1 Introduction

The Arabian shield and the Red Sea (Fig. 1) are areas with a huge potential in natural resources. One of the main targets in mineral and petroleum exploration is to characterize the structure of the sedimentary cover and the topography of the crystalline Precambrian basement [2]. During the last decade, different transect from the Arabian shield to the Red Sea have been subject of numerous geophysical surveys to unravel the crustal and lithospheric structure of the region [3,4,5,6]. All these studies show a variable resolution depending on the particular area subject to analysis and the method used. In our case, gravity modelling was combined with additional techniques and data sets in order to build the regional gravity and magnetic anomaly maps based on wavelength filtering and derivative techniques. The aim of this work is to provide a first-order estimate of the crustal and lithospheric mantle geometries of the transect Arabian Shield-Red Sea and bring new insights on the large-scale geodynamics of this segment.

Fig. 1
figure 1

Location of the study area with different geological domains

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2 Materials and Methods

In order to model the lithospheric structure of the studied area, two methods were referred to namely gravity and aeromagnetic methods.

  • Gravity method: The gravity data used in this work are from International Gravimetric Bureau (BGI), collected by United States geological survey (USGS) at a station density of 1 per 100 km2. The gravity survey was carried out in 1976, covering approximately 202,100 km2 and including a total of 2477 gravity stations. Free air and Bouguer gravity corrections were performed using sea level as a datum and 2.67 g/cm3 as a reduction density.

  • Aeromagnetic method: We used the aeromagnetic data from the International Gravimetric Bureau (BGI). Several aeromagnetic surveys were carried out from 1962 to 1983 by commercial companies under the auspices of the Ministry of Petroleum and Mineral Resources of the Kingdom of Saudi Arabia and supervised by BRGM and USGS. The surveys were conducted over individual blocks. They had round clearances of 150, 300, or 500 m and a line spacing of about 800 m [7].

3 Results

3.1 Gravity Results

We were able to identify the Bouguer anomaly (Fig. 2) with several anomalies in the transect indicating two major directions: NorthWest–SouthEast and North–South.

Fig. 2
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3D representation shows Bouguer anomaly superimposed over topographic model of the study area

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2.5D regional gravity model (Fig. 4) using a 3D forward modeling algorithm that calculates the gravitational attraction due to an ensemble of right, rectangular prisms.

Crust, lithospheric mantle and asthenosphere densities were estimated from average seismic velocities and previous studies [1, 5, 8] in order to convert them to densities using the Nafe–Drake relationship. Sediment thickness was extracted from a global sediment model. The model was broken up into three layers: Crustal structure (2.85 g/cc in the interior of the shield), lithospheric mantle (3.2 g/cc) and asthenospheric structure (3.12 g/cc) (Fig. 4). We chose 2.67 g/cc as a reference density. Gravity modeling indicates a high density heterogeneities within Asir province that represents the transition zone represented by a typical mafic crust below the coastal plain [9] and provides insight on mass distribution, lithospheric configuration and their significant changes. The model shows a mean global thickness of 45 km beneath the Shield areas that decreases to 12 km beneath the Red Sea.

We have managed to distinguish the delimitation in our study area (Red Sea, Asir tectonic province and Najd tectonic province) from the upper crustal structure (Figs. 3 and 4).

Fig. 3
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Results of the gravity modeling that provides some basic insight into the mass distribution and crustal configuration

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Fig. 4
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NorthEast–SouthWest cross-section of the 2.5D regional gravity model. The cross-section consists of three layers: asthenospheric structure, lithospheric mantle and crustal structure

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3.2 Aeromagnetic Results

A general examination of Reduction to the pole (RTP) anomaly (Fig. 5) shows the solid white lines coinciding with the High aeromagnetic axis, dashed white lines coinciding with the Low aeromagnetic axis, AP and AN are labels for the solid and dashed white lines, respectively. Contour interval is 10 nT, AP and AN are labels for the solid and dashed white lines respectively.

Fig. 5
figure 5

RTP anomaly map of the study area

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In comparison with the Bouguer gravity anomaly (Fig. 2) that reflects the northward shift in the positions of the inherited magnetic anomalies due to the elimination of the inclination and the declination effect of the magnetic field. Also, the sizes of the anomalies become larger and centered over their respective causative bodies.

4 Discussion

Based on gravity and magnetic analysis, there is evidence of several anomalies in two major directions: North West–South East and North–South. We interpret them as changes in bulk composition of rocks.

There are big differences in lithospheric buoyancy, thinner and denser beneath the Red Sea; thicker and more buoyant beneath the Arabian Shield. Our gravity model confirms the results of previous geophysical studies [1, 3, 5, 9, 10]. The Moho and Lithosphere, Asthenosphere Boundary (LAB) are shallowest near the Red Sea and become deeper towards the Arabian interior. Near the coast, the Moho is at a depth of about 12.5–15 km. Crustal thickening continues until an average Moho depth of about 40–45 km is reached beneath the interior Arabian Shield. The LAB near the coast is at a depth of about 35 km; however, it also deepens beneath the Shield to attain a maximum depth of 145 km. Signatures of rifting and crustal thinning are evident here: The crustal and lithospheric configuration derived from the gravity modeling for the entire transect supports the consensus view on Red Sea spreading for the region.

Our observations of lithospheric structure support a two-stage rifting history along the Red Sea and suggest that the Moho and LAB topography are the results of extension and erosion caused by asthenospheric flow where the thinnest lithosphere is coincident with the rift axis. Previous studies [1] speculated the existence of such topography and suggested that it may direct asthenospheric flow beneath the Arabian Shield and the Red Sea Rift.

5 Conclusions

The transect Arabian Shield-Red Sea represents an excellent example of passive margin with little relative complexity of the Bouguer and RTP anomaly controlled by structural complexity and variable lithology of the study area.

Finally, it is thought that further models (velocity model, aeromagnetic model) are required in order to give more insights on crustal configuration of the Red Sea margin.