Abstract The Oman Mountains contain one of the world's best- exposed and best-understood fold–thrust belts and the largest, best-exposed and most intensively studied ophiolite complex on Earth. This volume presents new international research from authors currently active in the field focusing on the geology of the Oman Mountains, the foreland region, the carbonate platforms of Northern and Central Oman and the underlying basement complex. In addition there is a particular focus on geoconservation in the region. The volume is divided into three main sections that discuss the tectonics of the Arabian plate using insights from geophysics, petrology, structural geology, geochronology and palaeontology; the petrology and geochemistry of the Oman Ophiolite and the sedimentary and hydrocarbon systems of Oman, drawing on the geophysics, structure and sedimentology of these systems. The volume is enhanced by numerous colour images provided courtesy of Petroleum Development Oman.

Abstract The Oman Mountains contain a number of unique and very special geological sites of World Heritage importance. Large-scale developments in Oman and UAE have already affected some of these sites and it is now crucial to preserve these sites for posterity in law before urban development destroys them forever. This paper outlines the geological importance of 50 sites as National GeoParks and Sites of Special Scientific Interest (SSSI). Three geological sites in Oman are so unique and so special that they warrant World Heritage status: the Semail Ophiolite, the world's largest, best exposed and most complete ophiolite; Jebel al-Akhdar; and the Musandam peninsula. Other world-class sites include the Sifah eclogites, the Oman Exotics, Majlis al-Jinn, sites of ‘Snowball Earth’ significance and many others. All these sites are of World Heritage quality and fundamental geological importance, not only for Oman but also for the world. The total acreage of all these GeoPark sites is <10% of Oman and the areas designated have little or no commercial value. The GeoParks will have long-lasting economic benefits as the post-oil and gas economy of Oman will have to rely on tourism and geo-tourism.

Abstract We use Pn -tomography to map lithospheric mantle velocity and anisotropy at the Arabia–Eurasia plate boundary, namely Makran and Zagros. We use catalogue events recorded by Oman, UAE, Saudi Arabia and Iran networks, the International Seismological Centre and the National Earthquake Information Center. Events of 1.8–16 degree distances were used for this Pn -tomography. In this study we show that the northeastern Arabia plate is characterized by cold and stable lithospheric mantle. Contrastingly, Eurasia is underlain by hot unstable lithospheric mantle. The Arabia–Eurasia lithospheric suture follows the Zagros collision surface suture within c. 70 km lateral proximity. At the southernmost Zagros collision, the Arabia lithosphere is inferred to extend further NE beneath Lut Block. This may be indicative of extended subduction of Arabia beneath Eurasia in southernmost Zagros. We find that eastern Makran shows typical subduction characteristics, with inferred oceanic lithosphere underlying the eastern Oman Sea and hot unstable lithospheric mantle below overriding Helmand Block. Contrastingly, the western Makran subduction zone including Arabia and Eurasia continental sides is underlain by a low- Pn -velocity anomaly, indicative of hot unstable lithospheric mantle. Surface evidence show that western, southern and eastern boundaries of western Makran low- Pn -velocity anomaly may represent a Late Neogene reactivated Precambrian terrane boundary in north Oman.

Abstract Widespread lava fields in the northern part of the Arabian platform are the subject of an open geodynamic debate on the origin of the intraplate volcanism. We present new K–Ar ages and whole-rock geochemical data for lava flows from Syria, which allow us to propose a new model of volcano-tectonic evolution highlighting how tectonics have controlled magma genesis in the region during the last 18 Ma. The Cenozoic Syrian lavas are alkaline and subalkaline rocks, typical of magma erupted in such a continental intraplate context. Basaltic samples from different Syrian volcanic provinces show significant variations in terms of incompatible trace element signatures. Crustal contamination plays a negligible role during magma migration and differentiation, and crystal fractionation is essentially restricted to olivine and clinopyroxene. Our results suggest that the Syrian lavas have been generated by variable degrees of partial melting ( c. 1–10%) from different levels of a locally heterogeneous lithospheric mantle. The light/medium rare Earth element (LREE/MREE) ratios not only illustrate how the degree of partial melting has changed spatially and temporally during the last c. 18 Ma, but also indicate that the degree and the style of extensional tectonics has changed through time. We conclude that the Cenozoic Syrian volcanism is a consequence of extensional tectonics, under periodical influence of the north- and eastwards convergence at the Arabia–Eurasia margin, which induces rotational tectonic styles. This controls the partial melting at various depths in the mantle. The volcanism of northern Arabia developed in the framework of the Red Sea rifting and initiated at the same time as the southern Red Sea volcanism. It extends up to historical time, progressively smoothed to the north in a contradictory relation with the compressional/extensional setting of the Arabia–Eurasia margin. Supplementary material: Further information on the analytical results is available at http://www.geolsoc.org.uk/SUP18681 .

Abstract A collection of terrestrial and freshwater snails from the Late Eocene to earliest Early Oligocene Zalumah Formation at Wadi Darbat, near Salalah, Oman is of importance on account of its taxonomic composition, its palaeoecological indications and its biogeographic affinities which are clearly tropical African; these are very different from the extant non-marine snail fauna of Oman which is typical of the Mediterranean belt. In addition, these fossil snails are by far the oldest-known representatives of their respective genera. The Late Eocene Zalumah deposits which yielded the molluscs accumulated just above sea level, as revealed by the admixture of marine, brackish water and freshwater taxa as well as fully terrestrial gastropods. These deposits now vary in altitude from near sea level at Taqah (Wadi Darbat) to over 930 m at Thaytiniti. Uplift of the Dhofar Plateau therefore must have occurred later than the Early Oligocene. The biogeographic affinities of the snails confirms that the Arabian Peninsula was part of the African continent at the time of deposition and that the opening of the Red Sea and the Gulf of Aden occurred later than the Early Oligocene.

Abstract New high spatial resolution secondary ion mass spectrometry (SIMS) U–Pb zircon data from the Sadh gneiss complex and the intruding Marbat granodiorite of the Marbat region, southern Sultanate of Oman, yield Cryogenian magmatic protolith ages for gneisses ranging from c. 850 to 830 Ma. Zircon ages record a c. 815–820 Ma period of deformation and migmatization, followed by intrusion of a hornblende gabbro/diorite and the undeformed Marbat granodiorite at c. 795 Ma. Following break-up and rifting of Rodinia at c. 870 Ma, crustal growth in the Marbat region occurred via arc accretion at c. 850–790 Ma, possibly in the easternmost part of the Mozambique Ocean based on earlier cessation of accretion here compared to the Arabian–Nubian Shield. Similarity of the new zircon geochronology to peaks of detrital zircon ages in the unconformably overlying Ediacaran Marbat sandstone suggests relatively local derivation from uplifted basement for the latter. Supplementary material: Detailed petrographic descriptions and photographs of hand specimens and thin-sections are available at http://www.geolsoc.org.uk/SUP18685 .

Abstract The northern Oman Mountains record the Late Cretaceous emplacement of the Semail Ophiolite and associated subduction trench and deeper-water sediments into a foredeep on the downwarped Arabian continental margin. In-sequence piggy-back thrusting was modified by later thrust sheets of Permian–Mesozoic continental slope sediments that breached the overlying allochthonous stack. Sections through these culminations show asymmetries linked to deep-seated faults perpendicular to the margin edge which created offsets (promontories) that influenced detachment. Seismic profiles across the foredeep indicate an unconformity at the top of the Mesozoic carbonate platform, the product of early flexural uplift, onlapped by sediments. A second onlap phase is linked with rapid subsidence during final emplacement of the allochthon. Passive margin conditions resumed during Maastrichtian time, conformable along the axis of the foredeep. A Paleocene–Eocene (Pabdeh) foredeep was initiated in the northern mountains, recorded on seismic profiles with another onlapping sequence. Subsidence and infill persisted into the Oligocene when compression, linked with the Zagros collision, resulted in uplift and en echelon folding, probably due to the reactivation of deep-seated faults. Post-Miocene erosion of one culmination exposed the cap of a gypsum/anhydrite intrusion. The origin remains uncertain, but derivation from the Neoproterozoic to Cambrian Ara Group salt is preferred.

Abstract Two localities where the inverted metamorphic sole of the Oman Ophiolite are best exposed, Sumeini Window and Wadi Tayyin, have been mapped and sampled in greater detail. In both areas an inverted pressure and temperature profile is exposed beneath the Semail Thrust, the base of the ophiolite, from garnet+clinopyroxene-bearing granulite to hornblende+plagioclase amphibolite down through epidote amphibolite and a variety of greenschist facies metasediments, dominantly cherts, marbles and quartzites. Thermobarometry on garnet- and clinopyroxene-bearing amphibolites immediately beneath the contact with mantle sequence harzburgites shows that the upper sole rocks formed at pressure–temperature ( P–T ) conditions of 770–900 °C and 11–13 kbar, equivalent to depths of 30–40 km in oceanic lithosphere. Heat for metamorphism can only have been derived from the overlying mantle sequence peridotites. Pressures are higher than can be accounted for by the thickness of the ophiolite (15–20 km). Timing of peak metamorphism was synchronous with the formation of the ophiolite gabbroic–trondhjemite crustal sequence and eruption of the pillow lavas (Cenomanian; 96–95 Ma). Metamorphic sole rocks have been structurally repeated by imbricate thrusting, casting doubt on previous estimates of thermal gradients. All the data support a subduction zone setting for metamorphism and a supra-subduction zone environment for ophiolite formation during the Cenomanian.

Abstract Based on detailed stratigraphy, petrology and geochemistry, the initial arc magmatism of the Oman Ophiolite consisting of tholeiitic lavas followed by boninite flows and tephras is studied in the Wadi Bidi area, northern Oman Mountains. An 1110-m-thick V2 sequence is divided into the lower 970 m (LV2) and upper 140 m (UV2) thick subsequences by a 1.0-m-thick sedimentary layer. Pahoehoe flows dominate in the lower part of the LV2, while the upper part consists mainly of sheet flows with sparse interbedded pelagic sediments and a cylindrical plug. In addition to the presence of a feeder conduit, the flow-dominant lithofacies with a few thin sedimentary interbeds in the LV2 indicates that the study area was the centre of a volcano grown in a short period. The UV2 is composed of boninite sheet flows overlain by a 2.0-m-thick pyroclastic fall deposit. A small amount of boninite lavas at the end of the V2 sequence overlain by thick pelagic sediments suggests that the subduction-related arc volcanism was short lived and terminated long before the ophiolite obduction. Supplementary material: Locations, mode of occurrence, phenocryst assemblages and bulk-rock major and trace element compositions of lavas in the Wadi Bidi area are available at http://www.geolsoc.org.uk/SUP18684 .

Abstract Detailed backscattered electron (BSE) and cathodoluminescence (CL) observations in an olivine gabbro from the Wadi Rajmi of the Fizh Block of the Oman Ophiolite revealed that this gabbro represents a perfect example of a partial melting reaction. The observed microtextures imply that water-rich fluids migrated on grain boundaries in a ductile regime, causing an incongruent partial melting reaction. All grain boundaries between primary plagioclases are characterized by c. 50 µm thick haloes of Anorthite-rich (An) plagioclase, with An contents which are c. 10 mol higher compared to the host plagioclase composition. Where these An-rich zones are in contact with the mafic primary minerals (olivine, clinopyroxene) in places, reactive rims of orthopyroxene and pargasitic amphibole around the primary mafic silicates have been formed. In situ analyses of 87 Sr/ 86 Sr isotope ratios on primary plagioclases and on An-enriched zones along grain boundaries revealed no significant difference, implying that the water/rock ratio was very low during the fluid–rock interaction. The observed features imply a model of hydrothermal activity proceeding on grain boundaries within the deep oceanic crust at very high temperatures (900–1000 °C) without any crack system, a prerequisite in current models to enable hydrothermal circulation. Interconnected porosity is generated by coupled dissolution–reprecipitation which allows pervasive fluid transport through the minerals at the grain boundaries, establishing a novel type of hydrothermal circulation maintaining water–rock interaction at very high temperatures in the ductile regime. The presence of isolated product phases as inclusions within the parent minerals emphasizes the importance of an interconnecting fluid establishing local equilibrium between parent and product phases of the incongruent melting reaction.

Abstract Wehrlitic intrusions constitute an important element of oceanic lower crust of the Oman Ophiolite, and numerous such dunite and plagioclase wehrlitic intrusions cut gabbro units in the northern Salahi Block of this ophiolite. For the first time, we describe a large wehrlitic intrusion (the Barghah complex) that has disturbed and folded surrounding gabbro units around this complex. The Barghah complex contains many gabbro blocks that record evidence of magma mingling. The crystallization sequence of the wehrlitic intrusions is olivine followed by co-crystallization of plagioclase and clinopyroxene. The forsterite content of olivine and the Mg# of clinopyroxene are more evolved than in rocks from the Moho Transition Zone (MTZ). TiO 2 and Na 2 O contents of clinopyroxene are similar to those of the MTZ and are characterized by wide compositional variability over a narrow range of Mg#, which is indicative of melt–mantle interaction. Compositional co-variation of plagioclase and olivine in the wehrlitic intrusions has a similar signature to that of V1 magmatism (ocean ridge stage). In light of these observations, we conclude that the wehrlitic intrusions are the product of off-axis magmatism. Inversion tectonics that led to a transition from spreading to compression may have accelerated the emplacement of the wehrlitic intrusions.

Abstract We report the major element compositions of constituent minerals in 278 harzburgites and of 101 whole rocks from the northern Fizh mantle section in the northern Oman Ophiolite to investigate the formation and evolution of oceanic lithospheric mantle. Olivine Fo varies from 90 to 92 whereas spinel Cr# (= Cr/(Cr+Al) atomic ratio) varies from 0.15 to 0.78. The Cr# of spinels in a large number of harzburgites exceeds 0.6, which is the upper bound for abyssal peridotites. In the northern Fizh mantle section, highly refractory harzburgites with spinel Cr# greater than 0.7 are distributed in a 3-km-wide band along a NW–SE-striking shear zone. We infer a two-stage depletion process in the northern Fizh mantle section. In the first stage, asthenospheric mantle was partially melted beneath a mid-ocean ridge, producing a harzburgitic residual column. In the second stage during detachment of oceanic lithosphere an H 2 O-rich fluid, released from the metamorphic sole due to thermal metamorphism of altered oceanic crust, extensively infiltrated the northern Fizh mantle section where the ridge segment boundary region was previously located. The residual harzburgites were subjected to flux melting, resulting in a highly refractory harzburgite zone with spinel Cr# greater than 0.7. Supplementary material: Locality map, names of the wadis and UTM coordinates and further analytical data are available at http://www.geolsoc.org.uk/SUP18679 .

Abstract Plagiogranites intrusive into the harzburgites of the mantle section of the Oman Ophiolite are compositionally distinct from plagiogranites in the crustal section of the ophiolite and are relatively enriched in MgO, Cr and Ni and in the incompatible trace elements K, Th, U, Ta, La and Ce. These geochemical features are used to explain the origin of the plagiogranites and it is suggested that they are the product of three-component mixing between melts from mafic, crustal and mantle sources. There is geochemical evidence to show that the plagiogranites interacted with their mantle host, although the geochemical signature of this interaction appears to be different from that produced during two-component mixing between a slab melt and the mantle wedge. This mantle interaction makes the Oman mantle plagiogranites a helpful analogue for understanding the extent to which Archaean tonalite–trondhjemite–granodiorite (TTGs) interacted with the mantle during their genesis. However, it is shown that the high Mg# in Archaean TTGs may be masked by later hornblende fractionation, obscuring the originally high Mg# of parental TTG magmas and reopening the possibility that Archaean TTGs might be slab melts.

Abstract Chromitite genesis in ophiolites is indirectly related to spreading rate, controlling mantle fertility and depth of melt crystallization. The most favourable conditions for chromitite concentrations are found in ophiolites generated at moderate spreading rates, where the world's largest chromitite deposits reside. A huge extension of the Oman Ophiolite permits an overview of chromitite distribution in relation to ridge tectonics, yielding constraints on ophiolitic chromitite genesis. Chromite genesis requires large instantaneous melt delivery, coupled with increasing oxygen fugacity produced by hydrous fluids. Chromitites in the Oman Ophiolite reside either in the Moho Transition Zone or at depth in the mantle, along wide shear zones. Both locations are domains of large melt transfer. Thinner crust on top of mantle diapirs and large shear zones limiting propagating segments represent particular domains where seawater circulation at depth is favoured. The thermal structure of these domains may explain the preferential formation of Al-rich versus Cr-rich chromites. A frozen equilibrium with parent mid-ocean ridge basalt (MORB) in the Al-rich chromites could result from rapid cooling of chromitites formed close to the ridge axis. Alternatively, equilibrium with primitive melt and a variable oxidation state is explained here by fluid circulation along the shear zones, active during transition from spreading to detachment.

Abstract A thick succession of Al Khlata glacio-lacustrine deposits, including diamictites, crops out in the Wadi Daiqa inlier and there are other possible Al Khlata outcrops in Wadi Amdeh and Wadi al Arabiyin. These outcrops in the Oman Mountains are 100 km north of where the Al Khlata had been thought to pinch out by non-deposition or erosion. Fossil spores and pollen from Wadi Daiqa are highly carbonized having been subject to greenschist facies metamorphism, but are still clearly identifiable as taxa from the Late Carboniferous 2159A zone of the oil-producing areas of interior Oman. This northernmost Al Khlata is sand rich and interpreted to be glacio-lacustrine. Previously the sand-dominated Al Khlata successions north of the Central Oman High have been considered to be glacio-fluvial outwash largely based on their context. The Al Khlata deposits in Wadi Daiqa and the underlying several-kilometre-thick Amdeh succession are preserved in Saih Hatat, probably in a continuation of the Ghaba Salt Basin that itself overlies an accreted terrane from the Pan African orogeny.

Abstract The Barremian(?)–Aptian Qishn Formation of Dhofar (Oman) is represented by eastward (landward) thinning strata that onlap Marbat palaeohigh basin margin. The formation includes Shabon, Hinna and Hasheer members, in ascending order. The Shabon Member consists of massive and cross-bedded, locally conglomeratic arkose and quartzarenite. The Hinna Member has a lower part of rhythmically arranged bioclastic wackestones/mudstones that grade to fine crystalline dolomudstone and an upper part of cyclic units of nodular, marly, rudistic floatstone/rudstone lithofacies capped by normally grading, intraclastic, bioclastic packstone/grainstone lithofacies. Mudcracks, teepee structures and microbial laminations occur in the lower part. The member terminates with 10-m-thick discoidal orbitolinid- and rudist-bearing marls envisaged as the deepest facies of the formation. The Hasheer Member consists of partly dolomitized strata of bioclastic packstones, grainstones and rudstones. Cross-bedding and erosional surfaces are present. The overall depositional system of the formation evolved from high-energy clastic-dominated, marginal marine environment through tidal carbonate mudflat and lagoonal setting to high-energy carbonate sand shoals. The overall stratigraphic arrangement of the formation suggests a third-order transgressive-regressive system with superimposed fourth- and fifth-order cycles. Shabon and Hinna members represent lowstand to transgressive systems tracts whereas Hasheer Member represents highstand to falling stage systems tracts. Terminal platform-wide exposure resulted in an unconformity at the Qishn–Kharfot contact.

Abstract Conglomerate sequences over 700 m thick were deposited subsequent to ophiolite emplacement during Late Cretaceous time in north Oman. The conglomerates were deposited by streams draining the allochthonous ophiolite and Hawasina complex after their obduction onto autochthonous Mesozoic and older Oman shelf sequences and subsequent uplift. The conglomerates belong to the Qahlah Formation of Late Cretaceous age, which is sandwiched between the Semail Ophiolite/Hawasina complex and Maastrichtian–Palaeogene carbonate rocks. The siliciclastics of the Qahlah Formation are the first sediments deposited over the obducted oceanic crust sequence of ophiolite and Hawasina lithologies. In five locations studied in north Oman, the thickness of the formation varies from 140 m to over 700 m and comprises interbedded conglomerate, sandstone and siltstone. The sediments were deposited in isolated segmented depressions each characterized by its source terrain depending on lithologies exposed in the source area. Lithofacies associations, clast sorting and grain roundness suggest deposition in stream-dominated alluvial fans. Clasts in the conglomerates range from subangular to subrounded pebbles to boulders with both grain and matrix (sandstone) support. Cross-bedded fining-upwards sequences in channelized conglomerate and sandstone suggest deposition by high-energy flows in the proximal to distal reaches of alluvial fans. High proportions of chert and ophiolite fragments in the conglomerates suggest rapid erosion of obducted oceanic crust. The presence of Loftusia -bearing carbonate beds and bivalve-bearing conglomerate beds in different sections indicates occasional interruption of alluvial deposition by marine transgressions.

Abstract Carbon capture and storage by mineralization is a potential method for storing anthropogenic CO 2 emissions, and is based on the reaction between Mg silicate and CO 2 to form Mg carbonate. The conglomerates of the Barzaman Formation exposed in the eastern United Arab Emirates represent an excellent natural analogue of this process. These conglomerates were deposited as a series of alluvial fans along the western margin of the Hajar Mountains, part of the Oman-UAE Ophiolite, and are composed largely of ultramafic and lesser-mafic clasts. The clasts and matrix have been extensively altered to dolomite during diagenetic processes. Analysis and interpretation of rock textures provide evidence for the various factors that influenced the diagenetic processes and shed light on the silicate–carbonate transformation process. All the reactions have taken place in the near-surface environment; the silicate–carbonate conversion reaction is exothermic and occurs spontaneously at near-ambient pressure and temperature, probably no greater than 50 °C. Estimates of the amount of CO 2 stored in this way can be obtained from considerations of outcrop area, formation thickness and percentage of dolomite replacement, and show that c. 150 billion tonnes (equivalent to about 4 years of worldwide CO 2 emissions at current rates) are stored.