ABSTRACT

Gravity measurements onshore and offshore of the United Arab Emirates (UAE) have been used to construct a new Bouguer gravity anomaly map of the region. The gravity data, which has been gridded at 2,700 m × 2,700 m interval, has been used to constrain the tectonic elements, major lineation trends and structures of the Neoproterozoic basement of the Arabian Plate and the distribution of infra-Cambrian salt basins. Advanced transformation techniques (including first vertical derivative, total horizontal derivative, tilt derivative and Euler deconvolution) were applied to identify gravity source edges as an aid to structural interpretation and geological modelling of the study area.

Three major structural provinces (fold-and-thrust belt, foreland and salt tectonic provinces) were identified based on the residual Bouguer gravity anomaly field. The eastern fold-and-thrust belt province is associated with short-wavelength positive gravity anomalies, which are attributed to the allochthonous series of the Semail Ophiolite and its related thrust sheets. The central foreland basin province is characterised by NNW-oriented negative gravity anomalies associated with deepening of the basement and thickening of Aruma and Pabdeh sediments in the foredeep basins and flexure of the top and base of the crust by the load of the Semail Ophiolite. The western salt tectonic province displays well-defined local gravity lows superimposed on a regional gravity high, which probably reflects the swelling of infra-Cambrian salt above a shallowing of the basement and thinning of the foredeep sediments. In addition, gravity modelling constrained by seismic and well data indicates the presence of substantial infra-Cambrian salt bodies in all basins of the UAE both onshore and offshore including the southern area of the Rub’ Al-Khali Basin.

An extensive array of previously unmapped N-S, NW- and SW-trending lineaments affecting the basement and possibly overlying sediments are mapped in the UAE. The N-S Arabian trending lineament represents the effect of a major structure, along which many important oilfields are located (e.g. Bu Hasa). The SW trend has regular spacing and is dominant in the southern and central part of Abu Dhabi, east of the Falaha syncline. The NW-SE lineament is the most striking and includes two well-defined trends that cross Abu Dhabi Emirate, which in this paper are named as the Abu Dhabi Lineaments. These lineaments are associated with a linear gravity high extending from the southwestern border with Oman to the offshore close to Zakum oilfield. They are probably related to the Najd Fault System.

INTRODUCTION

Gravity measurements have played a major role in understanding the structure and evolution of sedimentary basins. At short wavelengths, the data reflect the density changes associated with the nature of the basin fill, including facies changes, compaction and diapiric intrusions. At long wavelengths, the data reflect undulations in basement topography and lateral changes in its physical properties, including its density and thermal and mechanical properties. Accordingly, it is possible to use gravity data to delineate both the internal stratigraphic framework and nature of the basement structure in a basin (e.g., Bott, 1962; Karner and Watts, 1983; Best et al., 1990; Naouali et al., 2011). The basement relief of a sedimentary basin is formed by tectonic processes that generally control the basin

architecture and influences source rock distribution, heat flow, trap timing and sediment supply. In addition, the rheology and thermal and mechanical behaviour of the basement controls the rate of subsidence and sedimentary geometry of each phase of an evolving basin. The basement structures may propagate into the overlying sedimentary rocks and influence fluid flow and the distribution of hydrocarbon traps. For example, many oil and gas fields occur along fault-controlled linear trends. Therefore, the structural framework of the basement has a significant impact on basin evolution.

There have been few previous studies of the gravity field of the United Arab Emirates (UAE). Manghnani and Coleman (1981) presented an E-W gravity profile across the northern Oman Mountains from Shinas to Dubai. The profile shows two distinct positive residual Bouguer anomalies (75 mGal and 50 mGal respectively) separated by low residual gravity anomalies (2 mGal to 10 mGal). The residual Bouguer anomalies were correlated with imbricate ophiolite exposures detached by lighter allochthonous sediments. Shelton (1990) also reported the results of a gravity survey of the eastern side of the northern Oman Mountains from north of Wadi Jizi up to the border between UAE and Oman. The gravity data suggested that the ophiolite exposures consist of discrete nappes. Furthermore, Khattab (1993) compiled a Bouguer gravity anomaly map over parts of the Semail Ophiolite in the eastern UAE between Kalba, Dibba and Masafi. The data suggested three partially serpentinised nappes, two along the Gulf of Oman and another close to Masafi. In another study Khattab (1995) interpreted magnetic and gravity data in and around the Strait of Hormuz. The Bouguer gravity indicated a well-defined NE-SW negative gravity trend interpreted as an anomalous Hormuz Salt accumulation.

Ravaut et al. (1997) presented a detailed Bouguer gravity anomaly map of northern Oman Mountains including the eastern UAE. The gravity anomaly is characterised by a high-amplitude negative-positive couple. The negative anomalies were correlated to the Late Cretaceous and Tertiary foreland basins and the positive anomalies were linked to the Semail Ophiolite. More recently Ali and Watts (2009) compiled a Bouguer gravity anomaly map of the UAE and northern Oman Mountains from published and satellite-derived gravity data. The map showed that the northern Oman Mountains south of Dibba Fault Zone are associated with a NS-trending Bouguer gravity anomaly flanked on its western edge by a NS-trending gravity low. The high was correlated with the ophiolite outcrop and the low was attributed to the sediments that infill the foreland basin. Several other geophysical studies have documented a series of folds and thrust faults in eastern and northern UAE (Ali et al., 2008; Ali et al., 2009; Tarapoanca et al., 2010; Ali et al., 2013). Identification of the structural framework of the basement and overlying sedimentary cover is critical to hydrocarbon exploration of the UAE sedimentary basins.

The basement of the UAE is overlain by a deep sedimentary basin, which comprises a lower rifted passive margin sequence and upper foreland basin (Aruma and Pabdeh) sequences. The rifted margin sequence formed during the Middle Permian to Late Cretaceous following break-up of the Arabian Plate and the Cimmerian Terrane and the formation of Neo-Tethyan oceanic crust (e.g., Glennie et al., 1973; Searle, 1988b; Ruban et al., 2007). The Aruma foreland basin developed in the Late Cretaceous (Late Coniacian to Campanian) during ophiolite emplacement by flexural loading of an underlying rifted continental margin (e.g., Robertson, 1987a; Robertson, 1987b; Patton and O’Connor, 1988; Boote et al., 1990; Warburton et al., 1990; Ali et al., 2008; Ali and Watts, 2009). The Pabdeh foreland basin formed as a result of the Mid-Tertiary uplifting and thrust-culminating of Musandam shelf carbonates (Searle, 1988a).

The purpose of this paper is to compile all the available free-air gravity anomaly data for the UAE and to use this data to construct a new map of the Bouguer gravity anomaly for the region. In addition, the study aims to use the Bouguer gravity data to identify major lineaments and delineate main deep-seated structural basement features that may have acted as controls on the patterns of sedimentation in the region. In particular, mapping of prominent gravity highs and lows reveal a correlation between subsurface structural highs and salt basins. The different gravity anomalies and their geological significance are discussed in the following sections.

GEOLOGICAL SETTING

Basement terranes in the UAE are neither exposed nor imaged by available seismic and well data due to the thick sedimentary cover. However, these terranes form part of the Neoproterozoic compressional event (from ca. 715 to 610 Ma) of island-arc and microcontinent terrane accretion that led to the amalgamation and formation of the Arabian margin of Gondwana (Husseini, 1989; Loosveld et al., 1996; Al-Husseini, 2000; Sharland et al., 2001). The consolidation created a pronounced N-S structural fabric of sinistral shear zones in the crust of the Arabian Plate (Figure 1).

In the Neoproterozoic to Late Devonian (ca. 610 to 364 Ma) the Arabian Plate was located in an intra-cratonic setting within the continental interior of northeastern Gondwana bordering the Palaeo-Tethys Ocean (Loosveld et al., 1996; Al-Husseini, 2000; Sharland et al., 2001). This phase led to the development of a NW-SE shear fracture system (Najd Fault System) and a number of salt basins (Loosveld et al., 1996; Al-Husseini, 2000). Two major infra-Cambrian rift salt basins, Ara and Hormuz (Figure 1), developed in the eastern part of the Arabian Plate across Oman and the UAE. The Ara Salt Basin (Ediacaran–early Cambrian) is present across much of south-central Oman within a series of NE-trending basins. It comprises cyclic carbonates and evaporites, which form large diapirs that locally pierce the present-day surface and are associated with oil generation and reservoir development (Gorin et al., 1982; Mattes et al., 1990; Peters et al., 2003; Schroeder et al., 2003; Al-Siyabi, 2005; Reuning et al., 2009). The Hormuz Salt Basin represents time-equivalent rocks in the western UAE and in southern Iran (Kent, 1979; Edgell, 1991; Edgell, 1996).

In the late Carboniferous to mid-Permian (364 to 255 Ma) the Arabian Plate was located in a back-arc setting (Sharland et al., 2001; Ruban et al., 2007). The associated Hercynian events affected the area, creating regional uplift, widespread erosion and basement tectonics along the inherited, mechanically weak Neoproterozoic trends (Konert et al., 2001). An unconformity separates the Early Carboniferous (Berwath Formation) from older sediments of the overlying Permian Khuff and Unayzah deposits. In Middle to Late Permian (between 260 and 270 Ma), the Neo-Tethys opening began between the Cimmerian continental blocks (Central Iranian Plate and Lut Block) in the north and the eastern margin of the Arabian Plate in the south (e.g., Glennie et al., 1974; Searle and Graham, 1982; Béchennec et al., 1990; Blendinger et al., 1990; Searle et al., 2004; Ruban et al., 2007). By the Late Permian a vast stable shelf carbonate platform was established along the entire northeast Arabian Plate margin extending along the entire Zagros and Oman mountain region (Glennie et al., 1973). The mainly carbonate platform that developed on a rapidly subsiding Arabian Plate is represented by the Khuff and equivalent formation, Saiq Formation and Lower Mahil Member in Oman outcrops (Konert et al., 2001; Al-Husseini and Matthews, 2010).

The carbonate platform persisted for > 160 Myr, from the Triassic (Sudair, Gulailah and Minjur formations), through the Jurassic (Marrat, Hamlah, Izhara formations and Sila Group) to the Early Cretaceous (Thamama Group) and Aptian–Cenomanian (Wasia Group), as the margin subsided slowly due to a post-rift thermal subsidence (Sharland et al., 2001; Ziegler, 2001). By the end of the mid-Cretaceous, the region was a mature carbonate-dominated rifted margin in an expanding Neo-Tethys Ocean basin. The Semail Ophiolite (Figure 2) was formed within the Oman segment of Neo-Tethys during the Cenomanian (Tippit and Pessagno, 1979; Tilton et al., 1981; Warren et al., 2005). The Semail Ophiolite, Hawasina and Haybi complexes (Figure 2) were emplaced onto the Oman margin during the Late Cretaceous as a series of thrust sheets that, on a large-scale, preserve a proximal to distal stacking order up the structural pile.

The stacking of the obducted nappes caused loading, flexural subsidence and uplift and partial erosion of the underlying passive margin shelf carbonates (Wasia Group). This resulted in the development of the foreland basin (the Late Cretaceous Aruma Basin) and a peripheral flexural bulge at the western edge of the obducted allochthonous units (Robertson, 1987a; Patton and O’Connor, 1988). The Aruma foreland basin was infilled by up to 4.3 km of Turonian–Campanian sediments, which rapidly increase in thickness towards the northeast (Glennie et al., 1973; Robertson, 1987b).

Passive margin sedimentation returned during the Late Campanian–Maastrichtian (Nolan et al., 1990; Skelton et al., 1990). The margin remained stable until Oligocene time through the deposition of the successive transgressive Paleocene–Eocene Pabdeh Group (Umm Er Radhuma, Rus, Dammam and Asmari formations) and the Oligocene Asmari Formation (Nolan et al., 1990; Skelton et al., 1990).

Compressional deformation resumed during Late Oligocene–Miocene time. The deformation is evident in the frontal fold-and-thrust belt of the northwestern Oman Mountains and adjacent foreland basin, where it produced large-scale folding and the probable reactivation of deep-seated basement faults (Boote et al., 1990; Dunne et al., 1990; Ali et al., 2008). This deformation is interpreted as the beginning of the Zagros phase of continent-continent collision when the Musandam shelf collided with the central Iran continental block (Ricateau and Riche, 1980; Searle et al., 1983; Searle, 1988a, b).

A second, later foreland basin, the Pabdeh basin, was developed in front of the rapidly uplifting and thrust-culminating Musandam shelf carbonates (Searle, 1988b; Robertson et al., 1990). The Pabdeh Group (Paleocene–Oligocene) consists of deep-marine shales, marls and limestones. The basin extends into the eastern UAE, north of Al-Ain city, where it has a higher carbonate content (Alsharhan and Nairn, 1995). However, the Pabdeh basin tapers out to the south of the Musandam and is not present in the central Oman foreland basin (Searle, 1988a).

Another, much larger, flexural foreland basin is present in the Arabian Gulf, offshore of the UAE. The basin developed as a result of the closure of the Neo-Tethys ocean by the late Oligocene–Miocene continent-continent collision and the uplift of the Zagros Fold-and-Thrust Belt (Jahani et al., 2009). Furthermore, seismic profiles in the northern UAE and southeastern offshore Iran revealed an unconformity below the Fars Group (Jahani et al., 2009; Tarapoanca et al., 2010). This unconformity is related to the Oligocene–early Miocene uplift of the Oman Mountains (Jahani et al., 2009; Tarapoanca et al., 2010). Figures 1 and 2 show regional geology and structural map of the UAE and the northern Oman Mountains.

SOURCES OF THE GRAVITY DATA

The gravity data used in this study was acquired during a number of different surveys (Figure 3 and Enclosure I). The data from each survey has been acquired and processed using standard techniques. The data is not evenly distributed. There are also variations in the accuracy of the gravity base stations

used, the gravity values, coordinates and altitudes. The larger discrepancies, most apparent at the boundaries between sources, were usually the result of using different gravity base station values and/or various reference ellipsoids in raw data processing. These inconsistencies were carefully checked and adjusted in the compilation. The datasets were merged into one single digital set after visual comparison and adjustment of adjoining areas.

Although recent surveys have a better control on the gravity datum compared to earlier ADNOC (Abu Dhabi National Oil Company) surveys, we chose the ADNOC land gravity survey as the base survey due to the fact that it is the largest survey that covers most of the onshore UAE. All other surface, marine and airborne surveys were adjusted to tie to this survey. Table 1 shows the names of the fundamental absolute gravity stations used to tie-in the surveys, gravity value, datum and shift applied to all onshore and offshore surveys. Discrepancies between the various surveys were found to be not more than 13 mGal, and the boundaries between the various surveys cannot be seen on the final compiled map (Figure 4 and Enclosure I). Comparison of the various surveys after adjustment suggests that the discrepancy is, for the most part, less than 1 mGal.

The grid interval of the gravity data had to be small enough to capture the anomaly details where the data is recorded at a close spacing, but also large enough to interpolate areas of sparse spacing. In this study we used a linear minimum curvature gridding algorithm (Briggs, 1974; Swain, 1976). An optimal grid-cell size of 2,700 m × 2,700 m, 0.1% tolerance and 99% pass tolerance were chosen. No

tension was applied to the data in order to produce a true minimum curvature grid. We have tried smaller grid intervals (e.g. 2,000 m × 2,000 m), but found large gaps in the data coverage in particular shallow waters along coastal areas of western Abu Dhabi city.

Onshore Gravity Datasets

ADNOC Onshore Gravity Data

The land survey gravity data consist of more than 25,000 survey field data points and covers the entire country in a regular N-S and E-W profile mesh, except the northeastern emirates. R.H. Ray conducted the survey in the late 1940s and early 50s. The survey stations were laid out at about 1-km intervals along the survey lines, which were approximately 4 km to 10 km apart. Station intervals vary and they are not always in a straight line. In many cases, station sites were selected along existing roads and trails for ease of location surveying and to minimize terrain effects or other interference. Over most of the survey area, topographic irregularities are modest and terrain corrections are negligible except eastern portion where the magnitude of the correction may be as large as 5 mGal. The survey was tied to a gravity base station at Port Said in Abu Dhabi. There is no information on the gravity datum used at the base station. However, we speculate that the 1930 International Reference Spheroid (with no Potsdam correction) was used since the survey was completed well before the establishment of the International Gravity Standardization Net 1971 (IGSN-71). The data was provided with the free air anomalies and have been reduced to Bouguer anomaly using a density of 2,600 kg/m3.

The Petroleum Institute Gravity Data (Red data points, Figure 3 and Enclosure I)

Al Jaww Plain: The data was acquired from Al Jaww plain, which is located southeast of the city of Al-Ain, east of Abu Dhabi Emirate (Ali et al., 2008). Gravity measurements were acquired using a Scintrex CG5 gravimeter in April 2006 at 416 stations over an area of approximately 500 sq km. The survey was tied to the IGSN-71 datum in Sharjah (25°21.1′N, 55°24.2′E; g = 978,887.900 mGal). The gravity stations were spaced approximately 1.0 km apart, except in areas where logistical difficulties limited physical access. The data was reduced to Bouguer anomaly using an average density of 2,600 kg/m3. Terrain corrections were applied to obtain the complete Bouguer anomaly, which corrects for irregularities due to variations in terrain in the vicinity of the gravity stations. The data was shifted by -10 mGal to match the ADNOC onshore dataset.

Jabal Hafit: Gravity measurements were acquired in February and March 2007 at 1,327 stations over an area of approximately 200 sq km using a Scintrex CG5 gravimeter (Ali et al., 2009). The gravity stations were spaced approximately 200 m apart, but some variations in the sampling distance were occasionally required due to difficulties in accessing some areas. Measurements along main roads and trails provided generally good coverage at elevations below 500 m, whereas at higher elevations there were several large areas, which were totally inaccessible. This resulted in sparse data coverage over the summit of Jabal Hafit. The gravity data was corrected for drift, tide, elevation, latitude and terrain. The survey was tied to the IGSN-71 datum close to Seeb International airport in Oman. The data was reduced to simple Bouguer anomalies using an average density of 2,600 kg/m3. The data was shifted by -10 mGal to match the ADNOC onshore dataset (Table 1).

Jordan (2007): The data was acquired in January 2005 in conjunction with the University of Oxford. The main aim of the survey was to assess the rigidity of the northeastern Arabian continent (Jordan, 2007). Profiles were obtained along roads in the northern UAE to acquire data along the strike of the foreland basin in the region flanking the Semail Ophiolite with a LaCoste and Romberg model G land gravimeter (No. 645) (purple W-E profiles, Figure 3 and Enclosure I). Gravity values were tied to IGSN-71 global gravity network via an absolute base station close to Seeb International airport in Oman. The data has been corrected for instrumental drift, latitude, elevation, and terrain. The data was reduced to Bouguer anomaly using a density of 2,600 kg/m3. The data was not shifted since differences between the data and the ADNOC onshore dataset in the tie points is less than 1 mGal. The discrepancy of this survey with other recent surveys tied with the same absolute base station may be due to differences of geodetic reference ellipsoids used in raw data processing. Also, this survey used a LaCoste and Romberg model G gravimeter in contrast to the rest of recent surveys, which used Scintrex CG5 gravimeter. It is, therefore, possible that the discrepancy is due to errors in calibration constant of the gravimeter.

Savage (2007): Gravity data was collected along two traverses roughly perpendicular to the strike of the foreland basin and cover the edge of the foreland basin to the base of the northern Oman Mountains (Savage, 2007) (green W-E profiles, Figure 3 and Enclosure I). The data was acquired during December 2006, in conjunction with the University of Oxford using a Scintrex CG5 gravimeter. The survey was tied to the IGSN-71 datum close to Seeb International airport in Oman. The data has been reduced using a density of 2,600 kg/m3. The data was shifted by -10 mGal to match the ADNOC onshore dataset.

Lee (2008): Gravity measurements were acquired in January 2008 at 185 stations along four transects which in general correlate with the major roads along the northern UAE (Lee, 2008) (blue W-E profiles, Figure 3 and Enclosure I). The traverses are roughly perpendicular to the strike of the foreland basin and cover the edge of the foreland basin to the base of the northern Oman Mountains. The data was acquired in conjunction with the University of Oxford using a Scintrex CG5 gravimeter. The gravity data was corrected for instrumental drift, earth tides, elevation, latitude and terrain. The survey was tied to the IGSN-71 datum close to Seeb International airport in Oman. The data was reduced to Bouguer anomaly using a density of 2,600 kg/m3. The data was shifted by -8 mGal to match the ADNOC onshore dataset.

Northern Emirates: The data was acquired in October to November 2011 in order to survey areas in the northern emirates that have not previously been covered by gravity surveys (red data points, Figure 3 and Enclosure I). The data was collected along roads and tracks in the mountainous areas of Fujairah and Ras Al-Khaimah Emirates using a Scintrex CG5 gravimeter. Various corrections have been applied to the data including drift, latitude and elevation. The data was tied to the IGSN-71 gravity base station in Nazwa, UAE (25°59′44.02″N, 55°39′36.892″E; g = 978,846.767 mGal). The data was reduced to Bouguer anomaly using a density of 2,600 kg/m3. The data was shifted by -13 mGal to match the ADNOC onshore dataset.

Oman Gravity Data

RAK Petroleum

The gravity data was acquired in 2007 in two parallel lines across the border between Oman and UAE, north of Al-Ain city. Various corrections including drift, latitude, elevations and terrain correction were applied. The data was reduced to Bouguer anomaly using a density of 2,600 kg/m3. No information was provided on the gravity datum and absolute gravity base station in with the survey was tied to. The data was shifted by -2 mGal to match the ADNOC onshore dataset.

Indigo

The data was acquired in 2003 in Jabal Hafit area, along the border between Oman and UAE. In total nine lines were acquired along seismic profiles. The data was reduced to Bouguer anomaly using a density of 2,600 kg/m3. No information was provided on the gravity datum and absolute gravity base station in with the survey was tied to. The data was shifted by -3 mGal to match the ADNOC onshore dataset.

Ravaut and Warsi (1997)

The gravity data from the rest of Oman consists of Bouguer values digitised from a Bouguer anomaly map of Ravaut and Warsi (1997). The data was reduced to Bouguer anomaly using a density of 2,600 kg/m3. The survey was tied to the IGSN-71 datum close to Seeb International airport in Oman. The data was not shifted as differences in the tie points between the data and ADNOC onshore dataset was highly variable. Some of the points with high tie differences were removed.

Offshore Gravity Datasets

ADNOC Offshore Datasets

The ADNOC offshore data comprises of two parts: shipboard and sea bottom surveys.

Shipboard Survey: The surface marine survey consists of 79 lines of ship track data covering 2,350 km (light brown profile data, Figure 3 and Enclosure I). The survey was conducted in the summer of 1980 by GECO using the MV LONGVA II. The main purpose was to design the layout for a seismic survey. Most of the survey is made up of interconnected lines, which are widely spaced and randomly directed. The accuracy of the gravity data is probably around 1.0 mGal. The data was reprocessed to Bouguer gravity using a density of 2,600 kg/m3. The data was shifted by -3 mGal to match the Sanders dataset. The survey is tied to the absolute gravity datum through the operating base at the dock in Port Said, Abu Dhabi.

Sea Bottom Survey: The Geophysical Prospecting Company using the MV CALYPSO in 1953 and 1954 conducted the water bottom gravity survey. The survey consists of 400 stations ranging from about 6.4 km to 9.7 km apart in lines that range from about 8 km to 16 km apart (light brown data points, Figure 3 and Enclosure I). The coverage is absent in the shallow waters along the coast. The survey was merged with the shipboard survey. The data was reprocessed to Bouguer anomaly using a rock density of 2,600 kg/m3. The data was shifted by -7 mGal to match the Sanders dataset.

Sanders Dataset

Sanders Geophysics Limited conducted a high-resolution airborne gravity and magnetic survey for ADNOC in 2007–2008. The survey covered a major portion of the offshore region of the Emirate of Abu Dhabi in addition to a small onshore region (blue area, Figure 3 and Enclosure I). A total of 13,804 line kilometres were acquired. All lines were acquired with a traverse line spacing of 2,000 m and a control line spacing of 10,000 m. Various corrections including Eötvös, latitude, free air, Bouguer, curvature of the earth, terrain, static and level corrections were applied to the data. A density of 2,600 kg/m3 was assumed for the sediments that dominate the survey area and a density of 1,020 kg/m3 was employed for the seawater. Adjustments for terrain were computed using terrain data derived from Shuttle Radar Topography Mission (SRTM) data for the onshore. The bathymetric data used was taken from one-minute gridded bathymetric chart of the ocean (GEBCO). The dataset intersects the ADNOC onshore database and has been shifted by 10 mGal to match the onshore data.

Das and Dayina Land and Marine Datasets

The datasets were acquired and processed by Fugro Survey Middle East Limited for Abu Dhabi Company for Onshore Oil Operations (ADCO), in and around the Das and Dayina islands (two green areas, Figure 3 and Enclosure I). The objective was to acquire high-resolution gravity data to be used in the interpretation of the subsurface geology, especially to confirm the presence and define likely extent of salt domes thought to underlay each of the islands. Marine data was acquired at one-second interval. Land gravity stations on a nominal 250 m x 250 m grid were surveyed on each of the islands. However, due to restricted access on Das, the gravity stations were located on roads and tracks. In total 28 land stations were surveyed on Dayina and 71 stations on Das. The datasets were reprocessed to Bouguer anomaly using a density of 2,600 kg/m3. The Das and Dayina datasets have been shifted by -5 mGal and -4 mGal respectively to match the Sanders dataset.

ANALYSIS OF THE GRAVITY DATA

Bouguer Gravity Anomaly

The Bouguer gravity anomaly map of the UAE comprises a range of anomaly amplitudes and wavelengths (Figure 4 and Enclosure I). The most striking is the positive-negative Bouguer anomaly “couple” associated with the Oman Mountains and flanking UAE foreland basins. In the Oman Mountains south of the Dibba Fault Zone there is a NS-trending Bouguer gravity anomaly high, which correlates with the ophiolite outcrop and reaches its maximum value along the eastern coast of the UAE, suggesting that the ophiolite may extend offshore. Flanking the high on its western edge is a NS-trending Bouguer gravity anomaly low that reaches its minimum value over the UAE foreland basins. We attribute this Bouguer gravity anomaly “couple” to ophiolite loading and flexure of the rifted Neo-Tethyan margin of the Arabian Plate. The couple is flanked to the west by a well-defined long wavelength, lower amplitude Bouguer anomaly high trending SE-NW, which we believe delineates the present-day position of the outer bulge to this loading and flexure. In addition, this western portion of the Bouguer gravity map contains many less clear low-gravity closures, which may be associated with salt tectonics.

Regional-Residual Separation

The Bouguer anomaly map shows a number of significant short-wavelength anomalies that are superimposed on the long-wavelength anomalies. These anomalies are attributed to shallow sources in the upper crust. By removing the regional effects from the map, the resultant residual gravity map reflects the composite effects of density changes associated with these sources. Hence, a residual gravity map may reveal a better-defined image of local gravitational anomalies. This may include changes within Proterozoic basement. Thus, the residual gravity map would be expected to reflect the composite effects of density changes within basement, those originating near top of basement as well as those from within the sedimentary section.

We have used an upward continuation technique to determine the long-wavelength regional component of the gravity field that is then subtracted from the observed to obtain a residual short-wavelength anomaly. A number of upward continuation elevations (e.g. 15, 20 and 30 km levels) were tested. The regional gradient in the eastern UAE and northern Oman Mountains is different from that of the western UAE. Therefore, separation of regional and residual anomalies over the entire UAE is difficult to perform using a single filter. Nevertheless, the upward continuation at 20 km elevation appears to result in a smooth field that reflects well the effects of loading and flexure and their associated changes in density structures. The regional anomaly map indicates a gentle gravitational gradient increasing towards the south in the Mender-Lekhwair Arch. The gradient probably reflects the thinning of the crust and lithosphere towards the south. However, at this stage identification of any specific cause is subject to considerable ambiguity.

It is apparent from Figure 5 that in the eastern UAE and northern Oman Mountains a significant component of the local Bouguer gravity anomaly remains, and therefore residual anomalies are not well resolved in this region. For example, part of the long wavelength low and high anomalies defined on the Bouguer anomaly map are retained intact, which may emphasize the thickening of the sediments in the foreland and thickening of the ophiolite respectively.

Residual Gravity Anomaly

To highlight local anomalies, the regional component of the gravity anomaly field is commonly subtracted from the observed data, generating a residual gravity map. Figure 6 and Enclosure II show a residual gravity anomaly map, which was calculated by subtracting the upward continued gravity anomaly from the Bouguer gravity anomaly as presented in Figure 4 and Enclosure I.

The residual anomaly map (Figure 6 and Enclosure II) can be sub-divided into three major structural provinces, each having distinct gravity signatures. The eastern area of the UAE and northern Oman Mountains (fold-and-thrust belt province) exhibits short-wavelength positive gravity anomalies the outline of which appear to correlate with individual thrust sheets in the Semail Ophiolite. The central area (foreland basin) is characterised by a distinct NNW-oriented gravity anomaly low. We attribute the low to a deepening of the basement due to ophiolite loading and flexure and the infill of the resulting basin by thick Aruma and Pabdeh Group sediments. The low gravity is flanked to the west by a series of low-amplitude gravity highs. The highs are located in a region of generally high gravity anomaly that reflects the flexural bulge that flanks the UAE foredeep basin. The western area (salt tectonic province), in contrast, exhibits many well-defined gravity lows, which are probably caused by swelling of a basal Hormuz Salt layer as well as sedimentary depocentres associated with synclinal structures.

Interestingly, most of the major oilfields correlate with the flexural bulge and either circular negative gravity anomalies or elongated positive anomalies. For example, the Umm Shaif, Nasr, Mandous, Zakum and Hail oilfields are all located on well-defined closures of negative gravity anomalies. Most onshore oilfields (e.g. Bu Hasa, Bab, Jarn Yaphour structures), however, are defined by elongated positive gravity anomalies.

First Vertical Derivative

First derivatives maps enhance short-wavelength anomalies, discontinuities and tend to allow clearer image of shallow sources. For example, for a positive density body (i.e. basement uplift) vertical derivative map has a positive anomaly over the source body and zero contours over the body edges. However, the transformation can be noisy since it will amplify short wavelength noise. Figure 7 shows the first vertical derivative (vertical-gradient) of the residual Bouguer gravity anomaly. Compared to the residual gravity anomaly map (Figure 6 and Enclosure II), the vertical derivative, in effect, suppresses the gravity anomaly associated with deep sources and enhances the anomaly associated with shallow sources. In particular, the map shows clearly elongated NS-, NE- and NW-trending positive gravity anomalies as well as circular negative anomalies in the western salt tectonic province. In addition, the map suggests that the fold-and-thrust belt province is probably made of up of a number of discontinuous sheets of more dense igneous rocks that are separated by less dense, probably sedimentary, material.

Total Horizontal Derivative

The total horizontal derivative (Figure 8) is often effective at highlighting linear edge structures as positive maxima such as those associated with faults and contacts. The total horizontal derivative is expressed as (Cordell, 1979):

 

Total horizontal derivative = SQRT [(g/x)2+ (g/y)2]

where (∂g/∂x)2 and (∂g/∂y)2 are the derivatives of the gravity field (g) in x and y directions respectively. Total gradient derivative is always positive.

Total horizontal derivative maxima occur over the steepest gradients of the gravity anomaly relief’s slope indicating source boundaries, and minima over the flattest parts (Blakely, 1995). The maxima are related to the density contrasts and boundary depths (Cordell, 1979; Blakely and Simpson, 1986).

Figure 8 shows the total horizontal derivative of the residual Bouguer gravity anomaly. The map reveals both N-S and NW-SW trends, which we believe may reflect terrain boundaries. The map seems, however, to be noisy in places due to the amplification of short-wavelength noise.

Tilt Derivative

The tilt derivative (Miller and Singh, 1994; Verduzco et al., 2004; Fairhead et al., 2011) is a normalised phase derivative that uses first-order derivatives and has also been shown to be an effective method of mapping subsurface structural edges. It is defined as:

 

Tilt derivative = tan-1[VDR/THDR]

where VDR is the vertical derivative and THDR is the total horizontal derivative. Due to the nature of the arctan function, all amplitudes are restricted to values between (π/2 and –π/2) regardless of the amplitudes of the VDR and THDR. This fact makes this relationship function like an automatic gain control (AGC) filter and tends to equalise the amplitude output of gravity anomalies across a grid (Fairhead et al., 2011). The tilt derivative is independent of density (or magnetisation) since the VDR and THDR are functions of density (or magnetisation) of the subsurface. Therefore, it is a powerful derivative that determines subsurface structural edges (not density or magnetisation) from weak Bouguer or TMI anomalies (Verduzco et al., 2004).

Figure 9 shows the tilt derivative of the residual gravity field. Many lineaments and circular features can be clearly seen in the Figure. Lineaments directed NW-SE, NE-SW and N-S are more extended and appear with enhanced amplitudes. The tilt derivative map also illustrates the N-S trends behave as major, narrow and elongated ridges. In addition, a number of linear features can be seen on the eastern side over the fold-and-thrust belt, which can be an indication of lineaments in the basement. Furthermore, most of the oilfields occur either along linear or circular features. For example, Huwaila, Bu Hasa and Bida Al Qemzan oilfields lie along N-S uplifted basement structure. We have, therefore, found the tilt derivative to be the most useful technique in delineating structural trends.

Euler Deconvolution

The Euler deconvolution method is used to delineate trends and to estimate depth of sources from potential field data (Thompson, 1982; Reid et al., 1990; Mushayandebvu et al., 2001). The Euler’s deconvolution is expressed as (Thompson, 1982):

 

(xx0)gx+(yy0)gy+(zz0)gz=N(gB)

where (x0, y0, z0) is the coordinate position of the source, (x, y, z) is the coordinate position of the measurement, g is the gravity field, B is the regional field and N is the structural index (SI) which relates the rate of change of the potential field with distance. If the structural index of the source is known, its depth and location can be determined.

The quality of depth estimation depends mainly on the choice of a structure index (SI) as well as the window size (W) and the tolerance depth (TZ). For simple bodies the SI of gravity data is one less than the SI for the equivalent magnetic bodies and range from -1 to 2 (Stavrev and Reid, 2007). These parameters have to be carefully selected in order to obtain appropriate solutions. Use of an incorrect value of SI can result in a misleading depth (Reid et al., 1990; Stavrev and Reid, 2007). Depending upon SI, TZ and W, an extensive set of tests were performed through varying one parameter at a time while keeping the other ones constant. Suitable parameters were selected by inspection and comparison of the depths deduced from gravity data.

Figures 10 and 11 show results from Euler deconvolution. The results allow identification of the location and trend of contact faults and lineaments with a good clustering of the solutions along linear segments trending NW-SE, NE-SW and N-S. The estimated depth for these lineaments ranges from 2,000 m to > 8,000 m. The NW-trending lineaments are associated with the maximum depth, whereas the NS-trending lineaments correlate with the shallowest depth source. However, the well-expressed NW-SE lineament, located east of Abu Dhabi, exhibits a depth varying source between 2,000 m to > 8,000 m. It seems likely that these basement faults have controlled basin architecture, and that a number of these faults have been reactivated and offset sediments. Therefore, as density increases with depth due to compaction, varying source depths could be caused by density contrasts due to faulting in the basin.

It seems that the Euler deconvolution method fails to estimate correctly the depth of circular and elongated sources (e.g. salt diapirs and basements highs). The estimated source depths of these features is <2,000 m. However, available well and seismic data indicate that the sources of these anomalies are much deeper. An explanation of this discrepancy may be related to the SI value used in the depth estimates, which is probably not optimised for these sources. SI may be changing its value due to distance from source. In addition, the SI is a function of the geometry of the sources and the depths are biased if the wrong SI is given for any given source (Reid et al., 1990; Keating, 1998). It is expected that sources representing more than one SI are likely to be present in the study area. However, in this study we selected a SI of 0 in order to delineate faults and lineaments (Chenrai et al., 2010). Therefore, it is improbable that SI of 0.5 can determine correctly the depths of circular features such as salt diapirs.

Gravity Modelling with Geological and Geophysical Constraints

Two Bouguer residual gravity profiles (Figure 6) were forward modelled in 2-D using a line-integral method. Profile I is oriented N-S and crosses the Umm Shaif and Bab oilfields as well as Falaha syncline (Figure 12). Profile II is oriented NE-SW and crosses the western edge of the northern Oman Mountains, foredeep and Falaha syncline (Figure 13). The profiles were selected so as to compliment the structural interpretation of the basement configuration and to better define the spatial distribution of the infra-Cambrian salt basin.

The forward modelling of gravity anomalies is complicated by non-uniqueness (Blakely, 1995). In this study, our modelling of subsurface structures was constrained using different sources of geological and geophysical data. Outcrop geology (Figure 2) in the northern Oman Mountains provided the control for the outcrop widths, formation boundaries and the average dips of exposed rocks. Formation tops from available wells and depth-converted horizons from seismic profiles in offshore and onshore of UAE provided information on subsurface formation thicknesses. However, the basement in the UAE foreland basin is so deep that it was not reached by available oil exploration wells and is poorly imaged in seismic reflection profile data. Therefore, the depth to the Permian–Mesozoic shelf carbonates, Palaeozoic, Hormuz Salt and underlying crystalline basement were estimated from regional geological and geophysical studies in the UAE foreland basin and central Oman Mountains (Shelton, 1990; Ravaut et al., 1997; Al-Lazki et al., 2002; Peters et al., 2003; Ali and Watts, 2009; Reuning et al., 2009; Ali et al., 2013). Additionally, the densities assigned to the formations are based on available well log data (density logs from wells W1, W2, W3, W4, W5 and W6) and density analyses of previous studies in the region (Manghnani and Coleman, 1981; Ali et al., 2008; Ali et al., 2009; Searle and Ali, 2009). The structural geometries of the models were iteratively modified to derive a good fit between the calculated and observed residual Bouguer anomalies. The formation densities were assumed to remain constant along the profiles.

The gravity effect of the final subsurface models, which are presented in Figures 12 and 13, fit the observed residual Bouguer gravity anomalies well. The best-fit model solutions were determined by reducing the root mean square (RMS) error between calculated and observed gravity anomalies to less than 5.0% of the total dynamic range of the observed gravity anomaly data along each profile.

In Profile I, the observed residual Bouguer gravity anomaly shows two prominent gravity anomaly lows of up to -10 mGal at wells W1 and W3. The offshore anomaly coincides with the Umm Shaif oilfield which is located in a structural anticline that been interpreted as caused by deep-seated salt diapirism originating on the top of the basement (Alsharhan, 1989). The onshore anomaly occurs in the Falaha syncline. However, significantly, a low-density body, interpreted as salt, and deep basement are required to explain the full amplitude of the negative gravity anomaly. The model suggests that the Bab oilfield is underlain by a basement high that may have been reactivated during the Late Cretaceous.

In Profile II, the observed residual Bouguer gravity anomaly shows a gravity anomaly high of up to 60 mGal associated with the fold-and-thrust belt and a gravity anomaly low of up to -40 mGal associated with the flanking foredeep basin. Furthermore, the observed gravity anomaly data suggests that the central area of the profile is associated with a negative gravity anomaly, which correlates with the Falaha syncline (W3) as well as a thick salt body and basement graben.

The gravity models along Profiles I and II, therefore, indicate the presence of substantial infra-Cambrian salt bodies in all basins of the UAE, both onshore and offshore, including the southern area of the Rub’ Al-Khali Basin.

DISCUSSION

We have focussed in this paper mainly on gravity anomaly rather than magnetic anomaly data. Magnetic anomaly data has many advantages over gravity data in delineating basement structures. It can allow a better understanding of basement structure and fault geometry and hence enhance our understanding of the structural architecture and evolution of sedimentary basins. Unfortunately, aeromagnetic data of the UAE and Oman Mountains is not generally available at this time.

The residual gravity anomaly and derivative gravity anomaly maps (Figures 5 to 9) show a pattern of well-defined high-amplitude residual gravity anomaly lows in northwestern UAE. The northwestern offshore of the UAE contains a number of islands dominated by salt diapiric structures (e.g. Dalma, Zirkouh, Qarnain, Das, Sir Bani Yas, Arzana, Sir Abu Nuwair and Abu Musa). These islands are formed by basement faulting which has disrupted the infra-Cambrian Hormuz Salt, triggering deep-seated salt diapirism (Alsharhan and Salah, 1997). Furthermore, most of the offshore oilfields in Abu Dhabi are associated with salt-related traps. This is because basement faulting penetrated the infra-Cambrian Hormuz Salt, thus mobilising the deep-seated salt and producing domal oilfield structures. The Umm Shaif, Zakum and Hail oilfields are good examples of the salt-related traps as they lie exactly above the distinct gravity anomaly lows (Figure 6 and Enclosure II). Some of these oilfields are circular, such as Hail, whereas others are elongated, due to salt-wall diapirism, as in the case of Umm Shaif oilfield. However, in most of these oilfields exploration wells did not penetrate salt-related sediments in the crest of these structures. This has led to the interpretation that these oilfields are cored by deep-seated infra-Cambrian basement horst block, initiated possibly during the infra-Cambrian Najd rifting, and repeatedly reactivated during subsequent geological times (Edgell, 1996; Al-Husseini, 2000; Konert et al., 2001).

In the eastern UAE, patterns of alternating negative and positive gravity anomalies are interpreted to be associated with the foredeep and fold-and-thrust provinces respectively. Onshore central and western parts show well-defined distinctive negative residual gravity anomalies. The prominent NS-elongated negative gravity anomaly that extends offshore towards Hair Dalma oilfield is most likely related salt intrusions along the N-S Arabian trend. However, the negative anomaly centred on the axis of the Falaha syncline is interpreted as combination of a thick sedimentary cover as well as a thick salt body and deep basement depression. In addition, onshore central and southern parts of the UAE show strong positive gravity anomalies due to block uplift of the basement, which contrasts with the western offshore region where there are pronounced negative anomalies which have been interpreted as resulting from deep-seated salt diapirism that has been triggered by faulting. For example, the Bu Hasa and Bab structures are defined by elongated positive anomalies, which suggest that the basement is the most important factor in controlling structural styles in the overlying sediments. This is consistent with the results of gravity surveys conducted over onshore oilfields in NE Saudi Arabia, which generally show that most of giant onshore oilfields (e.g. Ghawar, Khurais and Abu Jifan) exhibit distinct positive gravity anomalies due to the presence of a dense uplifted basement beneath them (Aramco, 1959; Edgell, 1990). Therefore, there is a strong association of oilfields in the UAE with the centre of both negative gravity anomalies, interpreted as thick salt bodies as well as basement lows, and positive gravity anomalies, interpreted as basement ridges within the main sub-basin of the UAE foreland basin.

The gravity modelling presented in this paper indicates that infra-Cambrian salt bodies exist in all UAE basins. The presence of salt in the Rub’ Al-Khali Basin may indicate that the Hormuz Salt Basin and Ara (Fahud and Ghaba) Salt basins are connected. However, it is possible that locally salt may not occur on structurally high areas where basement is uplifted. Recently Cooper et al. (2012); Cooper et al. (2013) identified intrusions of gypsum and anhydrite in the core of the Jabal Qumayrah culmination.

The intrusions were emplaced after the Jabal Qumayrah nappe emplacement but before post-Eocene unroofing (Cooper et al., 2012; Cooper et al., 2013). The salt plugs have been interpreted to originate from evaporites within Ediacaran–Cambrian Ara Group. Major salt basins (Fahud, Ghaba and South Oman) occur to the south of Jabal Qumayrah, with surface piercing diapirs seen in the Ghaba Salt Basin (Peters et al., 2003; Reuning et al., 2009). It is, therefore, possible this basin extends further to the north and connects to the Hormuz Salt Basin as indicated by the gravity modelling.

The residual and derivative Bouguer gravity anomaly maps (Figures 6 to 9, and Enclosure II) clearly indicate three well-defined basement lineaments in the UAE that trend N-S, NE-SW and NW-SE. We believe these lineaments have played a major role in subsequent tectonic events and sedimentary basin evolution.

The N-S lineament trend (e.g. Figure 9) dominates the western part of the UAE and coincides with some of the onshore oilfields in Abu Dhabi Emirate (e.g. Bu Hasa oilfield). Reactivation of these trends has probably played an important role in the subsequent structural development of the region. Subsequent EW-trending extensional events, for example, have resulted in basement horsts and graben structures. The basement structures probably reflect the original Neoproterozoic tectonic elements of the Arabian Plate which have been periodically reactivated (Edgell, 1990).

The southern and central parts of Abu Dhabi Emirate to the east of the Falaha syncline (e.g. Figure 9) are dominated by NE-SW lineament trends. Structures of giant onshore oilfields of Shah, Asab and Sahil are located east of elongated NE-SW positive gravity anomalies. This may be related to deep-seated faults that displace the NE-SW trend of the uplifted basement blocks. Other Abu Dhabi onshore oilfields (e.g. Bab, Qusahwira and Mender) appear to be associated with uplifted basement blocks that are separated by negative gravity anomalies (e.g. Ghurab, Falaha and Hamra synclines). The crestal stratigraphic sequences of these oilfields indicate repeated rejuvenation of their associated underlying basement uplifts. The basement consists of alternating horsts and grabens. As a result, almost all the onshore oilfields of Abu Dhabi exhibit distinctive positive gravity anomalies due to the presence of denser uplifted basement beneath them. The trend may have been reactivated during Late Cretaceous, or possibly Palaeogene, following loading and flexure of the Arabian Plate associated with obduction of the Semail and Masilah ophiolites. The trend may also reflect a NE-trending Neoproterozoic compressional event that involved shear zones and basement thrust faults. Figure 14 shows regularly spaced, NE-trending lineaments that comprise in the central part of the UAE, which, we believe, reflects the Neoproterozoic tectonic elements of the Arabian Plate.

It is possible that the NE-SW trend extends further to the northeast where it intersects the Dibba Fault Zone. However, there is no clear evidence from either the residual or derivative gravity anomaly maps that the Dibba Fault crosses the UAE foreland basin. It seems to us that the NE-SW lineaments in central UAE are crossed by NW-trending lineaments, which makes the continuation of the NE-SW trend to the northeast difficult to map. Furthermore, it has been noted that the NE-SW trend corresponds to the main trend of the Hormuz and Ara Salt basins, and appears to control the distribution of infra-Cambrian salt basins of the Arabian Gulf and Oman (Husseini, 1988; Husseini and Husseini, 1990; Loosveld et al., 1996; Al-Husseini, 2000).

The NW-SE lineament is probably the most prominent trending lineament zone in the UAE region. The trend is present in the onshore central part and extends offshore towards the Zakum oilfield and possibly south across the Oman border. It seems that Jarn Yaphour oilfield is aligned along this trend. The trend can easily be traced along a high-amplitude trend of the first vertical derivative, total horizontal gradient and tilt derivative of the residual Bouguer gravity anomaly maps. These derivative maps show two narrow basement lineaments aligned along this trend. As Abu Dhabi Emirate is the region where it is most clearly defined (Figure 9) and presently the best documented, we propose to call these ca. 200 km long lineaments the Abu Dhabi Lineaments. The lineaments seem to be parallel to the NW-trending Najd Fault System. The Najd faults formed during a Gondwana-scale continental shearing event and comprise an array of major, crustal-scale sub-vertical NW-trending sinistral shear zones across northern Gondwanaland. The implied E-W to ESE-WNW principal compressional direction (Nehlig et al., 2002) is consistent with the development of conjugate NW-trending sinistrial (Najd Fault System) and NE-trending dextral shear zones (Al-Husseini, 2000). The NW-SE trend may also be related to the Tethyan rifted margin hinge zone, or shelf-slope break. The Tethyan hinge zone marks the boundary between unstretched and highly stretched continental crust where the depth to basement increases rapidly. There are no reliable constraints on regional crustal thickness so that backstripping of well and sediment isopach data will probably best constrain the geometry of the Tethyan margin of the Arabian Plate.

CONCLUSIONS

We draw the following conclusions from this paper:

  • A new compilation of gravity measurements onshore and offshore the UAE has allowed us to determine the structural elements of the basement of the northeast margin of the Arabian plate.

  • Three major structural provinces, each of which has a distinct Bouguer gravity anomaly signature, have been identified.

  • The fold-and-thrust belt province of eastern UAE and northern Oman Mountains is associated with short-wavelength gravity anomaly high, which we attribute to the relatively dense rocks that comprise the Semail Ophiolite and its related thrusts.

  • The foreland area of central UAE, in contrast, is characterised by a distinct NNW-SE oriented gravity low which we attribute partly due to the thick Aruma and Pabdeh Group sediments that infill the basin and partly to flexure of the top and base of the crust by the load of the Semail Ophiolite and its related thrusts.

  • The third major province is the salt province of the western UAE. This province is characterised by gravity anomaly lows, which we attribute to infra-Cambrian salt diaprism.

  • Most offshore oilfields (e.g. Umm Shaif, Nasr, Mandous, Zakum and Hail) of the UAE are located on circular negative gravity anomalies, which we attribute to deep-seated salt bodies. However, most onshore oilfields (e.g. Bu Hasa, Bab, Jarn Yaphour structures) are defined by elongated positive gravity anomalies, which we attribute to basement uplifts.

  • Gravity modelling indicates the presence of infra-Cambrian salt in all basins of the UAE including the Rub’ Al-Khali Basin.

  • Residual gravity anomaly maps, which have been constructed by separating the long wavelength gravity anomalies associated with the loading of the Semail Ophiolite and flexure from the observed Bouguer gravity anomaly, and transformed gravity anomaly data reveal the tectonic fabric of the basement.

  • Three well-defined short wavelength lineaments, which generally trend N-S, NE-SW and NW-SE, have been identified.

  • The NW-SE lineament is the most dominant and comprises two well-defined trends that cross Abu Dhabi Emirate and we have named the Abu Dhabi Lineaments. These lineaments are probably related to the Najd Fault System. N-S Arabian trends are observed throughout the UAE, but they are particularly strong in the western part. This trend coincides some of the onshore oilfields in Abu Dhabi (e.g. Bu Hasa oilfield). Finally, the NE-SW trend, which has a regular spacing is dominant in the southern and central part of Abu Dhabi, east of Falaha syncline.

  • The structural tends have clearly been inherited by subsequent extensional and compressional events in the Arabian Plate.

ACKNOWLEDGMENTS

We are grateful to The Petroleum Institute, Abu Dhabi for sponsoring this project, ADNOC (Abu Dhabi National Oil Company) for providing some of the gravity, seismic reflection profiles and well data used in this study, and James Small, Muhammad Iqbal and many students from the Petroleum Institute for gravity fieldwork assistance. Contour maps of the gravity data were constructed using the GeosoftTM Montage software and the gravity profiles were modelled using the 2.5-D GM-SYS program. We greatly acknowledge discussions with Mike Searle and David Cooper. We thank Derek Fairhead and an anonymous reviewer for their helpful comments on an early version of the paper. The final design and drafting by GeoArabia’s Production Co-manager Nestor Niño Buhay is much appreciated.

ABOUT THE AUTHORS

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

mali@pi.ac.ae

Anthony B. Watts has a BSc in Geology and Physics from University College, London, a PhD in Marine Geophysics from Durham University, and a DSc from Oxford University, UK. His current research is focused on the gravity field and its relationship to isostasy, lithospheric flexure, and the thermal and mechanical evolution of sedimentary basins. Other research interests include marine geology and geophysics and the deep structure of continental margins, seamounts and oceanic islands, and mid-ocean ridges. Anthony joined Oxford University in 1991 as a Professor of Marine Geology and Geophysics, previous to which he had been the Arthur D. Storke Memorial Professor of Geology at Columbia University, New York, USA. He is a fellow of the AGU, GSA and EGU.

tony@earth.ox.ac.uk

Asam Farid has Master of Science and Master of Philosophy degrees in Geophysics from Quaid-i-Azam University, Islamabad Pakistan. His current work is focused on seismic stratigraphy and seismic attributes of the Late Cretaceous sediments in the United Arab Emirates. Previous work included the seismic stratigraphic studies in Lower Indus and Offshore Indus Basins in Pakistan. Other interests include the mapping and characterization of the near-surface and depositional setting of alluvial basins. Asam joined The Petroleum Institute in 2010 and currently works as a Research Assistant. Previously, he worked with LMKR, Islamabad, from 2005–2007 and Fugro Middle East, Abu Dhabi, from 2008–2010. He is a member of SEG.

afarid@pi.ac.ae