Gravimetric Study of Geological Structures of Teboursouk Area , Northern Tunisia

Gravity data associated with surface geology in the Northern Tunisian Atlas offer better understand to the underlying structures in Teboursouk area and to highlight other deep or unknown structures in surface. The gravity study was based on qualitative and quantitative analysis including the construction of the gravity Bouguer anomaly, upward continuations, residual anomaly, and Horizontal gradient maxima maps. The main results display many positive and negative anomalies as the response of geological structures (J. Cheid Triassic structure, Khalled plain, El Aroussa plain). In addition, the horizontal gradient maxima integrated with geological and structural maps let the identification of major directions of gravimetric lineaments in the study area us NE-SW trending features at the boundaries of J. Cheid structure, NW-SE direction that limit Gaafour plain and Tabet Ech Cherif syncline, and N-S trending that bordered El Aroussa basin. Major results allowed the construction of a new structural map of the study zone.


Introduction
The geodynamic evolution and geometry of the Mediterranean basins has been related to different spreading rates along the Atlantic oceanic ridge [1,2].Since the Late Triassic to Present, the north African domain has undergone episodes of deformation that can be correlated to plate kinematics; these movements are fossilized in the sedimentary deposits [3,4].The Septentrional Atlas highlights paleogeographic continuity and records tectonic and sedimentary events follow the major orogenic phases, which are the consequence of the convergence movements between the African and Eurasiatic plates [1,[4][5][6][7][8][9][10].These are followed by compressive tectonic constraints beginning in the end of Late Cretaceous times [1,[11][12][13][14][15][16] and has created many chains and mountains in the Mediterranean borders.The various geological studies concerning Northern Tunisia and the Siculo-Tunisian strait [4,[17][18][19][20][21][22][23][24] shows the importance effect of the NE-SW, NW-SE and north-south faults on the control of sedimentation associated to the structuring of these domains (Figure 1).On the other hand, the Triassic rifting phase characterized by the rejuvenation of NE-SW hercynian faults was continued during the Jurassic and the Early Cretaceous time [4,25,26], and records in northern Tunisia the formation of grabens and synclines separated by horsts structures.Several other lineaments oriented NE-SW (Tunis-Ellès and El Alia-Teboursouk faults) has an important control on the sedimentary distribution during this same period.After a short Aptian-Early Albian transpressional period, NE-SW to ENE-WSW trending extensions prevailed during the Late Cretaceous.It is regionally identified through the Tunisian Atlas [3,4,20,[27][28][29].The tectonic movement with vertical component was associated with rising of the Triassic material along a basement opening major faults or fault systems.During Cenomanian period, is a second extensional event that has continued in the study domain [3,14,[29][30][31][32][33][34][35].The Late Cretaceous extensions affected the whole Northeastern African margin, originating NW-SE to NNW-SSE striking basins [3,14,36].From the Cenomanian to the Early Maastrichtian times, was the same regional NE-SW extension.The inversions started in Late Maastrichtian-Paleocene in Northern Tunisia, which constitutes a first compressional event.A second and major compressional event resulting in significant inversions of Mesozoic basins occurred in the Late Eocene (Atlassic phase).The Triassic domes and the diapirs zones are generally lengthened NE-SW [28,37].

Geological Setting
The study area (Teboursouk) is located in the northern Tunisian Atlas.Teboursouk map (Teboursouk 1/50,000 sc.Map) [38] contains major salt alignments (Figure 2) J. Cheid-Knana, which is the larger outcrop, Fej Lahdoum-Ain Jemmala, and Thibar structures.The studied area is composed of two main sedimentary series.The Mesozoic strata comprise Triassic material composed by chaotic deposits including salt layers [3,39,40].The Jurassic and the Lower Cretaceous deposits display no outcrops in the area.The Early Cretaceous strata are composed of Late Aptian-Early Albian sediments with black clayey marls.In addition, the Late Cretaceous is represented by Cenomanian (marls) to Senonian series (limestones and marls) [28,41].The Cenozoic deposits (Paleocene to Oligocene time) are composed of marls, limestones, and sandstones.The Neogene formation is composed of Miocene marls.The Pliocene-Quatenary series shows a conglomeratic beds on the base and of sandy clay ones on the top [35,[41][42][43][44][45][46].

Gravity Data
In our study, we used gravity data in the northern Tunisian Atlas collected by the Office National des Mines (ONM) [46] (gravimetric map of Teborsouk area).The studied area is covered by gravity stations on a 1 × 1 km 2 grid.The data were gridded at a spacing of 1 km using a minimum curvature technique Briggs, (1974) [47], and contoured at a 1 mGal interval to establish a complete Bouguer anomaly map.This map reflects generally the lateral variations of density in the subsurface associated to the geological structures.

Bouguer Anomaly
The complete gravity anomaly map (Figure 3) has much information on present discontinuities in the basement, which can be extracted by analyzing pressure gradients between anomalies [48][49][50].The complete Bouguer gravity anomaly values range from −20 mGals in the southeast to 18 mGals in the northeast.Indeed, Figure 3 shows positives anomalies such as: a NE-SW anomaly that coincides with Triassic bodies from the J. Cheid, a E-W anomaly that corresponds to a Cretaceous series of Tabet Ech Cherif, a NE-SE anomaly associated with a Triassic alignment of J. Cheid-Knana, and finally a cretaceous outcrops of Oued Arkou anticline and the south part of J. Thibar.These positive anomalies are bounded by two other negative anomalies associated with Gaafour, and Khalled plains composed of quaternary deposits.
In order to separate shallow sources, the residual ano-maly was obtained by subtracting the regional field to the Bouguer anomaly.For this reason, upward continuation method was used to constrain regional map [50][51][52][53][54].The upward continuation of the Bouguer anomaly to 20 km of altitude expresses a very smooth gravity pattern, a big positive anomaly in the NE part of the map (Cretaceous series) and negative anomaly in the south (plains with quaternary sediments) (Figure 4).

Residual Anomaly
The residual map (Figure 5) expresses various gravity anomalies with values ranging between 20 mGals in the northeastern part of the map, to −10 mGals in southeastern part that coincides with Gaafour plain.Positive anomalies are placed from the eastern part to western part of the study area, they corresponds to a Cretaceous series of Tabet Ech Cherif, Triassic alignment of J. Cheid-Knana, Oued Arkou anticline, and the south part of J. Thibar.Negative anomalies correspond to Gaafour and Khalled plains.NE-SW-direction curves indicate a gravity gradient decreasing laterally over the Khalled and Gaafour plains in quite coherence with the nature distributions in rock densities.

Horizontal Gradient Maxima
In order to delineate lineaments associated with boundaries of rock units or faults [49,[50][51][52] the horizontal gradient maxima method is applied on the Bouguer anomaly map.In this case, lineated contacts correspond to faults, while the circular contacts are the limits of diapirs or of intrusive bodies.Gravity anomaly over a vertical contact is realized by a curve having a minimum for a weak density rocks and a maximum for a high density rocks [48][49][50].The inflexion point of the curve is in the equilibrium of this contact showing the maximum of the horizontal gradient.If a contact has no vertical dip, the maxima of the horizontal gradient move in the dipening, but, these will stay near this contact for high dip.
Blackely and Simpson [53] (1986) proposed a new method to determine the horizontal gradient maxima.The dip of various contacts is determined by the upward continuation of Bouguer anomaly map.At every level, the horizontal gradient maxima are localized.Thus, the highest levels of continuation correspond to the deepest contacts and vice versa.If we have a vertical structure, all the maxima overlap.
The Horizontal gradient maxima map (Figure 6) reveals the existence of NE-SW and NW-SE direction, in the study area.This carte shows thin alignments; the most significant are those of the J. Cheid outcrop that reflect a net contrast of density between the Triassic and surrounding cretaceous series.Gradients are more im-portant in the Northern part of this structure, than the South part.In addition, two lineaments with NW-SE direction bordered El Aroussa plain and associated with quaternary deposits [54].This result is well confirmed by the Figure 7.This map (Figure 7) is obtained from superimposing the interpreted gravimetric faults to a geological map of the study zone.It reveals the existence of variety of directions that correspond to the boundaries of structure; NE-SW, NW-SE and N-S with dominance of NE oriented alignments.

Discussion and Conclusions
The superimposing of gravimetric lineaments map on the structural map of the study area shows an important fracturing in the sedimentary coverage especially to Tabet Ech Cherif, J. Cheid and in Ain Tounga zones (Figure 7).
Several directions were highlighted:  NE-SW Direction characterized by faults of great dimensions and an strong dip that limit especially J. Cheid (F1) and Ain Jemmala (F3) Triassic outcrops, and the south of Oued Siliana (F2). E-W Direction that constitutes a network of faults from the North to the South: the fault F4 of Oued Siliana with a dip towards the north, F5 cutting the J. Cheid Triassic outcrop, F6 from Koudiat Basina Serira to the North of J. Strassif and F7 in the south part of Gaafour plain (Figure 7). NS Direction which contains several faults: the fault F8 in El Aroussa plain and F9 between the two synclines of Tabet Ech Cherif and Ain Tounga. NW-SE Direction represented especially by the new lineament of Tabet Ech Cherif with a SW dip syncline (F10) and F11 that limit Gaafour plain (Figure 7).Additionally, the result map (Figure 7) shows that Triassic outcrops of J. Cheid and Kenana are separated by a depression.It is not a same structure.
Some gravimetric lineaments coincide perfectly with observed geologic faults.These faults put in contact grounds with different densities and are normally deep.Other gravimetric lineaments such as F4 and F9 are unknown and haven't signature in surface, represent one of the new contributions of this study, because these faults must be masked in the surface.

Figure 2 .
Figure 2. Simplified geological and structural map of the study area [38, completed and modified].

Figure 5 .
Figure 5. Map of the residual anomalies in the Teboursouk area.

Figure 6 .
Figure 6.Maxima of the horizontal gradient of the Bouguer anomaly and its upward continuation to different heights superimposed on the horizontal gradient, showing the interpreted gravimetric lineaments in the study area.

Figure 7 .
Figure 7. Interpreted structural map of Teboursouk area superimposed on the geological map of Perthuisot et al., 1978.