Group), beneath the Hawasina Nappes and Semail Ophiolite. Alternatively, evaporitic basins may have developed locally along the edge of the proto Neo-Tethyan margin during the earliest rifting phase, beneath what became the continental slope deposits, although there is little evidence for these elsewhere in the autochthonous shelf succession.

INTRODUCTION

The Oman Mountains preserve Mesozoic continental margin and deeper-water sediments that were emplaced onto the Arabian continental margin in the Late Cretaceous together with the Semail Ophiolite, a fragment of the Neo-Tethyan oceanic crust and upper mantle (Glennie et al., 1973, 1974; Rabu et al., 1993). Rifting in the Oman sector of Neo-Tethys started in the Middle Permian (De Wever et al., 1988; Blendinger et al., 1990; Chauvet et al., 2009) with the establishment of a carbonate platform, a proto-continental slope and an offshore basin. A major rifting event in the Mid to Late Triassic is linked to the start of sea-floor spreading the Neo-Tethyan Ocean and resulted in the creation of the “Hawasina Ocean”. The mainly continental slope deposits of the Sumeini Group passed into continental rise sediments of the Hamrat Duru Group which then passed oceanwards to a chain of intra-oceanic carbonate platforms and seamounts, now represented by the Kawr and Al Aridh groups, and deep-ocean cherts and shales of the Umar Group. The Hawasina Ocean subsequently closed during the Late Cretaceous, and initiation of NE-dipping subduction led to the formation of the Semail Ophiolite at ca. 95 Ma. Obduction of the ophiolite together with underlying trench deposits (Haybi Complex), Hawasina Ocean sediments (Hawasina Complex) and syn-obduction foredeep sediments (Aruma Group) onto the Arabian continental margin spanned the Late Cretaceous (Glennie et al., 1973, 1974; Bernoulli and Weissart, 1987; Robertson, 1987; Béchennec et al., 1990; Cooper, 1990; Robertson and Searle, 1990). Passive margin sedimentation resumed in the Late Cretaceous (Campanian–Maastrichtian) and continued episodically through the Tertiary (Nolan et al., 1990; Skelton et al., 1990).

The Jabal Qumayrah area (Figures 1 and 2) is a mountainous region located on the western side of the northern Oman Mountains, 40 km southeast of the cities of Al Ayn and Buraimi. It provides a well-exposed section through the allochthonous units of the Hawasina Complex and their overlying post-obduction cover. With the Dibba Zone, Jabal Sumeini, and the Hawasina Window (Figure 2 inset), it is one of a limited number of places where the predominantly in-sequence thrust stack has been modified by late-stage out-of-sequence thrusting and the development of culminations of Sumeini Group slope sediments which have punched their way through the overlying thrust pile. The Sumeini Group of Jabal Qumayrah forms a domed culmination some 15 x 25 km in area.

The Jabal Qumayrah area is also almost unique in the Oman Mountains in containing a late-stage salt intrusion, preserved as highly rotted remnants within the lowest structural elements. The only previous reference to the salt is in passing by Csontos et al. (2010), who briefly considered the only other reported instance of localised salt bodies in the Hawasina Window.

In this paper we document the structural evolution of the Jabal Qumayrah area from detailed mapping and field studies to constrain processes involved in the obduction of the Semail Ophiolite and Hawasina sediments and the subsequent evolution of the area.

TECTONO-STRATIGRAPHICAL FRAMEWORK OF THE JABAL QUMAYRAH AREA

Figure 3 compares the stratigraphy in the Jabal Qumayrah area of the Mesozoic sedimentary units of the Hawasina Ocean. The shelf carbonates of the Mesozoic Arabian continental platform, the Hajar Supergroup, are not exposed in this area, although they emerge as major culminations both to the north in the Musandam Peninsula, and to the south at Al Jabal Al Akhdar (Searle, 1988, 2007). Seismic and gravity investigations suggest they are present at depth (Boote et al., 1990; Ali et al., 2008; Searle and Ali, 2009).

Sumeini Group – Slope Sediments

The Jabal Qumayrah massif is formed from the predominantly continental slope deposits of the Jurassic and Cretaceous Mayhah, Huwar and Qumayrah formations of the Sumeini Group (Watts, 1987, 1990; Le Métour et al., 1991). The Mayhah and Huwar formations are divided into informal members, here referred to for brevity as the Mayhah 1 member, Mayhah 2 member etc. The Late Permian to Early Jurassic Maqam Formation, exposed to the north in Jabal Sumeini, is not seen in Jabal Qumayrah. It may be present at depth, but exposed detachment surfaces lie higher up the stratigraphical pile.

The Sumeini Group comprises over 700 m of limestone conglomerates and calcarenites derived from platform and slope material that are typically associated with well-developed turbidite structures, and platy, sometimes argillaceous calcisiltites and calcilutites with fine-grained rippled or laminated bed bases, interbedded with shales or marls. Stratigraphical sections in the western part of Jabal Qumayrah at jabals Fayyad (West), Gashnah and Sanqah (Figure 3, section A) are significantly thicker than those in the east at jabals Fayyad (East) and Al Huwar (Figure 3, section B), in particular in the Late Jurassic Mayhah 4 member. This member reaches 600 m thickness in the west, compared with about 180 m in the east. The Huwar Formation is only found in the east and centre of Jabal Qumayrah, where it starts with a characteristic interval of Tithonian to Early Cretaceous silicified limestones and cherts (the informal 1st member).

The lower part of the Qumayrah Formation comprises ca. 100 m thick of Cenomanian to Coniacian red cherts (Watts and Blome, 1990), shales and massive calcirudites overlain by a thick, highly deformed sequence of brown-weathering shales with thin silicified limestones and cherts and rarer quartz-rich sandstones that are most frequent in the highest stratigraphical levels. The Qumayrah Formation lies conformably above the Huwar Formation in the east, and on the Mayhah 4 member in the west. Le Métour et al. (1991) have suggested this reflects erosional down-cutting into the sediment pile as the margin edge collapsed. However, lateral variation may also play its part as the succession in Jabal Fayyad (West) appears essentially conformable, although there is an absence of corroborative dating evidence.

The sedimentology of the western jabals has been interpreted as representing an upper slope environment at the edge of the Arabian platform, whereas those in the east formed as a base-of-slope debris apron (Watts and Garrison, 1986; Watts 1990).

Continental Rise and more Distal Units of the Hawasina Ocean

The Hamrat Duru Group (Figure 3 section C) comprises a sequence of shales, calcarenite and rare calcirudite turbidites, and silicified limestones and cherts, the latter forming two distinctive units dated as Late Triassic (Carnian to Norian), and latest Jurassic to Early Cretaceous (Tithonian to Albian, Cooper 1986, 1990; Le Métour, 1991) correlating with the Huwar 1 member. The Late Cenomanian age of the Nayid Formation (Le Métour, 1991) suggests it is a lateral equivalent of the lower part of the Qumayrah Formation. Individual units show lateral variation on a kilometre scale, and the sedimentology indicates deposition in a comparatively sediment-starved continental rise location, with periods of higher off-margin sediment input leading to the development of carbonate fan lobes, and periods of subsidence below the calcite compensation depth (CCD) encouraging the formation of cherts (Cooper, 1990).

The more distal units of the Hawasina Ocean are exposed mainly in a north-south orientated ridge, the Hawrat al ‘Asan, to the south of Jabal Qumayrah. It consists of dismembered Upper Triassic reefal limestones overlain by Kimmeridgian to Valanginian condensed limestones (Kawr Group), locally associated with small exposures of pillow lavas and red cherts of the Umar Group. These have been interpreted as the remnants of a chain of seamounts within the Hawasina Ocean that foundered in the Late Jurassic (Glennie et al., 1974; Béchennec et al., 1990; Pillevuit et al., 1997). A smaller outcrop of these “Oman Exotic” limestones and basic volcanics is found at the northwestern end of Jabal Qumayrah.

Haybi Complex

The Haybi Complex is a distinct structural package; a thrust sheet above the Hawasina Complex and below the Semail Ophiolite (Searle and Malpas, 1980, 1982). In this sector, it is mainly exposed in the Asjudi Window to the east of Jabal Qumayrah. It contains dismembered elements of the Kawr and Umar groups, sedimentary and tectonic mélanges, and serpentinite sheets. The Haybi Complex also includes amphibolite and greenschist facies metamorphic rocks that became detached from the metamorphic sole of the ophiolite (Searle and Cox, 2002). The Haybi Complex has been interpreted as trench and subduction zone rocks that were accreted along the base of the ophiolite during initial oceanic displacement, and then incorporated into the distal units of the thrust stack (Searle and Malpas, 1980, 1982; Searle and Cox, 1999, 2002).

Semail Ophiolite

The Semail Ophiolite includes at least 5 km thickness of oceanic crust and over 10–15 km thickness of upper mantle harzburgites and dunites (Reinhardt, 1969; Glennie et al., 1974). The ophiolite is exposed along the northern, eastern and southern sides of the Jabal Qumayrah massif where it comprises a broad zone of mantle sequence rocks. The structurally lowest unit, lying immediately above the ophiolite sole thrust, is a distinct sequence of tectonically banded ultramafic rocks, mainly harzburgite and dunite (the Banded Ultramafic Unit), formed as a result of high-temperature ductile shear during the early emplacement history of the ophiolite (Searle and Malpas, 1980, 1982). It is usually separated from the underlying Haybi Complex by serpentinites, which also intrude along thrust planes in the Haybi Complex.

Maastrichtian and Tertiary Sediments

Maastrichtian and Tertiary sediments are seen along the western side of the Jabal Qumayrah area where they lie with a markedly angular unconformity on the allochthonous Hawasina Complex and Semail Ophiolite (Figure 2). They record events following final emplacement of the Semail Ophiolite and the filling of its foredeep, followed by peripheral margin effects from collision in the Zagros zone.

Initial sub-aerial erosion of the allochthonous units planed the imbricated Hamrat Duru Group and was followed by the deposition of a thin succession of poorly-exposed ferruginous sandstones that locally contain pebbles of the underlying Hawasina and ophiolite (the ?Late Campanian– Maastrichtian Qahlah Formation, Abdelghany, 2003). This was followed by a marine transgression, with rapid deepening and the deposition of Simsima Formation limestones, linked to continued isostatic adjustment of the margin to weight of the Semail Ophiolite (Nolan et al., 1990; Skelton et al., 1990), then by a shallowing linked to a global fall in sea level (Haq et al., 1987). The top of the Cretaceous and most of the Palaeocene is marked by a disconformity, which is widespread across the western part of the Oman Mountains (Nolan et al., 1990; Le Métour et al., 1991; Noweir and Eloutefi, 1997; Noweir et al., 1999; Noweir and Alsharhan, 2000).

A second period of carbonate-rich sedimentation started in the Late Thanetian (Late Palaeocene) and continued until the Bartonian (Eocene) with shallow-water, inner-shelf environment towards the south (Umm Er Radhuma Formation), passing northwards into deeper shelf to slope facies of the Muthayminah Formation. It ended with the tidal flat deposits of the Rus Formation in the south and, to the north, erosion surfaces and down-cutting limestone megabreccias of the upper Muthayminah Formation. These are overlain by the Eocene (Lutetian to Bartonian) Dammam Formation, which represents another transgressive phase followed by the establishment of a carbonate shelf, again with deeper-water facies towards the north (Le Métour et al., 1991).

Rocks younger than Bartonian are rare in the Jabal Qumayrah area, but a more complete succession is seen at Jabal Hafit, 20 km to the east. Here the Dammam Formation is overlain unconformably by the Oligocene Asmari Formation and, in the United Arab Emirates, a further unconformity separates that from the Miocene Fars Group, represented in isolated exposures through the foreland outwash gravels by the Barzaman Formation (Le Métour et al., 1991; Warrak, 1996; Noweir, 2000; Searle and Ali, 2009).

The Maastrichtian and Tertiary rocks contain little material from the Hawasina complex, limited to small pebbles of chert and ophiolite in the basal Qahlah Formation and chert pebbles locally in the Muthayminah and Barzaman formations (Le Métour et al., 1991).

STRUCTURE OF JABAL QUMAYRAH

The Jabal Qumayrah massif (Figures 1 and 2) has an inverted teardrop shape with an arcuate ridge of peaks up to 1,400 m high along its north and east side comprising Jabal Fayyad (West) and (East) and Jabal Al Huwar, and a NS-trending ridge up to 1,140 m high that forms the western side of the massif from its mid-point south of Wadi Sumer, comprising Jabal Ghashnah and Jabal Sanqah, separated by the Wadi Qumayrah Gorge. The main drainage is through wadis Sumer, Lisail and Milh. The Jabal Qumayrah culmination comprises three structural elements, the Huwar-Fayyad, Sumer-Milh and Ghashnah-Sanqah thrust sheets (Figure 2), here named after their main topographical features. The western side of Jabal Qumayrah is cut by the Sumer Fault, which forms a major structural divide across this part of the range. Figures 4 and 5 show detailed geological maps of the northern and southern parts of Jabal Qumayrah. Figure 6 contains cross sections across the area.

Huwar-Fayyad Thrust Sheet

On a mountain scale, the Huwar-Fayyad Thrust Sheet forms two, kilometre-scale west-facing anticline-syncline pairs, which have been domed so that fold axes plunge to both north and south. It can be considered in three parts, from southeast to northwest Jabal Al Huwar, Jabal Fayyad (East) and Jabal Fayyad (West). These represent in turn, the two major folds and the lower synclinal limb emerging to the west. Restoration of cross sections suggests the Ghashnah-Sanqah Thrust Sheet was linked to the Huwar-Fayyad Thrust Sheet from which it is now separated through faulting and erosion (Figure 7).

Jabal Al Huwar

The Jabal Al Huwar segment (see Figure 4) comprises a broad, 6 km wavelength asymmetrical west-verging anticline-syncline pair. The east-facing limbs typically dip between 25° to 40° and are comparatively undeformed. West-facing limbs are steep to vertical and more tightly folded en echelon on a 10–100 m scale (Figure 6 cross section C-C′, photograph Figure 8a). Fold axes are aligned N-S in the south of Jabal Al Huwar, but swing round towards NW-SE towards the north. The sole thrust of the Huwar-Fayyad Thrust Sheet runs along the eastern side of Wadi Milh and the Sumer Bowl, putting the Mayhah Formation over the upper Qumayrah Formation of the structurally underlying Sumer-Milh Thrust Sheet. The whole thrust sheet is domed and the northern and southern flanks dip away at between 30° to 60°.

The eastern side has been cut by two major NS-trending high-angle faults with throws of 50 to 200 m that have created a minor graben within the centre of the range. These cut across emplacement-related folds. Other late-stage, high-angle faults form a conjugate set trending NE-SW (035°) and ESE-WNW (110°) and focus on the salt deposits in Wadi Lisail. The sole thrust of the Huwar-Fayyad Thrust Sheet is also topographically lower in this area, and the preserved thrust sheet extends to the west on the north side of Wadi Lisail. The prevailing eastward dip of the thrust sheet is reversed adjacent to the Wadi Lisail salt. These features are discussed later with the Qumayrah salt.

The northern and eastern sides of this part of the Huwar-Fayyad Thrust Sheet are structurally overlain by sediments of the Hamrat Duru Group that are locally interleaved with thrust slithers of Qumayrah sediments. On the southern side of Jabal Al Huwar along Wadi Qumayrah, the Hamrat Duru and higher thrust sheets are not present and Qumayrah Formation sediments of the Sumeini Group are locally seen adjacent to serpentinites at the sole of the Semail Ophiolite.

Jabal Fayyad (East)

The Huwar-Fayyad Thrust Sheet at Jabal Fayyad (East) is folded into another mountain-scale NW-facing anticline-syncline pair (location Fe1 on Figure 4, Figure 6 cross section A-A′). The upper limb has been mostly eroded, but is seen along the northern edge of the Sumer bowl, where it forms high-level cliffs that are almost horizontally bedded. The fold plunges to the NE as a consequence of the doming of Jabal Qumayrah. The west-facing limb is vertical to steeply-overturned at wadi level, with higher stratigraphical levels folded into large (100 m) scale flat-lying recumbent folds, the limbs of which have been locally cut out by thrusting. Doming of the Jabal Qumayrah massif is also associated with a series of late-stage, low-angle top-to-the-north low-displacement thrusts along this northern side of the mountain.

The intensity of folding increases to the south and the major fold pair becomes recumbent at the level of the mountain peaks (location Fe2 on Figure 4). This inverted recumbent limb is subject to parasitic recumbent folding and thrusting. The eastern side of this fold has been truncated by later-stage faulting and shales of the upper Qumayrah Formation rest against the whole inverted sequence.

The lower terrain that separates Jabal Fayyad (East) from Jabal Fayyad (West) comprises tightly folded shales, cherts and conglomerates of the lower part of the Qumayrah Formation that forms an east-verging recumbent syncline (location Fe3 on Figure 4, Figure 6 cross section A-A′). Although Jabal Fayyad (East) has a full Huwar Formation stratigraphy and Jabal Fayyad (West) is characterised by its absence and a greatly thickened Mayhah 4 member (Figure 3), the implication is that these two areas are part of the same major thrust sheet.

Jabal Fayyad (West)

The mountainous area of Jabal Fayyad (West) comprises two separate elements, separated by the “Fayyad Fault” (Figures 2 and 4). This is a vertical fault and the largest of a number of broadly NE-orientated faults in the northern part of the Huwar-Fayyad Thrust Sheet that have lowered the northern side of Jabal Qumayrah. While the dominant movement was down-to-the-north, slickenslides preserve a complicated evolution path, with at least four separate phases of movement.

The structure of Jabal Fayyad (West) to the north of the Fayyad Fault is contiguous with that of Jabal Fayyad (East). Its eastern part (location Fw1 on Figure 4) comprises a west-facing anticline with bedding dipping approximately 30° to the NE, and vertical to steeply overturned to the west where it is associated with parasitic cascade folding. A vertical fault near the base of the lower Qumayrah Formation separates this part from the rest of the area to the north of the Fayyad Fault. This comprises a thick (30+ m) sequence of lower Qumayrah Formation conglomerates, coarse-grained limestone turbidites, cherts and shales that lie directly on the Mayhah 4 member. They are gently warped and dip to the north at between 10° and 30°.

The major part of Jabal Fayyad (West) lies to the south of the Fayyad Fault, with an entirely different structure. This extremity of the Huwar-Fayyad Thrust Sheet forms a broad U-shaped syncline that plunges about 15° to the north (location Fw2 on Figure 4, Figure 6, cross section A-A′). The base of the eastern limb is within the Mayhah 1 member, which dips steeply to the west. The western limb also steepens sharply towards the western edge of the Jabal, where it has been tightly folded. In contrast, the central part of the jabal is almost undeformed (photograph Figure 8b). This southern part of Jabal Fayyad (West) is structurally underlain by shales of the upper part of the Qumayrah Formation, which can be seen along the east, south and southwest side. However, along the western, leading edge of the thrust sheet, the Huwar-Fayyad Thrust Sheet has been emplaced over folded and inverted sediments of the Hamrat Duru Group.

Ghashnah-Sanqah Thrust Sheet

The Ghashnah-Sanqah Thrust Sheet lies to the south of Wadi Sumer and the west of Wadi Milh (Figures 2 and 5). It comprises jabals Ghashnah and Sanqah, separated by the Wadi Qumayrah gorge. Its stratigraphy is similar to that of Jabal Fayyad (West), with a greatly thickened Mayhah 4 member and absence of the Huwar Formation. The overall structure of the Ghashnah-Sanqah Thrust Sheet is a doubly-plunging west-facing anticline-syncline pair (cross section A-A′ in Figure 6).

The eastern side of the trust sheet is composed of Mayhah 3 member limestone conglomerates and calcarenites that are faulted against a slither of the Qumayrah Formation of the Sumer-Milh Thrust Sheet. Bedding is vertical in the north, in Jabal Ghashnah, but becomes progressively overturned and east-dipping at higher topographical levels and to the south so that, south of Wadi Qumayrah, inverted beds dip to the east at between 60° and 30°. Here, parasitic folding has exposed an elongate slither of the Qumayrah Formation of the structurally underlying Sumer-Milh Thrust Sheet in window through the Mayhah 2 and 3 members (location GS1 on Figure 5).

This is the western limb of a major west-facing syncline, cored by the Qumayrah Formation (location GS2 on Figure 5, photograph Figure 9a). The right-way-up western limb of the syncline dips evenly eastwards at about 35° and smaller-scale folding is almost entirely absent. This syncline becomes progressively cut out to the south by faulting which, on Jabal Sanqah, places inverted Mayhah 3 member limestones and a thin slice of the Mayhah 4 member against the Qumayrah Formation (location GS3 on Figure 5). This fault increases in throw to the south and eventually cuts out the whole of the inverted eastern limb of the syncline.

The western side of Jabal Ghashnah and Jabal Sanqah comprises the anticlinal frontal fold to the west of the central syncline. Bedding is much steeper, varying between 60° and vertical, with subordinate parasitic folding. Thrusting along this limb has locally cut out much of the Qumayrah Formation. This thrust dies out to the south as the west-facing anticline along the western edge of Jabal Sanqah plunges gently southwards so that only the conglomeratic lower Qumayrah Formation is exposed at the southernmost end of the range. As the southern end, the elongated anticline of Jabal Sanqah plunges southwards beneath the Hamrat Duru Group thrust sheets and the Hamrat Duru Group sole thrust is folded around the Sumeini Group anticline. Smaller-scale folding of the thrust stack is well-exposed along the western side of Jabal Sanqah, where the lower Qumayrah Formation has been locally re-thrust over the Hamrat Duru Group (location GS4 on Figure 5).

At the northern end of Jabal Ghashnah, the folded Ghashnah-Sanqah Thrust Sheet plunges northwards at 10° to 20° until it is truncated by the Sumer Fault, which sets it against the Sumer-Milh Thrust Sheet.

Sumer-Milh Thrust Sheet

The Sumer-Milh Thrust Sheet is the structurally lowest unit, located in central and south-central Jabal Qumayrah (Figures 2, 4 and 5). No sole thrust is seen, but internal imbrication indicates it is fundamentally allochthonous. It is structurally overlain in the north and east by the Huwar-Fayyad Thrust Sheet, under which the tight, to isoclinally folded and sheared upper Qumayrah Formation dips eastwards at 25° to 35°. Higher Hawasina nappes (Hamrat Duru Group, etc.) are not present between these Sumeini Group imbricates.

The geometry of the Sumer-Milh Thrust Sheet is influenced by the Sumer Fault. This runs 080° along Wadi Sumer and the northern end of the Ghashnah-Sanqah Thrust Sheet. It (or a branch of it) then crosses southeast into Wadi Lisail where it intersects with the Lisail salt. Smaller salt intrusions are also found along this section of the fault, discussed later. Branches then run 070° and 110° eastwards into the Huwar-Fayyad Thrust Sheet (Figures 2 and 4). Vertical displacement increases to the west, with different structural styles seen to the north and south.

North of the Sumer Fault, the Sumer Milh Thrust Sheet is folded into a broad west-facing anticline-syncline pair, mirroring the structure of the overlying Huwar-Fayyad Thrust Sheet (cross sections A-A′ and B-B′ in Figure 6). Increased large-scale complexity is seen in lower structural levels along the western side of the Sumer Bowl and south of Jabal Fayyad (West), where the central part of the thrust sheet, forming the steep, west-facing anticline limb has been dismembered by both forward-thrusting and back-thrusting that cuts through the Sumer Milh Thrust Sheet at least as deep as the Mayhah 3 member.

Along the northwestern side of the Sumer Bowl, a complexly folded imbricate is exposed through upper Qumayrah lithologies on the north, east and south sides. It forms a mountainous area 1.5 km x 0.7 km (location SM1 on Figure 4) with a north-east trend that contrasts with the general north-south grain of the area. It youngs to the southeast and is cored by tightly folded Qumayrah Formation cherts and shales. The Mayhah 4 member of the northwestern side is overturned, and dips as low as 60° to the north. The southeastern side is back-thrust to the southeast over the upper Qumayrah Formation, with increasingly lower stratigraphical levels exposed to the northeast.

A second major structure (location SM2 on Figure 4) lies immediately to the south of this overturned imbricate, and runs southwards for 1.5 km to Wadi Sumer. This is a back-thrust imbricate within the Sumer-Milh Thrust Sheet. Its eastern margin comprises the lower Mayhah 4 and, locally, Mayhah 3 member in a steep, to vertical contact with the upper Qumayrah Formation of the central Sumer Bowl. At its northern end, the bedding is attenuated and overturned, becoming steeply north-dipping along the SW-NE contact with the ‘SM1 imbricate’. Dips of this ‘SM2 imbricate’ decrease to the west and become essentially horizontal before a second back-thrust imbricate (location SM3 on Figure 4). This is developed as a tight east-verging syncline with an axial plane that dips gently to the northwest, and the lowest-exposed beds in the Mayhah 4 member are faulted against the lower Qumayrah Formation conglomerates and cherts of the imbricate immediately to the east.

The high ground to the west and north of the SM1 and SM2 structures comprises an isoclinally recumbent folded sequence of the upper Mayhah 4 member, and Huwar and Qumayrah formations which becomes increasingly inverted to the south. This is connected to the SM1 imbricate by highly attenuated Mayhah and Huwar limestones and cherts, and thus forms its SW extension. Similarly, although the contact with the northern part of the SM2 imbricate is faulted, this recumbent fold is linked to the northern end of the SM2 imbricate where it forms an overturned upper limb (photograph Figure 9b). The evolution of this area is considered in the Discussion section and in Figure 14.

Further to the west, the Sumer-Milh Thrust Sheet becomes structurally simpler (Cross section B-B′ in Figure 6). A west-directed, locally thrust-out anticline passes to the west into comparatively flat-lying upper Qumayrah Formation shales to the south of, and in thrust contact with the overlying Huwar-Fayyad Thrust Sheet on Jabal Fayyad (West). These shales are, in turn, thrust over mainly inverted, structurally overlying Hamrat Duru Group sediments. This fault has a high angle, a comparatively small displacement and is probably last-stage as it cuts through both the Sumeini and Hamrat Duru group thrust sheets.

South of the Sumer Fault (Figure 5, cross section C-C′ in Figure 6), the main part of the Sumer-Milh Thrust Sheet crops out as an east-dipping imbricate with Mayhah 4 member limestones thrust over a thin slither of the Qumayrah Formation, which is itself in fault contact with inverted Sumeini Group (Mayhah 3 member) limestones of the Ghashnah-Sanqah Thrust Sheet (location SM4 on Figure 5). The intensity of thrusting of this imbricate decreases to the north and the thrust tips out in a west-facing anticline in Wadi Lisail that plunges gently to the north.

The sole thrust of this imbricate also cuts up-section to the south, so by Wadi Qumayrah, the lowest exposed stratigraphical unit is the upper part of the Huwar Formation, and this disappears south of Wadi Qumayrah, where the Sumer-Milh Thrust Sheet is represented only by the Qumayrah Formation, faulted at about 60° with inverted Mayhah Formation of the Ghashnah-Sanqah Thrust Sheet on its west side. Qumayrah Formation sediments of the Sumer-Milh Thrust Sheet lie adjacent to the structurally higher Semail Ophiolite to the east – higher units of the Hawasina Complex and Haybi Complex are missing.

The Qumayrah Formation to the east of Wadi Milh is poorly exposed, but tightly folded red cherts and conglomerates attributed to the lower Qumayrah Formation crop out within the shale-rich sediments of the upper part of the formation. This implies significant folding and/or thrusting within this part of the Sumer-Milh Thrust Sheet. This area is also notable as it contains an arcuate, discontinuous band of salt outcrops that extend south from the main salt deposits in Wadi Lisail, which are discussed later.

STRUCTURE OF ADJACENT AREAS

The areas surrounding Jabal Qumayrah can be divided into three general parts, the Asjudi Window to its east, the Hamrat Duru Group imbricates to the west and southwest, and the Tertiary foreland folds along the western edge of the Oman Mountains (Figures 2 and 3). Their structures reflect the early evolution of the Jabal Qumayrah area and restacking centred on the culmination of the Sumeini Group linked to the emplacement of the Semail Ophiolite, and later peripheral impact of the Zagros Orogeny.

Asjudi Window

The Jabal Qumayrah massif is bounded on its eastern side by the Asjudi Window, which contains imbricates of the Hamrat Duru Group and Haybi Complex surrounded by the basal units of the Semail Ophiolite to the north, east and south, with the Sumeini Group culmination of Jabal Qumayrah to the west. The Haybi Complex comprises mainly dismembered amphibolites and greenschist facies metamorphic rocks, Late Triassic tholeiitic volcanics and mélanges at the base of, and structurally interleaved with serpentinites beneath the Semail Ophiolite (Searle and Malpas, 1980). They are notably absent from the north and south sides of the Jabal Qumayrah massif. The Haybi Thrust Sheet structurally overlies the Hamrat Duru Group thrust sheet. This is restricted to Al Jil and Matbat formation shales and cherts, representing the Triassic and lower Jurassic part of the Hamrat Duru Group stratigraphy. Higher sequences are absent, with the exception of a single band of the Guwayza to Nayid formations at the southeastern corner of Jabal Al Huwar (Le Métour et al., 1991). The Hamrat Duru Group is tightly folded and imbricated on a 50 m to 150 m scale. The Hawasina and Haybi thrust sheets have been folded around the lower, later culmination of Jabal Qumayrah.

Western Qumayrah Hamrat Duru Group

The Hamrat Duru Group along the western side of Jabal Qumayrah consists of parallel folded and imbricated thrust sheets, orientated NW-SE in the north which swing to N-S in the south part of the area. There are three major structural units, identified by their stratigraphy and structural arrangement (Figure 2).

The Wadi Suwar unit occupies much of the outcrop area. It comprises an imbricate fan in which individual thrust slices can be traced laterally for over 15 km and display restored widths perpendicular to their direction of transport that can exceed 2 km. The thickest imbricates have their sole thrust in the Matbat Formation shales, but many contain smaller-scale imbrication in higher stratigraphical levels (Figure 10). Imbricates generally dip eastwards at 45° to 60°; however those closest to the Sumeini Group of Jabal Sanqah have been folded into a broad syncline parallel to the culmination (Figures 4 and 6 cross section C-C′, photograph Figure 11a). The intensity of folding increases to the northeast where imbricates are inverted and east-dipping (Figure 6, cross sections A-A′ and B-B′). It is unconformably overlain to the west by Maastrichtian and Tertiary limestones.

The distinctive chert-rich succession of the easterly inverted imbricate of the Wadi Suwar imbricate fan has been overthrust by a belt of characteristic brown weathering coarser-grained folded Guwayza and Matbat rocks, the West Fayyad sequence in Figures 2, 4 and 6. These rocks are also mostly inverted. The geometry thus suggests they were originally structurally below the Wadi Suwar imbricate fan, but were re-stacked at late-stage during their emplacement. The Wadi Uqaybah area in the southeast contains tightly folded thrust sheets of more distal chert-rich Hamrat Duru Group facies that lie structurally below the Kawr and Umar Group of the Hawrat al ‘Asan. (Cooper 1986, 1990; Le Métour et al., 1991).

The exposed zone of Hamrat Duru Group imbricates is widest to the west of Jabal Qumayrah and narrows to both north and south, where these rocks are concealed beneath post-emplacement Maastrichtian and Tertiary sediments and the Semail Ophiolite. Seismic and gravity investigations suggest that the Hamrat Duru imbricate fan is buried at depth (Boote et al., 1990; Ali et al., 2008) in those areas. This suggests comparative uplift of the central part of the Hamrat Duru Group to the west of Jabal Qumayrah.

Maastrichtian and Tertiary Structures

Post-emplacement Maastrichtian and Tertiary sediments form a topographically high range along the western side of the Jabal Qumayrah area and rest with a marked angular unconformity on the planed Hamrat Duru Group surface. These predominantly shallow-water to deeper shelf limestones have been folded into a series of west-facing en echelon frontal folds (Figure 2). West-facing limbs tend towards the vertical while the upper, eastern limbs of folds are essentially horizontal with gentle warping (Figure 10, photograph Figure 11b).

Direct evidence of Tertiary folding within the Jabal Qumayrah massif is limited. A small area of Nummulitic (Eocene) limestones that lies unconformably over the Hamrat Duru Group crops out about 1.5 km west of from the frontal thrust of Jabal Fayyad (West) (Figure 2) has been tilted to the north and folded on a 50 m scale. Uppermost Cretaceous and earlier Tertiary rocks are missing, even though they are preserved just 2 km to the west, where they are gently folded into a north-plunging anticline. This is consistent with the general northward tilt of the Hamrat Duru imbricate fan and its lower topographical position within the Hamrat Duru area and also suggests that the Late Palaoecene disconformity affected the Hawasina Complex well to the east of what is now the main frontal fold belt.

There is no direct surface evidence of Tertiary thrusting in the Jabal Qumayrah area, even though this is seen to the north, where Sumeini Group rocks at Jabal Sumeini are locally thrust over Maastrichtian limestones, and to the south where, for example, thrusting in the Tertiary sequences to the south of Wadi Fatah around the town of Dhank has been attributed to the reactivation of Cretaceous emplacement-related thrusts (Le Métour et al., 1991).

The second major outcrop of Tertiary rocks lies beyond the boundaries of the Jabal Qumayrah area at Jabal Hafit, about 20 km to the west of the main mountain front. This is a doubly-plunging anticline about 22 km long and 4 km wide that exposes limestones and marls of Early Eocene to Miocene age. A secondary anticline-syncline pair is developed at its northern end. Unlike most structures in the area, the asymmetrical fold faces eastwards and is associated with a local, steep east-directed thrust fault (Searle and Ali, 2009). Its geology and evolution has been discussed by Warrak (1996), Boote et al. (1990), Noweir (2000) and Searle and Ali (2009). This structure was probably initiated during the Late Oligocene, but its main period of growth was post Miocene (Searle and Ali, 2009). It thus post-dates the main phase of folding along the western front of the Oman Mountains. Seismic investigations by Boote et al. (1990) suggest that reactivated basement faults also influenced the development of Tertiary folds along the western side of the northern Oman Mountains.

SALT INTRUSIONS IN JABAL QUMAYRAH

The centre of Jabal Qumayrah contains a number of isolated deposits (the “Qumayrah salt”) that are interpreted as highly weathered salt deposits (Figures 1, 2 and 11). Investigations into the Qumayrah salt continue and this discussion focuses on the relationship between the salt and the Sumeini Group of Jabal Qumayrah.

The salt deposits are characterised by masses of white to cream gypsum and anhydrite. Smaller outcrops are heavily weathered and the gypsum-rich matrix is largely structureless. Larger outcrops and, in particular, the main salt deposit in Wadi Lisail contain the elongate ghosts of bedding, and blocks and rafts of bedded fine-grained limestone, sandstone and shale that can exceed 100 m in length and 50 m thick. Some are clearly derived from the local Sumeini Group, but the origin of the rest is not known although a Qumayrah Formation source is likely.

The main salt deposit, in Wadi Lisail, is roughly L-shaped (Figure 12, photographs Figure 13). The central area has a diameter of about 400 m, extending for a further 200 m to the southwest and 500 m to the southeast. Boundaries with the Qumayrah Formation of the Sumer-Milh Thrust Sheet are steep to vertical and cut across bedding and all structures related to the folding and thrusting within the thrust sheet. Locally, coarse conglomeratic beds can be traced into the salt where they become fragmented, indicating in situ intrusion of the salt. The northern boundary of salt is against Mayhah Formation limestones of the Huwar-Fayyad Thrust Sheet. Where exposed, the boundaries are steep and the host rock has been steeply folded upwards. It is notable that, in this area, the regional NE dip in the Huwar-Fayyad Thrust Sheet has been reversed and, for an area up to 1 km around the salt, beds dip to the west at up to 20°. The along-strike altitude of the sole thrust of the Huwar-Fayyad Thrust Sheet has also been depressed, from about 1,000 m to the north and south, to about 650 m adjacent to the Wadi Lisail salt. The Wadi Lisail deposit lies at the focus of two sets of faults that cut into the Huwar-Fayyad Thrust Sheet and are orientated ENE and ESE, the main one being linked to the Sumer Fault.

Two lines of salt deposits extend out from the main Wadi Lisail centre, cropping out along mainly scree-covered hill sides in discrete, widely-separated bodies, which suggests that they may not be continuous at the surface, although it is likely that they are connected at depth. The “S” series on Figure 12, follows a straight, vertical fault orientated 307° for 1 km to reach the main axis of the Sumer Fault at the northern nose of Jabal Ghashnah. The “S3” outcrop lies where the faults meet and shows a well-exposed vertical boundary with highly folded Mayhah 4 Formation limestones of the Ghashnah-Sanqah Thrust Sheet. The folding does not affect the salt. Other outcrops follow the line of the Sumer Fault for 1 km as far as where Wadi Sumer debouches from the Sumer Bowl. The “M” series on Figure 12 lie on a 2.5 km gently arcuate south-trending line on the eastern side of the Wadi Milh valley. The outcrops are separated by poorly exposed Qumayrah Formation country rock. The intersection of the outcrop pattern and the topography suggests intrusion along a steeply W-dipping fault, although on a smaller scale more complex arrangements are seen, with both vertical plug and horizontal sill-like bodies seen at the “M2” locality.

DISCUSSION: TECTONIC EVOLUTION OF THE JABAL QUMAYRAH AREA AND IMPLICATIONS OF SALT INTRUSION

The structural configuration of the Jabal Qumayrah area primarily reflects the obduction of the Semail Ophiolite and Hawasina sediments onto the Arabian margin during the Late Cretaceous. The Semail Ophiolite was formed early in the Late Cretaceous (Tippet and Pessagno, 1979; Tilton et al., 1981; 95.3 ± 0.2 Ma from U-Pb dating of ophiolitic plagiogranite by Warren et al., 2005), coupled with NE-directed subduction of the Triassic and Jurassic Neo-Tethyan oceanic crust, evidenced by U-Pb zircon ages from sub-ophiolite amphibolites of 94.5 Ma (Warren et al., 2005) with cooling through hornblende closure temperature (ca. 500°C) at 92.6–94.9 Ma (Hacker, 1994; Hacker et al., 1996). Compression and loading linked to the onset of subduction caused the edge of the continental margin to subside to create the Aruma foredeep (Glennie et al., 1974; Robertson, 1987; Patton and O’Connor, 1988; Boote et al., 1990; Warburton et al., 1990). Erosion, subsidence and local collapse of the margin edge, possibly influenced by pre-existing rift-related faults, resulted in the Cenomanian–Coniacian debris sheets of the lower part of the Qumayrah Formation (Watts, 1990; Le Métour et al., 1991) and their Hamrat Duru Group equivalents (Cooper, 1990). While the timing and configuration of the subduction zones in the southern part of the Oman Mountains has been a topic of debate (for example Gregory et al., 1998; Gray et al., 2000, 2004; Gray and Gregory, 2003; Searle et al., 2003, 2004; Breton et al., 2004, 2005; Searle, 2007), there is no evidence from the Jabal Qumayrah area to suggest there was anything other than northeast-directed intra-oceanic subduction in this sector of the Oman margin.

Initial Emplacement

Subduction of more distal sediments of the Hawasina Ocean created the trench mélanges of the Haybi Complex and dismembered intra-oceanic platform and deep-ocean duplexes of the upper Hawasina nappes (Kawr and Umar groups). The mainly continental-rise sediment of the Hamrat Duru Group behaved more coherently. Initially, the Hawasina sole thrust propagated margin-wards within the Matbat Formation, creating the laterally extensive imbricates of the Wadi Suwar and Wadi Uqaybah imbricate fans. Footwall collapse then resulted in the imbrication of the stratigraphically and structurally lower Asjudi Window and west Fayyad successions. The Hamrat Duru Group duplexes were partly overridden by, and partly bulldozed ahead of the advancing ophiolite, in common with this sequence in the south-central Oman Mountains (Cooper, 1988, 1990, 2011). Relationships between the Hamrat Duru Group thrusts and the syn-emplacement foredeep sediments in the south-central Oman Mountains indicate that the Hawasina thrust sheets were emplaced onto the continental margin during the Santonian (Warburton et al., 1986, 1990). The present-day wrapping of the Hamrat Duru Group around Jabal Qumayrah indicates they were emplaced over the subsided continental slope as a comparatively thin-skinned imbricate fan.

As the mass of the Semail Ophiolite and its leading-edge wedge of mélanges and deeper-water Hawasina sediments were emplaced onto the depressed edge of the Arabian margin, the Hawasina sole thrust propagated into the Sumeini Group slope deposits. Searle et al. (1990) have suggested that the separate culminations of Sumeini Group thrust sheets at Jabal Sumeini, Jabal Qumayrah and in the Hawasina Window may reflect their palaeogeographical positions on promontories along the Oman margin and the impinging of the advancing ophiolite complex on those promontories during the early obduction phase. The sedimentology of the Mayhah Formation in the Jabal Qumayrah area offers further circumstantial evidence insofar as the successions in Jabal Fayyad (West) and jabals Ghashnah and Sanqah, containing a greatly thickened Mayhah 4 member and the absence of the Huwar Formation, restore to a position between the lower Sumer-Milh Thrust Sheet and the more distal parts of the Huwar-Fayyad Thrust Sheet of Jabal Fayyad (East) and Jabal Al Huwar. This variation trend is broadly perpendicular to the direction of nappe transport and the broad trend of the palaeo-Oman margin, which points towards an offset along the margin edge in the Jabal Qumayrah sector.

The absence of pre-Jurassic rocks (Maqam Formation) in the Jabal Qumayrah area suggests that the décollement surface developed towards the base of the Mayhah 1 member. This first detachment probably consisted of a single thrust sheet comprising what are now the Huwar-Fayyad and Ghashnah-Sanqah thrust sheets, given their structural positions above the Sumer-Milh Thrust Sheet.

The present-day distribution of the Hamrat Duru Group suggests that the imbricate fan in the Wadi Suwar area immediately to the west of Jabal Qumayrah had been thrust over the slope sediments before thrusting penetrated the latter. Thus, the absence of Hamrat Duru Group rocks between the structurally higher (distal) Huwar-Fayyad Thrust Sheet and structurally lower (proximal) Sumer-Milh Thrust Sheet suggests that the Huwar-Fayyad detachment did not breach the structurally overlying Hamrat Duru Group duplexes. Instead, the Hamrat Duru Group continued to move along its sole thrust over the as yet in situ Sumer-Milh Thrust Sheet ahead of and above the Huwar-Fayyad Thrust Sheet as initial imbrication in the Sumeini Group progressed. Restoration suggests that the leading edge of the Huwar-Fayyad Thrust Sheet moved at least 7 km relative to the underlying upper Qumayrah Formation sediments of what is now the Sumer-Milh Thrust Sheet and that it was emplaced as a comparatively rigid mass, with no significant internal imbrication.

Continued Compression

Continued compression during ophiolite obduction led to propagation of the sole thrust into the underlying slope sediment wedge of the Sumer-Milh Thrust Sheet. In the absence of high resolution sub-surface information, there are insufficient data to determine whether there are further detachments beneath the Sumer-Milh Thrust Sheet, although this is likely by analogy with seismic and gravity profiles to the north of the Jabal Qumayrah area (Searle and Ali, 2009; Tarapoanca et al., 2010). West-directed transport of the nappe pile resulted in the development of the kilometre-scale west-facing anticline that runs from north to south through the middle of Jabal Qumayrah, possibly as a consequence of ramp-development at depth. This folded the thrust plane between the Huwar-Fayyad and Sumer-Milh, and Ghashnah-Sanqah and Sumer Milh thrust sheets and, as compression intensified, resulted in the deeper-seated forward- and back-thrusting in the Sumer Milh Thrust Sheet along the western side of the Sumer Bowl and Wadi Milh.

Figure 14 illustrates the proposed evolution of these thrusts and associated folds in the Sumer Bowl. Initially, west-directed thrusting in the north created the ‘SM1’ imbricate, while to the south, back-thrusting created the ‘SM2 and SM3’ imbricates (Figures 14a and d). These were separated by a folded transfer zone. Continued east-west translation was accompanied by clockwise rotation of the ‘SM1’ imbricate along the northwestern side of the Sumer Bowl (Figure 14b). At the same time, it became progressively overturned, so that it is now preserved as a west-verging recumbent syncline and out-of-the-syncline back-thrusting placed the Huwar and Mayhah formations against the upper Qumayrah Formation along the southern edge of the ‘SM1’ imbricate (Figure 14c). Similarly, folding in the transfer zone between the ‘SM1’ and ‘SM2’ imbricates intensified, creating a second flat-to-overturned recumbent syncline.

The rotation of the ‘SM1’ imbricate is probably the result of a combination of lateral compression along the northern edge as deep-seated imbrication within the Jabal Qumayrah culmination was constrained by the Semail Ophiolite to the north and drag along the northern end of the imbricate as it was emplaced towards the west. This south-directed compression also explains the folding and northern termination of the ‘SM2’ seen in the present-day structural configuration (Figure 14d).

The Sumer Fault was also initiated during this stage of the evolution of Jabal Qumayrah as a scissors fault, broadly at right angles to the direction of nappe transport. Differential movement to the north and south of the Sumer Fault resulted in the present-day geometrical configuration. The crest of the major anticline through the centre of Jabal Qumayrah was off-set to the west by about 1 km on the north of the Sumer Fault. There is a similar off-set between the corresponding syncline that separates Jabal Fayyad (West) and Jabal Fayyad (East) in the north, and the syncline that runs through the centre of Jabal Ghashnah on the south side of the fault. North of the Sumer Fault, the Sumeini Group thrust sheets of Jabal Fayyad (West) are at a significantly higher topographical level and the folded western side of the Huwar-Fayyad Thrust Sheet reaches an altitude of almost 1,100 m. It, and the folded but broadly horizontal structurally underlying Sumer-Milh Thrust Sheet are exposed a further 4 km to the west compared with south of the fault, where the frontal fold of Jabal Ghashnah dives steeply beneath the wadi gravels and Hamrat Duru Group. The Sumeini Group thrust sheets to the north of the Sumer Fault also dip southwards towards the fault, but along-strike projection from the west front of Jabal Fayyad (West) still suggests vertical displacement on the Sumer Fault may exceed 500 m. High-angle thrusting of the Sumer-Milh and Huwar Fayyad thrust sheets over the Hamrat Duru Group along the western side of Jabal Fayyad (West) suggests that at least part of the comparative uplift of the area to the north of the Sumer Fault is the consequence of late-stage reverse faulting through the assembled thrust stack.

South of the Sumer Fault, continued compression resulted in the development of a kilometre-scale west-verging anticline-syncline pair along the western side of Wadi Milh and northwards into the Sumer Bowl as far as the Sumer Fault. As shortening increased, the eastern limb of the syncline along the eastern side of Jabal Ghashnah and Jabal Sanqah became overturned and the locus for out-of-sequence thrusting along the western side of Wadi Milh, in which imbricated the lower Sumer-Milh Thrust Sheet against a slither of the Qumayrah Formation and its original thrust contact with the structurally overlying Ghashnah-Sanqah Thrust Sheet. Similarly, the Hamrat Duru Group thrust sheets to the west of Jabal Ghashnah were also affected by this out-of-sequence thrusting, and west-directed folding. Smaller-scale re-imbrication of the Hamrat Duru – Qumayrah boundary also led to slithers of the latter being locally caught up between imbricates of the Hamrat Duru Group along the eastern side of Jabal Qumayrah. This late-stage breaching of the initial thrust stack and re-imbrication of Sumeini and Hamrat Duru group thrust sheets is seen elsewhere in the Oman Mountains, in the Jabal Sumeini area, 80 km to the north (Searle et al,. 1990; Searle and Ali, 2009), and the Hawasina Window, 70 km to the east-southeast (Searle and Cooper, 1986; Csontos et al., 2010).

Final development of the Sumer Fault in Jabal Qumayrah did not occur until a late stage during emplacement of these thrust sheets as it also truncates emplacement-related folds and thrusts, in particular along the northern end of Jabal Ghashnah.

The Sumer Fault did not penetrate westwards into the structurally overlying Wadi Sumer Hamrat Duru Group imbricate fan. Instead, a west-dipping overturned imbricate of the Hamrat Duru Group forms a continuous ridge across the line of the fault and the throw of the Sumer Fault was accommodated by folding and faulting within the poorly-exposed structurally underlying West Fayyad unit of the Hamrat Duru Group (see Figure 2). The thrust at its rear that restacks the West Fayyad Hamrat Duru Group unit probably reflects the surface expression of reverse faulting in the Sumeini Group to the west of the main culmination. Imbricates in these duplexes were rotated and overturned, suggesting that this boundary developed as a large-scale west-facing anticline-syncline pair that was breached by thrusting that preserved the overturned upper synclinal limb in the footwall.

The doming of Jabal Qumayrah affects the Huwar-Fayyad and Sumer-Milh stack and fold axes related to the initial emplacement of the Huwar-Fayyad Thrust Sheet are themselves folded around the dome on both the north and south sides. On the northern side of the Jabal Qumayrah massif, lateral forces linked to doming created small-displacement low-angle reverse faults in the Jabal Fayyad (East) sector. The Fayyad Fault was activated at this comparatively late stage in the emplacement sequence, and is the largest of a number of related normal faults with a down-to-the-north movement.

Post Emplacement - Tertiary Events

Emplacement of the Semail Ophiolite and Hawasina thrust sheets was completed by the Late Campanian to Early Maastrichtian, with deposition of the subaerial to shallow-water Qahlat and Simsima formations (Noweir and Eloutefi, 1997; Noweir and Alsharhan, 2000; Abdelghany, 2003). These lie unconformably on mantle sequence ophiolitic rocks to the north and south of the Jabal Qumayrah area, but on planed Hamrat Duru thrust sheets immediately to the west of the Jabal Qumayrah massif, suggesting that the thrust stack had undergone significant erosion during the emplacement process and that uplift was greatest adjacent to Jabal Qumayrah. This in turn indicates the Jabal Qumayrah culmination was a major Late Cretaceous event.

Passive margin sedimentation comprising Maastrichtian to Eocene limestone and marl-rich deposits built out over the largely infilled Aruma foredeep in a series of major shallowing cycles. Periods of emergence in the latest Maastrichtian to Early Palaeocene and Early Eocene can be attributed to contemporaneous recessions in global sea levels (Nolan et al., 1990; Skelton et al., 1990). A minor phase of compression and folding at the end of the Cretaceous, marked by a gentle unconformity identified locally in the northern Oman Mountains (Abou Sayed and Mersal, 1998) cannot be identified with any certainty in this area.

This cyclic sedimentation was curtailed by the major compressional phase that formed the west-facing en echelon frontal folds that dominate the western edge of the Oman Mountains between Jabal Qumayrah and Dhank. Seismic investigation by Boote et al. (1990) suggests this folding represents the shallow expression of deep-seated transpressional reactivation of high-angle basement faults. It is also associated with high-angle faulting that also affects the underlying Semail ophiolite between Dhank and Wadi Khubayb (Le Métour et al, 1991) and further north at Jabal Malaqat (Noweir and Eloutefi, 1997). This Late Tertiary compressional phase is likely to be related to the outlying effects of the collision of Arabia and Central Iran, focused on the Musandam Peninsula, and starting in the Late Oligocene with imbrication of the Musandam shelf along the Hagab Thrust (Searle et al., 1983; Searle, 1988). Folding further to the south, including in the Qumayrah area probably peaked in the mid to Late Miocene and was accompanied by uplift of the Oman Mountains of at least 3,000 m (Boote et al., 1990). Seismic profiles to the north of Jabal Qumayrah near Jabal Sumeini (Searle and Ali, 2009) and to the south of the Musandam Peninsula (Tarapoanca et al., 2010) indicate deep-seated imbrication of the northern part of the Oman margin during the Neogene with thrusts developing in the ‘autochthonous’ Mesozoic shelf sequence and propagating through the Palaeocene to Miocene sedimentary cover. While Hanna (1990) and Mann et al. (1990) have attributed elements of the folding within the Tertiary successions of the south-central Oman Mountains to gravity-driven compression associated with uplift of the mountains, there is no evidence to suggest this had a material impact on folding in the Jabal Qumayrah area.

The Jabal Qumayrah Salt

The position of the Jabal Qumayrah salt in the stratigraphical and structural sequence is poorly constrained. It is located in the lowest structural levels of the centre of the Jabal Qumayrah culmination and a cross-cutting relationship with folds and thrusts in the Sumeini Group host rocks indicates that salt intrusion was clearly a post-emplacement and thus post-Campanian event. This is reinforced by the presence of blocks of Sumeini Group limestones that had already been intensely folded and veined before they were incorporated into the salt. There are, however, no outcrop data to refine more precisely the timing of intrusion.

The origin of the Jabal Qumayrah salt intrusion is under investigation. The proximity of the Wadi Lisail salt centre to the topographical depression of the sole thrust to the Huwar-Fayyad Thrust Sheet and the reversal of bedding dips up to a kilometre from the salt intrusion may reflect significant subsidence in that area, possibly driven by the removal of salt at depth as it moved up through the allochthonous units. Its exploitation of pre-existing faults in the nappe pile, in particular the Sumer Fault and locus at the junction of a series of significant normal faults suggests a strong fault control to the surface distribution. However, the size and location of the salt outcrop pattern suggests that it is unlikely that the final culmination of Jabal Qumayrah was, itself, driven by salt tectonics. While the origin of the doming of Jabal Qumayrah cannot yet be determined unequivocally, it is more probably the product of deeper-seated oblique-lateral ramps in the Sumeini footwall and/or the stacking of smaller, laterally discrete imbricates at depth.

The age of the salt has not yet been established. The most significant salt deposits in northern and central Oman are found in the Ediacarian to Early Cambrian Ara Group, concentrated in large salt basins to the southwest of the Oman Mountains, which have undergone a considerable degree of halokinesis and diapir development (e.g. Heward 1990; Mattes and Conway-Morris, 1990; Peters et al., 2003; Schröder et al., 2003, Reuning et al., 2009). However, Ara salt has not been reported from northern Oman, nor has it been seen in the major windows into the Pre-Permian basement of the Al Jabal Al-Akhdar and Saih Hatat areas.

Evaporites, principally layers of gypsum and anhydrite, are developed within levels of the Late Permian Khuff Formation, linked to restricted basin development during the initial rifting of Neo-Tethys. These are well-developed in the United Arab Emirates, but their distribution in Interior Oman is of comparative limited extent (Hughes-Clarke, 1988; Lee, 1990) and the absence of salts such as halite militates against mobility through halokinensis. A review by Lee (1990) indicated that the equivalent Permian Hagil and Saiq formations of both the Musandam Peninsula to the north and Al Jabal Al-Akhdar and Saih Hatat to the southeast of Jabal Qumayrah comprise more open-marine carbonate platform facies which militates against these being sources of the salt.

Similarly, the Middle Triassic Jilh Formation in north and central Arabia and the Late Jurassic Hith and Arab formations of the western Rub al-Khali contain salt intervals, but these are thin and highly layered. None are diapiric and the equivalent intervals in the interior of Oman and the Oman Mountains (the Mahil Formation, and the upper Sahtan Group and Rayda Formation respectively) do not contain salt. This does not rule out more localised salt development. Csontos et al. (2010) have reported gypsum intrusions along faults within the Hawasina Complex of the Hawasina Window, 75 km to the east-southeast of Jabal Qumayrah. They suggested a possible origin at the base of the Sumeini Group with a Permian age, although without substantive evidence. The inference is that evaporitic basins could have developed along the site of what became the continental slope of the Oman margin during the early stages of Neo-Tethyan rifting. However, this remains very speculative.

Thus the presence of salt in Jabal Qumayrah raises the possibility that the Ara salt basins extend beneath the central and northern Oman Mountains or, possibly, that more localised Permian basins may have developed during early Neo-Tethyan rifting.

CONCLUSIONS

The structural evolution of the Jabal Qumayrah area primarily reflects the Late Cretaceous emplacement of the Semail Ophiolite onto the eastern edge of the Arabian Neo-Tethyan continental margin. An in-sequence thrust stack of distal to proximal units derived from subduction within the Hawasina Ocean has been modified by propagation of the sole thrust into deeper-level slope sediments after they had been partly overridden by the advancing nappe pile. Imbrication at this lower level of thrust sheets that were probably laterally discontinuous, and west-facing large-scale folding of the thrust pile resulted in the development of the domed culmination of Jabal Qumayrah around which wraps imbricate fans of the structurally higher Hawasina Complex. Passive margin sedimentation was reestablished at the end of the Cretaceous and persisted until the Late Tertiary, when compression linked to the early Zagros phase of orogeny in Central Iran resulted in uplift of the northern Oman Mountains and large-scale folding along their western edge. The presence of salt in the core of Jabal Qumayrah is a rare example within the Oman Mountains. The intrusion, preserved as a gypsum and anhydrite-rich matrix containing rafts mainly of Sumeini Group host rocks, occurred after the end of nappe emplacement. On-going gravity surveys will determine the extent to which salt underlies the Jabal Qumayrah structure.

ACKNOWLEDGEMENTS

We are grateful to The Petroleum Institute, Abu Dhabi, for partially sponsoring this project. We thank Deborah Rees for logistical assistance in the field, two anonymous referees whose comments have helped improve the original manuscript, the Editor of GeoArabia Moujahed Al-Husseini and the design staff of GeoArabia and in particular Nestor “Niño” A. Buhay IV for the final preparation of the Figures.

ABOUT THE AUTHORS

David J.W. Cooper is an independent consultant geologist working out of the UK. He obtained a BA in Geology from Oxford University (1982) and a PhD from Edinburgh University in the sedimentology of the Hamrat Duru Group in Oman (1986). This was followed by a NERC research fellowship at Leicester University working on the sedimentology and structure of the Neo-Tethyan continental margin and deepwater sediments in Ladakh, NW India. He has recently returned to geological research after a diverse career working for the UK tax authorities.

djwcooper@gmail.com

Michael P. Searle is a Lecturer in the Department of Earth Sciences, Oxford University, United Kingdom, and is a Senior Research Fellow at Worcester College, Oxford. He obtained his PhD in 1980 working on structures and metamorphism beneath the Semail Ophiolite in northern Oman. Since then he has worked along the length of the Oman Mountains from the Musandam to Masirah Island and the Batain coast. Michael has also worked over 25 years along the Himalayan and Karakoram Ranges in Pakistan, India, Nepal, Bhutan and Sikkim as well as in Tibet, Burma, Thailand and Vietnam. His research integrates field mapping, structural geology, metamorphic and igneous geology, and geochronology with the aim of unraveling the large-scale evolution of mountain belts.

mikes@earth.ox.ac.uk

Mohammed Y. Ali has a BSc in Exploration Geology from Cardiff University, an MSc in Geophysics from Birmingham University, a Postgraduate Certificate in Education from UWCN, and a PhD in Marine Geophysics from Oxford University, UK. His current research projects are focused on exploration geophysics in the areas of passive seismic, seismic stratigraphy and reservoir characterization and modelling. Other research interests include basin analysis, crustal studies, and the structure of passive margins. Mohammed joined the Petroleum Institute in 2003 and currently he is an Associate Professor of Geophysics. He is a fellow of the Geological Society of London and a member of the SEG, EAGE and AGU

mali@pi.ac.ae