NE Oman Research Articles

The complex continental margin of NE Oman: Fault-gouge dating of the Wadi Kabir Fault at the northern margin of the Saih Hatat window, Sultanate of Oman, and its wider implications

The complex structural geometry along the northern continental margin of Oman formed during polyphase deformation from the mid-Cretaceous to the Miocene. We focus on the kinematics and tectonics of the Wadi Kabir Fault, which represents an eastern segment of the large-scale, composite Frontal Range fault system that forms the northern boundary of the central Al Hajar Mountains. The most exhumed rocks outcrop in the Saih Hatat window of the Al Hajar Mountains, which occurs in the footwall of the Wadi Kabir Fault. To better understand the tectonic history of the Wadi Kabir Fault, we report three K-Ar illite fault-gouge ages. Sample FG16-1 from a shallowly north-dipping gouge zone of the eastern segment of the Wadi Kabir Fault yielded two consistent ages that overlap within 2σ uncertainties at ∼90 Ma. Sample FG16-2 from the central segment of the fault yielded a K–Ar age of 57.9 ± 2.0 Ma (2σ uncertainties), which is interpreted as a late Paleocene faulting event as it perfectly matches the cooling history of the high-pressure rocks of the Saih Hatat window in the footwall of the Wadi Kabir Fault. We present two tectonic interpretations for the ∼90 Ma fault-gouge age and prefer a synchronous kinematic relationship with the parallel and contractional mid-Cretaceous Yenkit Shear Zone that was active at the distal Arabian Platform since 114 Ma. Because the 57.9 ± 2.0 Ma age is associated with pronounced footwall cooling, we interpret this age to be related to the formation of the Bandar Jissah Basin in the hanging wall of the Wadi Kabir Fault. We conclude that the Frontal Range fault system likely formed during the Paleocene and that its young (≤∼43 Ma) history was accommodated along the western segment of the fault, as there is no evidence of this Eocene fault interval at the Wadi Kabir Fault.

Radix rufescens (J. E. Gray, 1822) (Gastropoda: Lymnaeidae), a new species for Oman and Arabian Peninsula

The fauna of freshwater gastropods of the Arabian Peninsula, a desert land, is neither rich nor extensively studied. Only five species of the family Lymnaeidae have been recorded from this subcontinent so far. In February 2023, a few specimens of Radix rufescens (J. E. Gray, 1822) have been collected from the stream at Wadi Tiwi, NE Oman. The shell, female part of the reproductive organs, prostate, paraeputium and phalloteca are presented. Cytochrome oxidase subunit I (COI) sequences confirmed the assignment of these specimens to R. rufescens, known from Nepal, Bangladesh, and Myanmar, not from the Arabian Peninsula. Either R. rufescens is the sixth lymnaeid species inhabiting Arabia, or at least some of the records of R. natalensis (Krauss, 1848), whose shells are hardly discernible from the ones of R. rufescens, should be interpreted as misidentified R. rufescens. Thousands of years of the trade relations between Oman and India suggest accidental transportation of this snail by trade vessels.

Tectonics and kinematics of an Alpine shear zone near As Sifah (NE Oman)

Tectonics and kinematics of an Alpine shear zone near As Sifah (NE Oman)

Triple folded surface morphology of Neoproterozoic rocks (Jabal Akhdar Dome, Oman Mountains) – Insights into buttressing effects and regional tectonics

• Two Pre-Permian deformation events affected Neoproterozoic rocks of NE Oman. • An inherited horst controls the deformation style of Neoproterozoic rocks. • The Jabal Akhdar is a prime example for studying buttressing effect. • Structural geology controls the morphology of a regional-scale carbonate surface. • The Shuram-Khufai boundary is an unconformity . Northern Oman was obducted by allochthonous rocks, including the Semail Ophiolite, during the Cretaceous, which generally hampers field investigations on autochthonous rocks. However, central parts of the Jabal Akhdar Dome provide insights into sub-allochthonous deformation patterns from northern Oman because autochthonous Arabian rocks are exposed. Field survey and satellite image analysis reveal triple-folded autochthonous Neoproterozoic rocks. In the Neoproterozoic succession, carbonates are more resistant to erosion than the under- and overlaying siliciclastics and shape present-day morphology in the superbly exposed surface of the Hat Plateau, where anticlines and synclines coincide with ridges and troughs, respectively. Previously unrecognized F1 folds are NNE-verging, overturned and tight with amplitudes of 10–100 s of meters, have ESE-oriented fold axes, and display a gently to moderately SSW-dipping penetrative axial-plane cleavage. F1 structures are re-folded by open-tight upright kilometric F2 folds, with NE/NNE-oriented fold axes and a penetrative NE/NNE-striking sub-vertical axial-plane cleavage. A younger ∼WNW-trending broad anticline (F3) exhibits a widely spaced sub-vertical ESE-striking axial-plane cleavage. The F2 deformation style is heterogeneous. In the West, the NE/ENE-oriented F2 folds are ∼3 km in amplitude and gently plunging. In the East, the NNE-oriented folds are <1 km in amplitude and non-plunging. The change ensues abruptly along a NNE-oriented zone at the western end of the Hat Plateau. The pre-existing NNE-oriented western flank of the Makarem-Mabrouk High/Horst in the subsurface controlled this change, exerting a buttressing effect with less deformation in the East.

Thermal maturity of the Hawasina units and origin of the Batinah Mélange (Oman Mountains): Insights from clay minerals

The Oman Mountains developed during Cretaceous to Cenozoic time by obduction of the Semail Ophiolite on top of the Arabian rifted margin. The tectonic pile of the orogenic system is composed of three major domains, which from bottom to top are: (i) the proximal domain of the Arabian rifted margin; (ii) the Hawasina Nappe, including rocks pertaining to the distal portions of the Arabian rifted margin; (iii) the Upper Cretaceous Semail Ophiolite. However, in NE Oman (Batinah Coastal Plain), exposures of Hawasina rocks are resting above the ophiolite.What is the origin of the uncommon structural position of Hawasina rocks in the Batinah Coastal Plain? To address this question, an extensive dataset of X-ray diffraction analyses from rocks of the Hawasina units has been used to reconstruct their thermal and burial history since the Late Cretaceous. Temperature-dependent clay minerals indicate that the Hawasina units experienced different levels of thermal maturity from early diagenetic to anchizone conditions depending on their structural position during orogenic build-up. Rocks from the Hawasina units resting above the ophiolite contain random-ordered mixed layer illite-smectite (I–S) with an illite content between 30 and 45% displaying low levels of thermal maturity reflecting early diagenetic conditions that were acquired because of limited sedimentary burial (600 m) and minor heating during the early Pliocene. Deep-water rocks of the Hawasina units below the ophiolite are characterized by long-range ordered mixed layer I–S with an illite content between 80 and 90% and by a mineralogical assemblage of rectorite and pyrophyllite displaying more evolved levels of thermal maturity in deep diagenetic/anchizone conditions that were acquired during the obduction of the 4100 to 5500 m thick Semail Ophiolite. We finally propose a new evolutionary scheme for the genesis of the Batinah Mélange, that was not buried by the ophiolite but was transported by gravity-driven mass transport on top of the ophiolite. Our results may further improve the assessment of thermal maturity of Arabian passive margin deposits for hydrocarbon exploration purposes.

Geological and seismic evidence for the tectonic evolution of the NE Oman continental margin and Gulf of Oman

AbstractLate Cretaceous obduction of the Semail ophiolite and underlying thrust sheets of Neo-Tethyan oceanic sediments onto the submerged continental margin of Oman involved thin-skinned SW-vergent thrusting above a thick Guadalupian–Cenomanian shelf-carbonate sequence. A flexural foreland basin (Muti and Aruma Basin) developed due to the thrust loading. Newly available seismic reflection data, tied to wells in the Gulf of Oman, suggest indirectly that the trailing edge of the Semail Ophiolite is not rooted in the Gulf of Oman crust but is truncated by an ENE-dipping extensional fault parallel to the coastline. This fault is inferred to separate the Semail ophiolite to the SW from in situ oceanic Gulf of Oman crust to the NE. It forms the basin margin to a “hinterland” basin formed atop the Gulf of Oman crust, in which 5 km of Late Cretaceous deep-water mudstones accumulated together with 4 km of Miocene and younger deep-water mudstones and sandstones. Syndepositional folding included Paleocene–Eocene folds on N-S axes, and Paleocene to Oligocene growth faults with roll-over anticlines, along the basin flank. Pliocene compression formed, or tightened, box folds whose axes parallel the modern coast with local south-vergent thrusts and reversal of the growth faults. This Pliocene compression resulted in large-scale buckling of the Cenozoic section, truncated above by an intra-Pliocene unconformity. A spectacular 60-km-long, Eocene(?) to Recent, low-angle, extensional, gravitational fault, down-throws the upper basin fill to the north. The inferred basement of the hinterland basin is in situ Late Cretaceous oceanic lithosphere that is subducting northwards beneath the Makran accretionary prism.

Supradetachment to rift basin transition recorded in continental to marine deposition; Paleogene Bandar Jissah Basin, NE Oman

AbstractA transition from supradetachment to rift basin signature is recorded in the ~1,500 m thick succession of continental to shallow marine conglomerates, mixed carbonate‐siliciclastic shallow marine sediments and carbonate ramp deposits preserved in the Bandar Jissah Basin, located southeast of Muscat in the Sultanate of Oman. During deposition, isostatically‐driven uplift rotated the underlying Banurama Detachment and basin fill ~45° before both were cut by the steep Wadi Kabir Fault as the basin progressed to a rift‐style bathymetry that controlled sedimentary facies belts and growth packages. The upper Paleocene to lower Eocene Jafnayn Formation was deposited in a supradetachment basin controlled by the Banurama Detachment. Alluvial fan conglomerates sourced from the Semail Ophiolite and the Saih Hatat window overlie the ophiolitic substrate and display sedimentary transport directions parallel to tectonic transport in the Banurama Detachment. The continental strata grade into braidplain, mouth bar, shoreface and carbonate ramp deposits. Subsequent detachment‐related folding of the basin during deposition of the Eocene Rusayl and lower Seeb formations marks the early transition towards a rift‐style basin setting. The folding, which caused drainage diversion and is affiliated with sedimentary growth packages, coincided with uplift‐isostasy as the Banurama Detachment was abandoned and the steeper Marina, Yiti Beach and Wadi Kabir faults were activated. The upper Seeb Formation records the late transition to rift‐style basin phase, with fault‐controlled sedimentary growth packages and facies distributions. A predominance of carbonates over siliciclastic sediments resulted from increasing near‐fault accommodation, complemented by reduced sedimentary input from upland catchments. Hence, facies distributions in the Bandar Jissah Basin reflect the progression from detachment to rift‐style tectonics, adding to the understanding of post‐orogenic extensional basin systems.

Late-stage tectonic evolution of the Al-Hajar Mountains, Oman: new constraints from Palaeogene sedimentary units and low-temperature thermochronometry

AbstractMountain building in the Al-Hajar Mountains (NE Oman) occurred during two major shortening stages, related to the convergence between Africa–Arabia and Eurasia, separated by nearly 30 Ma of tectonic quiescence. Most of the shortening was accommodated during the Late Cretaceous, when northward subduction of the Neo-Tethys Ocean was followed by the ophiolites obduction on top of the former Mesozoic margin. This shortening event lasted until the latest Santonian – early Campanian. Maastrichtian to Eocene carbonates unconformably overlie the eroded nappes and seal the Cretaceous foredeep. These neo-autochthonous post-nappe sedimentary rocks were deformed, along with the underlying Cretaceous tectonic pile, during the second shortening event, itself including two main exhumation stages. In this study we combine remotely sensed structural data, seismic interpretation, field-based structural investigations and apatite (U–Th)/He (AHe) cooling ages to obtain new insights into the Cenozoic deformation stage. Seismic interpretation indicates the occurrence of a late Eocene flexural basin, later deformed by an Oligocene thrusting event, during which the post-nappe succession and the underlying Cretaceous nappes of the internal foredeep were uplifted. This stage was followed by folding of the post-nappe succession during the Miocene. AHe data from detrital siliciclastic deposits in the frontal area of the mountain chain provide cooling ages spanning from 17.3 to 42 Ma, consistent with available data for the structural culminations of Oman. Our work points out how renewal of flexural subsidence in the foredeep and uplift of the mountain belt were coeval processes, followed by layer-parallel shortening preceding final fold amplification.

Gondwana accretion tectonics and implications for the geodynamic evolution of eastern Arabia: First structural evidence of the existence of the Cadomian Orogen in Oman (Jabal Akhdar Dome, Central Oman Mountains)

This work describes two early Cambrian folding events within Cryogenian to earliest Cambrian rocks of the western Jabal Akhdar Dome (NE Oman). This sequence is truncated at an angular unconformity and overlain by a Permo-Mesozoic succession that is folded into a wide, gentle anticline, that differs in style and intensity of deformation from that below the unconformity. An older Paleozoic deformation (D1) have been identified within limestone of the Hajir Formation, in which F1 metric folds reflect ductile deformation affecting Neoproterozoic-Cambrian rocks with low dip-angle axial planes. A younger event (D2) has refolded the F1 folds with open to close F2 folds with sub-vertical to steep axial planes dipping towards NNW and fold axes plunging either ENE-wards with ~50°, or SW-wards with ~30°, at the northern and southern flanks of the Jabal Akhdar Dome, respectively. The F2 folds have been discussed by previous authors as possibly Hercynian in age, while the F1 folds are here firstly presented, revealing a uniform NE-vergence of F1 folds after restoration at pre-D2 geodynamic conditions.First structural evidence of a tectonic event in eastern Arabia dates between ~542 and 525 ± 5 Ma (D1), due to the convergence between Arabia and microcontinents and/or oceanic subduction of the Proto-Tethys Ocean. D1 and D2 are related to NE-SW and ~NW-SE main compressional directions, respectively. Based on geometric and time considerations D1 correlates with the Cadomian and D2 with the Angudan orogenies. Our evidences directly question former discussions of the presence of the “Hercynian Orogen” in eastern Arabia.

Taxonomic notes on Convolvulus sect. Serospinescentes (Convolvulaceae)

In the present study the taxonomic status of the species belonging to Convolvulus L. sect. Serospinescentes Boiss. (Convolvulaceae) is presented using morphological characters of specimens from the authors’ expeditions and other herbarium collections. Most species of the section are endemic to Iran. C. sect. Serospinescentes is geographically restricted to the Oman Sea margin, S and SE Iran, NE Oman, Mediterranean Sea margin, NE Egypt, Palestine and Turkey. Two new taxa, C. bazmanensis Ranjbar, Ezazi &amp; F.Ghahrem. and C. oxysepalus Boiss. subsp. neyrizensis Ranjbar &amp; Ezazi, are described and illustrated. The former species is collected from Sistan and Baluchestan Province and the latter from Fars Province. C. virgatus subsp. subaphyllus Boiss. is reported as a new record for the flora of Iran. The subsection Serospinescentes is raised to the rank of Convolvulus sect. Serospinescentes. In addition, a taxonomic key, detailed morphometric descriptions, illustrations and distribution maps for all species of the section are provided. The conservation status of taxa is evaluated according to the IUCN Red List Categories and Criteria.

3D geometry and kinematic evolution of the Wadi Mayh sheath fold, Oman, using detailed mapping from high-resolution photography

The Wadi Mayh sheath fold in north-eastern Oman is one of the largest and best-exposed sheath folds known, and presents a unique opportunity to better understand this somewhat enigmatic style of deformation. We undertook high-resolution photographic surveying along Wadi Mayh to document the sheath fold in 61 georeferenced panoramic photomerges. Here we present ten such images that provide a structural interpretation of the sheath fold and surrounding structure. We resolve this structure in a simplified three-dimensional model and in two orthogonal cross sections, and propose a kinematic evolution to explain the geometry. The Wadi Mayh sheath fold is the most prominent example within what we suggest is a composite sequence of sheath folds, which is itself enclosed within a SSW-closing recumbent syncline at the base of the major Saih Hatat nappe. Sheath folding is accommodated within Permian Saiq Formation limestones showing carpholite assemblages (6–8 kbar; 275–375 °C). A major discontinuity separates this sequence from enveloping older rock units. The sequence formed during progressive top-to-north, ductile shearing as the overlying nappe migrated northwards with respect to the underthrusting Hulw unit. This process occurred during SSW-directed exhumation of partially subducted continental crust in NE Oman, approximately 15 Ma after obduction of the Oman ophiolite initiated.

Origin and tectonic evolution of the NE basement of Oman: a window into the Neoproterozoic accretionary growth of India?

AbstractThe Omani basement is located spatially distant from the dominantly juvenile Arabian–Nubian Shield (ANS) to its west, and its relationship to the amalgamation of those arc terranes has yet to be properly constrained. The Jebel Ja'alan (NE Oman) basement inlier provides an excellent opportunity to better understand the Neoproterozoic tectonic geography of Oman and its relationship to the ANS. To understand the origin of this basement inlier, we present new radiogenic isotopic data from igneous bodies in Jebel Ja'alan. U–Pb and 40Ar/39Ar geochronological data are used to constrain the timing of magmatism and metamorphism in the jebel. Positive εHf and εNd values indicate a juvenile origin for the igneous lithologies. Phase equilibria modelling is used to constrain the metamorphic conditions recorded by basement. Pressure–temperature (P–T) pseudosections show that basement schists followed a clockwise P–T path, reaching peak metamorphic conditions of c. 650–700°C at 4–7.5 kbar, corresponding to a thermal gradient of c. 90–160°C/kbar. From the calculated thermal gradient, in conjunction with collected trace-element data, we interpret that the Jebel Ja'alan basement formed in an arc environment. Geochronological data indicate that this juvenile arc formed during Tonian time and is older than basement further west in Oman. We argue that the difference in timing is related to westwards arc accretion and migration, which implies that the Omani basement represents its own tectonic domain separate to the ANS and may be the leading edge of the Neoproterozoic accretionary margin of India.

Mapping of high pressure metamorphics in the As Sifah region, NE Oman using ASTER data

The high pressure metamorphic zone of As Sifah area in the Saih Hatat window, NE Oman plays a vital role to study global tectonic setting, metamorphism, transport direction and age of initial ophiolite emplacement. Scientists and researchers are keen in determining the protolith, structural evolution, and timing and metamorphic conditions of the Saih Hatat area. In this study, mapping of the metamorphic zone and discrimination of associated rock formations occurred in the As Sifah region is carried out using visible and near infrared–shortwave infrared (VNIR–SWIR) spectral bands of Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and well known image processing methods such as image decorrelation, Principal Component Analysis (PCA) and Spectral Angel Mapper (SAM). The study delineated the region of metamorphic zone and discriminated the different metamorphic and carbonate rocks of the study area characterized by hydroxyl (OH) and carbonate bearing minerals. The assessment of accuracy for the occurrence and spatial distribution of major lithological units provided the overall accuracy of 96.06% with Kappa Coefficient=0.95 in the matrix of Maximum Likelihood (ML) and compared with the Spectral Angle Mapper (SAM) and Spectral Information Divergence (SID) algorithms. The occurrence of such minerals are detected and confirmed by SAM supervised classification method. The study also describes the spectral character of metamorphic and carbonates minerals. The metamorphic zone and associated rock types interpreted over the images are verified in the field and checked for their occurrences and spatial distributions. The occurrence of CO3 bearing carbonate minerals and OH bearing metamorphic minerals are confirmed further under microscope, XRD analysis and PIMA spectral measurements in the laboratory studies. The study proved sensor capability of ASTER to discriminate metamorphic rocks from associated formations and detection of different mineral occurrences. It is recommended to use the studied technique to map metamorphic zone occurred in the rugged topography, where it is difficult to do conventional geological mapping in similar arid regions.

Impact of sewage and mining activities on distribution of heavy metals in the water-soil-vegetation system

Several samples of groundwater and soils and plants have been collected from Sohar (Batina region, NE Oman), which is affected by various activities such as mining, agriculture, and sewage. To characterize quality of groundwater, As and Cu concentrations have been investigated in waters collected from different wells. Comparison of data with local and international standard values revealed that groundwater in Sohar region is characterized by lower concentrations in Cu and As compared with standards. In soils collected from the same area, concentrations of heavy metals have been measured in different fractions in order to investigate the mobility of such elements and risk of vulnerability in this area. A sequential extraction procedure has been applied to surface sediments to determine the partitioning of Zn, Cu, Co, Mn, Fe, Pb, Cr, Cd and Ni among (1) exchangeable and acid-soluble phases, (2) Fe–Mn oxides, (3) organic matter and sulphides and (4) resistant phases. The results showed that the mobile fraction in the sewage area accounts only for 10 % of the total concentration in sediments while in the Cu mining area, the contribution of the mobile fraction may exceed 10 %, especially for Pb, Mn, Cd, Cu and Co. Investigation of concentrations in As and Cu in plants collected from mining and sewage areas revealed an important accumulation of these elements in leaves and may explain enrichment of As in shallow groundwater relative to deep groundwater. This investigation also showed that Cu is more available in sewage area than in mining zone, as opposed to As.

Evolution of a very deeply subducted metasediment from As Sifah, northeastern coast of Oman

Near the coastal village of As Sifah, NE Oman, eclogite-facies rocks occur in the Saih Hatat window. We investigated a metapelite from this area, which is composed of mm-sized garnet and greenish phengite and minor epidote, blue amphibole, paragonite, albite, quartz, rutile, opaque phases, barite, and carbonate. Garnet exhibits a chemical zonation with Gro17Alm66Pyr6Spe11, Gro22Alm72.5Pyr5Spe0.5, and Gro25Alm65Pyr8Spe2 as inner core, mantle and outermost rim compositions. Inner portions of phengite have Si contents of up to 3.6 per formula unit (pfu), whereas rims are poorer in Si (3.2–3.4 pfu). We constructed a P-T pseudosection in the system Na2O-K2O-CaO-FeO-O2-MnO-MgO-Al2O3-SiO2-TiO2-H2O for the bulk-rock composition of the studied metapelite and contoured it by isopleths of various parameters such as the molar fractions of garnet components. Based on this contouring a P-T path was derived that starts at ultrahigh-pressure conditions. Garnet began to form at 25kbar and 490°C. Subsequently, temperatures increased and pressures decreased to finally reach P-T conditions of 13kbar and 565°C at which low-Si phengite, the outermost rim of garnet, Na-amphibole, epidote, quartz, magnetite, and rutile were in equilibrium. The P-T path is related to events in a subduction channel where the top of subducted oceanic crust, including the studied metasediments, was involved in an upwards-directed mass flow, resulting in the release of about 3wt.% H2O by garnet formation from hydrous minerals such as chlorite and lawsonite. In order to get hints at the interaction of such hydrous fluids, we have analyzed the trace and minor elements in phengite. The contents of B, Rb, Cs, and Tl (20, 397, 6.7 and 1.7ppm, respectively) are nearly constant over the entire Si range of potassic white mica. In contrast, the contents of Ba and Sr increase from 900 and 0.5 to 10500 and 14ppm, respectively, with decreasing Si content in phengite. We hypothesize that this result reflects early leaching of mobile elements during subduction and the later approach of the rock to a barite deposit within a melange ascending in the subduction channel.

Interpreting high-pressure phengite 40Ar/39Ar laserprobe ages: an example from Saih Hatat, NE Oman

New single grain fusion and core-rim 40Ar/39Ar laserprobe phengite data from the Saih Hatat high-pressure terrane in NE Oman show that individual samples yield a range of apparent ages which is similar to that previously reported from across the entire terrane. The majority of the determined ages are older than the previously reported U-Pb zircon peak metamorphic age. Core to rim age variations within individual grains range from no discernible difference across the grain to grains with older cores, or, rarely, older rims; some samples manifest all three patterns. Numerical diffusion modelling shows that due to the peak temperature of ca. 550°C, the measured apparent ages cannot be explained by simple cooling or by partial retention of crystallisation or detrital ages in an open system. The age variability is better explained by spatially and temporally variable open or closed system behaviour at the mm-cm scale coupled with pervasive and heterogeneously distributed excess argon. Anomalously old eclogite phengite 40Ar/39Ar ages are due either to internally derived 40Ar inherited from a K-bearing precursor, or externally derived 40Ar distributed by grain boundary fluids. Mica-rich schists within the eclogite boudins yield younger phengite ages, suggesting excess argon was absent or diluted. Pelites hosting the eclogite appear to have been affected by later fluid ingress during deformation and greenschist-facies overprint and yield very variable ages commonly with apparently older rims on younger cores. The grain- and sample-scale age variations measured in Saih Hatat indicate that the grain boundary network in eclogite pods was not an efficient transfer pathway for argon transport, whereas the grain boundary network in the surrounding pelites acted as a more efficient pathway on the timescale of the metamorphic cycle.

Ophiolite emplacement in NE Oman: constraints from magnetotelluric sounding

SUMMARY Magnetotelluric (MT) data have been acquired across the Samail Ophiolite, Oman mountains, along a 115 km transect in January 2005. Twenty-five MT stations were deployed approximately every 4 km along a profile perpendicular to the coastline near Muscat. The profile extends from the Huw'l/Meeh subwindow in the northwestern part of the profile and crosses the Saih Hatat window, a Tertiary domal culmination that folds a major NE-facing recumbent fold nappe, and ends along the southern boundary between the Dasir and Ibra ophiolite blocks. The survey aimed to investigate the tectonic evolution of the emplacement of the Samail Ophiolite by delineating major faults and geological boundaries on a crustal scale. The data shows complex behaviour with the influence of static shift effects and anomalously high phases exceeding 90°. Phase tensor analysis and 3-D forward modelling hint that the upper plate–lower plate (UP–LP) shear zone has a 3-D effect on the data in the northern part of the profile. Rotated MT impedances were inverted using a 2-D code. Major resistivity interfaces coincide with the location of the UP–LP shear zone near the surface, and dip towards the Arabian margin suggesting a southwest orientated underthrusting prior to the ophiolite emplacement. The para-autochthonous and less deformed upper plate is a resistor, whereas the strongly deformed lower plate is more conductive.

The Eocene Rusayl Formation, Oman, carbonaceous rocks in calcareous shelf sediments: Environment of deposition, alteration and hydrocarbon potential

Paralic carbonaceous series intercalated among calcareous shelf sediments have seldom been investigated. During the early Eocene, calcareous and siliciclastic sediments were deposited on a wide shelf in front of low-reliefed hinterland in the Al Khawd region in NE Oman. The siliciclastic-calcareous sediments originated from strongly reworked debris of the Arabic Shield. The underlying Semail Ophiolite did not act as a direct source of debris but provided some heat to increase the maturity of carbonaceous rocks and modify the isotope signal of the calcareous minerals in the Rusayl Formation. A multidisciplinary approach involving sedimentology, mineralogy, chemistry, coal petrography and paleontology resulted in the establishment of nine stratigraphic lithofacies units and provides the reader with a full picture from deposition of the mixed carbonaceous-calcareous-siliciclastic rocks to the most recent stages of post-depositional alteration of the Paleogene formations. The calcareous Jafnayn Formation (lithofacies unit 1) developed in a subtidal to intertidal regime, influenced episodically by storms. Deepening of the calcareous shelf towards younger series was ground to a halt by paleosols developing on a disconformity (lithofacies unit 11) and heralding the onset of the Rusayl Formation. The stratigraphic lithofacies units III and IV reflect mangrove swamps which from time to time were flooded through washover fans from the open sea. The presence of Spinozonocolpites and the taxon Avicennia, which today belong to a coastal marsh vegetational community, furnish palynological evidence to the idea of extensive mangrove swamps in the Rusayl Formation [El Beialy, S.Y., 1998. Stratigraphic and palaeonenvironmental significance of Eocene palynomorphs from the Rusayl Shale Formation, Al Khawd, northern Oman. Review of Palaeobotany and Palynology 102, 249-258]. During the upper Rusayl Formation (lithofacies units V through VII) algal mats episodically flooded by marine incursions make up a greater deal of the sedimentary record than mangrove swamps. Terra rossa paleosols mark the end of accumulation of organic material (OM) and herald supratidal conditions at the passage of Rusayl Formation into the overlying Seeb Formation. In the subtidal-supratidal cycles of lithofacies unit VIII the terra rossa horizons are thining upwards and become gradually substituted for by deep-water middle ramp sediments of lithofacies unit IX Framboidal pyrite, (ferroan) dolomite with very little siderite are indicative of an early diagenetic alteration stage I under rather moderate temperatures of formation.

Age and Stratigraphic Relationships of Structurally Deepest Level Rocks, Oman Mountains: U/Pb SHRIMP Evidence for Late Carboniferous Neotethys Rifting

Igneous zircons in felsic schist, or metatuff, infolded with mafic schist, calcschist, and quartz mica schist of the As Sifah lower plate window, NE Oman, have yielded a U/Pb SHRIMP crystallization age of $$298\pm 3$$ Ma. The metatuff was previously considered to represent Permian bimodal volcanism, but these data indicate that the magmatism is older and suggest that Neotethys intracontinental rifting on the Oman part of the Gondwana northern margin was initiated in the Late Carboniferous. This, combined with δ13C values of calcschists indicating a predominantly pre–Late Permian age for the basal lower plate metacarbonates, suggests that passive margin carbonate ramp development occurred in the Late Carboniferous/Early Permian during Neotethys oceanic spreading and not just in the Late Permian, as previously thought. The restricted occurrence of the metatuff to the Saih Hatat lower plate, a unique lower plate stratigraphy, and the unique δ13C values of the calcschists all suggest that the lower plate was certainly distal, and possibly exotic, to the Arabian Peninsula. The felsic schist sample was entrained within an eclogite megaboudin at As Sifah. The age relationships therefore suggest that the high‐P rocks that reached pressures of up to 20 kbar and are now part of the lower plate of Gregory and coworkers were either part of a subducted Florida‐style promontory as part of the attenuated leading edge of the Arabian platform or part of a subducted microplate that was originally caught in the developing southern Neotethys ocean some 1300 km from the margin.

Transition from subduction- to exhumation-related fabrics in glaucophane-bearing eclogites, Oman: evidence from relative fabric chronology and 40Ar/ 39Ar ages

Controversy over the age of peak metamorphism and therefore the tectonic evolution of the Arabian margin relates to the polydeformed and polymetamorphosed nature of glaucophane-bearing eclogites from the Saih Hatat window beneath the allochthonous Samail ophiolite in NE Oman. These eclogites contain relicts of earlier fabrics, structures and metamorphic assemblages and provide a record of change from subduction to exhumation. The eclogites are part of a mafic layer that was disrupted into boudins up to 0.5 km in length within a lower plate shear zone (As Sifah shear zone). The megaboudins not only preserve the relicts of the highest grade of metamorphism but also an early ENE-trending lineation and sheathlike isoclines enveloped by the flat-lying schistosity. The boudin-bearing layer is isoclinally folded with calc-schist, mafic schist and quartz–mica schist, where the regional folds have axes parallel to the NE-trending stretching lineation (a-type folds). Textural evidence suggests multiple growth events for garnet and clinopyroxene, requiring polymetamorphism of the mafic layers that formed the eclogite megaboudins. The surrounding calc-schist and quartz–mica schist are both intensely deformed with transposition foliation containing an NE-trending lineation in phengite and asymmetric shear indicators such as C′-type shear bands and asymmetric pressure shadows around garnets, that give top-to-the-NE sense of shear. Although consistent ENE-trending lineations in all the boudins suggest that they have largely acted as passive, nonrotating rigid bodies, the presence of NE-vergent asymmetric mesofolds, extensive dynamic recrystallisation, multiple generations of phengites and a range of 40Ar– 39Ar apparent ages within the megaboudins suggest, however, that they have not acted entirely passively during the later deformation. Phengites isolated from the high- P/low- T fabrics show groupings in 40Ar– 39Ar apparent ages interpreted as distinct metamorphic/cooling intervals at ∼140–135, ∼120–98 and ∼ 92–80 Ma. Microstructural relations suggest that age groupings younger than 100 Ma reflect phengite growth during exhumation with the top-to-the-NE shearing. The older ages (∼120–110 Ma) from fabrics that give top-to-the-S shear sense may reflect growth during the subduction phase. The combination of groupings of apparent argon ages older than the crystallisation age of the Samail Ophiolite, the suggestion of different geothermal gradients, and superposed metamorphism suggest that the eclogites and garnet blueschists formed as a result of underthrusting along a break that was not directly related to the metamorphic sole of the ophiolite. The glaucophane–eclogites are interpreted as having formed at different times under varying pressure–temperature conditions during underthrusting with variations in the rate of underthrusting, allowing thermal equilibration and/or rapid cooling at different crustal levels.