Structural analysis and basement topography of Gabal Shilman area, South Eastern Desert of Egypt, using aeromagnetic data

1 Introduction

Magnetic method considers one of the common beneficial accessible tools that assist in the identification of the surface and subsurface geology. The goal of the utilization of the aeromagnetic analysis is to help in explaining the problems of provincial geologic mapping and structure (Eldosouky, 2019; Sehsah et al., 2019; Eldosouky and Mohamed, 2021; Melouah and Pham, 2021), representation of buried contacts, position of the presumable fields of rock differentiation, mineralization (Eldosouky et al., 2017, 2020a, 2021; Ekwok et al., 2019) and density of sedimentary cover. The magnetic enhancement has an interest that it is a quick and adequate technique for investigating the subsurface geologic structure, geothermal potentials, crustal studies, and outlining the basement fractures (Ibraheem et al., 2018; Eldosouky et al., 2020b; Sehsah and Eldosouky, 2020; Pham et al., 2020, 2021; Melouah et al., 2021a,b).

Interpretation of magnetic data can be used to confirm the association between tectonics of and the structures within the sediments that overlying the basement. The linear features enhancement in potential field data is greatly helpful as it enables dykes, faults, and other features to be presented more obvious, thus assisting the geologic understanding process (Cooper, 2003).

In the present work, we deal with accurate mapping of geological structures and depth to basement rocks for more understanding of the structural framework of the study area. To achieve these objectives, we will apply 1st VD, AS, HGM, and SPI to RTP aeromagnetic data of G. Shilman area, SED, Egypt for well understanding of structural regimes and basement depth of the study area.

2 Geology of the study area

G. Shilman area situated to the southeast direction of Aswan in the south eastern desert (SED) of Egypt between latitudes of 22° 35′ 00″ and 22° 50′ 00″ N and longitudes of 33° 37′ 50″ and 33° 52′ 50″ E (Fig. 1). The Northern Sudan, Egyptian Eastern Desert (EED), and western part of Saudi Arabia have been collectively named the Arabian–Nubian Shield (ANS), which is distinguished by four principal rock associations: (i) an arc association; (ii) an ophiolite association; (iii) a gneiss association; and (iv) intrusions of granite (Abdel Rahman, 1995).

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Fig. 1

Location and geology of G. Shilman area.

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The foremost rock types in G. Shilman area are represented by ophiolites, syn-orogenic granodiorite, metasediments including marble, and metavolcanics (Fig. 1). The northwestern extension of the Allaqi-Heiani belt is defined by the ophiolitic rocks (Kröner et al., 1987). They are intruded by granodiorite and overlain by the arc metavolcanic–metasediment sequence. The ophiolites include completely talc-carbonates, serpentinized peridotite, amphibolites and metagabbros (Abd El-Naby et al., 2000; Abdeen and Abdelghaffar, 2011). The most considerable serpentinite block occurs at G. Shilman producing a thrust nappe, which is also connected with a large metagabbro source. Older granites appear in the eastern part of G. Shilman area while younger granites occupy small portion at the northwestern part. Metavolcanic rocks are represented in the central part trending in the NW-SE direction. Metasediments cover the most northwestern part of the study area with small traces to the east of metavolcanics (Abd El-Naby et al., 2000; Abdeen and Abdelghaffar, 2011) as shown in Fig. 1.

Abd El-Naby et al. (2000) studied G. Shilman area and its surroundings and concluded that the study area revealed the remnants of back-arc oceanic crust, which causes a metamorphic sole underneath an allochthonous ultramafic source, which was overthrusted as a hot body.

3 Data and methodology

3.4

3.4 Source parameter imaging (SPI) technique

Thurston and Smith (1997) developed the SPI method (SPI technique is called local wavenumber) because all the parameters that make up the body which include susceptibility contrast, dip, and depth are computed from the complex AS. Fairhead et al. (2004) linked the source depth to the SPI (k) of the magnetic field which can be obtained from the computed HGM and VD of the RTP grid. The SPI method acts well at all magnetic latitudes which gives it the advantages for G. Shilman area. The SPI is given by:

(4)

$$\mathrm{k}=\frac{\frac{{\partial }^2\mathrm{M}}{\partial \mathrm{x}\partial \mathrm{z}}\frac{\partial \mathrm{M}}{\partial \mathrm{x}}-\frac{{\partial }^2\mathrm{M}}{\partial {\mathrm{x}}^2}\frac{\partial \mathrm{M}}{\partial \mathrm{z}}}{{\left.\left(\frac{\partial \mathrm{M}}{\partial \mathrm{x}}\right.\right)}^2+{\left.\left(\frac{\partial \mathrm{M}}{\partial \mathrm{z}}\right.\right)}^2}$$

For the dipping contact, the K maxima are positioned directly overhead the contact edges and are independent of the remanent magnetization, strike, dip, declination and inclination of the magnetic data. The depth is determined at the edge of the source from the reciprocal of the SPI. $$\mathrm{D}\mathrm{e}\mathrm{p}\mathrm{t}\mathrm{h}(\mathrm{x}=0)=\frac{1}{{\text{K}}_{\text{max}}}$$

The methodology of our study is illustrated in Fig. 2.

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Fig. 2

Flowchart of the applied methodology.

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4 Results

In our existing work, aeromagnetic data are employed to analyze the structural trends and basement topography of G. Shilman area. To perform this target, the reduced to the pole (RTP) map (Aero-Service, 1984) was enhanced (Fig. 3). The RTP map (Fig. 3) shows magnetic highs and lows. There are many positive NW-trending magnetic anomalies in the central and western parts and NE-trending positive anomalies in the north-eastern part of the study area. The southern part of the area is characterized by negative anomalies trending in the NW and N-S directions.

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Fig. 3

RTP map of G. Shilman area.

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RTP map of the study area (Fig. 3) shows magnetic intensities (positive and negative anomalies) vary from −260 nT (Lows) to 250 (Highs) nT. It is clear to notice that the high magnetic values are related to ophiolitic serpentine and granites while low magnetic values are associated with metavolcanic and metasediment rocks.

The first step of the structural analysis is to identify the high- and low-frequency elements of the data that are related to surface/subsurface geology. To enhance local features for outlining the edges of anomalous sources from the data, 1st VD map (Fig. 4a) was computed from the RTP grid. The 1st VD map shows that most of the local anomalies were in the N-S, NNE and NW directions.

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Fig. 4

a) 1st vertical derivative; b) Analytic signal; and c) Horizontal gradient magnitude maps of G. Shilman area.

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The AS map (Fig. 4b) emphasize the differences in the magnetization of the magnetic bodies in the study area and delineate discontinuities and anomalies texture. On the other hand, the HGM filter is applied to the RTP data to detect the contact locations of the bodies at depths as shown in Fig. 4c.

To distinguish the tectonic trends affecting G. Shilman area, the dominant magnetic structures on the 1st VD, AS and HGM maps (Fig. 4a, 4b and 4c) were traced and presented in the structural maps in figures (5a, 5b and 5c respectively) and statistically interpreted taking into record the azimuth and number of the structural trends in each 10° and displayed in the form of rose diagrams (Fig. 6a, 6b and 6c) respectively.

5 Discussion

Lineaments that describe fracture/fault zones of G. Shilman area (Fig. 5a and 6a) from 1st VD revealed that the N-S, NW and NNE directions are the main structural trends controlling the shallow and local sources while the NW, NE, N-S, and ENE directions are the dominant ones revealed from the AS maps (Fig. 5b and 6b) and HGM maps (Fig. 5c and 6c). This indicates that the structural regime affecting the study area causing the N-S direction is the most recent while the NW direction the predominant structural tectonic trend at G. Shilman area (Eldosouky and Elkhateeb, 2018).

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Fig. 5

The structural lineament maps of the G. Shilman area derived from: a)1 st VD map (Fig. 3a); b) AS map (Fig. 3b); and c) HGM map (Fig. 3c).

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According to Zoheir and Klemm (2007), Zoher et al. (2017) and Elkhateeb et al., 2021, the deformation history of the central part of the Allaqi-Heiani suture indicated that the is an intense crustal shortening took place throughout the collision of east and west Gondwana (640–550 Ma). As a result, the Allaqi-Heiani suture was deformed by the post-accretionary N–S trending Hamisana Zone and related NW–SE sinistral and NE–SW dextral transpressional faults, during the Late Pan-African Najd orogen (e.g., Abdeen and Abdelghaffar, 2011).

The SPI depth map of G. Shilman area (Fig. 7) derived from the RTP data (Fig. 3) shows depth ranges from 0.1 km to about 1.5 km. The light to deep blue colors with depth ranges from 0.1 to 0.271 km show regions of shallow lying magnetic bodies. Red to pink colors with depth ranges from 0.6 to 1.4 km show areas of thicker sediments or deep-lying magnetic sources. SPI map (Fig. 7) maps basement topography well in G. Shelman area.

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Fig. 6

a) Lineament rose diagram of .1st VD map; b) Lineament rose diagram of .AS map; and c) Lineament rose diagram of .HGM map.

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Fig. 7

Source parameter imaging depth map of G. Shilman area.

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6 Conclusion

The existing work deals with the analysis and interpretation of structural trends and mapping the basement topography of G. Shilman area, SED of Egypt, using aeromagnetic data. A variety of enhancement techniques were applied to RTP data of G. Shilman area. 1st vd, AS and HGM were applied to analyze the structural trends while SPI technique was applied to map basement topography.

1st VD map of the study area outlined the structural features related to local sources (short-wavelength components) and illustrated that the N-S, NW and NNE directions were the most tectonic trends controlling the study area. Moreover, the 1st VD mapped well the edges of shallow sources. AS and HGM maps delineated the tectonic structural trends that controlling G. Shilman area. Structural and statistical analysis of these maps represented that the NW, NE, N-S and NNE directions were the most structural trends and the NW direction was predominant. Furthermore, HGM delineated edges of the magnetic sources more obviously than the edges which were represented by AS. The SPI depth map of the study area showed depths varied from 0.1 to about 1.5 km. The applied enhancement techniques in the present work can be employed in alike rugged areas in the Egyptian Eastern Desert and other similar parts of the world for the structural analysis and depth estimation purposes. The present study shows the role of magnetic data in accurate detection of structures and depth to the basement that can be applied for mapping the structures in different complicated areas around the world.