Structural style of the southern part of the N-S axis in the Central Tunisian Atlassic foreland: tectonic implications

Field work

Geological mapping and lithostratigraphy nomenclature in this work are based on well data and published literature (Burollet, 1956; Khessibi, 1978; Boukadi, 1985; M’rabet, 1987; Mahjoub & M’hadhbi, 2005; Ouali, 2007; Dhahri & Boukadi, 2017). Unfortunately, the outcrop thicknesses are approximate due to the presence of several discontinuities in this area, as well as facies variations between the structures of the Central Atlas and the N-S axis. The investigated wells, W1 and W2, are located in the Central Atlas (Maknassy plain) and Eastern Platform (Mezzouna plain), respectively (Fig. 1C). An east-west correlation between these two wells was conducted to observe the stratigraphic variations across the N-S axis. To perform geological mapping, three geological sections were examined (see fig 1C for locations).

In the Meknassy-Mezzouna region the mountainous ranges provide plenty of exposures promising detailed structural analysis, geological mapping and geologic cross sections. Nevertheless, the plains between these mountains are covered by Quaternary deposits hampering direct observation. The use of geophysical methods is thus mandatory to bridge the gap of data in the area where observations and analysis at the outcrop are not allowed. In this study, two available regional seismic sections cross-cutting the N-S axis are used together with a good coverage of gravity data with the aim to decipher the structural setting of the study area.

Seismic data

To analyze the subsurface structure and the salt features in the study area, we used 2D seismic reflection profiles, crossing perpendicularly the studied structures (Fig. 1C). The quality of the seismic data is generally medium to poor in the study area, especially at depth. This depends on the parameters of acquisition and processing of seismic data, as well as the structural pattern of the region and halokinesis. We had also noted some problems of vertical offset between some lines. This shift, generally low to medium, could be attributed to the use of different processing parameters, including static correlations. Seismic sections were calibrated with well data and combined within geological maps and outcrops. The interpretation of these sections allowed the establishment of geoseismic cross sections with the delineation of the main brittle and halokinetic structures within the Mesozoic-Cainozoic cover.

Gravity data

The gravity data used has been acquired from previous works (Sandwell et al., 2009, 2013, 2014), where raw data were extracted from satellite and ground surveys. Bouguer Anomaly map was developed by Kriging interpolation, which is a linear interpolation method that guarantees a minimum variance. In this study, data step is approximately 2000 m and the gridding spacing was accordingly set to 500 m. To take better advantage of the gravity data, advanced interpretation for structural analysis was applied to the gravity field following a flow chart procedure including three successive fundamental steps: (1) regional-residual separation (2) horizontal derivative for standing out the major structural lineaments, and (3) Euler deconvolution for extracting the structural lineament parameters. For a better regional-residual separation, we used three techniques to remove the long wavelength amplitudes and emphasise the short/medium wavelength features, which are the polynomial regression, the upward continuation and the Gaussian filter. The quantitative interpretation of gravity data will be based mainly on the Spectral Analysis and the Euler Deconvolution solution. To facilitate a comprehensive interpretation of the gravity data, we opted to expand the boundaries of the study area. This broader scope allows for a comparative analysis of the local structures with neighboring geological features, particularly those located further north. Furthermore, it minimises the need for interpolation, a technique that can introduce distortions into the interpretation and final results.

Stratigraphy

The Late Triassic evaporitic series are the oldest lithostratigraphic outcrops in the Meknassy-Mezzouna region (Fig. 1C and 2A). They crop out on the eastern side of jebel Zebbeus along a ~12 km long N-S ridge extending from jebel Gouleb to the north, to the Meknassy plain to the south with 150 m average mappable width. They are also widely exposed to the west of the Zebbeus structure where their western limit is mainly sealed by Quaternary materials. Other extensive Upper Triassic outcrops can be seen along the NW-SE corridor of Meknassy-Mezzouna, especially in Jebel Dribika El Hamra, Jebel Nadour, Fej Ezzmoul, jebel Kohl and Jebel Mezzouna anticline. All these outcrops are associated to the fault segments of the N-S axis. In contrast, Triassic rocks do not crop out within other structures such as those of Bouhedma-Boudouaou and Melloussi even though Jurassic rocks form the core of the Bouhedma anticline (Bahrouni et al., 2016). Farther northwestward, the Upper Jurassic-Lower Cretaceous, represented by the Sidi Khalif formation, occupies the core of Jebel Melloussi. This Formation (Fig. 2A) is characterised by alternating sequences of clays and marly limestones (M’rabet, 1987). Notably, their uppermost 120 metres consist primarily of green clays with occasional bioclastic and often shaly limestone interbeds, and it ends with a reddish dolomitic bar. Wells W1 and W2 provide geological evidence for the presence of a complete Jurassic succession (Lower, Middle, and Upper) at depth within the Central Atlas and Eastern Platform (Fig. 2B). The Lower Cretaceous stratigraphic units are the prominent outcrops within the Bouhedma and Melloussi structures.

Fig. 2 Stratigraphic correlation logs of two wells W 1 and W 2, spaced 40 kilometers apart, showing a central Triassic salt diapirism intrusion.

The lithostratigraphic correlation between wells W1 and W2 confirms a continuous deposition of Lower Cretaceous sequences throughout the Atlassic domain (Central and Eastern Platform) at depth (Fig. 2B). However, these sequences tend to thin eastward. Their absence in surface outcrops within the southern part of the N-S axis indicates that they are still buried at a relatively considerable depth.

The Late Cretaceous series, mainly comprising the Zebbag, Aleg, and Abiod Formations, are widely exposed across the study area (Figs. 1C and 2A). The Zebbag Formation is well developed in Khanguet Zebbag locality in Jebel Melloussi and so in the Jebel Gouleb part of the N-S axis. While absent in areas like Dribika El Hamra, Nadhour, and Jebel Fej Ezzmoul, it crops out southward in the cores of several structures, such Jebel Mezzouna and Bou Douaou. It also forms the outer flanks of Jebel Bouhedma. Subsurface data reveal a stratigraphic variation: Well W1 intersects the Lower and middle Zebbag layers near the surface, while Well W2 crosses the full sequence where the top of the formation rests at-480 m depth.

The Aleg Formation, Turonian–Campanian in age (Burrolet, 1956), crops out within all major structures in the study area, including both flanks of Zebbeus and Jebel Kef Ennsour synclines, and to the west of Jebel Mezzouna, and Bou Douaou. It comprises thick clay sequences with interbedded limestone occupying also wide lowlands of the region’s plains where they are partially sealed by Quaternary deposits. The Abiod Formation, lower Campanian to lower Maastrichtian in age, is well exposed in Oued el Abiod (its type locality), as well as at Jebel Zebbeus, Fej Ezzmoul, and Jebel Mezzouna. It comprises three members: a basal member made of chalky limestones and green marls, a middle member made of green marls, and an upper member made of white limestone and dolomite (Khessibi, 1978). The Abiod formation records frequent facies variations in the study area such as intraformational conglomerates and slump structures reflecting the floor instability along with the N–S Axis during the Late Cretaceous. The Haria Formation, late Maastrichtian–Paleocene in age (Compte & Dufaure, 1973), is a clayey unit that exhibits local thickness variation in the study area. This formation is locally eroded in W1 and W2 wells. It crops out to the east of Zebbeus structure (Fig. 1C), and it is expected to be sealed by recent deposits in the synclinal plains.

Paleogene series are extensively exposed in the study area. They crop out along the N-S axis corridor where they are folded as perched synclines (e.g, Kef Ennsour and Zebbeus) and monoclines (Jebel Abdallah) and they occupy also wide areas in Regueb plain eastward (Fig 1C). The lower Eocene sequence in the study area includes a clayey bottom unit followed by a rich phosphatic deposition topped by dolomitic and gypsum strata all considered lateral equivalent of Chouabine and Kef Eddour formations of Gafsa Basin farther westward (Burrolet, 1956; Fournié, 1978). This sequence exhibits significant variations in facies and thickness across the study area, particularly within Jebel Zebbeus. This is due to some synsedimentary normal fault affecting the southwestern part of the structure and to the rise of Triassic evaporite at its eastern edge. The Late Eocene sequence is represented by the Jebs Formation, made by thick gypsum units intercalated with dolomitic and clayey intervals. This formation crops out widely across the region, with its type locality at Jebel Zebbeus, where it forms the core of the perched syncline.

The Oligocene-Lower Miocene successions crop out especially within Dribika El Hamra, Kef Ennsour and Mezzouna structures and generally in the eastern side of the N-S axis (Fig. 1C) Data from W2 well indicates that the Oligocene rocks consist of fine-grained sands interbedded with clays, evaporites, and minor carbonates. The Miocene-Pliocene series consists of detrital continental facies exposed downslope of the major topographic features within the study area and they fill also the synclinal structure of kourkmia (Fig. 1C).

Structural data analysis

A notable observation is the occurrence of Triassic evaporites along the North-South Axis (Fig 1B and C). In the southern part of the N-S axis, Triassic evaporites are extruded on both flanks of the Eocene-cored perched synclines of Zebbeus and Kef Ennsour and they seem to occupy the interior of the Meknassy-Mezzouna corridor where they crop out in an inner position such as in Dribika El Hamra and Nadour Diapir while they underline only the eastern side of Jebel Mezzouna. The cross-sections and fields photos presented in figures 3 and 4 illustrate the position and the geometry of Triassic rocks within the structures of the N-S axis. The Zebbag Formation is the oldest series in contact with Triassic evaporites in the study area. Its contact with Triassic rocks along the eastern flank of the Zebbeus Syncline matches with a N-S segment of the N-S axis, while it rests unconformably over the Triassic diapir in Jebel Kohl to the west of Mezzouna (Figs 3 and 4A). Overall, in the study area, Late Cretaceous to Eocene strata in contact with the Triassic show significant thickness reduction and the ones that still uneroded appear as isolated remnants.

Fig. 3 Four geological cross-sections labeled A to D displaying folded, faulted strata with a color-coded formation legend.

Fig. 4 Geological outcrop photos and interpretive cross-sections illustrating Triassic salt structures and the surrounding sedimentary formations.

The structural map (Fig. 5) highlights a zone of predominantly folded and brittle structures across the central (N-S Axis) and western (Central Atlas) parts of the study area. These align into distinct orographic trends. From north to south, structures from the meridional part of the N-S Axis are as follow: The northern part is characterised by the Jebel Gouleb anticline, a fold with a submeridional axis. Late Cretaceous outcrops dominate the lithology of this anticline. The Cenozoic cored, Zebbeus perched syncline, is located to the south of Gouleb anticline. Its almond-shaped, non-cylindrical fold widens to over 250 metres in the south and narrows northward, formed between two converging faults—one striking N0° and the other N25°. Their intersection at a 25° angle controls the syncline’s northern closure. The axial plane is slightly curved, trending around N10° (Sebai et al., 2021).

Fig. 5 Structural map showing geological faults, synclines, anticlines, and Triassic salt deposits across the Central Atlas and Eastern platform.

Further south along the N–S Axis lies the ‘Triassic complex’ (Boukadi, 1985), marked by a dominant NW–SE structural trend. It begins with the Dribika el Hamra anticline, exposing Triassic rocks, and connects to Jebel Nadour and the Miocene-cored Kourkmia Syncline (Figs. 1C and 5). Jebel Nadour, a small faulted anticline with Triassic core and Campanian–Maastrichtian flanks, represents a key tectonic intersection between the N20° trend of Jebel Bou Douaou and the NW–SE Mezzouna corridor (Boukadi, 1985; Sebai et al., 2021) (Fig. 4A and B). The elliptical Kourkmia Syncline (Figs. 1C 3B and 5), located between Dribika el Hamra and the Kef Ennsour syncline, is bordered to the west by a suspected NW–SE fault, where outcrops of the Jebs and Abiod Formations abruptly end. The Kef Ennsour is a 3 km cylindrical elongated perched syncline that exhibits external edges affected by erosion and by directional and oblique faults. Toward the south, the disappearance of the Jebs formation makes the Triassic rocks underneath crop out widely (Fig. 3 C). The Mezzouna and Oued Abiod plains lay respectively to the east and to the west of the corridor (Fig. 5). South of Kef Ennsour, the El Kohl structure consists of an isolated dolomitic blocks attributed to Upper Zebbag that rest unconformably over the Triassic rocks (Figs. 4C and D). Jebel Mezzouna connects to the Njilet–Rouijel folds—two sub-meridional structures sharing similar Lower Cretaceous sequences, fold geometries, and orientations. The Rouijel anticline exposes a complete stratigraphic sequence from the Aleg to the Segui Formations. Jebel Njilet forms a north–south monocline and it is segmented by faults causing block tilting and collapse (Figs. 1, 3D and 5). Further south, the Khetatil and Chabita monoclines are aligned along a major E–W fault stretching 13 km from Jebel Njilet to Jebel Bouhedma.

Out of the N-S axis corridor, Jebel Bouhedma, Jebel Boudouaou, and Jebel Melloussi illustrate various styles of Atlassic folds. Jebel Bouhedma is a classic anticline affected to the west by the NW-trending El Mech fault (N110–120) and extends toward the east along the Boudouaou–Njilet structures. Its Albian–Cenomanian outer strata dip 45° and over 80° in their northern and southern flanks, respectively. Jebel Boudouaou is an asymmetric NE–SW trending anticline (N20°), measuring approximately 12 by 5 km, and is cut by multiple faults (Figs. 3C and 5). Jebel Melloussi trends E–W, it has a near-vertical southern flank indicative of its vergence towards the south.

As shown in Figure 5, the study area is structurally segmented by four main fault systems: N–S, NW–SE, NE–SW, and E–W, all mapped within the Cretaceous and Tertiary hard rocks that are well exposed along the mountain ranges. However, the existing structural map lacks detail, particularly within the wide plains filled by recent detrital materials. Therefore, additional investigations using seismic and gravity data are mandatory to better characterise the fault network and its structural impact across the area.

Seismic interpretation

Seismic lines A and B (Figs 6 and 7), both oriented NE-SW, provide key insights into the subsurface structure of the study area. Line A, near the Mezzouna structure, reveals a ~2 km wide breccia zone interpreted as a Triassic salt wall intruding between two major subvertical faults, oriented NW-SE, underlying the Meknassy-Mezzouna corridor. Similarly, Line B, near Dribika El Hamra, shows a large diapiric body nearly 5 km wide. Both sections show significant lateral thinning of Jurassic and Lower Cretaceous units toward the east, in the Regueb plain. A remarkable difference between the two sections is the opposite dip sense of the Jurassic-Cretaceous sequences: towards the west in A and toward the east in B. Possible interpretations for this setting is that the post-Triassic strata collapsed after a welding phenomenon in the western block after Triassic evaporites migration to the diapiric ridge of the Meknassy-Mezzouna corridor and/or the dissolution of evaporites underneath such as described above within the northeastern edge of the Boudouaou structure that collapsed near the corridor. These seismic profiles reveal also some faults buried beneath Neogene–Quaternary cover. The two seismic sections reveal a huge diapiric activity along the N–S Axis and highlights its major role in controlling the deposition and the deformation in the region during Meso-Cenozoic times. This is evident in the lateral thinning of Jurassic and Lower Cretaceous sequences toward the diapirs. Salt tectonics and associated processes such as evaporite dissolution and welding seem to be also implicated; high-quality seismic data can provide support for further detailed interpretation.

Fig. 6 Seismic reflection profile and geological interpretation of Line A, showing a central Triassic salt wall piercing younger sedimentary strata.

Fig. 7 Seismic profile and geological interpretation of Line B, showing a central Triassic salt diapirism structure piercing sedimentary strata.

Gravity analysis

The Bouguer anomaly map established for the Meknassy-Mezzouna region shows that the anomalies values range from -12.5 to 21.7 mGal, with a general gradual increase from west to east (Fig. 8). This map displays at least five positive anomalies labelled PA1–PA5 and three negative anomalies, labelled NA1–NA3. The positive anomalies match with the uplifted folded terrain cored mainly by Cretaceous rocks, whilst, the negative ones correspond to the synclinal plains of Meknassy (NA1), to the west, and Regueb (NA2 and NA3), to the east. These plains are separated by the Meknassy-Mezzouna corridor underlined by parallel NW-SE isovalue lines of ~5–7 mGal. The positive anomaly PA1 has an ENE-WSW orientation, and it exhibits relatively high amplitudes of 14 mGal. It corresponds to the Bouhedma-Boudouaou structure truncated to the east by the western border of the Meknassy-Mezzouna NW-SE corridor. The NW-SE-oriented PA2 anomaly exhibits an average amplitude of 6.3 mGal. It is associated to a subsurface Triassic salt dome crossed by the Mez1 petroleum well at the depth of 2682 m. PA3 anomaly displays a N-S orientation and exhibits high amplitude, reaching 20 mGal. It corresponds to the N-S lineament of Zebbeus-Gouleb belonging to the N-S Axis. The latter seems to prolongate toward the NE by the NE-SW oriented PA4 anomaly associated with the Khcham anticline (Fig. 8). NA2 and NA3 anomalies displaying both an amplitude of -12.5 mGal form together a probable minibasin between the N-S axis to the northwest and the Mezzouna and Bir Ali Ben Khlifa domal uplifts to the southeast, which appears to be subdivided by a NW-SE accident parallel to the Meknassy-Mezzouna corridor.

Fig. 8 Geophysical gravity anomaly map with labeled positive and negative anomalies, regional structures, petroleum wells, and a milligal scale.

Analysis of the regional residual separation in Figure 9 allows us to subdivide the subsurface geological units within the study area by four major stratigraphic discontinuities. The first one is situated at 8.5 km, corresponding to the deep basement sources (Fig. 9). This depth is probably related to the Paleozoic or the Precambrian basement. However, there is no petroleum well that can confirm this hypothesis. The second discontinuity exhibits an average depth near 3.5 km. This depth correlates with the non-uplifted top of the Late Triassic strata. This hypothesis could be confirmed with the two petroleum wells W1 and W2. In fact, away from salt walls and domal/diapiric structures the Late Triassic evaporites are at 3500 to 4000 metres depth. The third and the fourth discontinuities represent the anomalies from shallower sources. They could represent the Late Cretaceous-Paleocene at 1.2 km depth, and the Neogene-Pliocene contact at 250 m depth. Figure 10 presents the regional gravity anomaly map. By comparing various polynomial trend surfaces with the upward continuation map at 16 km (Jacobsen, 1987) (Fig. 10D), a strong similarity is observed between the third-degree polynomial and the upward continued map in terms of isogal patterns and overall trend. Based on this similarity, the third-degree polynomial surface was selected to represent the regional field and will be subtracted from the Bouguer anomaly map to produce the residual anomaly map.

Fig. 9 A power spectrum plot showing depth estimations from deep basement, 8.5 kilometers, to very shallow sources, 250 meters, and noise.

Fig. 10 Four color-coded gravity anomaly maps labeled A to D showing regional and residual subsurface trends, each with an milligals color scale.

The map of residual anomalies was developed analytically following a subtraction of a regional represented by a third-degree polynomial from the original complete Bouguer anomaly grid. Accordingly, after smoothing the effect of deep sources, the Residual anomalies map (Fig. 11) shows only anomalies, which are due to intermediate and shallow sources. Comparing Bouguer and Residual maps, several anomalies change their form and spatial extension. Additionally, many anomalies appear. In concordance with the Bouguer map, high amplitude maxima are highlighted within Jebels Kbar, Zebbeus and Khcham in the northern part of the map, as well as within the Bouhedma-Boudouaou structure to the south. We notice that PA2 changes its direction from NE-SW in the Bouguer to N-S in the residual map. In addition, PA5 (Jebel Kbar) changes direction from ~E-W to ENE-WSW in the residual map. This direction may coincide with the direction of the Mio-Pliocene outcrops around Mezzouna locality. A new PA6 corresponding to jebel Melloussi appears in the residual map. On the other hand, NA2 changes direction from NE-SW to N-S. Furthermore, NA3 appears to be subdivided into two separate anomalies; NA3’ to the north, oriented NE-SW and parallel to jebel Khcham, and NA3” to the south, oriented N-S and parallel to NA2. The MGH map (Fig. 12) shows areas that have high gravity gradients. Geologically, these gravity lineaments can be correlated with density discontinuities, which are globally related to structural discontinuities. This map shows three main directions that are N-S, E-W and NE-SW. The NE–SW faults are located on both sides of the N-S axis. However, E–W faults appear as three main parallel lineaments within Bouhedma, to the north of Bou Douaou and within the Melloussi structure. The N-S direction is related to the N-S axis major fault. In addition, the Euler map (Fig. 13) shows a very important fault within the Triassic corridor zone-oriented NW-SE.

Fig. 11 A gravity anomaly map highlighting positive and negative subsurface anomalies with regional structures and a milligal scale.

Fig. 12 A gravity lineament map depicting subsurface structural faults, regional geological trends, petroleum wells, and a milligals-per-meter color scale.

Fig. 13 An Euler deconvolution map displaying color-coded depth points along geological structures, from deep basement to shallow subsurface features.

Tectonic style

The inventoried fracture network shows the four main directions: N-S, E-W, NW-SE, and NE-SW consistent with the structural framework of Tunisia reported in previous studies (Caire, 1970; Martinez et al., 1991; Piqué et al., 1998; Ouali 2007). Most of the recognised faults in the eastern platform were detected especially thanks to seismic and gravity data interpretation since they are buried or not well expressed at the outcrop due to the detrital character of the Quaternary deposits. The significant depths of the faults confirm that the structuring of the cover of the Tunisian margin is largely influenced by pre-existing discontinuities.

The crustal rooting of the N-S axis fault suggests it may be an ancient structure inherited from the Hercynian or even the Pan-African orogeny (Burollet, 1956, 1981). This regional structure comprises deep-rooted, multidirectional faults anastomosed within a narrow tectonic corridor mainly oriented N-S (Abbes, 2004; Ouali, 1984, 2007; Dhahri et al., 2015). This corridor was home of extension and/or transtension that initiated during the Mesozoic Tethyan opening followed by shortening and/or transpression during the Alpine orogeny. During the Mesozoic extension, a progressive and continuous subsidence of the western compartment has favored the development of a subsiding area with relatively thick Jurassic and Lower Cretaceous deposition (Haller 1983; Ellouz 1984; Touati 1985; Ouali 2007; Dhahri et al., 2015). E–W synsedimentary faults were also described in central Tunisia by Ouali (1984) and Gourmelen (1984). NE-SW and NW-SE trending faults are however associated with the present-day structure of the N-S axis engendering dextral and sinistral offsets. Since the N-S faults displayed sinistral strike-slip kinematics during the NE-SW alpine shortening, NE-SW segments, perpendicular to the main stress developed thrust and folds, however NW-SE (e.g., Meknassy-Mezzouna corridor) were home of transpression (Boukadi, 1985; Dhahri et al., 2015).

The influence of the N-S faults is particularly evident in the Zebbeus area, where the eastern flank is outlined by a major N-S fault intruded by Triassic materials. The NW-SE trend is manifested by the border faults bounding the Meknassy-Mezzouna corridor. These segments of the N-S axis can be mapped by following the geological contact between the Triassic rocks and the intruded Cretaceous-Cenozoic sedimentary cover. Farther southeast, the Meknassy-Mezzouna corridor is replaced by only one main fault near jebel Mezzouna (Fig. 5) that acquires a sub-meridional direction further south near the Jebel Rouijel structure.

The most remarkable E-W fault segments interpreted in the study area are those of the Bouhedma-Khetatil lineament in the south and the Jebel Melloussi in the northwest of the study area. Both structural lineaments are interpreted to be fault-propagation anticlines with vertical to slightly overturned southern forelimbs. In fact, regional E-W faults were described in both the southern and central Tunisian Atlas playing a significant role in the geological evolution of the Tunisian Atlassic chain (Dhahri et al., 2015; Tanfous et al., 2017). Most of the paleogeographic domains identified for the Jurassic are oriented generally E-W, but other directions exist, such as NE-SW (Elmi, 1996; Tanfous et al., 2017). The Orbata-Bouhedma chain, which is structurally connected to the inherited E-W fault system, lays along the southern margin of the so-called Kasserine Island, established during the Late Cretaceous (Marie et al., 1984; Marco et al., 2014). And it is regarded as a tectonic boundary separating two distinct structural domains: the Central Atlas and the Southern Atlas, each exhibiting its own structural style. Further northwards, the Mrihla-Cherichira fault, part of the E-W fault network, intersects the northern edge of the N-S axis at Cherichira locality (Fig. 1B). This fault delineates the northern limit of the Kasserine Island and marks the transition to a relatively more subsident domain to the north.

The superposition of the Euler and residual maps (Fig. 14) shows that faults delimit generally the positive anomalies from the negative ones. The structural map deduced from gravity data (Fig. 15) shows another interesting feature. The multidirectional fault systems in the study area created several zones of fault intersection and delimited several polygonal tectonic blocks. These weakened zones and the stretched borders of the polygonal blocks enabled Triassic salt extrusion especially during the Jurassic-Cretaceous extension and evolved in response to the Atlassic orogeny to complex salt tectonic structures. The interference of the NW-SE direction with the NE-SW and N-S directions in the Meknassy Mezzouna zone generates a huge load of salt at this location. Supporting this interpretation, seismic line B crosses this fault interference, revealing a huge diapiric salt structure approximately 5 kilometres wide. Further evidence comes from Dribika el Hamra, the closest area to this fault intersection. This structure exhibits the most significant halokinetic activity observed in the study area. The intersection of N-S, E-W, and NE-SW faults at Jebel Rhéouis weaken considerably the sedimentary cover and created a prominent zone for Triassic evaporites extrusion, the most evident in the Central and Southern Atlas (Figs. 14 and 15). Our observations suggest a strong correlation between fault intersection and the diapirism of Triassic evaporites. These salt deposits appear to be most concentrated within these fault intersection zones, as evidenced by the cases of Jebel Rhéouis and Dribika El Hamra. A key finding of this study is that the most significant halokinetic movement occurs precisely at these fault intersections, highlighting their critical role in localizing salt mobilisation. The interference of multiple fault systems creates more space within sediments layers, potentially enhancing the conditions for halokinesis. This allows for easier movement of the buoyant Late Triassic salt.

Fig. 14 A combined gravity anomaly map overlayed with color-coded Euler deconvolution depth points, outlining subsurface faults and structural trends.

Fig. 15 A structural sketch detailing gravity lineaments, anticline and syncline axes, petroleum wells, and pink halokinetic material zones.

The available geological data show that the Triassic rocks movement, in the study area, occurred within two main fault zones, oriented N-S (Jebel Zebbeus) and NW-SE (Meknassy-Mezzouna corridor). Gravity data processing demonstrated the deep rooting of these fault zones to about 5000-8500 and 3500 metres, respectively. Extensional Mesozoic fault zones reach the Upper Triassic layer buried at an original depth of about 3000-4000 metres so they were home of salt halokinesis. In this study area, faults with depths of 3000 metres could potentially initiate thin-skinned deformation, which can contribute to the upward migration of salt and the formation of salt structures. Gravity data also shows that the NE-SW lineaments impact sedimentary layers from 250 to 1500 metres, with potential deeper influences in some localised areas. This suggests a mechanical decoupling between this shallow cover and its basement that coincides seemly with the Upper Triassic evaporitic sequence. Jurassic and Lower Cretaceous sediments show a lateral thickness decrease toward salt diapirs. Thus, the Upper Triassic evaporites controlled the Jurassic and the Early Cretaceous sedimentation by its early mobility. This interpretation deals with the results of Tanfous et al. (2005) and Azaiez et al. (2008). It seems that the N–S axis corresponds to a steeply-dipping major weak zone established in central Tunisia before Mesozoic times. The loading and piercement of the Triassic evaporites along this weak zone are key factors in the thinning of the Mesozoic-Cenozoic sedimentary cover along this paleogeographic discontinuity. Deep faults segment of the N-S Axis guided salt intrusion, inducing diapir formation. Obviously, the Meknassy-Mezzouna region corresponds to a major weak zone where several stages of Triassic salt rise occurred during Jurassic-Cretaceous times and gave several salt tectonic structures that crop out today all along this range.

As known, diapirism can be triggered by a variety of mechanisms including buoyancy contrasts (Trusheim, 1960), differential loading (Ge et al.,1997; Warsitzka et al., 2015), and extension or contraction driven by tectonics or gravity gliding (Vendeville & Jackson, 1992a, b; Nilsen et al., 1995). In this case, diapir growth seems to have initiated at least since the Early or the Middle Jurassic (early Liassic rifting event) in agreement with the results of Tanfous-Amri et al. (2005) and Azaiez et al. (2008). This means that this extension generated normal faults affecting the salt overburden and probably reactivating the basement faults. Among the above mentioned diapirism triggering factors, tectonic mechanisms should have the major role in Central Tunisia. Vendeville & Jackson (1992) delivered a series of experiments on diapiric rise during extension. They suggest that diapirism typically occurs in three stages: reactive piercement, active piercement, and passive piercement. During the initial stage, diapirs slowly pierce overlying grabens or half-grabens in response to localised thinning of the overburden caused by normal faulting during regional extension. The second stage of diapirism is rapid active piercement, which is largely independent of regional extension. Fluid pressures at the diapir’s crest become sufficient to lift and displace the overlying thin roof. The final stage of diapir growth is passive piercement, beginning when the diapir reaches the surface. Thus, diapirism in the study area seems to follow these three stages (reactive, active and long-lived passive diapirism).

One important point that we have to raise is the role of inherited faults in the sub-salt series. Sebai et al. (2021) delivered a series of experiments about Kef Ennsour and Zebbeus perched synclines. They modeled only the impact of pure thin-skinned deformation above a mobile evaporitic layer by maintaining a flat, horizontal subsalt base. The final result was similar to structures from the southern part of the N-S Axis. Such an approach, which consists of considering that the salt and its overburden are mechanically and kinematically decoupled from the subsalt series, was also tested in Zidi et al. (2024) while studying the Labaied-Trozza lineament (part of the major E-W Mrhila-Cherichira fault). These examples support the hypothesis of thin-skinned tectonics in central Tunisia. This does not eliminate the possibility of thick-skinned tectonics being at play. Further investigation of both mechanisms is crucial to validate (or not) either hypothesis.

Tectonic implications

During Jurassic time, the Tunisian realm was controlled by an extensional regime. This tectonic established multidirectional fracturing induced by the reactivation of deep-rooted faults, mobilised for the first time in the Triassic following the early stages of Tethyan rifting, then in the Jurassic (Ouali, 2007). Accordingly, the Jurassic deposition was controlled by an extensional E-W and a N-S transfer fault network.

The Upper Cretaceous outcrops delivered several tectono-sedimentary records that permitted the delineation of the geological evolution of Central Tunisia. In Jebel Boudinar, to the north of the study area, a network of NW-SE and N-S normal faults have controlled the Cenomanian-Turonian sedimentation. This permitted Soyer (1987) to define an extensional setting, characterised by a N-S to NNE-SSW stretching axis for the Cenomanian-Turonian period. Dhahri & Boukadi (2017) described also NE-SW extension in Central and eastern Tunisia during the Late Cretaceous that triggered several diapiric movements along N-S and NW-SE tectonic corridors of the N-S Axis. Diapirism engendered consequent necking and thinning of the overburden. The corridor borders are underlined by tectonic contacts between Triassic evaporites and overburden rocks, however, in their central parts, with a Triassic elevated ridge, relatively thin post-diapirism deposition occurred “unconformably” over the diapir crest and evolved later to allochthonous shreds. The extensional movement, initiated in the Cenomanian, resumed in the late Turonian before being dampened in the late Senonian, as reported by numerous studies in various structural domains of Tunisia: (Haller 1983; Ellouz, 1984; Touati, 1985) in the Sahel platform, Martinez et al. (1991) in the Saharan platform, Chikhaoui & Turki (1995) in the northern Atlas and Khéssibi (1987), Ouali (1984), Gourmelen (1984), Soyer (1987), and Rabhi (1999) in Central Tunisia and the N-S Axis.

In the southern part of the N-S axis (Maknassy-Mezzouna region), the Campanian-Maastrichtian sedimentation is controlled by two essential phenomena: Halokinetic movements and tectonics. In fact, during the uppermost Cretaceous the tectonic regime is not well defined; there was a transition from extension to shortening and the basin floor was unstable. Both extensional and shortening structures were described along central and eastern Tunisia during the Mesozoic-Tertiary transition (Abbes, 2004; Dhahri & Boukadi, 2010). However, a notable thickening of the Abiod Formation is observed in the southern periclinal closure of Zebbeus structure testifying it was a subsiding area by the end of the Cretaceous. This extensional setting persisted until the Eocene, as indicated by the local thickening of the Ypresian phosphatic deposits and the Lutetian–Priabonian gypsum sequence, both controlled by synsedimentary NE–SW-oriented normal faults.

At regional scale, the Tertiary tectonic regime was a NW-SE shortening; the N-S deep rooted faults were reactivated as sinistral strike-slip. Locally, the NW-SE-oriented relay of the Meknassy-Mezzouna acted as an extensional bend that stretched the southern part of the Zebbeus structure toward the south and engendered the local extension that forced the diapir to fall as described in Sebai et al. (2021). In fact, the local extension mentioned in the southern part of Zebbeus structure seems to have started at the end of Cretaceous and this is documented by some synsedimentary normal faults that have controlled the thickening of the Upper Cretaceous-Eocene deposits. In the N-S Axis we believe that the accumulation of relatively thick sediments of the upper Eocene deposit is a response to transtensional movements during the NW-SE shortening phase.

In the study area, the Oligocene is represented by the Fortuna and Messiouta formations, exposed only in sparse and discontinuous outcrops east of Jebel Er Rouijel. These limited exposures do not allow for a detailed tectonic analysis, but they indicate a relatively subdued deformation phase. The Langhian Aïn Grab Formation, also discontinuously exposed east of Jebel Rouijel and south of Jebel Nejilet, unconformably overlies the Jebs, Haria, and Abiod formations, reflecting renewed activity along inherited faults. Regionally, the Oligocene–Langhian interval corresponds to a transitional tectonic regime in Tunisia, where both extensional and shortening domains coexisted: extension and synsedimentary faulting dominated in northern and eastern Tunisia (Perthuisot, 1978; Haller, 1983; Chihi, 1984; Philip et al., 1986; Touati, 1985), while localised shortening prevailed in central regions associated with the early stages of Atlas uplift (Letouzey & Témolière, 1980). Within this broader framework, the study area records only subtle evidence of fault reactivation, suggesting that deep-seated structures continued to influence sedimentation despite limited surface deformation.

The Segui Formation, deposited during the Tortonian–Pliocene interval, consists of continental detrital facies that accumulated downslope of major topographic highs and across broad flat plains in central Tunisia. In the study area, this formation displays an angular unconformity with the underlying units along the N–S Axis. The NW–SE-directed late-stage shortening regime has reactivated pre-existing faults with either right- or left-lateral strike-slip movement, depending on their orientation (Letouzey & Trémolière, 1980). In northeastern Tunisia, this phase generated horsts and grabens oriented N140-160, indicating an extensional and strike-slip tectonic regime (Ben Ayed, 1993; Belguith et al., 2011, 2013; Booth-Rea et al., 2018). This local extension propagated into the central Tunisia domain during the Messinian-Pliocene (Belguith et al., 2011, 2013). However, extension did not affect southern Tunisia (Philip et al., 1986).

From the Villafranchian, a NW-SE to NNW-SSE shortening led to the formation of folds in eastern Tunisia and accentuated the earlier folding in central and northern Tunisia. Similarly, the NW-SE-oriented basins continued to extend in a NE-SW direction.

During this late-stage shortening, Triassic salt structures, originally initiated during Mesozoic extension, were rejuvenated. This renewed mobilisation and deformation in response to shortening, has reshaped their geometry and influenced the surrounding sedimentary layers.