Structural Geometry, Style and Timing of Deformation in the Hawasina Window, Al Jabal Al Akhdar and Saih Hatat Culminations, Oman Mountains

Home , Basement high, Rusayl, Rustaq, Tectonostratigraphy, Wadi Kabir, Yiti

GeoArabia, Vol. 12, No. 2, 2007 Structural deformation, Oman mountains Gulf PetroLink, Bahrain

Structural geometry, style and timing of deformation in the Hawasina Window, Al Jabal al Akhdar and Saih Hatat culminations, Oman Mountains

Michael P. Searle

ABSTRACT

The Al Jabal al Akhdar and Saih Hatat culminations in the central Oman Mountains expose the complete mid-Permian to Late Cretaceous (Cenomanian) passive shelf and margin carbonate sequence beneath the allochtonous slope (Sumeini Group), basin (Hawasina complex), distal ocean-trench (Haybi complex) facies rocks, and the Semail ophiolite thrust sheets that were emplaced from NE to SW during the Late Cretaceous. Reconstruction of the pre-thrust sequences shows that time- equivalent rocks occur in successively stacked thrust sheets from shelf to slope to basin. The Al Jabal al Akhdar structure is a 60 km wavelength anticline plunging to the northwest beneath the Hawasina Window and with a fold axis that curves from WNW-ESE (Jabal Shams) to NNE-SSW (Jabal Nakhl). The structure shows little internal deformation except for minor intra-formational thrust duplexing within the Cretaceous shelf stratigraphy along the northern margin. The upper structural boundaries around the flanks of the shelf carbonate culminations have been re-activated as late stage normal faults. The Semail thrust formed a passive roof fault during late-stage culmination of al Al Jabal al Akhdar such that the ophiolite rests directly on Wasia Formation top-shelf with the entire Sumeini, Hawasina and Haybi thrust sheets displaced around the margins. NE-directed backthrusting and intense folding in the northern part of the Hawasina Window affects all allochtonous units and is related to a steep ramp in the Late Cretaceous shelf margin at depth.

The Saih Hatat culmination is another 40 km half-wavelength anticline in the central Oman Mountains, but shows extreme deformation in the form of recumbent folds, sheath folds with NNE-trending axes and thrusting along the northern margin. High-pressure carpholite, blueschist and eclogite facies rocks are exposed at successively deeper structural levels, separated by high-strain normal sense shear zones. There is no evidence for a separate ‘North Muscat microplate’ or an intra-continental subduction zone, as previously proposed; all high-pressure units can be restored to show their pre-deformation palaeographic positions along the northern margin of the Arabian Plate. Both Al Jabal al Akhdar and Saih Hatat are Late Cretaceous culminations, folded after obduction of the Hawasina, Haybi and Semail ophiolite thrust sheets from northeast to southwest during the period Turonian to Campanian-Lower Maastrichtian. Maximum compressive stress along the central Oman Mountains was oriented NE-SW, parallel to the ophiolite emplacement direction, but a second compressive stress axis was oriented WNW-ESE, either concurrently or slightly later in time, resulting in a dome and basin structural geometry. The biaxial fracture pattern in the foreland, southwest of the Oman Mountains could be explained as a result of the WNW- directed emplacement of the Masirah ophiolite belt and Batain mélange during the Campanian-early Palaeocene.

Both Al Jabal al Akhdar and Saih Hatat were positive topographic features at the end of the Cretaceous with Upper Maastrichtian and Palaeogene sediments onlapping both flanks. In Jabal Abiad, these Palaeogene sediments have been uplifted by at least 2 km since the Late Miocene-Early Oligocene associated with minor NNE-SSW compression. Tertiary shortening, folding and thrusting increases to the north in the Musandam peninsula where the first effects of the Arabian Plate-Eurasian Plate (Zagros belt) continent-continent collision are seen.

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INTRODUCTION

Three major culminations of Permian - Cenomanian shelf carbonate rocks occur along the Oman Mountain range, from NW to SE the Musandam, Al Jabal al Akhdar and Saih Hatat culminations (Figure 1). Each is a very large-scale antiformal fold that also affects the structurally overlying Sumeini, Hawasina, Haybi and Semail ophiolite thrust sheets. Palinspastic reconstruction of the Oman continental margin has shown that the allochthonous thrust sheets contain time-equivalent units but of progressively more distal facies structurally up-section (Figure 2; Glennie et al., 1973, 1974; Searle et al., 1983; Rabu et al., 1993). The reconstructed shelf margin and basin for the Central Oman Mountains is shown in Figure 3. Several basin-wide stratigraphic markers have been traced from the proximal Sumeini Group to the most distal Hawasina radiolarian cherts (Searle et al., 1983; Figure 3).

56°E 57° 58° TURKEY59° Caspian Sea N 0 km300 Semail Ophiolite Med SYRIA Sea IRAN Metamorphic sole IRAQ 26° 26°N JORDAN Musandam Amphibolites, Greenschists KUWAIT Arabian Peninsula Gul Haybi, Hawasina thrust sheets f BAHRAIN Permian-Mesozoic shelf carbonates QATAR EGYPT Arabian UAE Shield OMAN Dibba High pressure metamorphic units

Hagab Thrust SAUDI ARABIA Asimah-Masafi Carpholite zone Red SUDAN Sea

YEMEN Bani Hamid Garnet, glaucophane, blueschists Arabian Sea granulites ERITREA

Wadi Ham 25° N Semail Thrust 0 50 Wadi Hatta km Sumeini

Wadi Jizi

Sohar Haybi

Figure 12 Hawasina Window 24° Figure 11 24° Figure 13 Figure 14 As Judi

Al Ajaiz Muscat

As Sifah eclogites

Al Jabal Hulw Hawasina al Akhdar Saih Hatat ap G Jabal Nakhl ail Thrust Jabal Abiad 23° Sem 23°

Ras Al Sem Hadd ail Thrust OMAN

Jabal Ja’alan

56° 57° 58° 59° Figure 1: Geological map of the Oman Mountains, after Glennie et al. (1974). Locations of the three cross-sections of the Hawasina Window, Al Jabal al Akhdar and Saih Hatat are shown.

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Interior Oman and Stratigraphy Shelf Slope Hawasina Basin Haybi Trench Al Jabal al Akhdar Exotics Maastrichtian Campanian Juweiza Santonian Fiqa Group Aruma Qumayrah Fm Late Coniacian Muti Turonian Natih Cenomanian Wasia Nayid Fm Albian Group Nahr Umr Safil Aptian Shu’aiba D Barremian Kharaib

CRETACEOUS Lekhwair Hauterivian Early Habshan C Group Valanginian Kahmah Salil

Berriasian Rayda B Sidr Fm Tithonian Jubaila Nadan Kimmeridgian Hanifa Late Oxfordian Tuwaiq Callovian Bathonian Bajocian Dhruma A Middle Aalenian

Toarcian Sahtan Group JURASSIC

Mafraq Guwayza Formation Wahrah Formation Pliensbachian Mayhah Formation Sandstone - Limestone Sinemurian Early Hettangian Unconformity Rhaetian Jabal Wasa Fm Triassic Norian

Hamrat Duru Group Exotics Sumeini Group Misfah Late E Fm (limestone)

Carnian n (limestone)

Dhera Fm Haybi Ladinian Mahil Volcanics OIB D Haybi Volcanic Group

Middle Anisian TRIASSIC WPB Zulla Fm Maqam Formatio Olenekian C Akhdar Group

Early Induan B

3 A Baid Fm Saiq/ 2 Yankul PERMIAN Khuff Exotics 1 Al Jil Fm OIB Figure 2: Generalised Permian and Mesozoic stratigraphy of the Oman Mountains including the shelf carbonates (Glennie et al., 1973, 1974; Sharland et al., 2004) and allochthonous slope and basin facies rocks (Robertson and Searle, 1990; Blechschmidt et al., 2004). WPB – Within-plate volcanics; OIB – Ocean island volcanics.

Regional mapping of the Oman Mountains was carried out first by the Shell team of Glennie et al. (1973, 1974) who also defined the stratigraphy, and proposed a thrust emplacement model for the Semail ophiolite and Hawasina complex rocks, a model that is still widely accepted today. During the 1970–1980s, a USGS team mapped a strip across Saih Hatat and the Ibra ophiolite block from Muscat south to Ibra (Bailey, 1981) and an Open University team mapped much of the northern Oman Mountains, concentrating mainly on the Semail ophiolite complex (Lippard et al., 1986). The BRGM carried out detailed mapping at 1:100,000 scale of much of the Oman Mountains and summaries of their work have been published by Rabu et al. (1993) and Le Métour et al. (1990). The northern Saih Hatat region has been studied in some detail following the discovery of high-pressure (HP) eclogites, blueschists and carpholite-bearing rocks (Le Métour et al., 1986; Goffé et al., 1988; El-Shazli et al., 1990; Searle et al., 1994, 2004; Miller et al., 1998, 1999). Recently, debate has centered on the timing of uplift of the Al Jabal al Akhdar and Saih Hatat culminations (Late Cretaceous or Tertiary) and the tectonic evolution of the Oman Mountains and the foreland.

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Downloaded from http://pubs.geoscienceworld.org/geoarabia/article-pdf/12/2/99/4570063/searle.pdf by guest on 24 September 2021 Searle Northeast e x e 95 Ma s oceanic crust Semail comple ophiolit p), basin (Hawasina greenschists amphibolite u Metamorphic sol h s c Triassi (Umar basin) Haybi complex Haybi volcanic distal ocean-trenc )

n c + c e i

Cenomania Mid Jurassi U. Triassi Nadan Fm (chert Kawr-Misfah platform facies Triassic alkal volcanic substrat s

y Off-axis alkaline volcanics Na

ayza Fm

w Chert horizon Sidr Fm

Gu Hawasina complex (Hamrat Duru Group) distal slope and basin facies 100 facies Hallstatt km slumps (slope facies conglomerates) approximate scale only and Haybi complexes) are time-equivalent stratigraphic units bounded by major thrusts. Fm slope facies Fm Sedimentary profile of the restored Mesozoic Arabian continental margin, Central Oman Mountains Maqam Sumeini Group Huwar Mayhah Qumy 0 Mahil Natih Haushi Sahtan Saiq/Khuff Formation Nahr Umr Shu’aiba Lekhwair L U Shelf carbonates Al Jabal al Akhdar Period CEOUS System CRETA- PERMIAN TRIASSIC JURASSIC Figure 3: Palinspastic reconstruction of the Oman shelf, slope and basin in the Late Cretaceous. The shelf, slope (Sumeini Gro Figure 3: Palinspastic reconstruction of the Oman shelf, slope and basin in Late Cretaceous. The (Sumeini 92 Ma Age 149 182 257 Southwest

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Two recent models in particular have drawn attention to the key questions of timing of high pressure metamorphism and subduction, and the polarity of the subduction system. Gregory et al. (1998), Gray and Gregory (2000) and Gray et al. (2000) proposed a new tectonic model involving ‘nascent’ SW-directed subduction of the Muscat microplate (As Sifah and Hulw units) at 130–95 Ma prior to, and unconnected with ophiolite emplacement. Sedimentation along the Oman shelf during this time (Kharaib, Shu’aiba, Nahr Umr and Natih formations) was clearly related to passive margin sedimentation, and there is no evidence of any igneous activity as would be expected above an active subduction zone. This model was largely based on old (pre-ophiolite) 40Ar/39Ar ages of white micas from the high pressure units and the occurrence of NE-directed shear zones and NE-facing folds. Arguments against this model have been put forward by Searle et al. (2003, 2004) and Warren et al. (2005). They have questioned the validity of the older Sm-Nd garnet ages and 40Ar/39Ar phengite ages in the light of new and precise U-Pb ages of zircons from the ophiolite and amphibolite metamorphic sole.

Breton et al. (2004) presented a tectonic model involving two NE-dipping subduction zones, an outer one beneath the Semail Ophiolite and an inner intra-continental subduction zone beneath the ‘North Muscat microplate’ comprising the outer part of the Arabian platform (As Sifah unit), the slope and the Hawasina basin. This model, like the Gregory et al. (1998) one, requires a suture zone to separate the As Sifah eclogite unit from overlying structural units. Structural mapping and restoration clearly shows that there was no suture zone above the As Sifah rocks, which are in fact outboard, time- equivalent units to the inner shelf (Searle et al., 2004, Figures 3 and 4).

This paper describes the structures in the Al Jabal al Akhdar, Hawasina Window and Saih Hatat culminations with the aim of unraveling the Late Cretaceous and Tertiary structural evolution. Maastrichtian and Tertiary rocks are only exposed around the margins of each culmination, not over the main structures, so the Tertiary jabals are also examined along the flanks of the mountains. Firstly, a brief history of the evolution of the Oman shelf and margin is described.

Evolution of the Oman shelf

Permian-Triassic Rifting Phase

The earliest rifting deposits along the shelf margin are dated as Early Permian in the Jabal Qamar exotic in the Dibba zone United Arab Emirates (UAE) and mid-Permian (Saiq Formation) around the Al Jabal al Akhdar (Figure 5a,b,c) and Saih Hatat shelf carbonate culminations (Glennie et al., 1973, 1974). The Jabal Qamar exotic in the Dibba zone is the only exotic in Oman to show a pre-Permian basement (Ordovician Rann Formation, and Lower Carboniferous Ayim Formation) indicating that it was a rifted block of the Arabian margin (Hudson et al., 1954; Pillevuit et al., 1997). The earliest sedimentary rocks in the slope facies Sumeini Group in the Jabal Sumeini area are late Permian - Triassic Maqam Formation (Watts and Garrison, 1986; Watts 1990), equivalent to the Khuff-Saiq Formation on the autochthonous shelf. The oldest radiolarian cherts in the Hawasina complex are also late Permian (255 Ma; De Wever et al., 1988; Cooper, 1990), suggesting that the shelf-slope-basin distinction along the Oman margin was already in place by Late Permian time (ca. 268 Ma).

The Middle Permian unconformity around Al Jabal al Akhdar and Saih Hatat abruptly truncates fold axes and bedding in the Late Proterozoic - Early Palaeozoic rocks below, indicating a mid-Palaeozoic deformation event (without a major metamorphic imprint). Early Permian continental siliciclastics of the Gharif Formation thin to the north and are overlapped by the Late Permian transgression (Blendinger et al., 1990). Drowning of the Arabian margin occurred as the ‘Fusulinid sea’ covered the rifted continental margin across Oman (Glennie et al., 1974; Rabu et al., 1990). Whereas in Al Jabal al Akhdar, the Permian is inner shelf facies, around Saih Hatat the facies is more outer-shelf and continental-slope facies (Pratt and Smewing, 1990). Mildly alkaline rift-related volcanic sills and flows occur in the Saiq 1 Formation in Saih Hatat (Le Métour et al., 1986). These volcanic flows are interpreted to form the protolith of the As Sifah eclogites at the deepest structural levels of the Oman margin exposed today (Searle et al., 1994, 2004).

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Tectonic Stratigraphy Lithology and Description Interpretation Ma

Lutetian Rusayl Formation (limestone) 50 EOCENE J5 Ypresian J4

J3 Thanetian shallow marine Jafnayn shelf 60 Formation J2 (limestone) alkali Danian basalt PALAEOCENE J1

2 onlap Simsima shallow Formation marine (limestone) Maastrichtian emergence Laterite Al Khod 70 1d Fm

Juweiza Formation 70 Ma

1c emplacement Upper Fiqa of ophiolite plus Campanian p Formation Hawasina over Aruma

80 1b Aruma Grou Lower Fiqa Aruma Formation

Semail foredeep

Santonian 1a onlap ophiolite emplacement Muti Fm LATE CRETACEOUS

Coniacian emplacement collapse Hawasina-Haybi of shelf 90 margin Turonian

Wasia-Aruma Break e

95 Ma

Qumayrah e Wasia Group K-Ar biotit Formation ocean crust Cenomanian subduction Natih Formation (slope facies) formation e U-Pb (shelf facies) K-Ar muscovit plus Ar/Ar amphibolite Albian 100 ophiolit

Metamorphic U-Pb zircon sole Figure 4: Late Cretaceous time chart for shelf and foreland basin (Aruma Group) in Central Oman, based on seismic and well data, adapted after Warburton et al. (1990), showing thrusts cutting into the Fiqa and Juweiza formations during the Late Cretaceous emplacement (stage 1a to 1d). Also shown are timing of ophiolite formation and emplacement. Stage 2 is a further phase of gentle folding affecting Maastrichtian Simsima Formation rocks prior to 15 my of stable shallow-water deposition during the Paleocene - Eocene.

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The considerable thickness of Upper Permian along northern Saih Hatat indicates that the true continental margin must have been located some way outboard (NE) of the present coastline. Permian exotic limestones are exposed in the thrust sheets beneath the Semail ophiolite associated with minor alkali basalts, interpreted as within-plate ocean-island-type volcanics (Searle et al., 1980). The most prominent Permian exotic block is the Baid exotic south of Saih Hatat, an isolated piece of rifted Late Permian platform underlain by alkali basalts along the base. Permian rifting was followed by sea-floor spreading in the Hawasina (Tethys) Ocean. The earliest pelagic radiolarian cherts and red ammonoid limestones at the base of the allochthonous Hamrat Duru Group (proximal Hawasina complex) are Late Permian (De Wever et al., 1990; Béchennec et al., 1990). These deep-sea sediments were probably deposited on alkaline oceanic crust (Searle et al., 1980) rather than stretched continental crust (Béchennec et al., 1990; Rabu et al., 1993) or true mid-oceanic-ridge basalt (MORB) oceanic crust (Glennie et al., 1974; Bernoulli and Weissart, 1987; Pillevuit et al., 1997).

Permian-Triassic Volcanism

Permian volcanic rocks occur within the shelf carbonates (Saiq 1 Formation) in Saih Hatat, along the base of many of the Hawasina thrust sheets, and in the distal Haybi complex (Searle et al., 1980; Maury et al., 2003). The lavas and flows within the shelf sequence and in the proximal Hawasina are all high-titanium alkaline lavas related to the rifting of the Arabian margin. True MORB volcanics are not present and it is unlikely that true MORB oceanic crust was formed until at least Late Triassic. Maury et al. (2003) related the Permian volcanism in Oman to a mantle plume, but the widespread distribution extending over a distance of 350 km from the Dibba zone in the UAE to the eastern Oman Mountains, and the palaeogeographic distribution from the shelf margin outboard to Haybi complex preclude a plume origin. It seems far more likely that the Permian volcanism was related to continental rifting and break-up, with more localised within-plate, off-axis volcanism related to ocean-island guyots formed off the passive continental margin during the Late Permian and Late Triassic (Glennie et al., 1974; Searle et al., 1980).

In the most distal Haybi complex thrust sheets nephelinites, ankaramites, alkali basalts and trachytes represent extremely alkaline lavas, with rare, but widespread intrusions of highly alkali peridotites (jacupirangites, biotite wehrlites) and kaersutite gabbros beneath exotic limestones (Searle, 1984). These rocks are characteristic of present-day ocean islands offshore passive continental margins, such as the Comores, Tristan da Cunha, Canary Isles etc. A similar tectonic position is thought to be comparable for the origin of the Oman volcanics. The Haybi volcanics formed the protolith of the sub- ophiolite metamorphic sole amphibolites (Searle and Malpas, 1980, 1982). Thus the footwall of the ophiolite was old, cold Triassic and Jurassic ocean floor (Searle and Cox, 1999, 2002), not Cretaceous ocean floor equivalent to the Semail ophiolite (Boudier et al., 1988).

Oman Exotics: Baid and Misfah Platforms

Two types of ‘Oman Exotic limestones’ occur in the allochthonous stack structurally above the shelf carbonates. The first group includes large-scale, well-bedded carbonate banks frequently with an alkali basalt basement. Examples are Jabals Misfah, Kawr and Misht (Figures 5c and 6a) around the SW flank of Al Jabal al Akhdar, and the Baid exotic south of Saih Hatat. The second group includes numerous smaller exotics, frequently intimately associated with within-plate, highly alkaline volcanics, often occurring in mélanges and imbricate slices immediately beneath the Semail ophiolite (Searle et al., 1980; Searle and Graham, 1982; Pillevuit et al., 1997).

The Baid platform carbonates comprise Late Permian platform limestones, Triassic (Hallstatt facies, red ammonite-rich limestones) overlain by thin cherty limestones and radiolarites of Early Jurassic age (Blendinger et al., 1990; Pillevuit et al., 1997). This platform sequence probably rifted away from the Oman continental margin as it occurs northeast outboard of the true shelf and the proximal Hawasina rocks in the palaeogeographic reconstruction (Bernoulli and Weissart, 1987; Bernoulli et al., 1990). These exotics have been thrust to the SW above Hamrat Duru (Hawasina complex) thrust sheets that are structurally above the Cretaceous shelf and its cover of Aruma Group, and immediately beneath the Semail ophiolite.

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Southwest a Wasia Jabal Shams (3,009 m)

Kahmah

Sahtan

Mahil

Saiq Fm

Mu’aydeen (Pre-Permian)

Northeast b

Jabal Shams Summit

Wasia

Kahmah

Sahtan

Wadi Nakhr Grand Canyon

Figure 5: (a) The full section of Late Permian up to Cenomanian shelf carbonates in Al Jabal al Akhdar; the north face of Jabal Shams (3,009 m), top of Wadi Sahtan, view towards the southwest. (b) The shelf carbonates along the southern margin of the Al Jabal al Akhdar anticline showing lack of internal deformation, Wadi Nakhr, view towards northeast.

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Southwest c Jabal Kawr Jabal Misfah

Wasia

Kahmah

Wadi Nakhr

Sahtan

Figure 5 (continued): (c) The shelf carbonate sequence in Al Jabal al Akhdar exposed along the deeply incised Wadi Nakhr, Grand Canyon of Oman, view towards southwest. Triassic exotic limestones of Jabals Kawr and Misfah in the distance rest on thrusts along the top of the Cretaceous shelf.

In Jabals Misfah (Figure 5c), Kawr and Misht (Figure 6a), structurally above the Al Jabal al Akhdar shelf sequence, 700–800 m thick Late Triassic platform limestones (with Late Dogger Ammonico Rosso facies) up to 1,000 m thick, were deposited in 10–15 my. These limestones are similar to the Triassic shelf (Mahil Formation), but overlie an alkaline basaltic substrate indicative of off-axis, within-plate ocean-island volcanic rocks (Searle et al., 1980; Searle and Graham, 1982). On the Jabal Misfah exotic, foundering of the Triassic seamount occurred during the Late Oxfordian to Tithonian with deposition of thin deep-water pelagic limestones and cherts of the Nadan Formation at the top (Pillevuit et al., 1997). Thermal subsidence following sea-floor spreading during the Late Triassic, resulting in subsidence and drowning of the Triassic exotic carbonate platform.

These exotic limestones have been thrust over proximal Hamrat Duru Group and Al Aridh Formation (Hawasina complex) rocks and occur immediately beneath the Semail ophiolite, indicating their palinspastic restoration position well outboard of the shelf and shelf margin (Figure 3).

Triassic Ocean-Island ‘Exotic’ Limestones

Numerous Late Triassic exotic limestones occur throughout the Haybi complex thrust sheet immediately beneath the ophiolite (Glennie et al., 1973, 1974; Searle and Graham, 1982). These rocks were later termed part of the Umar Group (BRGM mapping; Béchennec et al., 1990; Rabu et al., 1993); however they span Late Permian to Middle Cretaceous, are time-equivalent rocks to the Hawasina and Sumeini thrust sheets and the autochthonous shelf, and are bounded by thrust faults, so the structural term Haybi complex is retained here. Many Triassic exotics show spectacular Megalodon sp. limestones representing reef limestones, with flanking mega-breccias representing ocean-island slope deposits. Blocks of shallow-water Triassic exotic limestone are sometimes encased in deep- water radiolarian cherts indicating base of slope facies.

The presence of alkali basalts, and some highly alkaline peridotite-gabbro intrusions into the Late Triassic exotic basements suggest that their origin was on a series of isolated within-plate, off-axis

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North a

Jabal Misht

Haybi volcanics

Al Ayn Fm

West b Jabal Nakhl

Semail Gap

Harzburgite

Gabbro

Figure 6: (a) 1,000 m cliff face of Jabal Misht showing Triassic exotic limestones above an alkali basalt substrate, thrust above cherts and shales of the Hamrat Duru and Al Ayn thrust sheets (Hawasina complex). (b) Aerial view looking west across the Semail ophiolite to the Jabal Nakhl shelf carbonates. The Semail Gap is interpreted as a normal fault contact above the shelf carbonate anticline. Ophiolite, Haybi and Hawasina thrust sheets have been down-faulted to the east during culmination of Jabal Nakhl - Al Jabal al Akhdar.

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North South c

Yiti duplex

Figure 6 (continued): (c) South-vergent duplex developed in Triassic Mahil Formation shelf carbonates at Yiti, northern Saih Hatat. The duplex represents localised internal structural thickening within the shelf carbonates.

oceanic volcanic island seamounts. These Triassic oceanic floor rocks of the Haybi complex restore to a palaeogeographic position immediately SW of the Semail thrust subduction zone (Figure 3). The restoration shows that old, cold Triassic - Jurassic oceanic crust was subducted beneath the Semail ophiolite to form the amphibolite-greenschist facies metamorphic sole, very different from the Late Cretaceous of the Semail ophiolite (Searle and Malpas, 1980, 1982; Searle and Cox, 1999, 2002).

Late Jurassic - Early Cretaceous Drowning of Platform

The Jurassic - Cretaceous boundary (145 Ma) is marked by a hiatus along the Oman platform and preceded a drowning event that lasted ca 20 my (Pratt and Smewing, 1990). Shallow-water shelf carbonate deposition was resumed during the Albian-Aptian (Rabu et al., 1990). Increasing subsidence across the shelf and slope into the Hawasina basin may have occurred in response to stretching during sea-floor spreading. Along the southeastern margin however the NNE-trending Huqf High remained a positive feature into the Cretaceous. This basement high probably extended north into the eastern Oman Mountains where basement rocks exposed at Jabal Ja’alan crop out beneath the Tertiary unconformity. This basement high may have had a profound effect on the subsidence within the foreland basin and may also have divided the dominant NE-SW compressive stress regime to the NW from the WNW-ESE compressive stress regime along the Masirah-Batain coast.

Late Cretaceous Platform Margin Collapse

A large-scale gravity low over the Oman Mountains foreland has been interpreted in terms of elastic flexure of the Arabian foreland beneath the allochthonous thrust sheets (Ravaut et al., 1997). The Late Cretaceous Aruma foredeep basin includes up to 3.5 km thickness of Santonian - Campanian Fiqa Formation with up to 1.5 km thickness of Late Campanian Juweiza Formation thickening to

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the NE. Major changes in platform margin sedimentation occurred following the Cenomanian when the whole continental margin collapsed as recorded by large-scale megabreccias, intra-formational unconformities and transgression to deep water clastic sediments of the Qumayrah Formation along the margin and the time equivalent Aruma Group (Robertson, 1987; Watts, 1990). Collapse of the shelf margin and beginning of the foreland basin deposition following the ‘Wasia-Aruma break’ (Turonian) was accompanied by uplift along the peripheral bulge. Up to 1,100 m of uplift and erosion of the Natih Formation on the Lekhwair High may have been caused by flexural bending of the foreland (Warburton et al., 1990; Loosveld et al., 1996; Filbrandt et al., 2006) along the Suneinah Trough (Patton and O’Connor, 1988; Boote et al., 1990).

The Aruma (Suneinah) basin shows a rapid increase in thickness towards the NE with Coniacian- Santonian Muti Formation conglomerates passing up to deeper water shales and siltstones of the Campanian Fiqa Formation (Figure 4). The Late Campanian Juweiza Formation shows the first debris eroded from the Semail ophiolite (Glennie et al., 1974). Seismic sections across the foreland show the Hawasina thrust tips extending up into the Fiqa Formation (Warburton et al., 1990; Boote et al., 1990). The entire thrust process was completed by 70 Ma (Lower Maastrichtian) when the mountain belt just breached sea-level. Thin laterite soils overlie the ophiolite erosional surface in the southeast Oman Mountains.

Age constraints from the allochthon

Age constraints from the Semail ophiolite and the amphibolite-greenschist facies metamorphic sole are summarised in Figure 7. The age of crustal formation of the Semail ophiolite is known from the U-Pb zircon ages of plagiogranites (95.4–94.5 ± 0.5 Ma; Tilton et al., 1981, and 95.3 ± 0.2 Ma; Warren et al., 2005). The ages are consistent with Cenomanian radiolarian ages from cherts within the lower pillow lavas (Geotimes unit or V1) and Turonian ages from the middle volcanic member

Pelagic sediments (Cenomanian - Turonian) V2 V3 - Alley, Salahi V2 - Lasail IAT Pillow lavas V1 - Geotimes MORB

Sheeted dykes 5 km mush Plagiogranites Z = 94.5 - 95.4 (93.5 - 97.3 Ma) Isotropic gabbros H = 93.6 - 94.6 (92.3 - 96.3 Ma) Wehrlite intrusions

Layered gabbro-peridotites Moho

Late biotite granite sills Mantle Crust

Gabbro dykes

Garnet + clinopyroxene amphibolites

Peridotite tectonite Amphibolites H = 92.6 - 95.7 Ma harzburgite, dunite 200 m Epidote amphibolites

Greenschists M = 89.5 - 96.5 Ma Banded ultramafic unit K-Ar

Metamorphic sole M = 90.9 - 94.6 Ma Semail thrust 40 39 Metamorphic sole Ar - Ar B = 88.4 - 92.5 Ma Haybi, + Hawasina complex

Figure 7: Tectonostratigraphy and age constraints from the Semail ophiolite and metamorphic sole; see text for sources of data. Mineral age data is from Z (zircon); H (hornblende); M (muscovite) and B (biotite).

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(Lasail unit or V2). The ophiolite complex has an inverted metamorphic sole along the base of the mantle sequence harzburgites and dunites comprising garnet and clinopyroxene amphibolites overlying greenschist facies metasediments (Searle and Malpas, 1980, 1982; Searle and Cox, 2002). These rocks are metamorphosed Triassic and Jurassic basalts and sediments subducted to 35 km depth (10–12 kbar) beneath the mantle at the same time as the ophiolite crust was forming (40Ar-39Ar ages of hornblendes of 94.9–92.6 ± 0.6 Ma; Hacker, 1994; Hacker et al., 1996).

The geochemistry of the ophiolite lavas (Pearce et al., 1981) and the synchroneity of the ophiolite ages with the metamorphic sole ages strongly suggest that the ophiolite formed above a NE-dipping subduction zone (Searle and Malpas, 1980; Lippard et al., 1986; Searle and Cox, 1999, 2002), and not at a mid-ocean ridge (Boudier et al., 1988; Nicolas et al., 1988). The lack of any trace of an active continental margin during the Cretaceous along the Oman Mountains also strongly suggest that this subduction zone dipped to the NE away from the continent (e.g. Lippard et al., 1986; Searle et al., 1994, 2003; Searle and Cox, 1999, 2002; Breton et al., 2004), not to the SW beneath the continent (Gregory et al., 1998; Gray et al., 2000; Gray and Gregory, 2004).

During the latest stages of ophiolite obduction, the northeastern leading edge of the Arabian Plate was subducted beneath an oceanic (ophiolitic) hanging-wall. The entire northeast margin of Saih Hatat has been affected by high-pressure metamorphism with increasing pressure and temperature (P-T) conditions with depth. The deepest structural levels are exposed around As Sifah (Figure 1) where Ordovician Amdeh quartzites, and overlying Permian limestones with basaltic flows (Saiq 1 Formation) have been metamorphosed to eclogite facies (Searle et al., 1994, 2004). Crinoid ossicles are still preserved within eclogite facies calc-schists at As Sifah. Gray et al. (2005) obtained a U-PB SHRIMP (Sensitive High-Resolution Ion MicroProbe) age of 298 ± 3 Ma from felsic schist within an eclogite boudin, suggesting that the protolith rocks were Early Permian - Late Carboniferous.

A U-Pb zircon age of 78.95 ± 0.13 Ma from the As Sifah eclogites, interpreted as dating peak high- pressure metamorphism (Warren et al., 2003, 2005), shows that the leading edge of the Arabian continental shelf was subducted to ca 80 km depth beneath the oceanic crust and mantle during the Campanian, some 15 My after formation of the ophiolite (Figure 8). 40Ar-39Ar ages of phengites from the upper slab rocks in northern Saih Hatat 80-70 Ma are consistent with Campanian High- Pressure metamorphism and cooling (Miller et al., 1999, 2002). The geological and geochronological constraints (Figure 8) are consistent with a single NE-dipping subduction zone, starting as an intra- oceanic subduction zone during the Cenomanian along which old, cold Triassic - Jurassic crust was subducted beneath the ophiolite (Searle and Cox, 1999, 2002; Searle et al., 1994, 2004). When the passive continental margin reached the subduction zone, the Triassic - Cretaceous upper slab decoupled and folded, forming the NE-directed backfolds and backthrusts of northern Saih Hatat. The Permian and older lower slab was subducted rapidly down and back up the same zone.

Maastrichtian – Tertiary evolution

The Late Cretaceous period of thrusting and ophiolite obduction ended by early Maastrichtian time with a regional unconformity over all allochthonous units (Figure 9). Conglomerates and shales of the Qahlah and Al Khod formations are overlain by shallow water rudist-bearing limestones, indicative of a return to stable shelf conditions (Simsima Formation). The abundance of branching corals, rudists, bivalves and gastropods in the Simsima Formation across all of north Oman is testament to the fact that tectonism had ceased by 70 Ma. North Oman became emergent again at the end of the Maastrichtian, due largely to a global eustatic fall in sea-level. Minor deformation and uplift of the mountains is indicated by the outcrop pattern of the two unconformities below and above the Simsima Formation. During the Late Palaeocene stable shallow-water limestone deposition resumed and lasted for 25 My until the Oligocene (Nolan et al., 1990; Mann et al., 1990).

A second orogenic phase began during the Late Oligocene - Miocene with renewed shortening and folding of the Palaeogene limestones. The style of folding in the Tertiary is generally large-scale open folds or box folds with kink band sets. These folds may have steep, even overturned limbs but flat tops characteristic of box folds with prominent kink-band sets. Shortening is minimal but uplift of at least 2,000 meters is known from Jabal Abiad, east of Saih Hatat (Figure 10a,b). The magnitude and extent of Tertiary folding increase to the north along the Oman foreland (Figure 10c). The Musandam

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Stratigraphy Lithology and Description Tectonic Interpretation

Ma Semail Continental ophiolite Margin 60 Tertiary Danian limestones

Culmination

PALAEOCENE Saih Hatat + uplift + erosion

Maastrichtian Ar/Ar micas Upper Plate ? out-of-sequence 70 thrusting + normal faulting

folding upper plate

As Sifah Campanian eclogite

Rb/Sr phengite Continental 80 78 Ma subduction/ eclogite Ruwi exhumation schist Ar/Ar micas eclogite Lower Plate metamorphism/ Santonian -Hulw, Wadi Mayh exhumation Thin-skinned SW-thrusting

LATE CRETACEOUS Coniacian

90 Turonian n Ar-biotite sole

Cenomanian 40Ar/39Ar- Oceanic subduction 40Ar/39Ar- muscovite hornblende sole U-Pb-zircon sole Crustal formatio Semail ophiolite 100 Albian

Figure 8: Late Cretaceous time chart for the Saih Hatat area showing age constraints and tectonic interpretation, after Searle et al. (2004). See text for sources of data.

peninsula shows large-scale re-folding and re-thrusting of the entire Pre-Permian basement up to Palaeogene cover with minimum east to west translation of 14 km along the Hagab thrust (Searle et al., 1983; Searle, 1988a,b). This is interpreted as the beginning of the Zagros phase of continent– continent collision when the Musandam shelf first collided with the central Iran continental block (eastward extension of the Zagros suture zone).

HAWASINA WINDOW CULMINATION

The Al Jabal al Akhdar anticline axis continues NW along the Hawasina Window (Figure 1), where complex folding and thrusting in Sumeini slope facies, Hamrat Duru proximal basin facies, and Haybi complex distal basin facies rocks is exposed (Searle and Cooper, 1986). The cross-section (Figure 11) shows that the central part of the window is dominated by steep NE-verging back-folding and backthrusting, affecting all allochthonous units. Early SW-directed thrusts are folded by later NE-directed backfolds and thrusts. Following SW-directed emplacement of the allochthon, the Semail thrust has

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SE Oman Iran-SE UAE Oman NE Oman Stratigraphy Batain Coast Zagros Foreland Foreland (SW) Batinah Coast Indian Oman + Dhofar Tectonics Tectonics Mountains Ma Ocean-Dhofar Bakhtyari Zagros folding 3 Pliocene Aghajari Upper

10 UAE Nar foreland Folding of Mishan Middle

Upper Aden, Uplift Dhofar margin Miocene Adawnib Ocean-floor spreading Gulf of Fars Dam ? 20 Gachsaran Lower Sawadi + Lower

A3 Mughsayl Hagab thrust Musandam Upper

Asmari Culmination of Asmari A2

30 graben faulting A1 2 Oligocene Al Jaww Ashawq Lower Deepening of Dhofar margin, Zalumah

D4 Ru TERTIARY Upper Senaiya w Seeb ayda 40 D3 Aydim

Dammam h Hafit D2 - Fahud Pabdeh - Dammam Middle Jahrum Rusayl

Eocene D1

50 Rus J5 Rus sedimentation Rus Jafnayn U J4 Hadramaut Group Neo-autochthonous Lower shallow marine along Umm Er

M flanks Oman Mountains Umm Er Saah Radhuma Gentle subsidence shallow marine Radhuma J3 L Mundassa Upper

Muthaymimah Jafnayn J2 60 Malaqet J1 d Paleocene Lower

Simsima Simsima Thaqab Simsima 70 Qahlah Qahlah Al Khod Maastrichtian e

Gurpi Filim

Juweiza Ras Madraka ophiolites Upper 80 Semail CRETACEOUS

Group Fiqa Aruma Group Emplacement of Masirah Island an

ophiolite emplacement Aruma Campanian End of ophiolit Fiqa 1 Figure 9: Stratigraphy of the post-emplacement neo-autochthonous Maastrichtian - Tertiary sequence in the SE Zagros Mountains, Oman and the UAE.

been folded and inverted such that Haybi complex rocks have been re-thrust NE over the Semail ophiolite. Along strike to the NW, normal SW-vergence is resumed in the Haybi corridor (Searle, 1985). Along the northeastern half of the window recumbent nappes in the Hamrat Duru Group at Jabal Milh are NE-facing, whereas along the southern half of the window all folds are SW-facing, creating a divergent ‘pop-up’ structure (Searle and Cooper, 1986; Figures 10, 13). The deepest structural levels are three culminations of Sumeini Group slope facies limestones (Jabals Rais, Rastun and Mawq). These jabals are all affected by NE-verging late-stage backthrusts that cut up-section as far as the Semail thrust (Figure 11).

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Southeast Northwest a Jabal Abiad

Ash Shab beach

West East b

Seeb Fm

Wadi Dhank

Figure 10: (a) Northern flank of Jabal Abiad, eastern Oman Mountains showing cascade box folds along the flank of the 40 km wavelength fold affecting Palaeocene and Eocene shallow water limestones lying unconformably above Semail ophiolite. View southeast from Fins beach – Ash Shab coast. Note four prominent raised beaches above the beach (arrows). (b) WSW-vergent cascade box-fold along the SW margin of Jabal Awaynah - Jabal Dhank, west margin of central Oman Mountains, north of Ibri. Palaeocene - Eocene limestones lying unconformably above Hawasina and Haybi complex rocks show a second post-Eocene phase of folding.

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West East c

Seeb Fm

Jafnayn Fm

Simsima Fm

ophiolite mantle harzburgite

Wadi Tiwi

Figure 10 (continued): (c) Wadi Tiwi in the northeastern Oman Mountains showing the full sequence of Late Maastrichtian, Palaeocene and Eocene neo-autochthonous limestone cover. Exposures of Semail ophiolite mantle harzburgites can been seen along the wadi bottom.

The Hawasina Window rocks form a large-scale duplex bounded by the Hawasina thrust along the top of the shelf carbonates and the Semail thrust beneath the ophiolite. Whereas around the Al Jabal al Akhdar shelf most of the Haybi and Hawasina rocks have been faulted out by the late-stage normal faulting, culmination of Al Jabal al Akhdar may have displaced these rocks laterally to the NW beneath the passive roof fault defined by the Semail thrust. A lateral (NW-SE) section shows how the Hamrat Duru nappes have been squeezed out laterally, sandwiched between the Sumeini culminations below and the Semail thrust above (Figure 12). Searle and Cooper (1986) explained the structural geometry of the Sumeini backthrusts by the presence of a steep frontal ramp at depth possibly along the shelf edge. The structural evolution of the Semail thrust around the northern Hawasina Window is complicated; it must have started (stage 1) as an early SW-vergent thrust carrying the ophiolite above Haybi and Hawasina thrust sheets, then (stage 2) became re-activated as an out-of-sequence late- stage thrust truncating stratigraphy in the footwall (Searle, 1985). During culmination of the Sumeini Group structures at depth and folding of the Hamrat Duru rocks in the Jabal Milh nappes (stage 3) it became a normal fault bringing footwall Hawasina Window rocks up from beneath the ophiolite. Finally during further NE-SW compression (stage 4) the deep-level Haybi and Hawasina rocks were backthrust NE over the ophiolite in the central part of the Hawasina Window.

AL Jabal AL Akhdar culmination

The Al Jabal al Akhdar culmination (Figure 13) shows a single very large-scale 30–35 km half- wavelength anticline with a flat top (Saiq plateau). Dips increase outward from the axis such that the style is one of a gentle rounded box fold. Along the southern flank dips are gentle (15–20oS) but increase towards the NW-plunging nose, where a distinct overturning, verging SW is apparent. This may be a kink band that progressively developed into a steep thrust from Al Jabal al Akhdar towards the Hawasina Window at depth. Along the northern flank dips are 25–35oN. The upper contact of the shelf carbonates is presently a listric normal fault along the north and south flanks of Al Jabal al Akhdar, and the west and east flanks of Jabal Nakhl (Figure 6b). Ophiolite mantle sequence rocks rest directly on Wasia Formation top shelf with most of the Haybi and Hawasina thrust sheets faulted down. Throw on these late-stage normal faults could be as much as 4–5 km (Figure 6b).

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Formation; Semail thrust Semail Semail ophiolite and SW-facing folds in -Tertiary box folding

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Continuity of Hawasina, Haybi and Semail ophiolite thrust sheets over Al Jabal al Akhdar prior to folding is apparent from the presence of ophiolite all around its south, west and northern flanks (Bahla – Sint – Hawasina – Rustaq ophiolite blocks) and south and eastern flank (Nizwa – Semail ophiolite blocks). West of the Wadi Nakhr ‘Grand Canyon’ and Jabal Shams summit (Figure 5c), Haybi thrust slices comprising Triassic alkali volcanics and Oman exotics (Jabals Kawr, Misfah and un-named exotics along the crest of the fold) are more-or-less continuous over the southwestern limb of the Al Jabal al Akhdar fold.

The large scale of the Al Jabal al Akhdar fold resulted in extensive bedding plane slip during flexure. Along the south flank ofAl Jabal al Akhdar, little or no internal folding or thrusting has been observed in many of the deeply incised wadi sections (e.g. Wadis Nakhr, Tanuf, Mu’aydeen). Some small-scale intra-formational thickening occurred by internal duplexing locally along the north flank ofAl Jabal al Akhdar (Breton et al., 2004; figure 7). These are very small-scale folds and thrusts, restricted to the Cretaceous part of the shelf sequence and are not continuous up to the underlying shelf or overlying allochthonous units. These duplexes are not regional north-verging folds (Gray and Gregory, 2004), but local features related to thickening along the flanks of the main Al Jabal al Akhdar fold. Similar internal thrusts and folds in the shelf sequence of northern Saih Hatat are south-vergent (for example Yiti duplex; Figure 6c). The north-vergent backfolds of the Hawasina Window are much larger scale, late-stage structures affecting the overlying Semail ophiolite thrust sheet as well (see next section).

Bouger gravity data suggest a crustal thickness beneath Al Jabal al Akhdar of 47 km (Ravaut et al., 1997), or as much as 48–51 km (Al-Lazki et al., 2002). This thick crustal root is surprising, being only 75 km inland from the present-day coastline. Even beneath the Batinah coast the crustal thickness is as much as 39–42 km (Al-Lazki et al., 2002). Because of this, it is suggested that the Arabian continental crust must have extended a long way offshore buried beneath the thick Tertiary sedimentary sequence. This would increase the distance of obduction of the Semail ophiolite and underlying thrust sheets even more than proposed from surface geology.

Two possible models for the structure of Al Jabal al Akhdar have been proposed. The first model is a simple fold with no thrusting at depth (Glennie et al., 1973, 1974; Al-Lazki et al., 2002). The second model shows the anticline as a thrust culmination, either with a flat-lying detachment at depth ramping up in front of the Jabal Salakh frontal fold (Searle, 1985; Figure 11; Bernoulli and Weissart, 1987; Figure 2; Hanna, 1990; Figure 11; Cawood et al., 1990, Figure 11), or with steeper basement faults (Mount et al., 1998). Although the dips of the Al Jabal al Akhdar range are relatively gentle there is a SW-verging asymmetry to the fold towards the west. Geometrical constraints suggest that a deep flat within the pre-Permian basement is likely albeit with little horizontal translation. Although the Al Jabal al Akhdar culmination must have been a positive feature during the latest stages of Late Cretaceous deformation, the main period of growth was probably Oligocene - early Miocene. Fission track apatite data suggests that Oligocene uplift and erosion was followed by slower cooling in the Neogene (Poupeau et al., 1998; Mount et al., 1998).

Saih Hatat culmination

The Saih Hatat culmination is the most complex of the three major shelf carbonate folds (Figure 14). Systematic mapping of northern Saih Hatat was carried out by Le Métour et al. (1986), Searle et al. (1994, 2004) and Miller et al. (2002). The structures shown here are based on the cross-section of Searle et al. (2004) for the north and from BRGM mapping around the Ibra ophiolite block to the south. The restored section (Figure 15) shows that the shelf-slope section is still ca 3 km thick in the north, indicating that the true Mesozoic shelf margin remains buried offshore or is missing. The Saih Hatat culmination is a 40 km half-wavelength fold that affects the entire basement up to Tertiary cover sequence. A prominent middle Permian unconformity crops out all the way around the Saih Hatat window, and separates a folded and mildly metamorphosed pre-Permian basement from the Permian - Cretaceous shelf carbonate sequence. Along the southern margin of Saih Hatat the shelf carbonates are conformable and unaffected by major internal folding or thrusting. Several thrust and normal faults do cut the section along Wadi Dayqah, but offsets are minor. However, along the northern margin of Saih Hatat, the basement rocks (Late Proterozoic Hatat schists, Hijam dolomite and Ordovician Amdeh quartzites) and the Permian - Jurassic shelf carbonates are severely affected by folding and thrusting.

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Mu Kh Sa Ma Sq across Ha Ha NE. edge Al Haja Hatat thins Arabian the to ) the Saih high-pressure leading emplaced Am of Upper/Lower plate discontinuity Hulw Unit cuts down-structural section to SW and Hi The + folds suggest NE-vergent thrustin from Sq 1 to Am kinematic indicators . carbonates part across outboard SW 14

partially

1 2 shelf to

lower

Ha Ha

Ha Ha Ha 3, 3, Ha NE further subduction Lc calc-schists are metamorphosed Saiq Fm Lv meta-volcanics are ? metamorphosed Saiq volcanics (mid-Permian Lqms schists are metamorphosed Am quartzites Figure already Sahtan lay of from were part time Pre-Cambrian basement Campanian stratigraphicaly Kahmah, that sheets to at sia, and northern Cenomanian Wadi Kahza prior Wa thrust

to Restored section across Arabian continental margin, Saih Hatat, Oman Mountains (at ~ 90 Ma) the margin of sequence Permian Muti Fm Saiq Fm Mahil Fm 5 4 faults, Wasia Gp Sahtan Gp Kahman Gp riassic T sheets); shelf folds in pre-Permian Am Am Am 1-3 Hijam Sarin Amarat Jahlut Saadi (Proterozoic) the (base Sq 1) truncates Permian unconformity

and Hawasina

Amdeh 4 3 2 1

volcanics comprise 50–100 m of basaltic lavas and are protolith rocks of the Hulw blueschists metavolcanics (Lv). volcanics comprise 50–100 m of basaltic lavas and are protolith rocks the Hulw blueschists Hatat - section ? 2 Hi the Kh Sa Sq Mu Ma Wa was from Jabal Tayin Jabal Abyad in Jahlut member of Hatat Fm Up to 60 m thickness of pillow thrust lavas and tholeiitic basalt flows U U U U U zones that Haybi Restored shear and eclogites up-section ophiolite 15: margin Saih Hatat CRETA- CEOUS JURASSIC TRIASSIC PERMIAN LATE PROTE- ROZOIC from central and SW ORDO- VICIAN Stratigraphic thickness suggesting Sifah 0 1 2 3 4 5 10 (km) Southwest Depth Figure late-stage ophiolite of the and right are constrained by the thicknesses from Glennie et al. (1974) Le Métour (1986). Note thickness NE cutting As Semail thickens to NE. Saiq

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The structurally highest units exposed show basement and Permian cover rocks folded around a 20 km long NNE-aligned sheath fold, exposed along Wadi Mayh (Figure 16). The compressive stress axis was primarily aligned NNE-SSW, but a second major axis was oriented WNW-ESE. Folds are NE-facing throughout. Above a major detachment termed the ‘Upper plate – Lower plate discontinuity’ (Miller et al., 1998, 2002) the Permian limestones are highly deformed by internal folding and imbrication. Below the discontinuity, metamorphism increases with depth through carpholite-bearing assemblages (ca 7–8 kbar, 280–350oC; Goffé et al., 1988) and blueschist facies (ca 12–15 kbar, 450–540oC) to deepest eclogite facies (As Sifah unit: 15–20 kbar, 540oC) rocks (Searle et al., 1994, 2004). Major normal-sense ductile shear zones separate these metamorphic zones. A 20 kbar (>80 km) depth profile is condensed to ca 7 km structural depth today, reflecting very large strains during rapid exhumation from the subduction zone at 78.95 ±0.13 Ma (U-Pb age of zircon from eclogite; Warren et al., 2005).

The southern flank of Saih Hatat along Wadi Tayyin is a large-scale, listric normal fault down-throwing the Semail ophiolite mantle sequence against the top shelf Wasia Formation. The lower part of the ophiolite thrust sheet together with the underlying Haybi and Hawasina complex thrust sheets has been down-faulted to the south. This normal fault is not a gravitational ‘collapse’ fault structure, but a normal fault operative during active compression. As the shelf carbonates were pushed up along the basal detachment the antiformal culmination was forced up through the overlying ophiolite. Little or no horizontal extension was involved in this process.

The minimum distance of emplacement of the Hawasina, Haybi and Semail ophiolite thrust sheets was 140 km from the tip line in the foreland to the present-day coast, but internal shortening within each duplex, and the unknown missing amount of continental margin offshore, must also be taken into account. Cooper (1988) estimated over 400 km of emplacement of the Hawasina complex rocks from palinspastic reconstructions. The protolith of the As Sifah unit eclogites were lateral equivalents of the Permian Saiq Formation limestones. Deepest-level quartz mica schists may be metamorphosed Ordovician Amdeh quartzites. The protoliths of the eclogites were basaltic flows and sills intruded into Permian Saiq 1 Formation limestones (Searle et al., 2004; Figure 14). There is no evidence for suture zone rocks above the As Sifah unit as required by the model of Gregory et al. (1998), Gray et al. (2000) and Gray and Gregory (2000). There is also no evidence of an active plate margin along the Cretaceous shelf in Oman, also required by their model of early nascent SW-directed subduction beneath the continental margin. Instead, passive margin sedimentation existed along the Oman margin throughout the Mesozoic up until ca 95 Ma when the ophiolite, Haybi and Hawasina thrust sheets were emplaced above the shelf into a rapidly deepening foreland basin.

Most workers agree that a single NE-dipping subduction zone existed offshore Oman from 95 Ma, and thrusting progressed generally in-sequence from NE to SW, from deep to shallow during the Late Cretaceous (Searle and Malpas, 1980; Lippard et al., 1986; Goffé et al., 1988; Searle and Cox, 1999, 2002; El-Shazli et al., 2001; Searle et al., 2003, 2004). There is no need for the double subduction zone postulated by Breton et al. (2004), as it can be proven that protoliths of the As Sifah unit were Permian and older rocks of the Arabian continental margin (Figure 15), and not an exotic plate.

Thrust emplacement of the Semail ophiolite, Haybi and Hawasina complexes onto the stable passive- margin foreland, exhumation of the deeply subducted continental margin and late-stage culmination of Saih Hatat was completed by the Upper Maastrichtian (Figures 8, 9). Upper Maastrichtian Qahlah and Simsima formations and Palaeocene - Eocene shallow water limestones unconformably overlie all allochthonous units around Saih Hatat. These shallow water limestones have been uplifted by about 600 meters along the southern margin of Saih Hatat and up to 2,000 meters on Jabal Abiad to the east of Saih Hatat (Figure 1). Jabal Abiad shows a giant box fold with cascade-type folds verging away from the fold axis along the flanks (Figure 10a). Post-Eocene shortening in the eastern Oman Mountains was very small. Along the northern margin of Saih Hatat in the Ras al Hamra area near Muscat, Tertiary folds show gentle folding along NNE-aligned axes, mimicking the Late Cretaceous sheath fold orientation. Post-Eocene normal faulting is also apparent along the Wadi Kabir fault at Ruwi where Late Cretaceous Ruwi mélange (with high pressure carpholite assemblages) and the Semail ophiolite have been down-faulted to the north against Triassic Mahil Formation limestones (Figure 14).

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Wadi Mayh

Truck

b c

Bus

d e

Figure 16: Photos showing Wadi Mayh sheath fold structure, northern Saih Hatat. Red lines are thrust faults; blue lines outline bedding planes. Note the internal duplexing and boudinage structures aligned NNE along the sheath fold axis within beds of the Permian limestones. A bus and truck in Wadi Mayh give scale of cliffs.

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a Hayb 57°E 58° 59° 24°N i As Suwaiq Hawasina Gulf of Oman Batinah coastal plai

n Barka Muscat Window Yanqul Fanjah Dhan As Sifah k Rustaq Nakhl 3,009m Saih Al Jabal Hatat Ibri Semail Quriyat Jabal al Akhdar Shams

Saiq Wadi Tay Baid 23° 23° Nizwa yin Bahla Semail Gap

Hamrat Duru Hubat range

Ibra

Natih Anticlin Maradi Faul

Salakh Arch Fa e t h ud Anticlin Adam N Jabal Madar 0 50 e 57° 58° km

Postnappe cover (Maastrichtian Semail Nappe (Cretaceous oceanic Late Permian to Cenomanian shelf to Miocene sediments) crust and upper mantle) and Turonian-Santonian basin sediments Hawasina Nappes (Permian- Anticline axis and plunge Cretaceous slope and basin Neoproterozoic to Ordovician sediments, volcanic basement) sediments and volcanics

Al Jabal al Akhdar Late Cretaceous Late Cretaceous b fold axis c d Foreland South of Al Jabal al Akhdar nticline Anticlin A Oman Mountains yncline 035° S e

Basin Synclin nticline A N e Riedel shears 100° Dome and joint sets

Anticline 125° Maximum compressive 125° 155° Saddle stress axis 135° 035° Maradi fault trend Dome Figure 17: (a) Map showing the Hawasina Window, Al Jabal al Akhdar and Saih Hatat culminations, ophiolite blocks in structural depressions and the fold interference patterns creating a large-scale dome and basin structure. (b) Geometry of the finite strain ellipse for the Central Oman Mountains. The shaded area represents the range of fold axes orientations generally oriented 125±10°. Minor east-west or WNW-ESE aligned fractures on Al Jabal al Akhdar have a small component of sinistral shear (Filbrandt et al., 2004, figure 35). The Miocene to present day NE-SW compressive stress axis orientation mimics the Late Cretaceous NE-SW maximum compressive stress axis for the Oman mountains. (c) Geometry of the finite strain ellipse for the Oman foreland, south and southwest of the Oman Mountains, after Filbrandt et al. (2004). Faults and fractures in the foreland follow two sets of Reidel shear planes oriented NNW-SSE (sinistral) and WNW-ESE (dextral). The latter set of WNW-ESE aligned fractures have been reactivated as dextral strike-slip faults, although offsets are relatively minor. The conjugate set of fractures and strike-slip faults in the foreland are regional, extending all across the Arabian foreland and are consistent with a NW-SE maximum compressive stress axis in the foreland (Filbrandt et al. 2004). The Maradi fault, oriented 155°, follows a sinistral Reidel shear band, possibly reactivated by dextral shear during late Cenozoic inversion. (d) See facing page.

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FOLD ORIENTATIONS AND STRESS FIELD

Two major orogenic events dominate the stress field in Oman during the Late Cretaceous, firstly the emplacement of the Semail ophiolite and underlying thrusts sheets from NE to SW onto the passive margin of Arabia, and secondly the emplacement of the Masirah ophiolite and underlying Batain mélange from ESE to WNW along the southeast Oman coast. The resultant bi-axial compressive stress field resulted in a dome and basin type fold pattern along the North Oman Mountains (Figure 17). The major anticline axis swings around from NW-SE in the Hawasina Window to ENE-WSW along Al Jabal al Akhdar, then swings around through 90o along the Jabal Nakhl NNE-SSW trend before swinging back to WNW-ESE along Saih Hatat. Four main NNE-SSW aligned anticline axes at right angles to the main trend are apparent from the map pattern: (1) Wadi Bani Umar–central Hawasina; (2) Rustaq to Bahla; (3) Saiq to Jabal Nakhl; and (4) Hubat-Baid-Ras al Hamra (Figure 17). Where two anticline axes meet, a structural dome is present (Hawasina Window, Al Jabal al Akhdar, Saih Hatat). The ophiolite blocks of Rustaq, Bahla and Semail are located in structural depressions where syncline axes intersect, in-between the shelf carbonate domes.

The bi-axial fold interference pattern is undoubtedly late stage because it affects all the allochthonous thrust sheets after their emplacement onto the shelf margin. Mid-Tertiary fold axes orientations are parallel to the Late Cretaceous fold axes indicating two phases of NE-SW compression. Folds in the Tertiary jabals along the southwestern flank of the mountains (e.g. Fahud, Natih, Ibri, Dhank structures; Figure 17) have a similar orientation to the Late Cretaceous structures along the mountains. West of Muscat in the Ras al Hamra area, fold axes in Tertiary rocks are aligned NNE-SSW parallel to the Late Cretaceous sheath folds in the Saih Hatat Window.

Semail Gap

The Semail Gap is the lineament of the Semail thrust, reactivated as a down-to-east normal fault during the uplift of Al Jabal al Akhdar-Jabal Nakhl shelf carbonates (Figure 1). The geometry of the main anticline along the northern end of Jabal Nakhl suggests that some sort of basal detachment must exist in the pre-Permian basement at depth. This detachment is likely to be similar to the one proposed at depth beneath Saih Hatat. Both deep detachments have more displacement in the north than the south, and may ramp up to higher levels along a dominant kink band set, as seen, for example, in the SW flank of Al Jabal al Akhdar and the Wadi Tayyin area of southern Saih Hatat. The Semail Gap fault would therefore be related to a deeper-level lateral ramp parallel to the main transport direction. Up to 3–4 km of throw occurs along this fault which juxtaposes ophiolite mantle sequence rocks above Wasia Formation shelf carbonates.

The Semail Gap fault is not a strike-slip fault because it does not laterally offset any geology, and does not extend south into the foreland. Instead it is a S-shaped transfer between the Al Jabal al Akhdar and Saih Hatat domes. The fault is most likely a listric normal fault curving around the Wasia Formation top shelf, decreasing in throw to the north and south away from the central sector. The lateral ramp would therefore have been active during the latest part of the Late Cretaceous tectonic event and enhanced by some unquantifiable amount of late Tertiary uplift. Prominent NNE-trending fold axes also occur along the Baid corridor, where the Semail ophiolite has been folded around a N-S- aligned fold axis, along the NNE-aligned Jabal Qirmadil and the Wadi Mayh sheath folds around the northern flank of the Saih Hatat culmination.

Figure 17 (continued): (d) Model showing present-day map pattern of a biaxial fold interference pattern. A NE-SW aligned maximum compressive stress axis (Oman Mountains trend) and a secondary compressive stress axis aligned NW-SE (Masirah-Batain coast trend) results in domes where two anticline axes meet, and basins where two syncline axes meet. The maximum compressive stress axis swings around from NE-SW in the central mountains (Hawasina Window) to NNE-SSW in Saih Hatat. The secondary compressive stress axis oriented WNW-ESE in Saih Hatat resulted in NNE-aligned fold axes (Jabals Nakhl, Qirmadil, Hubat-Baid, Wadi Mayh sheath folds).

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Foreland Fracture Systems

The fracture pattern in the Oman foreland south and southwest of the mountains is very different from the Oman Mountains (Figure 17a,b,c). The Late Cretaceous foreland fracture pattern, interpreted from 3-D seismic data as the result of NW-SE to WNW-ESE compressive stress (Filbrandt et al., 2006) is also very consistent across nearly 1,000 km from the Arabian Gulf to the Indian Ocean. South of the Hawasina basal thrust in the foreland there is very little evidence of S- or SW-directed thrusts or fractures associated with the Oman Mountains trend (Filbrandt et al., 2006). In the foreland, SW of the Oman Mountains, from Lekhwair to the Ghaba salt basin, two dominant fault sets are oriented WNW-ESE and NNW-SSE (Filbrandt et al., 2006). The highest fault density occurs in the competent Mesozoic shelf carbonate unit, whereas the clastic dominated Aruma Group absorbs strain by buckling and internal structural thickening. Several prominent faults, such as the Maradi and Burhan faults, developed during the Campanian and form en-echelon faults showing sinistral shear. Many of the north Oman Shu’aiba and Natih oil traps formed during the Campanian and are associated with large steep faults that were active during deposition of the Aruma foredeep sediments (Turonian to Campanian). The Fahud structure was formed by thickening of the Fiqa Formation towards the SW-dipping normal fault, and subsequent folding of the Fiqa Formation and overlying Palaeogene limestones (Filbrandt et al., 2006). Tertiary contraction caused local inversion of earlier faults such as the Fahud fault and reversal of slip along the Maradi fault, which shows a dextral sense of shear today. These faults may have provided vertical conduits for migration of hydrocarbons from deeper reservoirs (for example: Gharif Formation) up into shallower reservoirs (for example: Shu’aiba, Natih formations).

Filbrandt et al. (2006) suggested that the NW-SE to WNW-ESE maximum horizontal stress present along the Oman foreland was a result of an oblique collision of the Indian continent with the Arabian Plate during the Santonian-Campanian. Although there is certainly a fracture pattern that is consistent with this stress field, the fractures have very small or no offsets and several faults have beenre- activated during an evolving stress regime. The deformation in the foreland and along the SE coast of Oman is not compatible with a continental collision such as presently seen around the northern margins of the Indian Plate (hundreds of km of shortening, regional metamorphism etc). Instead, it is suggested here that the fracture pattern in the Oman foreland is more likely to be related to WNW- directed emplacement of the Masirah ophiolite and Batain mélange.

Magnetic, seismic and structural data suggest that the Masirah ophiolite is part of an elongate ‘basement’ ridge of ophiolite that extends along the SE coastal margin of Oman (Mountain and Prell, 1990). This ophiolite belt, together with the Batain mélange was emplaced towards the WNW during the late Maastrichtian-early Palaeocene (Shackleton and Ries, 1990; Shackleton et al., 1990; Peters et al., 1997; Schreurs and Immenhauser, 1999). This timing slightly post-dates the final stages of emplacement of the North Oman ophiolite belt (ca 95–67 Ma), although the earlier stages of obduction of the Masirah ophiolite remain unknown, as the base is never seen. It is possible that the NNE-SSW trending fold axes in the North Oman Mountains were related to the Masirah ophiolite emplacement event. In this case, the biaxial fold interference pattern in Al Jabal al Akhdar-Saih Hatat could have been the result of refolding of the earlier NW-SE aligned folds (Al Jabal al Akhdar trend) by a slightly later set of NNE-SSW aligned folds (Jabal Nakhl-Semail Gap trend).

SUMMARY AND Conclusions

The structural evolution of the Oman Mountains can be related to the Late Cretaceous phase of thrusting of the Semail Ophiolite and underlying Haybi and Hawasina thrust sheets from NE to SW onto the passive continental margin of the Arabian Plate. This process closed an ocean at least 400-450 km wide (Cooper, 1988) and lasted ca 27 My from 95–68 Ma. Minimum convergence rates were of the order of 17 mm/year (Searle et al., 2004). The Semail ophiolite was formed above a NE-dipping subduction zone at 95 Ma, and thrusting propagated towards the SW with time from Semail to Haybi to Hawasina thrusts. Many of these thrusts were locally reactivated during the later phases of deformation as normal faults or backthrusts. There is no evidence of SW-dipping subduction beneath the Oman margin during the Middle Cretaceous as claimed by Gregory et al. (1998), Gray et al. (2000) and Gray and Gregory (2000). There are clearly no Andean-type granites or volcanic rocks typical

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of these active margins, and the Middle Cretaceous was a time of relatively stable passive margin carbonate sedimentation (Kharaib, Shu’aiba, Nahr Umr, Natih Formations) along north Oman. There is also no evidence of a separate plate origin for the ‘North Muscat microplate’ (Breton et al., 2004) or ‘Lower plate’ rocks of the Hulw and As Sifah units (Gregory et al., 1998; Gray et al., 2000). No suture zone rocks occur above the As Sifah eclogites or Hulw unit, and the protoliths rocks were attached to the Arabian margin, although show a slightly deeper stratigraphic level (Ordovician to Carboniferous -Permian protoliths).

The two giant shelf carbonate culminations of Al Jabal al Akhdar and Saih Hatat were both formed during the late stages of Late Cretaceous obduction, after thrusting of the Semail ophiolite, Haybi and Hawasina thrust sheets across the Arabian shelf margin. Thrusting along the northern margin of Al Jabal al Akhdar (Le Métour et al., 1990; Breton et al., 2004) is restricted to the Cretaceous (Natih and Nahr Umr formations) part of the shelf carbonate and is very minor intra-formational flexural slip- thickening associated with late-stage, large-scale folding of Al Jabal al Akhdar. Bedding is continuous below and above each duplex. In contrast, NE-vergent folds and NE-directed backthrusts across the Hawasina Window affect the Sumeini Group (Jabals Rais, Mawq, Rastun), the Hawasina and Haybi thrust sheets, as well as the Semail thrust (Searle and Cooper, 1986) and must be late-stage structures formed after emplacement of the entire allochthon onto the Arabian margin above a large-scale ramp along the basal detachment. The fact that these NE-facing folds affect all units rules out the model of early NE-directed thrusting associated with presumed craton/SW-directed subduction (Gregory et al., 1998; Gray et al., 2000; Gray and Gregory, 2003).

Saih Hatat is another 40 km half-wavelength anticline affecting all units from pre-Permian basement up to Semail ophiolite. Whereas the southern margin of the shelf carbonates is relatively undeformed, the northern margin shows extreme deformation: (1) in the form of major ductile shear zones, placing lower grade rocks onto higher grade rocks; (2) a series of large-scale asymmetric recumbent folds; (3) sheath folds with highly attenuated limbs; and (4) NE-facing folds affecting upper crustal rocks. These NE-directed folds and shear zones are late-stage backfolds (Searle et al., 1994, 2004) and not related to early SW-directed subduction as claimed by Gregory et al. (1998), Gray et al. (2000) and Gray and Gregory (2003).

The ‘North Muscat microplate’ comprising ‘lower plate’ Hulw and Sifah structures (Gregory et al., 1998; Gray et al., 2000; Breton et al., 2004) does not exist. The Hulw and Sifah structures are not exotic plates, but were connected to the Arabian continental margin prior to its attempted subduction (Searle et al., 1994, 2004). There is no evidence for a suture zone between the ‘lower slab’ Hulw-Sifah rocks and the overlying thrust sheets. The carbon isotope differences between the Sifah unit and the ‘upper slab’ rocks (Gray et al., 2005) are attributed simply to different stratigraphic levels, not to different structural ‘plates’. The As Sifah unit eclogite and blueschist facies rocks are metamorphosed Carboniferous-Permian limestones with basaltic flows and sills, that were part of the leading edge of the Arabian Plate (Searle et al., 2004). Quartz mica schists in the lowest structural level of the As Sifah unit are probably metamorphosed Ordovician Amdeh quartzites (Miller et al., 2002; Searle et al., 2004).

All stratigraphic, structural and U-Pb geochronological data suggest that a single NE-dipping subduction zone was operative during the Late Cretaceous (Searle and Malpas, 1980, 1982; Lippard et al., 1986; Searle and Cox, 1999, 2002; Searle et al., 1994, 2003, 2004; Warren et al., 2003, 2005). Protoliths of the As Sifah eclogites are Permian volcanic rocks intruded into the leading edge of the Arabian Plate, subducted to 20 kbar (ca 78–80 km depth) at 78.95 ± 0.13 Ma (U-Pb zircon age; Warren et al., 2005) and exhumed back up the same subduction zone (Searle et al., 1994, 2004).

Both the Al Jabal al Akhdar and Saih Hatat culminations may be underlain by deep blind-thrust flats, which may ramp to the surface along the foreland fold-thrust belt (Jabal Salakh-Madmar-Adam folds). Both culminations have large-scale listric normal faults bounding their flanks. These normal faults were active during uplift of the footwall shelf carbonates through their allochthonous cover (Hawasina-Haybi-Semail) and were operating in a wholly compressional stress field. The faults were likely active during the latest phase of Late Cretaceous deformation associated with ophiolite emplacement. It is also possible that the flank-bounding normal faults could have accommodated some amount of mid-Tertiary uplift of the Al Jabal al Akhdar and Saih Hatat culminations.

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A second phase of folding occurred after the Late Eocene - Early Oligocene as evidenced by the gentle box fold of Jabal Abiad, east of Saih Hatat (Figure 10a,b), and the box folded Tertiary jabals along the SW flank of the Oman Mountains (Jabals Hafit, Awaynah, Dhank (Figure 10c), Ibri, Fahud, Natih, etc). Eocene limestones are now at 2,000 m altitude on the Jabal Abiad structure and up to 4,000 m of post-Eocene uplift may have occurred in northern Saih Hatat. Apatite fission track data is consistent with a compressional tectonic event beginning in the Burdigalian (Early Miocene) and culminating in the Late Miocene - Pliocene (Poupeau et al., 1998).

The three main shelf carbonate culminations in the Oman Mountains (Musandam, Al Jabal al Akhdar, Saih Hatat) all show different structural geometries, degree of thrusting and internal folding, and timing of culmination and uplift. The Musandam culmination resulted from deep level Miocene thrusting affecting the pre-Permian basement up to the Eocene - Early Oligocene neo-autochthonous cover. At least 15 km of westward thrusting of the complete sequence occurred along the Hagab Thrust, during the initial affects of collision between Arabia and the Central Iran Plate (Searle et al., 1983; Searle, 1988a,b).

ACKNOWLEDGEMENTS

Much of this work was funded by NERC grant NER/K/S/2000/00951. I thank John Smewing, Dave Cooper, Jon Cox, Clare Warren, Tom Jordan, Mohammed Ali, Tony Watts, Randall Parrish and Hilal Al-Azri for discussions, and Dave Sansom for excellent cartography. I am grateful to Dr. Hilal Al-Azri and Dr. Salim Al Bu-Saidi from the Omani Directorate General of Minerals (Ministry of Commerce and Industry) in Muscat for their support and to Judy and David Willis for their hospitality in Bawsher. Detailed reviews from Pete Jeans and two anonymous reviewers greatly improved the manuscript. The design and drafting of the final graphics by Gulf PetroLink is appreciated.

REFERENCES

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ABOUT THE AUTHOR

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. [email protected]

Manuscript received April 20, 2006

Revised August 10, 2006

Accepted September 9, 2006

Press version proofread by author January 24, 2007

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56°E 57° 58° TURKEY59° Caspian Sea Tertiary limestones N Ruwi schists 0 km300 Upper-Lower plate discontinuity, unconformity overlying Semail Ophiolite Med SYRIA P 3-6 kb Sea folded above Hulw Window Muscat peridotite IRAQ IRAN T 350-400°C Metamorphic sole 26° 26°N JORDAN N-S fold axes Musandam Amphibolites, Greenschists KUWAIT Arabian Peninsula Haybi, Hawasina thrust sheets Gulf Structural geometry, style and timing of deformation in the BAHRAIN Depth Permian-Mesozoic shelf carbonates QATAR (km) Wadi Jabal Saih Hatat Jabal EGYPT Arabian Wadi Mayh Shield UAE Wadi Hawasina Window, Al Jabal al Akhdar and OMAN 10 Jabal Al Hammah Wadi Bu Taymah Tayyin Aswad Qirmadil Dibba High pressure metamorphic units sheath fold Qanu Hagab Thrust SAUDI ARABIA Wadi Sarin Red Saih Hatat culminations, Oman Mountains Asimah-Masafi Carpholite zone SUDAN Sea Eocene Ruwi Muscat YEMEN Bani Hamid Garnet, glaucophane, blueschists Arabian Sea Palaeocene ERITREA Foreland Michael P. Searle granulites U. Maastrichtian 5 fold-thrust belt Tertiary unconformable on ophiolite extrapolated from east 5 CRJR Wadi Ham 25° TR GeoArabia Volume 12, Number 2, 2007, Pages 99-130 Pm N crust Upper Semail Thrust 0 50 0 Wadi Hatta Semail Moho L km CR Haybi Mantle owe Published by Gulf PetroLink Sumeini r P JR Hawasina Hulw late P.O. Box 20393, Manama, Bahrain TR Pm 4 E-mail: [email protected] Wadi Jizi -5 Haybi www.gulfpetrolink.com P 7-8 kb Hawasina Sohar T 380-420°C 3 Haybi Mahil Saiq Carpholite 0 10 As Sifah 2 Figure 12 Hawasina Window 1 -10 km Blueschist 24° Figure 11 24° P 12-15 kb Northwest Southwest Eclogite Figure 13 Figure 14 Wadi Sarin normal fault T 450-550°C As Judi 2-3 km throw down-to-S Wadi Tayyin normal fault imbricated thrust slices of (BRGM Quriat sheet) mostly Inferred ramp-flat in SW-facing folds in Out-of-sequence motion along 3 recumbent folds in Jabal Abiad Semail ophiolite ca 2 km throw down-to-S Al Ajaiz Muscat Hamrat Duru/ Wahrah Fm Late Cretaceous fault cuts up basal detachment As Sifah eclogites Haybi corridor beneath Semail thrust truncates Hamrat Duru Group Triassic exotics normal faulted inferred thrust ramp 4-5 km thickness placing Semail mantle proximal Hawasina in to Maastrichtian cover beneath Saih Hatat P 15-20 kb folded Semail thrust stratigraphy in footwall Jabal Milh nappes Haybi complex directly above shelf As Sifah in basement and shelf Semail ophiolite mantle over top shelf (Wasia Group) eclogites foreland fold-thrust belt window T 550°C beneath ophiolite beneath Ibra dome beneath Moho km Jabal Rais Jabal Milh Hawasina Thrust Al Jabal Hulw 3 Semail ophiolite al Akhdar Saih Hatat 5 Wadi Kabir normal fault

Jabal Nakhl Figure 14: NNE-SSW cross-section of the Saih Hatat culmination, Southeastern Oman Mountains; location of section on Figure 1. The northern part of section is a composite section combining data from along strike across northern Saih Hatat (after 4 Yenkit shear zone Jabal Abiad 23° Searle et al. 2004). The southern part of the section utilizes mapping from BRGM. Semail Gap 23° 3 Al Khuyran shear zone

Semail thrust 2 As Sheik shear zone 2 Ras Al 1 As Sifah shear zone Semail Thrust Ex Ex Hadd N Haybi Thrust OMAN S G Z Sumeini S N Al Jabal Z Group G al Akhdar Jabal Southwest Restored section across Arabian continental margin, Saih Hatat, Oman Mountains (at ~ 90 Ma) Northeast culmination Ja’alan 1 Hawasina thrust Nayid Fm 56° 57° 58° 59° Mayhah Fm CR Wasia Group Sidr Fm Figure 1: Geological map of the Oman Mountains, after Glennie et al. (1974). Locations of the three Jabal Tayin Wadi Kahza Al Hajar Wadi Mayh Wadi Qanu Wadi Kabir Al Bustan coastline Hamrat Duru Maqam Fm Guwayza Fm cross-sections of the Hawasina Window, Al Jabal al Akhdar and Saih Hatat are shown. Jabal Abyad CR Kahmah Group nappes 0 Zulla Fm 0 1 Sumeini thrust JR Sahtan Group km TR Mahil

Figure 12: WNW-ESE lateral section across the Hawasina Window culmination to the nose of Al Jabal al Akhdar showing the lateral differences in duplex thickness and the Unconformity at base Muti Fm Approximate stratigraphic late-stage normal faults placing Semail ophiolite directly above Cretaceous shelf carbonates. Location of section shown in Figure 1. Note: N - Nayid Formation; S - Sidr cherts; cuts down-section to NE thicknesses northern Saih Hatat G - Guwayza Formation; Z - Zulla Formation, all part of the Hamrat Duru Group (Hawasina complex). progressively, cutting out Wadi Kabir the Wasia, Kahmah + fault-post Palaeocene Sahtan Gp carbonates normal fault down-to-NE Stratigraphic thickness Permian unconformity Southwest Northeast from central and SW (base Sq 1) truncates margin Saih Hatat folds in pre-Permian Maastrichtian-Tertiary unconformity above Hawasina cherts Triassic exotics SW-facing folds in Hawasina NE-facing backfolds of Jabal Milh sequence SW-directed shearing + Hawasina folded by mid-Tertiary box folding Haybi volcanics in Yankul melange along southern window Hamrat Duru Group Semail thrust folded by Upper/Lower plate discontinuity thrusting placing Sq 3 and Ruwi schists (lawsonite NE vergent backfold; cuts down-structural section to SW Ma over older rocks meta-basalts; carpholite- northern margin from Sq 1 to Am kinematic indicators metasediments) are ≡ more Hawasina Window + folds suggest NE-vergent thrusting deeply buried Haybi complex Depth inc Triassic-Jurassic Haybi J. Rais J. Mawq J. Abiad km Al Khyran Yenkit volcanics HyV Depth shear zone 5 Tight upright folds in shear zone (km) 4 deep Sumeini Group Blocks of Mu Muti Fm 3 Ibri Jabal Awaynah Dhank duplexes Batinah coast 0 Crustal Wadi Hawasina and U Mu Hawasina Complex Semail Ophiolite Wa Haybi Complex 2 Haybi complex Wasia Gp sequence Bani Umar U Ruwi Semail Semail 1 Miocene Cretaceous Kh Muti Fm 1 Kh Kahman Gp Ophiolite Moho 0 U Sa Mantle Sequence -1 Eocene Jurassic Sa Sahtan Gp Hv U. Palaeocene Ma Ma -2 Ma Ma harzburgites 2 Mahil Fm Batinah melange Ma 3 -3 Haybi Sahtan Triassic Sq Sq 3 + dunites Radiolarian cherts 3 2 3 Haw -4 crust Sq Wasia-kahmah Permian Saiq Fm -5 Saiq Mahil Sq 2 Sq Hawasina 3 U Upper Plate -6 1 2 Mantle Moho v Pre-Permian Semail Am 5 Tertiary thrust Haybi Sq1 Semail ? basement 1 ophiolite 4 Am Lower Plate Am Hulw Haybi Listric normal fault Hawasina Metamorphic sole Ha 3, 4 Am ≡ Lqms Sifah Sq2v 0 10 ? Hi Unit Hawasina along south margin of Grt + Cpx amphibolites Ordovician Am 4 Unit Lqms Amdeh Ha 2 Hi km Possible blind thrusts in window downthrowing P 10 - 12 kb 5 Semail Thrust Muti basement and shelf carbonates T 750 - 800°C Haybi Thrust ? ? ophiolite ~4km south Ha 1 Lc/Ldt Am 1-3 Sq1v Figure 11: NE-SW cross-section of the Hawasina Window culmination, partly after Searle and Cooper (1986); location of section on Figure 1. Lv U Ha1 Hawasina Thrust Hi Hijam Pre-Cambrian basement Ha 1 Ha2v Southwest Northeast 4 Sarin 0 2 Lqms Late 3 Amarat km Wadi Ghul: Hatat SW-verging Sint Syncline: Klippe of Triassic Wadi Sahtan-Rustaq Proterozoic Sifah Unit Listric normal fault Jahlut fold and thrust Klippe of ophiolite Exotic and Haybi Listric normal fault 2 Lc + Ldl calc schists are along front of mantle sequence above downthrowing volcanic above downthrowing ophiolite Haybi + Exotics Cenomanian Blocks of Haybi and ≡ metamorphosed Saiq Fm Jabal Kawr Exotic Jabal Kawr-Misfah Exotic Jabal Shams against shelf 1 Hulw Unit against top shelf radiolarian cherts Hawasina rocks in Saadi Lv Sifah eclogites are ≡ to Saiq 1 volcanics. in V2 volcanics Batinah melange ? Lc calc-schists are metamorphosed Saiq Fm above ophiolite Lv meta-volcanics are ≡ metamorphosed Saiq Lqms quartz-mica schists are ≡ 10 volcanics (mid-Permian) metamorphosed Amdeh quartzites and Late Exhumation Depth Jabal Kawr Sint Jabal Shams Sahtan Lqms schists are metamorphosed Amdeh quartzites lower units ≡ Hatat schists related faults (km) Up to 60 m thickness of pillow 3,009 m bowl Wadi Sahtan 5 lavas and tholeiitic basalt flows imbricated Early Subduction 4 ophiolite in Jahlut member of Hatat Fm related faults/ Hamrat Duru Group Semail Ophiolite 3 Muti (Proterozoic) shear zones 2 Muti CR JR Batinah coast 1 Nadan Wasia-kahmahSahtan PMTR Kawr Misfah Mid 0 HT Mahil -Perm Aruma Pre-Permian ian EoceneMiocene Figure 15: Restored section of the northern part of Figure 14 across Saih Hatat (to ca 90 Ma) from Searle et al. (2004) showing projections of the major late-stage shear zones and faults, prior to Campanian subduction and high-pressure metamorphism, and subsequent culmination Saiq unco U. Palaeocene -1 Wasia I x Mistal nfor Batinah Sahtan mbri mple Muti basement mity Moho pillow lavas of Saih Hatat. The Semail ophiolite and Haybi - Hawasina thrust sheets were already partially emplaced across the Saih Hatat shelf at this time. The stratigraphic columns on the left and right are constrained by the thicknesses from Glennie et al. (1974) and LeMetour et al. (1986). -2 cated Hawasina Co Huqf Gp Mantle crust melange Mahil sheeted dykes -3 gabbros Note the thickness of the Permian-Triassic section in the NE suggesting that the shelf margin at that time lay further outboard to the NE. Note also the basal thrust of the Semail – Haybi – Hawasina allochthon cutting up-section from Triassic to Cenomanian from NE to SW. The Saiq Cr Semail Thrust -4 Jr leading edge of the Arabian margin was subducted to the NE and the protolith of the As Sifah eclogites was the Permian and stratigraphicaly lower part of the Arabian margin. Muti basin thickens to NE (foredeep migrating SW in-front of Semail ophiolite thrust sheets); Wasia, Pre-Permian 0 10 -5 Tr Haybi Thrust Kahmah, Sahtan shelf carbonates thins towards NE nearer to continental margin shelf edge; and Saiq Formation thickens to NE. Saiq2 volcanics comprise 50−100 of basaltic lavas and are protolith rocks of the Hulw blueschists metavolcanics (Lv). Pm Al Jabal al Akhdar anticline Hawasina Thrust Metamorphic sole -6 km Haybi Complex Volcanics Figure 13: NE-SW cross-section of the Al Jabal al Akhdar culmination, central Oman Mountains; location of section on Figure 1.

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