The present study is based on qualitative and quantitative analysis of the gravity data to delineate both deep and shallow basement structures in the study area.

The gravity data used for this study were obtained from the Office National de Mine (ONM). All the data were merged and reduced using the 1967 International Gravity formula. Free Air and Bouguer gravity corrections were made using sea level as a datum and 2.34 g/cm³ as a reduction density. The gravity anomaly data were gridded at 1km spacing and contoured to produce a Bouguer gravity anomaly map.

For gravity field, it is commonly used to upward continue the data in order to determine regional anomaly patterns. To determine the regional gravity anomaly, upward continued the Bouguer gravity field. The resulting Residual gravity map (Bouguer gravity field minus Regional gravity field) shows some well defined structures. Detailed inspection allows us to identify, in the study region, some new regional-scale features which represent significant basement discontinuities that have controlled the tectonic setting and the subsidence history in the region.

The complete Bouguer gravity anomaly (Figure 6) shows a regional variation in the Northeastern Tunisia ranged from −4 mGal (negatives anomalies) to 36 mGal (positive anomalies). These gravity anomalies are related to crustal thickness [29,30], thick piles of low-density sedimentary rocks [20,31], and lithospheric thickness or densities [32].

The Bouguer anomaly map (Figure 6) shows five gravity lows situated over depression zones with recent Cainozoic-Quaternary series and five gravity highs, associated with Mesozoic series uplifts. We can distinguish:

1) Low-gravity anomalies (negative anomalies) 1 to 5: associated with the low density sediments (clay, thin sand beds and gypsum) are showed in Jebel Messeftine (1), extreme south-western part of study area (2), southeast part of Jebel 2) High-gravity anomalies (positive anomalies): 6 to 10 associated with high density sediments (dolostones and limestones) are showed in Bechateur region (6), Jebel Ichkeul and Mateur region (7), west part of Jebel Nadhour (8), Jebel Ammar (9) and west part of Jebel Lansarine (10).

Baouala (3), south part of Jebel Ammar (4) and south part of Utique region (5).

The horizontal derivative of Bouguer gravity field maps at several directions: N0 (a), N90 (b), N45 (c), N135 (d) show several lineaments (Figure 7).

The Horizontal Gradient is a simple approach to locate

linear structures, such as contacts and faults from potential field data. The Horizontal Gradient Magnitude (MGH) is calculated using north-south and east-west directional derivations. The Horizontal Gradient Magnitude (MGH) method has been used intensively to locate contacts of density contrast from gravity data [33] and the squared magnitude of the horizontal gravity gradient HG (x,y) is given by Equation (1) :

The partial derivatives of gravity with respect of the x and y directions were obtained in the wave number domain following described [34,35] stated that the horizontal gradient of gravity anomaly because it trends to place narrow ridges over edges of abrupt changes in density [36,37]. The procedure HMG, which is illustrated the location of the maxima of the horizontal derivative defined on a regular grid by comparing the value at a centre point of a 3 × 3 grid window to the surrounding eight points along four main directions: horizontal, vertical, and both diagonals.

Peaks in the horizontal gradient magnitude of gravity field are used to locate the vertical contacts (Figures 8(a) and (b)) and estimate their strike directions (Figure 8(c)).

The horizontal gradient magnitude is also robust to delineation either shallow comparison with the vertical gradient, which is useful only for the shallower structures.

Figure 9 represents the superimposition of Horizontal gravity gradient (HGG) and this map shows alignments outlining the contacts. The several faults determined by geophysical analysis are interpreted as lineaments oriented E-W, N-S, NE-SW and NW-SE.

The Bouguer anomaly map is really constituted by two regional and residual fields. The regional term remains rather subjective. For our study, one has no information on the shape and the depth of the substratum. The selection of the degree of the polynome of regional anomaly

map is based on the upward continuation (Figure 10).

Figure10 shows the Bouguer anomaly and its upward continuation at several heights, from 500 m (Figure 10(a)) to 20,000 m (Figure 10(h)). The upward continuation of Bouguer gravity field at 20,000 m map (Figure 10(h)) represents the regional gravity field of the study area because the gravity effect of the most prominent structure related to the Jebel Ammar anticline disappear totally.

Figure 11 shows the distribution from the horizontal derivative of the Bouguer anomaly and its upward continuation at several heights, up to 500 m to 20,000 m. These maxima represent the major deep contacts in the study area. In the case of a dipping contact, the maxima move down dip with increasing continuation height.

The Euler method has been applied to the residual data using a moving window of 20 m × 20 m. We have assigned several structural index values and found that structural index of 0.25 gives good clustering solutions.

3D form of Euler’s equation can be defined [38] as Equation (2)

where, and are the derivatives of the field in the x, y and z directions,

is the structural index value that needs to be chosen according to a prior knowledge of the source geometry. By considering four or more neighboring observations at a time, source location (x₀, y₀, and z₀) and b can be computed by solving a linear system generated from Equation (2). Then by moving the operated window from one location to the next over the anomaly, multiple solutions for the same source are obtained.

Figure 12 shows the results of the Euler deconvolution method from the gravity data. The Euler solutions (Figure 12) indicate that the E-W, N-S, NW-SE and NESW trends characterize the shallower structure setting. The Euler solutions indicate that:

•  33% of contacts where are found to be about 0 to 1000 m.

•  45.4% of contacts where are found to be about 1000 to 2000 m.

•  19.1% of contacts where are found to be about 2000 to 3000 m.

•  2% of contacts where are found to be about 3000 to 4000 m.

•  0.5% of contacts where are found to be about 4000 to 500 m.

These results confirm that the regional-scale faults characterized the shallower structure setting of the Northeastern of Tunisia, probably affected the basement. The proposed model as a mosaic of quadratic blocks

bounded by probably deep basement E-W and NW-SE faults characterized the deep structural scheme of the study area.

In the study area, the Mesozoic and Cenozoic deposits is based on seismic interpretation of 2D seismic reflection lines (L1 and L2 (Figure 2)) are obtained by ETAP company. The migrated seismic reflection lines were analyzed to determine correlations between stratigraphic horizons correlated from the boreholes W1 to W6 and seismic reflection marks.

The NW-SE Residual gravity profile L1 (Figure 13(a)), Horizontal gravity gradient (HGG) profile L1 (Figure 13(b)) and interpretation seismic profile L1 (Figures 13(c) and (d)) showing some results:

-The major deep faults at several direction affected Mesozoic and Cenozoic deposits;

-Important thickness of Miocene and Pliocene deposits;

-Lateral facieses change; the thickness of the Pliocene deposit varies on both sides of the basin in the part north of Utique region and the thickness of the Miocene deposits reduced towards the El Mabtouha Lowlands.

The NE-SW Residual gravity profile L2 (Figure 14(a)), Horizontal gravity gradient (HGG) profile L2 (Figure 14(b)) and interpreted seismic profile L2 (Figures 14(c) and (d)) showing:

•  The presence of depressions (syncline) in Utique region and Southwestern part of Mateur region;

•  The uplifts zones in Jebel Kechabta and southeastern part of Mateur region;

•  The uplifts and depression structures are controlled by major deep faults at several directions;

•  Thick Miocene unit deposits;

•  Thin Miocene deposits towards the Utique region and southwestern part of Mateur region.

The interpretation of two seismic profile L1 and L2 reveals that the uplifts are limited by listric deep normal faults and reverse fault at several direction (Figures 13 and 14) and Thickness variation of the and lateral facieses change of Mesozoic and Cenozoic deposit. These listric faults delimited the study region in different blocks. The Southeastern part of Mateur region block is marked by eventail configuration (Figure 14) attesting for tilted blocks geometry as a consequence of normal component movement of limited listric faults during Miocene.