Consideration of Al-Wajid graben tectono-stratigraphy by analysis of geophysical data, Saudi Arabia

Journal Pre-proof Consideration of Al-Wajid graben tectono-stratigraphy by analysis of geophysical data, Saudi Arabia

Ahmed Salem, Christian Hofmann, Dumitru Ion, Carlos Planchart, Emad Muzaiyen, Mesbah Khalil PII:

S0926-9851(19)30429-X

DOI:

https://doi.org/10.1016/j.jappgeo.2020.103941

Reference:

APPGEO 103941

To appear in:

Journal of Applied Geophysics

Received date:

9 May 2019

Revised date:

30 September 2019

Accepted date:

3 January 2020

Please cite this article as: A. Salem, C. Hofmann, D. Ion, et al., Consideration of AlWajid graben tectono-stratigraphy by analysis of geophysical data, Saudi Arabia, Journal of Applied Geophysics(2019), https://doi.org/10.1016/j.jappgeo.2020.103941

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Journal Pre-proof Consideration of Al-Wajid graben tectono-stratigraphy by analysis of geophysical data, Saudi Arabia Ahmed Salem, Christian Hofmann, Dumitru Ion, Carlos Planchart, Emad Muzaiyen, and Mesbah Khalil

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Dhahran, Saudi Aramco, Saudi Arabia

Journal Pre-proof Abstract Al-Wajid graben is one of the largest Precambrian fault-bound basins in Saudi Arabia. Seismic reflection data show a deep-seated dome structure in the middle of the graben, which can be interpreted as a salt dome, an inversion structure, or even an igneous pluton. This ambiguity makes it difficult to understand the tectono-stratigraphy of Al-Wajid graben. In this paper we sought

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to understand the tectono-stratigraphy of Al-Wajid graben by analysis of various

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remote sensing geophysical data including vertical seismic profile (VSP), gravity,

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magnetic and seismic reflection data.

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Determination of depth to basement from the magnetic data suggests that the dome structure is non-magnetic since the depth solutions were obtained at

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the bottom of the structure. Processing of the VSP data was useful in estimating

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some physical parameters for the structure in question such as acoustic impedance, velocity, and density. The estimated density is 2700 kg/m3, which

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suggests that the structure is unlikely to be salt and most likely to be a dense

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mafic or metamorphic of non-magnetic rocks. This interpretation is consistent with extension and subsidence of the graben. The integrated interpretation of seismic, VSP, gravity, and magnetic data proposes that Al-Wajid graben is a deep trough intruded by dense magmatic rocks before opening the graben. The cooling of these igneous rocks has caused local densification and continuous subsidence that implied downward bending of the crust, creating the basin as a cratonic graben. Since Precambrian, the graben

Journal Pre-proof has been affected by several tectonic phases of extension and compression, ending with positive inversion.

Introduction Al-Wajid graben is located in Saudi Arabia (Figure 1). Relatively few publications

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deal with the tectono-stratigraphy of Al-Wajid graben (Konert al., 2001; Ziegler, 2001;

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Dyer and Husseini, 1991; Lange, 2006; Stewart, 2016). Seismic reflection data (Figure

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2) shows a dome-shaped structure in the middle of Al-Wajid graben. There is

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uncertainty about the nature of the dome structure as to whether it is due to a salt

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dome, an inversion structure, or even an igneous pluton (Stewart, 2016). This ambiguity makes it difficult to understand the tectono-stratigraphy of Al-Wajid graben.

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In this paper, we attempt to understand the tectono-stratigraphy of Al-Wajid

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graben by analysis of existing multidisciplinary geophysical data including surface seismic, gravity, magnetic, and vertical seismic profile (VSP) data. In our approach, we

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start with the qualitative analysis of the magnetic data to extract structural and lithologic trend information of the basement rocks, whereas density and magnetization of the basement rocks are investigated using 2D inverse and forward modeling of gravity and magnetic data separately. Then, we process the existing VSP data to investigate the deep reflectors and associated densities. Finally, we integrate all information from geophysical and geological data to build a tectono-stratigraphic model of Al-Wajid graben.

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Regional geology and tectonic settings The East Arabia Basin (Figure 1) is a part of the wide Paleozoic passive margin that was uplifted during the Hercynian tectonic phase (Late Devonian) then developed in the Mesozoic basin cycle. A multi-component crust acted on the East Arabia Basin,

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including Al-Wajid graben, creating sub-highs and lows during basin filling, which

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resulted in basin segmentation. There are two basic interpretations for the active tectonic events during the Infracambrian. The first is a collisional model (Davies, 1984;

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Hughes, 1988) and the second is a transform-extensional model (Stern, 1985). The

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transform extensional model, deliberated in more details by Husseini (1987), is

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consistent with the data presented by Dyer and Husseini (1991) and links the Najd fault system of the Arabian Shield (Figure 1) with the formation of rift basins in the northeast

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Egypt and the Sinai peninsula, as well as the development of the extensive salt basins

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in Oman, the Arabian Gulf, Iran, and Pakistan (Dyer and Husseini, 1991).

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In the Neoproterozoic to Late Devonian epoch (610 to 364 Ma), the Arabian Plate was found in an intracratonic location within the continental interior of northeastern Gondwana bordering the Palaeo-Tethys Ocean (Al-Husseini, 2000; Sharland et al., 2001). During this period, a NW–SE shear fracture system (Najd Fault System) and a number of salt basins were developed (Loosveld et al., 1996; Al-Husseini, 2000). During 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, generating regional uplift, widespread erosion, and basement tectonics along the Neoproterozoic trends (Konert et al., 2001). During Middle to Late

Journal Pre-proof Permian (260 to 270 Ma), the Neo-Tethys opening started between the Cimmerian continental blocks (Central Iranian Plate and Lut Block) in the North, and eastern margins of the Arabian Plate (Blendinger et al., 1990; Searle et al., 2004; Ruban et al., 2007). By the end of the Mid-Cretaceous, the region was a matured carbonatedominated rifted margin in an expanding Neo-Tethys Ocean basin. The Semail ophiolite

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was formed within the Oman segment of Neo-Tethys during the Cenomanian (Tippit

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and Pessagno 1979; Tilton et al., 1981; Warren et al., 2005). Passive margin

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sedimentation returned during the Late Campanian-Maastrichtian (Nolan et al., 1990;

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Skelton et al., 1990). During the Late Oligocene-Miocene time, compressional

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deformation resumed. This deformation is interpreted as the beginning of the Zagros phase of continent-continent collision, when the Musandam shelf collided with the

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central Iran continental block (Searle et al., 1983; Searle, 1988).

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Gravity and Magnetic Data

The Bouguer gravity data over Al-Wajid graben (Figure 3) was acquired at a station density of about 1 km along irregular profiles, spaced 5 to 7 km apart. The total magnetic intensity data used in this study was acquired by a High Resolution AeroMagnetic (HRAM) survey, flown in 2002 at a nominal flight altitude of 120 m over topography, along 500 m spaced traverse flight lines, oriented NE-SW, and 1500 m spaced tie lines, oriented NW-SE. Common corrections including diurnal, IGRF removal, leveling, and reduction to the pole (RTP) were applied to the measured magnetic data. Figure 4 shows the RTP magnetic data over Al-Wajid graben.

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Lineaments Analysis Magnetic response in HRAM data is slightly affected by sediments in comparison with volcanics and basement rocks. Only minerals located in depth shallower than the Curie isotherm might influence the magnetic field. By contrast, the gravity response

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included in the Bouguer anomaly is an integrated effect of density variations within the

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crust and also deeper in the mantle, where temperature variations result in mantle density variations. Consequently, magnetic data has a significant advantage over

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gravity data in mapping more directly basement structural changes, especially when

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there are limited other constraints. Therefore, considering the sediments as transparent

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to magnetic field, we used the magnetic data to delineate both lineaments and basement structures. A wide range of data enhancement techniques based on the

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derivatives of the magnetic field are available to define the boundaries of basement

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structures. Such derivatives can complement each other in enhancing the detectability

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of edges and corners of basement blocks that have distinct magnetic properties or structural variations.

The horizontal derivative is used as an edge detector, while the application of the first vertical derivative reduces the effects of long wavelength anomalies and enhances short wavelength anomalies caused by relatively shallow magnetic sources (Salem and Ali, 2015). In general, the vertical derivative sharpens up anomalies over bodies and allows clearer imaging of the causative structures.

Journal Pre-proof Cascading several derivatives may outline differently specific structural elements, for example focusing on a depth range. The combination of the derivatives can be classified into two categories depending on their shape and amplitude above the magnetic sources (Salem and Ali, 2015). One class of the combined derivatives exhibits a maximum absolute value over magnetization contrasts (i.e., contacts), whereas the other class is from derivatives that change from negative to positive over

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the magnetic contact, and therefore their zero-contour values map the magnetic

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contacts.

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An example of the first class is the total horizontal derivative of the magnetic field,

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which is defined as the square root of the squared derivatives of the magnetic field in x-

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and y-directions. The main advantage of the total horizontal derivative is its usability and stability in the presence of noise (Nabighian et al., 2005). The second class of the

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enhanced derivatives includes the second vertical derivative (Hospers and Rathore,

(Salem et al., 2005).

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1984), the tilt derivative (Miller and Singh, 1994), and the vertical local wavenumber

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Derivatives of the magnetic data can enhance the short wavelength noise in the data, which has similar attributes to magnetic response of shallow sources. Therefore, applying an upward continuation to the data before edge detector derivatives may effectively attenuate the noise while preserving the depth reference for the deeper sources. Alternatively, one may apply a low-pass filter instead. However, such a filter damages the depth information for all magnetic sources. In this study, lineaments analysis was carried out on the derivatives of the RTP magnetic data, along with supporting information from published geologic data. To

Journal Pre-proof reduce the effect of noise and enhance the structural signature within the data, a 500 m upward continuation was applied to the RTP magnetic data. The tilt derivatives map of the upward continued data (Figure 5) indicates the regional signature of the magnetic basement rocks, outlining the contacts between basement blocks by following the zerovalue contour lines. The width of the tilt derivatives anomaly can be related to the depth of the associated basement block: the narrower the tilt derivative anomaly, the

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shallower the basement block.

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The anomaly maxima of the total horizontal derivative of the upward continued RTP

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(Figure 6) clearly outline the separation between basement blocks (lineaments).

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Relative vertical offset of these blocks can be qualitatively inferred by integrating the interpretation of tilt derivatives with the total horizontal derivatives analysis.

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Structural interpretation was derived by combining the coherent lineaments from all

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enhanced derivatives of the magnetic data. This process involved observing the directions of increasing anomaly amplitude and alignment of offsets between

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features in the enhanced derivatives maps. Commonalities and differences

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between the structure patterns were identified to form a coherent interpretation of the lineaments and geology of the area. A simplified interpreted basement block map based on the qualitative interpretation of the magnetic data in Al-Wajid graben is depicted in Figure 7. The lineament analysis of this study suggested that the basement structures are mainly trending in the N-S, SSE-NNW, SE-NW, and SSW-NNE directions. These lineaments indicate the main edges of basement blocks and the structural trends. We suggest that they have played a major role in subsequent tectonic events and graben evolution. Gravity and magnetic data in areas close to Al-Wajid

Journal Pre-proof graben show North-South and northwest-southeast trends (Stewart, 2016), matching the mapped Precambrian faults. The SSW-NNE trend is clearly seen in major offshore oil field structures in Saudi Arabia (Edgell, 1990). The basement rocks in the east and west of the graben are characterized by low magnetic anomalies, which may be produced by granitic rocks. In the western flank of the graben, a high magnetic anomaly exists and it may be created by basement rocks

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similar to those deeper in the center of the graben. Exposed Precambrian units in the

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Arabian Shield, the west of Al-Wajid graben, consists of low to medium-grade volcanic,

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plutonic rocks (Johnson and Stewart, 1995).

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volcaniclastic and epiclastic rocks, and large amounts of mafic, intermediate and felsic

The boundaries along the eastern flank of the graben interpreted from the

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derivatives of the magnetic data match those interpreted from seismic data. In the

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western flank, the interpretation from the two data sets are not identical, and so the orientation of the graben system is more complex. In the south direction, the graben

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appears to have a north-west orientation roughly parallel to the Najd fault trend. The

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orientation of dike swarms observed on the shield in the vicinity of the Najd fault might be used to predict the orientation of graben bounding faults in the absence of seismic data (Dyer and Husseini, 1991). The Arabian Plate underwent a period of extensional tectonics that occurred together with the main period of movement along the Najd fault system of the Arabian Shield (Dyer and Husseini, 1991). The foregoing orogenic processes, which resulted in the growth of the Arabian Plate, are likely responsible for the initiation of movement along the Najd faults zone and its dominant northwestsoutheast orientation (Husseini, 1987).

Journal Pre-proof Dyer and Husseini (1991) suggested Al-Wajid graben to be likely formed concurrently with the major period of NW-SE movement on the Najd fault system. They found it is difficult to reconcile the extensive carbonate and the evaporite depositional environment which existed during the Infracambrian with a collisional model for the Najd fault system trend because continued collisional tectonics would have maintained the Arabian plate as a prominent highland. They were able to identify characteristics of

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extensional tectonics from seismic data, including tilted fault blocks, proximal sediment

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wedges downthrown to major fault blocks, and post-depositional erosions.

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In the northern part of the area, Al-Wajid graben appears to be oriented roughly

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north-south, which does not lie along a Najd fault projection. Beydoun (1991) pointed out that N-S basement structural trends were created due to the consolidation of the

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Afro-Arabian craton in the latest Proterozoic and Early Cambrian. The N-S pattern is

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variably referred to as the “old grain” of the Arabian Peninsula, the “El Nala trend” (Ayres et al. 1982), the “Arabian Trend” (Edgell, 1990), or “Rayn anticlines”

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(Al-Husseini, 2000). Geophysical investigations in many areas in Saudi Arabia revealed

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that the N-S trend is underlain by faulted Precambrian basement blocks comprising horsts and grabens (Edgell, 1990). To account for the extension of the north-south trending, graben faults must have had a dextral component of movement (Dyer and Husseini, 1991).

Magnetic depth determination Several interpretation techniques have been developed as efficient methods for determination of depth to top of magnetic sources from aeromagnetic data. For a

Journal Pre-proof complete review, see Blakely (1995), and Nabighian et al. (2005).

Some of the

techniques are based on statistical models developed by Spector and Grant (1970) and have been used widely in the interpretation of magnetic anomalies. Other techniques assume that the magnetic anomaly is produced by simple models (e.g., contact) then estimate the source location parameters of the model, for example Werner method (Werner, 1953; Ku and Sharp, 1983), Euler method (Thompson, 1982) and Source

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Parameter Imaging (SPI) method (Thurston and Smith, 1997). Here, we investigated the

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depth to basement in the Al-Wajid graben using Werner, Euler, and SPI techniques.

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The final depth solutions were derived by combining the coherent solutions from the

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different methods used. These selected solutions are displayed as black filled-in circles in the lower panel of Figure 8.

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Time interpretation of the available seismic line was converted into depth using a

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velocity model computed from many wells. Note that the estimated depth to basement from magnetic data agrees with the one from seismic interpretation. No depth solutions

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were obtained at the top of the dome structure observed in seismic data (Figure 2).

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The depth solutions obtained from the magnetic data are deeper than the top of the dome structure, and they are supported by a long wavelength anomaly of the horizontal derivative on the analyzed RTP profile (the upper panel of Figure 8). Generally, horizontal derivative’s peaks and troughs are located above the edges of basement blocks. The width of horizontal derivative anomaly is qualitatively related to the depth of the magnetic source: the wider anomaly, the deeper source. The lack of shallower magnetic depth solutions associated with the top of the dome structure however can have different interpretations. We may for example

Journal Pre-proof consider the dome structure is of nonmagnetic source (e.g., salt structure) or it could be a basement high with low magnetic susceptibility. It is important to recall that in the context of this paper, basement is crystalline basement, which comprises volcanic, medium-to-high grade metamorphic, and/or plutonic rocks. Although the estimated top of magnetic basement is coincident with the interpreted top basement from seismic data, in-house drill-hole information suggests

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that magnetic sources are probably below the top of the Precambrian basement over

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large areas in Saudi Arabia, where a considerable thick sequence of low-susceptibility,

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magnetically transparent, metasedimentary rocks are present in the stratigraphic

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column. To further investigate this ambiguity, an additional quantitative analysis has

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been performed using the existing VSP data.

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Look Ahead VSP Data Vertical seismic profile (VSP) data collected in boreholes are currently used

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to calibrate and enhance the resolution of surface seismic data. Its acquisition geometry

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allows recording of downgoing and upgoing wavefields. These wavefields are separated from each other and used within a deterministic deconvolution in order to obtain the reflectivity free of multiples in the interval section that was penetrated by the well. One more important aspect of VSP is the ability to look ahead of the trajectory along the well, below the deepest receiver. As the receivers are closer to the reflectors below the total depth (TD) of the well, a better reflectivity can be obtained which can be used to estimate rock properties. In our case, the existing VSP at the available well was processed to investigate the deep reflectors. The data were processed with a

Journal Pre-proof conventional VSP

workflow including

gain compensation, wavefield separation,

downwave deconvolution and generation of corridor stack. As a reminder, the corridor stack is a single trace obtained from summing portion of data selected near the first arrivals. The evaluation of the VSP data in two-way time is crucial to verify the reliability on the events ahead of the well. Figure 9 displays the two-way time VSP with original frequency, two way time

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VSP after band pass filter 6-30 Hz and its corresponding corridor stack. The two way

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time VSP shows two important groups of reflectors denoted by events A and B and

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highlighted in red arrows, especially in the deep interval below the total depth of the

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well (TD). These events are nearly horizontal in the time section. Other dipping events are also identified below TD. We assumed that any non-horizontal events in the VSP

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two-way time were coming from out of the plane and therefore considered as noise.

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The lateral continuity of the events A and B indicate that they are most probably true reflections detected by most receivers. The corridor stack was carefully chosen to keep

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the events A and B. The tie between the corridor stack and surface seismic data shows

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that the events A and B are corresponding to the interpreted top and base of the dome structure in the surface seismic data (Figure 10). After the successful seismic tie, the VSP data was used to estimate the acoustic impedance beneath the well from the inversion of selected data window (a corridor stack). The corridor stack represents the true reflectivity along the well, directly above and ahead of the TD. Assuming that the VSP reflectivity is free of noise and multiples, both velocity and density can be estimated by combining the acoustic impedance from the VSP inversion and Gardner’s equation (Gardner et al., 1974). Sparse Spike

Journal Pre-proof Inversion was applied to transform the seismic trace from the corridor stack to a broadband acoustic impedance trace. This process requires an estimate of the reflection coefficient and velocity associated with a dominant reflection in the corridor stack. This estimate is extracted from the well log data and used to scale the seismic trace in the reflectivity range. The scaled reflectivity is then converted to acoustic impedance using a method developed by Oldenburg et al. (1983). The inverted density

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of the domed structure is shown in Figure 11. The estimated density for the domed

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structure was approximately 2700 kg/m3. Modeling of gravity profiles across Al-Wajid

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graben (Stewart, 2016) indicates a best fit average density of 2660 kg/m3 for the graben

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fill. The estimated density from VSP data suggests the dome structure is unlikely to be salt. This density value could represent low-porosity siliciclastics, or interbedded

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siliciclastics and volcanics, or igneous rocks. Although carbonate rocks can have similar

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densities, it is unlikely for the carbonate to form dome structure at these deeper depths. Consequently, we presume that the dome structure is likely to be produced by igneous

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rocks with low magnetic contrast.

Tectono-stratigraphic model

The main goal of this study is to understand the tectono-stratigraphy of Al-Wajid graben. To achieve this goal, we need to quantify changes in rock properties at depth and to decipher a reasonable geologic scenario for the graben. Gravity and magnetic data can be helpful in developing a reasonable geologic picture of the subsurface using forward or inverse modeling. Modeling potential fields data is non-unique as several models can produce the same observed response. The ambiguity in the modeling

Journal Pre-proof process can be reduced by using simple models (Roy, 1962) and some constraints such as depth, density, and susceptibility of the subsurface rocks. The interpreted structures (Figure 7) can be assumed to be 2D in nature, which can be approximated by 2D models. Generally, 2D models are adequate to provide a firstorder understanding of the subsurface. We performed constrained 2D inversion to estimate only magnetization and density

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values for the basement rocks. Most elements of the model (as previously described in

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Figure 2) such as depth to the top of sedimentary and basement horizons and densities

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of sedimentary layers are constrained by all available data as well as those obtained by

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qualitative analysis of potential fields data and density derived from VSP data for the domed structure.

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As previously stated, the basement was not reached by any of the available wells

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within the area. To constrain depth to basement, we depended on the corresponding estimates from seismic and magnetic data. The densities of the sedimentary layers

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were taken from average densities values observed in the density logs of the wells. The

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density of the domal structure body was defined as 2700 kg/m3 based on the inversion results of the VSP data.

To constrain the gravity response of Moho surface, we calculated depth to Moho using Airy isostatic model constrained by earthquake seismic receiver functions. The estimate of depth to Moho under Al-Wajid graben is in a good agreement with the published estimates of depth to Moho in Central Arabia based on earthquake seismic refraction data (Rodgers et al. 1999; Al-Damegh et al. 2005). Central Arabia has a thick lithospheric root relative to the rest of Arabia and North Africa, according to

Journal Pre-proof interpretation of Rayleigh wave tomography (McKenzie et al., 2015). We assigned a density of 3300 kg/m3 to the upper mantle. Then, we divided the basement unit into subunits. The edges between these subunits were selected based on the locations of maxima absolute horizontal derivative of the gravity data. We inverted the gravity data to obtain the density of each subunit of the basement rocks. The lower panel of Figure 12 shows the estimated density for the basement rocks in Al-Wajid graben. In the upper

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panel of Figure 12, we represent a reasonable fit between the observed Bouguer

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gravity and the calculated response. No attempt was made to obtain close fit as such

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target would require inserting more basement units in the model. We favored to keep

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the number of subunits in the basement rock at a minimum. The obtained density values within the center of the graben are relatively high with respect to the values in

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the shoulder areas, indicating a dense igneous body was intruded before opening the

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graben. The cooling of this igneous body caused local densification that implied downward bending of the crust as indicated by the relatively deeper Moho depth. Even

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higher density values for the basement block at the base of the graben would be

was salt.

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required to compensate for the gravity anomaly if we assume that the domed structure

To estimate the magnetization of the basement rocks, we assumed that the measured magnetic response is produced by the basement rocks only, with no contribution from the sedimentary layers. Similar to the inversion practice of the gravity data, we divided laterally the basement units into subunits. The edges between adjacent subunits were selected at the locations of the maxima of the absolute horizontal derivative of the magnetic data. The lower panel of Figure 13 illustrates the estimated

Journal Pre-proof magnetization for the basement rocks in Al-Wajid graben along the analyzed profile AB. The fit between the observed RTP magnetic data and the calculated response is displayed in the upper panel of Figure 13. The estimated magnetization in the center of the graben is higher than on the flanks, which provides further support to the scenario of mafic composition for the base of the graben.

Figure 14 shows the simplified

tectono-stratigraphic model of Al-Wajid graben based on the integrated interpretation of

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surface seismic, potential fields and well data and previous geologic studies. Our

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model suggests that the area was intruded with a dense igneous body before opening

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the graben. Later, the igneous body cooled and caused local densification which

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initiated downward bending of the crust. Then the area underwent local higher subsidence rate due to the densified crust, thus creating the basin as a cratonic graben.

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The graben was subjected to continuous subsidence that was interrupted by several

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Precambrian to present.

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tectonic phases of extension and compression, ending with positive inversion since

Conclusions

The main objective of this work is to understand the tectono-stratigraphy of AlWajid graben by integrating all available geophysical data including gravity, magnetic, seismic, and VSP data. The derivatives of magnetic anomalies were very useful in mapping regional variations of the magnetic properties of the basement. Based on the interpretation of the lineaments delineated on magnetic data, four magnetic terranes (rock units) were outlined. The terrane associated with Al-Wajid graben is characterized by high amplitude magnetic anomalies produced by mafic rocks. The lineaments

Journal Pre-proof obtained from the analysis of the magnetic data suggest that the basement structures are mainly trending in the NS, NNW, NW, and NNE directions. The processing of the existing VSP data in the available well was useful in estimating some physical parameters of the dome structure such as acoustic impedance, velocity, and density. The estimated density for the domed structure from the VSP data was 2700 kg/m3, which suggests that the graben is unlikely to be salt basin. This density value can be

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associated with carbonates or igneous rock. It is unlikely for carbonates to build a dome

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structure at deeper depths. As a result it is most likely that the domal structure is formed

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by low magnetized igneous rocks, possibly metamorphic rocks associated with deeper

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igneous mafic body. From the integrated interpretation of seismic, VSP, gravity, and magnetic data, we can conclude that Al-Wajid graben is intruded by a dense igneous

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body before opening the graben. Local densification and continuous subsidence

creating a cratonic graben.

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There is no conflict of interest

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associated with the cooling of the igneous body caused downward bending of the crust,

Acknowledgments

The authors are grateful to Saudi Aramco for permission to publish this work. The authors thank Simon Stewart and Nizare El Yadari for their constructive comments. We are grateful for comments by Dhananjay Ravat and two anonymous reviewers that helped to improve the manuscript, as did extensive suggestions by associate editor, Afif Saad.

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Journal Pre-proof Figure 1: Tectonic setting of the Arabian plate (modified from Stern and Johnson, 2010)

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Figure 2: Seismic interpretation section cutting across Al-Wajid graben Figure 3: Bouguer gravity map of Al-Wajid graben Figure 4: Reduced to the pole (RTP) magnetic map of Al-Wajid graben Figure 5: Tilt derivative of the RTP magnetic data over Al-Wajid graben Figure 6: Total horizontal derivatives of the RTP magnetic data over Al-Wajid graben Figure 7: Interpreted basement lineaments from analysis of the RTP magnetic data. Colors indicate different basement terranes. Figure 8: Total horizontal derivatives the RTP magnetic data of Al-Wajid graben (upper panel). Depth to basement from seismic and magnetic data (lower panel) Figure 9: VSP data recorded at the well in Al-Wajid graben Figure 10: Comparison between surface seismic and the corridor stack of VSP data

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Figure 11: VSP inversion

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Figure 12: Density variations within the basement rocks in Al-Wajid graben

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Figure 13: Magnetization variations within the basement rocks in Al-Wajid graben

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Figure 14: Simplified Tectonostratigraphic model of Al-Wajid graben

Journal Pre-proof Highlights In this paper we sought to understand the tectono-stratigraphy of Al-Wajid graben by analysis of various remote sensing geophysical data including vertical seismic profile (VSP), gravity, magnetic and seismic reflection data. Determination of depth to basement from the magnetic data suggests that the dome structure is non-magnetic since the depth solutions were obtained at

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the bottom of the structure. Processing of the VSP data was useful in estimating

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some physical parameters for the structure in question such as acoustic

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impedance, velocity, and density. The estimated density is 2700 kg/m3, which suggests that the structure is unlikely to be salt and most likely to be a dense

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mafic or metamorphic of non-magnetic rocks. This interpretation is consistent

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with extension and subsidence of the graben.

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The integrated interpretation of seismic, VSP, gravity, and magnetic data proposes that Al-Wajid graben is a deep trough intruded by dense magmatic

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rocks before opening the graben. The cooling of these igneous rocks has caused

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local densification and continuous subsidence that implied downward bending of the crust, creating the basin as a cratonic graben. Since Precambrian, the graben has been affected by several tectonic phases of extension and compression, ending with positive inversion.

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