Yizi Chen

Edwin Carlinet

Joseph Chazalon

Clément Mallet

Bertrand Duménieu

Julien Perret

Combining Deep Learning and Mathematical Morphology for Historical Map Segmentation

Keywords: Deep Learning, Convolutional Neural Networks, Mathematical Morphology, Historical Map Segmentation, Object Extraction

The digitization of historical maps enables the study of ancient, fragile, unique, and hardly accessible information sources. Main map features can be retrieved and tracked through the time for subsequent thematic analysis. The goal of this work is the vectorization step, i.e., the extraction of vector shapes of the objects of interest from raster images of maps. We are particularly interested in closed shape detection such as buildings, building blocks, gardens, rivers, etc. in order to monitor their temporal evolution. Historical map images present significant pattern recognition challenges. The extraction of closed shapes by using traditional Mathematical Morphology (MM) is highly challenging due to the overlapping of multiple map features and texts. Moreover, stateof-the-art Convolutional Neural Networks (CNN) are perfectly designed for content image filtering but provide no guarantee about closed shape detection. Also, the lack of textural and color information of historical maps makes it hard for CNN to detect shapes that are represented by only their boundaries. Our contribution is a pipeline that combines the strengths of CNN (efficient edge detection and filtering) and MM (guaranteed extraction of closed shapes) in order to achieve such a task. The evaluation of our approach on a public dataset shows its effectiveness for extracting the closed boundaries of objects in historical maps.

Introduction

The massive digitization of archival collections carried out by heritage institutions provides access to huge volumes of historical information encoded in the available documents. Among them, maps are unfortunately still little exploited. Yet they are a gold mine of geographic data that allows to reconstruct and analyze the morphological and social evolution of a place over time [START_REF] Perret | Roads and cities of 18th century france[END_REF][START_REF] Dietzel | Spatio-temporal dynamics in california's central valley: Empirical links to urban theory[END_REF]. Topographic maps, in particular, engrave many geographical features: their distribution in space, their topological relationships and various information encoded (a) Some geographical entities typically depicted in city maps: building blocks (orange), roads (green) and rivers (blue). by the map legend or by text labels [START_REF] Leyk | Saliency and semantic processing: Extracting forest cover from historical topographic maps[END_REF][START_REF] Chiang | Efficient and robust graphics recognition from historical maps[END_REF]. Transforming such graphical representations of geographic entities into discrete geographic data (or vector data) is a crucial step for numerous spatial and spatio-temporal analysis purposes. Such a transformation is most often manually retrieved by historians or with the help of crowdsourcing tools. This is extremely time-consuming, non-reproducible, and leads to heterogeneous data quality. Automating this tedious task is a key step towards building large volumes of reference geo-historical data.

Unfortunately, historical maps exhibit characteristics that hinder standard pattern recognition approaches and make them relatively inefficient at extracting data of good quality, i.e., that do not need to be manually post-processed. Unlike modern computer-generated maps which follow roughly the same semiotic rules, these maps vary in terms of legend, level of generalization, type of geographic features and text fonts [START_REF] Leyk | Saliency and semantic processing: Extracting forest cover from historical topographic maps[END_REF]. They also usually lack texture information, which creates ambiguities in the detection of objects. For instance, building blocks and roads have very similar textures despite being of completely different nature (Figure 1a). Popular semantic [START_REF] Long | Fully convolutional networks for semantic segmentation[END_REF][START_REF] Badrinarayanan | Segnet: A deep convolutional encoder-decoder architecture for image segmentation[END_REF][START_REF] Ronneberger | U-net: Convolutional networks for biomedical image segmentation[END_REF] and instance [START_REF] Chen | Hybrid task cascade for instance segmentation[END_REF][START_REF] Romera-Paredes | Recurrent instance segmentation[END_REF][START_REF] Bai | Deep watershed transform for instance segmentation[END_REF] image segmentation algorithms detect objects based on textures and are prone to fail in our context. Color is not a relevant cue either: the palette is usually highly restricted due to the technical limitations and financial constraints of their production. Objects in maps are often overlapping, some are thus partially hidden and hardly separable. Occlusion happens with overlayed textual and carto-geodetic information in particular (Figure 1b, rectangles (1) and ( 2)). Last, preservation conditions of historical maps play a role as stains, folds or holes might cause gaps in the cartographic information. Such artifacts may lead to incorrect object detection (Figure 1b, rectangle (3)).

Historical Map

Deep Contour Detection

Edge Probability Map

Filtering & Watershed

Closed Shapes Fig. 2: Overview of the approach presented in the paper: we combine an efficient edge detection and filtering stage using a deep network with a fast closed shape extraction using mathematical morphology tools.

Our contributions in this paper are as follows. After reviewing the limitations of the current approaches for segmenting maps in section 2, we propose a simple pipeline (fig. 2) that combines deep networks and mathematical morphology for object detection in maps. It takes benefit from their complementary strengths, namely image filtering and strong guarantees with respect to closed shapes. We derive edge probability maps using a multi-scale deep network approach depicted in section 3 and then leverage mathematical morphology tools to extract closed shapes as explained in section 4. Eventually, in section 5, the second contribution lies in a thorough evaluation of the relevance of the mathematical morphology stage with novel visualizations and metrics to objectively assess our approach and better identify the strengths and weaknesses of each stage and of the workflow.

Approaches for map segmentation

We target to recover geometric structures from scans of historical maps. As mentioned above, due to the limited texture and colour content of such data sources, standard semantic segmentation approaches of the literature would fail for most cases. Instead, we cast our problem as a vectorization challenge that can be turned into a region-based contour extraction task. Such a problem is traditionally solved through a two-step approach: the detection of edges or local primitives (lines, corners) followed by the retrieval of structures based on global constraints [START_REF] Zhang | Superedge grouping for object localization by combining appearance and shape informations[END_REF]. Recent works have shown the relevance of a coupled solution [START_REF] Favreau | Extracting geometric structures in images with delaunay point processes[END_REF]. They remain tractable and efficient only for a limited number of structures. Region-based methods (e.g., based on PDEs [START_REF] Orzan | Diffusion curves: A vector representation for smooth-shaded images[END_REF]) may lead to oversimplified results and will not be further analysed here. The main issue of two-step solutions is the edge detection step. This low-level task is achieved by measuring locally pixel gradients. Due to the amount of noise (overlapping objects, map deformation), this would result in many tiny and spurious elements that any global solution would manage connecting. Instead, we focus on boundary detection, i.e., a middle-level image task that separates objects at the semantic level according to different geometric properties of images. This offers two main advantages: (i) a limited sensitivity to noise in maps and (ii) the provision of more salient and robust primitives for the subsequent object extraction step. We do not focus on a primitive-based approach since shapes on maps cannot be simply assumed.

Recently, among the vast amount of literature, convolution neural networks (CNN) have shown a high level of performance for boundary detection [START_REF] Xie | Holistically-nested edge detection[END_REF][START_REF] He | Bdcn: Bi-directional cascade network for perceptual edge detection[END_REF]. However, they only provide probability edge maps. Without topological constraints, image partitioning is not ensured. Conversely, watershed segmentation techniques in mathematical morphology can directly extract closed contours. They run fast for such a generation, but may lead to many false-positive results. Indeed, using only low-level image features such as image gradients, watershed techniques may not efficiently maintain useful boundary information [START_REF] Bai | Deep watershed transform for instance segmentation[END_REF]. Consequently, we propose here to merge the CNN-based and watershed image segmentation methods in order to benefit from the strengths of both strategies [START_REF] Xie | Integrating deep convolutional neural networks with marker-controlled watershed for overlapping nuclei segmentation in histopathology images[END_REF]. A supervised approach is conceivable since we both have access to reference vectorized maps and CNN architectures pre-trained with natural image.

Deep Edge Detection

We detail how we selected the network architecture used to detect and filter edges, with illustrations of the strengths of such approach, and describe the training procedure we followed to use the selected network (BDCN) on our dataset.

Network architecture Contour detection was first addressed with the design of handcrafted features based on brightness, color, textures [START_REF] Martin | A database of human segmented natural images and its application to evaluating segmentation algorithms and measuring ecological statistics[END_REF]. Then, improvements lied in their efficient group through mono-or multi-scale attributes retrieving micro-structures: textons are a salient example [START_REF] Zhu | What are textons? Intl[END_REF]. Afterwards, main methods focused on combining all available cues, such as [START_REF] Arbelaez | Contour detection and hierarchical image segmentation[END_REF]. They used a global probability boundary by learning the weights of manually selected features (gradients and textons as features in several image scales) in order to detect contours and form better closed boundaries to represent the objects in images. Since CNNs have proved their relevance to extract and combine meaningful image features, a large amount of research has focused on detecting contours. The most famous one is the so-called Holistically edge detector (HED) [START_REF] Xie | Holistically-nested edge detection[END_REF], which is an end-to-end multi-scale deep learning network. The novelty consisted in using skip-connections to merge different levels of features and learn different losses from intermediate layers of VGG-16 [START_REF] Simonyan | Very deep convolutional networks for large-scale image recognition[END_REF]. This allowed to recover multiscale representations of image features. Eventually, He et al. [START_REF] He | Bdcn: Bi-directional cascade network for perceptual edge detection[END_REF] proposed a so-called Bi-Directional Cascade Network (BDCN) by designing a scale enhancement module (SEM) on top of HED to enhance multiscale spatial contexts in images resulting in a better performance than humans in the BSDS500 dataset.

One advantage of BDCN is that the multiscale representatives combine semantically meaningful features to efficiently filter out the image textures and text information while maintaining useful contours and lines in the images. It is particularly suited for handling noise in our maps. Another advantage is that learnable dilated convolutions in SEM can learn fine-grained features with larger receptive fields that are beneficial when we want to separate the texts with object Fig. 3: BDCN produces an Edge Probability Map (right) with texts and textures removed from the input (left). contours accurately. It is because building contours have much longer pixel continuity than text, resulting in higher activation after dilated convolution. After several iterations, the probability of text pixels will vanish, leading to their removal, similarly to texture, as shown in fig. 3. However, the BDCN network works only at the pixel level and cannot guarantee the required topological properties in predicted edge probability maps without additional topological constraints [START_REF] Clough | Explicit topological priors for deep-learning based image segmentation using persistent homology[END_REF], thus the current solution requires knowledge of the number of structures to be retrieved.

Training Annotated historical maps are used to train a BDCN network. The final prediction which is a probability map where each pixel in the maps contain values in range [0, 1] (zero means the pixel does not belong to a contour, one that it does). We train our network from scratch instead of using transfer learning on the edge weights learned from BSDS500 (dataset developed for image boundary detection and segmentation tasks): the features in natural images are very different from our historical map images.We need to filter out most of the texts in our maps, but the network trained on the BSDS dataset does not provide any useful features related to geometric filtering tasks. In order to handle data imbalance during training, we proceed as follows. We define our input image as x ∈ R H•W and ground truth label y ∈ 0, 1 H•W . The output of predicted image is ŷ = f (x, w) ∈ 0, 1 H•W and every element of ŷ is interpreted as the probability of pixel i having label 1: ŷ ≡ p(Y i = 1|x, w). Since the edge detection is a binary classification task, binary cross entropy loss is used as loss function between predictions and ground truths. Due to highly imbalanced edge (97.5%) and nonedge (only 2.5%) classes, extra parameters α, β are used as weights to re-balance the binary cross entropy loss, as

L BCE = -α j∈Y-log(1-ŷj )-β j∈Y+ log( ŷj ) where Y + is the set of indices of edge pixels, Y -is the set of indices of non-edge pixels, α = (λ • |Y -|/(|Y + | + |Y -|))
is the percentage of edge pixels in each batch of historical map image and

β = (|Y + |/(|Y + | + |Y -|))
) is percentage of non-edge pixels. An extra λ = 1.1 factor is used to enhance the percentage of edge pixels in order to give extra weights for edge responses. We build our code based on the BDCN code repository to train our historical map dataset from scratch with a few modifications. We evaluate the loss for every epoch and also for choosing the best training weights. To make the network converging faster, we replace SGD with ADAM optimizer. The initial learning rate is set to 5 × 10 -5 with 0.9 momentum and 0.002 weight decay.

From the Edge Probability Map, we then need to extract boundaries of the objects. In Mathematical Morphology, the Watershed Transform [START_REF] Meyer | Topographic distance and watershed lines[END_REF] is a de-facto standard approach for image segmentation. It has been used in many applications and has been widely studied in terms topological properties [START_REF] Cousty | Watershed cuts: Thinnings, shortest path forests, and topological watersheds[END_REF][START_REF] Roerdink | The watershed transform: Definitions, algorithms and parallelization strategies[END_REF], in terms of algorithms and in terms on computation speed [START_REF] Roerdink | The watershed transform: Definitions, algorithms and parallelization strategies[END_REF][START_REF] Couprie | Quasi-linear algorithms for the topological watershed[END_REF].

It has two well-known issues: the over-segmentation due to the high number of minima, and the gradient leakage that merges regions. There is a third general issue with the watershed that concerns the separation of overlapping or touching objects, but this is not a problem in our case since the map components do not overlap.

Solutions to the over-segmentation problem The first problem is generally solver by filtering the minima first. In [START_REF] Soille | Morphological image analysis: principles and applications[END_REF], the h-minima characterize the importance of each local minimum through their dynamic. When flooding a basin, it actually refers to the water elevation required to merge with another basin. Attributes filter, filters by reconstruction [START_REF] Salembier | Flat zones filtering, connected operators, and filters by reconstruction[END_REF] also allow to eliminate some minima based on their algebraic properties: size, shape, volume. . . Another efficient approach consists in first ordering the way the basins merge to create a hierarchy of partitions and then performing a cut in the hierarchy to get a segmentation with non-meaningful basins removed [START_REF] Beucher | Watershed, hierarchical segmentation and waterfall algorithm[END_REF][START_REF] Perret | Removing non-significant regions in hierarchical clustering and segmentation[END_REF][START_REF] Barcelos | Exploring hierarchy simplification for non-significant region removal[END_REF].

Solutions to the early leakage problem The second problem lies in the quality of the gradient. It has been noted [START_REF] Perret | Evaluation of hierarchical watersheds[END_REF], that (hierarchical) watersheds have better results on non-local supervised gradient estimators. The idea of combining the watershed with high performance contour detector dates back to [START_REF] Arbelaez | Contour detection and hierarchical image segmentation[END_REF].

The relevance of a simple closing by area and dynamic on the edge map produced by our deep-learning edge detector combined with the watershed for this application lies in three points.

First, the minimum size of the components is known. Indeed, the document represents a physical size, and regions whose area is below 100 m 2 are not represented in the map. Thus, we have a strong a priori knowledge we want to inject in the process, the minimum size of the regions (in pixels). This type of constrain is hard to infer in a deep-learning system and we cannot have such guarantees from its output. Having hard guaranties about the shapes and their size is at the foundation of the granulometries in Mathematical Morphology. Moreover, the connected (area) filter used for filtering the edge image ensure that we do not distort the signal at the boundaries of the meaningful regions.

Second, the watershed segmentation method does not rely on the strength of the gradients to select the regions. Even if the edge response is low (i.e, the gradient is weak), the watershed is able to consider this weak response and closes the contour of the region. We do not depend on the strength of the edge response from BDCN which is difficult to calibrate and normalize.

Last but not least, not only the watershed outputs a segmentation but some implementations also produce watershed lines between regions. In our application, watershed lines are even more important than regions because we need to extract polygons for each shape. Event if we could extract boundaries from shows the ability to recover weak boundaries. This sensitivity is not desirable in some cases as it leads the over-segmentation of the 2nd row. The third row suggests that the over-segmentation can be prevented by a stronger filtering but would also lead to a lower shape detection.

regions, it avoids an extra processing step. The watershed lines produced by the algorithm is one pixel-large and are located where the edges are the strongest, i.e., where the network has the strongest response on thick edges. The watershed lines form closed boundaries around regions which is a guarantee we cannot have from the output of a network. Figure 4 shows the strength of the watershed to recover the boundaries of objects even on weak edge responses that would be lost by thresholding the EPM. This is especially visible in the first row where the boundaries of "Place du Châtelet" are leaking; nevertheless they are recovered in the segmentation. On the downside, this ability to recover weak edges is also a bottleneck that can create false-boundaries as shown in the middle row where the place around "Eglise Notre-Dame" is over-segmented because of some detection noise.

The filtering parameters (dynamic h and area λ) are important to control the trade-off between the fact we want to recover small/leaking regions (somewhat related to the recall ) and the false-detection of boundaries (somewhat related to the precision). This is illustrated with two sets of parameters A and B where B has more restrictive filtering parameters. The third row of Figure 4 shows that B has less over-segmentation but in the two first rows, it misses some boundaries.

The decision to merge objects is actually very context-dependant, as it does not depend on the size of the component, neither its volume, nor its shape. The watershed "does its best" to create the missing boundaries and, at the moment, we have not managed to find better rules (e.g., with extinction values of some attributes) to filter out the basins of the watershed.

Evaluation

To assess the performance of the proposed approach, we conducted a series of experiments on a fully manually annotated map sheet. We report here details about this dataset we created and used, the experimental protocol as well as the calibration procedures we followed, the metrics we designed and used, and discuss some results.

Dataset Among the multiple map sheets of the collection of Paris atlases, our work focuses on the particular sheet representing a central area of the city from year 1925 [START_REF]Préfecture de la Seine[END_REF]. We encourage the reader to refer to the supplementary material of this paper for a full-size view of this image. Indeed, such map sheets are large by nature and were digitized with high resolution, resulting in a 8500×6500 image for the area of interest. We carefully annotated the original image by creating line vector information for each edge of each object of interest in the map. It should be noted that only a subset map strokes should be kept as many objets are not relevant for our current study: underground lines and railways, for instance, should be discarded. The resulting vector information was rastered to produce: i) a reference edge map (a small dilation was applied so the resulting edges have a thickness of 3 pixels); ii) a reference label map identifying each shape to be detected. We divided the image into three disjoint subsets: a training set (rows 0 to 3999); a validation set (rows 4000 to 4999); and a test set (rows 5000 to 6500). These areas were divided into 228 disjoint tiles of 500×500 pixels.

Protocol Our goal in the evaluation protocol we designed was the assess the impact of the watershed stage in our pipeline. We compared compared the performance of a baseline system, without watershed, with our proposed approach: the same baseline augmented by a watershed stage (see fig. 2). The baseline (without watershed) consists in a deep edge detection stage using the BDCN network presented in section 3. This stage produces an edge probability map (EPM) as previously explained. The network was trained on the training set using the validation set as control set during training. To generate closed shapes, we simply thresholded the EPM and extracted the connected components. We selected the best performing threshold value (9) on the validation set for fair comparison. The proposed approach (baseline plus watershed) consists in adding a joint filtering on area and dynamic of the EPM followed by a watershed. This approach produces a label map, i.e. a usable set of closed shapes, as detailed in section 4. We selected the best performing values for area (λ) and dynamic h parameters on the validation set. validation and test sets merged) but the performance indicators were computed exclusively on the test set by masking other areas. Metrics While it is common in segmentation challenges to evaluate the quality of object detection by evaluating the precision and recall of edge detection at pixel, such an approach would only evaluate the process halfway to our target application: closed shapes detection. To evaluate shape detection, we need to identify pairs of matching shapes between a reference set (R) and a predictions set (P ). Because, in our particular case, shapes are disjoint among R and also among P (by construction), we can leverage the following property: as soon as the intersection over union (IoU ) between r i ∈ R and p j ∈ P is strictly superior to 0.5, then we know that no other element r k ∈ R, i = k can have a higher IoU with p j ∈ R than r i ∈ R, and reciprocally. As soon as a pair of shapes (r i , p j ) ∈ R × P exhibits IoU(r i , p j ) = area( ri∩pj ri∪pj ) = T > 0.5 then we count a successful match under the threshold constraint T . We introduce this threshold value so as to consider all possible values between 0.5 (exclusive) and 1 (inclusive) and create a global indicator of the system under all potential quality requirements. This allows us to count the number of correctly detected shapes (true positives or TP), missed shapes ((false negatives or FN)), and wrongly predicted shapes ((false positives or FP)) for every operating characteristics. We derive from this set of measures two analysis tools. First a precision ( TP TP+FN ), a recall ( TP TP+FP ) and a F1 score (

) curves for all possible threshold values. They offer a condensed view of the behavior of a system under all possible operating characteristics. We suggest to use the area under the F1 score curve to compute a single performance indicator. The values of this indicator ranges from 0 (worst) to 0.5 (best). The second tool is a pair of visualization maps: a precision map which associates for each predicted shape p j ∈ P the maximal IoU value b pj such as b pj = argmax ri∈R (IoU(r i , p j )), and a recall map which associates for each expected shape r i ∈ R the maximal IoU value b ri such as b ri = argmax pj ∈P (IoU(r i , p j )). Each pixel of each shape is then assigned a color indicating the value of the maximal IoU: red to yellow for values between 0 and 0.5, and yellow to green for values between 0.5 and 1. The darker the green the better the match in both maps. The darker the red, the more serious the false positive (resp. negative) in precision (resp. recall) map.

Results and Discussion

We report here the results for the best calibrated variant of each of the two systems (baseline+connected component labeling vs baseline+watershed) under test. Figure 5 (left) compares the evolution of the F1 score indicator for both systems under each possible IoU threshold. Figure 5 (right) details the different indicators for several key values of IoU thresholds. We can see from those results that the watershed post-processing consistently and significantly improves the quality of the results. The precision and recall maps presented in fig. 6 illustrate the benefits that the watershed post-processing bring to the deep edge segmentation: it adjusts the border of the shapes (improves precision and recall); it also removes small noise (improves precision); and it also efficiently recovers some weak boundaries (improves recall).

Conclusion

In this paper, we propose an efficient combination of convolutional neural networks and mathematical morphology to address the problem of closed shapes extraction in historical maps. Convolutional neural networks (BDCN) allow us to efficiently detect edges while filtering unwanted features (text for instance). Mathematical morphology is applied to the edge probability map created by BDCN to create closed shapes reliably. The efficiency of our approach is shown by testing it on an open dataset. We believe such a method will make the digitization process of historical maps faster and more reliable.

  Challenges in historical maps: (1) planimetric overlap, (2) text overlap, (3) paper folds.

Fig. 1 :

 1 Fig. 1: Contents of a 1925 urban topographic map along with an overview of their challenging properties for automatic feature extraction.

Fig. 4 :

 4 Fig.4: Some failures and some success stories of the watershed segmentation. The parameter sets are A: h=3, λ = 250, and B: h=7, λ = 400. The first row shows the ability to recover weak boundaries. This sensitivity is not desirable in some cases as it leads the over-segmentation of the 2nd row. The third row suggests that the over-segmentation can be prevented by a stronger filtering but would also lead to a lower shape detection.

Fig. 5 :

 5 Fig. 5: Left: comparison of the evolution of the shape detection F1-score across all possible IoU threshold with and without the watershed stage. Right: evaluation metrics with and without watershed.

Fig. 6 :

 6 Fig. 6: Precision and recall maps without and with watershed.

Acknowledgements

This work was partially funded by the French National Research Agency (ANR): Project SoDuCo, grant ANR-18-CE38-0013.

https://github.com/soduco/


Mannoubi Khelil
email: khelilmannoubi@gmail.com

Sami Khomsi

François Roure

Jaume Vergés

Fouad Zargouni

Structural styles of the Tellian fold-and-thrust belt of Tunisia based on structural transects: Insights on subsurface pre-salt play system and future prospectivity

Keywords:

We present several structural transects throughout the internal part of the Tunisian Tell focusing on some representative structures and domains. We used also an interpreted seismic crosssection and a 3-D structural view to highlight the styles within the Tellian Units in the Fernana-Adissa allochthon domain. The structural style is mainly represented by active roof-thrust and duplex structures affecting the Mesozoic-Cenozoic sequences belonging respectively to the Infra-Kasseb and Kasseb Units. The main decollement thrusting layer is located in the Triassic evaporites while the roof thrust corresponds to the Upper-Cretaceous or to the Paleocene shaly levels. The structural transects allow to view that the Numidian Flysch unit presents different structural configurations and relationships with its subsurface supporting units; (1) it rests on an apparent stratigraphic contact above the underlying Kasseb nappe or even over pre-existing Triassic salt structures while elsewhere (2) it is detached above these infra-Numidian units reworking an originally downlap contact. The area is also characterized by important halokinetic movements controlling the overall architecture of the Tell fold and thrust belt that

Introduction and problematics

The Tellian fold-and-thrust belt (Fig. 1) is a collisional belt resulting from the lower Miocene docking of the Kabylian block against the African north-western plate margin [START_REF] Durand-Delga | Le cadre structurale de la Méditerranée occidentale[END_REF][START_REF] Wildi | La chaîne tello-rifaine (Algérie, Maroc, Tunisie): structure, stratigraphie et évolution du Trias au Miocène[END_REF][START_REF] Bouillin | Le "bassin maghrébin"; une ancienne limite entre l'Europe et l'Afrique à l'ouest des Alpes[END_REF]among others). Its internal part is essentially constituted by thick turbiditic series of Oligocene-lower Miocene age [START_REF] Carr | Overthrust emplacement of Numidian Flysch complex in the westernmost Mogod Mountains, Tunisia[END_REF][START_REF] Rouvier | Géologie de l'extrême-nord tunisien: tectonique et paléogéographies superposées à l'extrêmité orientale de la chaîne nord-maghrébine[END_REF]Khomsi et al., 2009b and2021). The relation between the Numidian series, widely outcropping (Fig. 1) and broadly studied [START_REF] Rouvier | Géologie de l'extrême-nord tunisien: tectonique et paléogéographies superposées à l'extrêmité orientale de la chaîne nord-maghrébine[END_REF][START_REF] Yaich | Correlation stratigraphique entre les unites oligo-miocenes de Tunisie centrale et le Numidien[END_REF][START_REF] Talbi | Le Numidien de la Tunisie septentrionale: Données stratigraphiques et interprétation géodynamique[END_REF][START_REF] Riahi | Stratigraphy, sedimentology, and structure of the Numidian flysch thrust belt in northern Tunisia[END_REF][START_REF] Riahi | Age, Internal stratigraphic architecture and structural style of the Oligocene-Miocene Numidian Formation of Northern Tunisia[END_REF][START_REF] Riahi | Sedimentological and stratigraphic constraints on Oligo-Miocene deposition in the Mogod Mountains, northern Tunisia: new insights for paleogeographic evolution of North Africa passive margin[END_REF][START_REF] Thomas | A constrained African craton source for the Cenozoic Numidian Flysch: Implications for the paleogeography of the western Mediterranean Basin[END_REF][START_REF] Guerrera | Tectono-sedimentary evolution of the "Numidian Formation" and lateral facies (southern branch of the western Tethys): Constraints for central-western Mediterranean geodynamics[END_REF][START_REF] Belayouni | Paleogeographic and geodynamic Miocene evolution of the Tunisian Tell (Numidian and post-Numidian successions ), bearing with the Maghrebian Chain[END_REF]among others), and its meso-cenozoic substratum led since a long-time to strong discussions, ending in various and often opposite interpretations. Many authors, i.e., Gottis (1953), [START_REF] Gottis | Stratigraphie, structure et évolution structurale de la Kroumirie et de ses bordures[END_REF], [START_REF] Kujawski | Contribution à la connaissance stratigraphique de base de «flysch» oligocène de l'extrême nord tunisien[END_REF], [START_REF] Crampon | Aperçu structural de la bordure des Mogods, du pays de Bizerte et du nord des Hédil (Tunisie septentrionale)[END_REF], and De [START_REF] Jong | Gravity tectonics or plate tectonics: example of the Numidian Flysch, Tunisia[END_REF], concluded that both the Tellian Mesozoic to Eocene sedimentary units and overlying Numidian Complex represent a conformable sequence of deposits and considered that the Numidian is transgressive over the underlying deformed Tellian series, with a sedimentary hiatus occurring between them during the Priabonian. By contrast, [START_REF] Jauzein | Contribution à l'étude géologique des confins de la dorsale tunisienne[END_REF] had proposed that only the Numidian is allochthonous. Otherwise, [START_REF] Carr | Overthrust emplacement of Numidian Flysch complex in the westernmost Mogod Mountains, Tunisia[END_REF], [START_REF] Rouvier | Géologie de l'extrême-nord tunisien: tectonique et paléogéographies superposées à l'extrêmité orientale de la chaîne nord-maghrébine[END_REF], El [START_REF] El Euchi | Deformation, fluid flow and reservoir appraisal in foreland fold-and-thrust belts[END_REF] and Khomsi et al. (2009b), emphasized the existence a set of "thrust sheets" characterized by the pile of various allochthonous units where the Numidian series are thrusted over the Infra-Numidian units represented by Upper Cretaceous as well as Eocene pelagic limestones of the Kasseb Units. It was shown that the Numidian nappe was defined as an ''out of sequence thrust'' [START_REF] Morley | Out-of-sequence thrusts[END_REF]. Since then, several authors did not admit the existence of nappes in the Tell [START_REF] Tlig | Tectogenèses alpine et atlasique: deux événements distincts dans l'histoire géologique de la Tunisie. Corrélation avec les événements clés en Méditerranée; Alpine and Atlasic tectogenesis: distinct phases of the geological history of Tunisia[END_REF][START_REF] Alouani | Le Jurassique du Nord de la Tunisie. Marqueurs géodynamiques d'une marge transformante : turbidites, radiolarites, plissement et métamorphisme[END_REF][START_REF] Jaafari | Géodynamique, magmatisme basique et minéralisations des Mogods: pétrologie, géochimie et inclusions fluides[END_REF][START_REF] Melki | Tectonique de l'extrémité nord-est de la Tunisie[END_REF][START_REF] Melki | Nouvelles données sur l'évolution structurale de l'extrémité nord-est de la Tunisie (région de Bizerte)[END_REF][START_REF] Talbi | Le Numidien de la Tunisie septentrionale: Données stratigraphiques et interprétation géodynamique[END_REF]. Although the structural styles of the Tellian fold-and-thrust belt is still widely debated, many interpretations from previous work and literature remain controversial. More recently [START_REF] Khomsi | Surface and subsurface architecture of the Kasseb structures: implications for petroleum exploration beneath the Tellian allochthon, the easternmost portion of the Maghrebides[END_REF] emphasized the structural relationships between the Numidian Flysch and the underneath structures of Kasseb, showing strong evidences of an onlapping and stratigraphic contact of the Oligo-Miocene Numidian flysch over the Kasseb Units in a piggy-back configuration.

In this paper, we aim to present the configuration of the structural units in the inner part of the Tellian fold-and-thrust belt along the Tunisia-Algeria borders, deciphering their structural relations and allowing to propose a revised deformational model. The timing and tectonic evolution of this part of the Maghrebides belt are also discussed below. In addition, the Mejerda Basin and the overall structural evolution of the Northern Tunisian Atlas will be also presented, discussed, and integrated in the regional tectonic evolution. This study is based onto the interpretation and integration of all the available surface and subsurface geological and geophysical data by the use of regional structural surface-subsurface transects.

The structural transects in the Tell FABT presented in this work and others [START_REF] El Euchi | Deformation, fluid flow and reservoir appraisal in foreland fold-and-thrust belts[END_REF][START_REF] Aissaoui | Petroleum assessment of Berkine-Ghadames Basin, southern Tunisia[END_REF][START_REF] Tari | Is the Pre-salt sequence underexplored regionally in North Africa?[END_REF][START_REF] Khomsi | A review of the crustal architecture and related pre-salt oil/gas objectives of the eastern Maghreb Atlas and Tell: Need for deep seismic reflection profiling[END_REF]2021) emphasize the interest to undertake oil/gas exploration in the Maghrebides. In fact, two major petroleum systems are potentially productive in the Tell: the Pre-salt Paleozoic system and the Meso-Cenozoic system [START_REF] Khomsi | A review of the crustal architecture and related pre-salt oil/gas objectives of the eastern Maghreb Atlas and Tell: Need for deep seismic reflection profiling[END_REF]2021). In a precedent paper, [START_REF] Khomsi | Surface and subsurface architecture of the Kasseb structures: implications for petroleum exploration beneath the Tellian allochthon, the easternmost portion of the Maghrebides[END_REF] discussed the interest of the Cenozoic-Mesozoic petroleum system underneath the Tellian allochthon units, emphasizing its importance and high potentialities (see [START_REF] Khomsi | Surface and subsurface architecture of the Kasseb structures: implications for petroleum exploration beneath the Tellian allochthon, the easternmost portion of the Maghrebides[END_REF]. Thus, in this paper we will focus only on the Paleozoic system and the pre-salt traps.

Tectonostratigraphy and geology of the Tunisian Tell

The Tunisian Tell (Fig. 1) occupies the eastern part of the Tell-Rif orogenic system [START_REF] Caire | Chaînes alpines de la Méditerranée centrale (Algérie et Tunisie septentrionales, Sicile, Calabre et Apennin méridional)[END_REF][START_REF] Gélard | Géologie du nord-est de la Grande Kabylie (un segment interne de l'orogène littoral nord-africaine[END_REF][START_REF] Rouvier | Géologie de l'extrême-nord tunisien: tectonique et paléogéographies superposées à l'extrêmité orientale de la chaîne nord-maghrébine[END_REF]Khomsi et al., 2009b and2016) and results from the progressive closure of the Maghrebian Tethys during the Miocene (Frizon de Lamotte et al., 2009;[START_REF] Khomsi | The Late Eocene and Late Miocene fronts of the Atlas Belt in eastern Maghreb: integration in the geodynamic evolution of the Mediterranean Domain[END_REF]. The compressional deformational pulses reached their climax in the late Miocene period, resulting in the development of a south-verging accretionary prism in front of the AlKaPeCa (for Alboran, Kabylies, Peloritan and Calabria; [START_REF] Bouillin | Le "bassin maghrébin"; une ancienne limite entre l'Europe et l'Afrique à l'ouest des Alpes[END_REF] domain until its collision/docking against northern Africa [START_REF] Catalano | Paleogeography and structure of the central Mediterranean: Sicily and its offshore area[END_REF][START_REF] Doglioni | On the interference between the early Apennines-Maghrebides back arc extension and the Alps-Betics orogen in the Neogene geodynamics of the western Mediterranean[END_REF]Vergès and Sàbat, 1999;[START_REF] Jolivet | Mediterranean extension and the Africa-Eurasia collision[END_REF][START_REF] Benaouali-Mebarek | Post-Cretaceous kinematics of the Atlas and Tell systems in central Algeria : Early foreland folding and subduction-related deformation[END_REF][START_REF] Roure | Alpine inversion of the North African margin and delamination of its continental lithosphere[END_REF][START_REF] Van Hinsbergen | Origin and consequences of western Mediterranean subduction, rollback, and slab segmentation[END_REF][START_REF] Khomsi | The Late Eocene and Late Miocene fronts of the Atlas Belt in eastern Maghreb: integration in the geodynamic evolution of the Mediterranean Domain[END_REF].., 2019). Vergés Mebarek

The Tell of northern Tunisia (Fig. 1) is characterized by several structural units detached and transported toward the southeast. Previous works refer to a geographically variable nomenclature, leading to confusions (see [START_REF] Khomsi | Surface and subsurface architecture of the Kasseb structures: implications for petroleum exploration beneath the Tellian allochthon, the easternmost portion of the Maghrebides[END_REF], with the definition of more than 7 structural units in northern Tunisia, based only on typology, lateral facies variations, paleoenvironment records and biostratigraphic contents. However, and in line with Khomsi et al., (2021) we emphasize that the Tell series of Tunisia can be better subdivided into three major tectono-stratigraphic packages [START_REF] Khomsi | A review of the crustal architecture and related pre-salt oil/gas objectives of the eastern Maghreb Atlas and Tell: Need for deep seismic reflection profiling[END_REF]2021), which account for the overall stratigraphic and structural evolution of the Tell domain of Tunisia and its tectonic history: (i) the allochthonous units represented by the Kasseb Units and their lateral equivalents supporting the Numidian Flysch deposits. The Kasseb Units rest tectonically on the Langhian Miocene deposits pertaining to the foreland domain of the Tell, (ii) the foreland structures of the Tell represented by the Meso-Cenozoic deposits ending with the major angular/erosional unconformity at the base of the Langhian deposits set up during the post-Eocene compressions.

(iii), the foredeep synkinematic layers of the late Miocene-Quaternary deposits issued from the recent erosion of the rejuvenated structures with in many transported mini-basins of more than 1000 m of continental deposits.

The following part deals with the description of the lateral facies changes observed between the various structural units, resulting in our subdivision which are therefore based on field observations, geological mapping, and key cross-sections. They are described from external to internal part of the Alpine chain of northern Tunisia.

Décollement levels

We have used field data supported by seismic and wells data, to localize and identify individual detachment levels (Fig. 2). Obviously, salt is constituting mechanically weak detachment horizon. Therefore, Upper Triassic series, which are made up thick salt layers of hundred meters, acted as the main décollement level at the sole and within the Tellian fold-and-thrust system. Thick Cretaceous and Paleogene marls series constitute secondary potential décollement levels. Examining the lithostratigraphic column, we have identified numerous weak horizons within the Lower Cretaceous, Coniacian-Santonian, Paleocene, middle to upper-Eocene and infra-Numidian series (Fig. 2).

Importance of the Triassic salt

Both in the onshore as well as in the offshore of the Tellian domain in Tunisia, seismic interpretations [START_REF] El Euchi | Deformation, fluid flow and reservoir appraisal in foreland fold-and-thrust belts[END_REF]Khomsi et al., 2009b and2019) show a structural style characterized by Numidian thrust sheets detached over the Triassic salt. whereas the underlying para-autochthonous series were affected by inherited faults and tilted blocks which were subsequently inverted during Eocene and late Miocene compressional events. The Numidian thrust faults are deeply rooted within the major décollement level represented by the interface between allochthonous Triassic salt and the underlying Mesozoic-Cenozoic paraautochthonous cover. [START_REF] Vila | La chaîne alpine de l'Algérie orientale et des confins algéro-tunisiens[END_REF], and more recently, [START_REF] Amri | Interconnection salt diapir-allochthonous salt sheet in northern Tunisia: The Lansarine-Baoula case study[END_REF] and [START_REF] Khelil | Late Miocene-Quaternary thrusting in the Utique-Kechabta foreland basin of the Tell, northern Tunisia[END_REF] recognized the occurrence of salt canopies in the foreland domain of the Tell in Algeria and Tunisia, respectively, that might correspond to the long Morocco-Algeria-Tunisia diapiric province that was active since the Early Jurassic as described in [START_REF] Moragas | Jurassic rifting to postrift subsidence analysis in the Central High Atlas and its relation to salt diapirism[END_REF].

On the other hand, it is worth noting that the evaporitic facies of the Upper Triassic series change laterally from SW to NE into dolomites/carbonates when approaching the coastline and in the northern part of the Gulf of Tunis. In fact, in the northern part of the Tell, the Triassic series become more dolomitic with a decrease in the evaporite proportion leading possibly to pure dolomitic series in the Gulf of Tunis (Fig. 3).

Fig. 3

Present distribution of the Late Triassic facies in Tunisia, Sicily channel, and Sicily [START_REF] Casero | Neogene deformations at the Sicilian-north African plate boundary[END_REF][START_REF] Catalano | Paleogeography and structure of the central Mediterranean: Sicily and its offshore area[END_REF][START_REF] Stefano | Late Paleozoic-Early Mesozoic stratigraphy and paleogeography of Sicily. Timescales and basin dynamics: Sicily, the adjacent Mediterranean and other natural laboratories: Palermo[END_REF][START_REF] Kamoun | Triassic palaeogeography of Tunisia[END_REF][START_REF] Soto | Permo-Triassic salt provinces of Europe, North Africa and the Atlantic margins: tectonics and hydrocarbon potential[END_REF]in Khelil et al. 2021).

Otherwise, and despite the fact that few petroleum wells have been drilled in the Tell, either in the onshore or the offshore, the Triassic series encountered in the wells of the Gulf of Tunis are rather thick with dolomitic facies in their upper part [START_REF] Melki | Tectono-sedimentary events and geodynamic evolution of the Mesozoic and Cenozoic basins of the Alpine Margin, Gulf of Tunis, northeastern Tunisia offshore[END_REF]. By contrast, the Mejerda well shows a predominance of the evaporitic component [START_REF] Troudi | Styles of salt tectonics in central Tunisia: An overview, Permo-Triassic salt Provinces of Europe, North Africa and the Atlantic margins: Tectonics and hydrocarbon potential[END_REF]) (Fig. 2).

These changes are important in terms of structural control of the deformations and the occurrence of alternative detachment layers, which do not involve here the Triassic-Jurassic interface as in the onshore, but rather involve shallower detachment layers situated within Upper Cretaceous and Paleogene in the offshore.

The southeastern edge of the Tell corresponds to a complicated foredeep [START_REF] Khomsi | A review of the crustal architecture and related pre-salt oil/gas objectives of the eastern Maghreb Atlas and Tell: Need for deep seismic reflection profiling[END_REF][START_REF] Khelil | How to build an extensional basin in a contractional setting? Numerical and physical modelling applied to the Mejerda Basin at the front of the eastern Tell of Tunisia[END_REF]2021) tectonic domain in front of the Tell, in the footwall of the South Tellian thrust front. This area comprises Triassic salt plugs associated with thrusting that occurred during strong late Miocene-Quaternary Alpine compressions [START_REF] Khomsi | A review of the crustal architecture and related pre-salt oil/gas objectives of the eastern Maghreb Atlas and Tell: Need for deep seismic reflection profiling[END_REF][START_REF] Khelil | Late Miocene-Quaternary thrusting in the Utique-Kechabta foreland basin of the Tell, northern Tunisia[END_REF]. The South Tellian Front is also located in this important deformation zone, along which the Tell domain overthrusts the Neogene foredeep deposits. In fact, this foredeep basin, parallel to the Tellian fold-and-thrust belt, presents locally a more than 1300 m-thick Miocene sedimentary sequences, mainly constituted by continental molasse and evaporiticlacustrine late Miocene deposits. It constitutes the uppermost part of the sedimentary infill of the Mejerda Valley, reworking some allochthonous Tellian tectonic units and lying directly on top of the underlying Triassic halokinetic series (Khomsi et al. 2009b[START_REF] Khomsi | The Late Eocene and Late Miocene fronts of the Atlas Belt in eastern Maghreb: integration in the geodynamic evolution of the Mediterranean Domain[END_REF]. The Mejerda well (Fig. 2), drilled in the central part of the basin, shows a highly complicated setting with late Miocene-Pliocene continental deposits overlying unconformably the Triassic evaporites. The latter are locally thrusted on top of the tectonic pile, but elsewhere the Triassic salt remains buried beneath the Upper Cretaceous-Cenomanian strata (Fig. 2).

The tectono-stratigraphic units

2.3.1-The Infra-Kasseb unit

The Infra-Kasseb tectonic unit (Figs 2 and4) comprises the entire Mesozoic succession with an average thickness of 2500 m. Its lower part is made up of Triassic salt and breccia which pass to Jurassic carbonates upwards. The Early Cretaceous is characterized essentially by thick clay strata, sandstone at the bottom and carbonate turbidites at the top of the sequence. The upper part of the Cretaceous succession is made up of interbedded shales and carbonates recording deep-water paleo-depositional environments [START_REF] Khomsi | Surface and subsurface architecture of the Kasseb structures: implications for petroleum exploration beneath the Tellian allochthon, the easternmost portion of the Maghrebides[END_REF]. The top of this tectonic unit exhibits shales of Paleocene age, which also form the lower part of the Kasseb unit ensuring also the decoupling of the Kasseb Units from the underlying Cretaceous structures.

2.3.2-The Kasseb unit

The Kasseb unit (Fig. 2 and4) is cropping out mainly at the front of the Numidian structures.

It comprises Paleocene marls and predominantly lower Eocene limestones (Globigerina facies),

upper Eocene marls and Oligocene glauconitic sandstones. Worth noting that these Oligocene sandstones rest unconformably on top of the Paleogene succession and form a lateral equivalent of the Numidian succession with a shallower water paleo-depositional environment. The Kasseb unit of few hundred meters is thrusted mostly on top of Miocene strata.

2.3.3-The Numidian unit

The Numidian (Fig. 2 and4) is an important structural unit of the Tellian fold-and-thrust belt.

It consists of an alternation of mudstones and quartzarenites with rhythmic character, up to 2500 m thick. The authors agree on the deep-water paleo-depositional environment of the Numidian domain and the turbiditic character of the sedimentation (e.g., [START_REF] Riahi | Stratigraphy, sedimentology, and structure of the Numidian flysch thrust belt in northern Tunisia[END_REF][START_REF] Riahi | Age, Internal stratigraphic architecture and structural style of the Oligocene-Miocene Numidian Formation of Northern Tunisia[END_REF][START_REF] Riahi | Sedimentological and stratigraphic constraints on Oligo-Miocene deposition in the Mogod Mountains, northern Tunisia: new insights for paleogeographic evolution of North Africa passive margin[END_REF]. In northern Tunisia, it is a clay-sandstone formation that crops out over wide areas. It is considered as the shallowest structural unit, resting at the top of the tectonic pile over underlying Infra-Numidian units, Triassic breccia, and younger upper Miocene deposits.

It is worth noting that the Numidian unit crops out along more than 2000 km, from Spain to Italy, across the northernmost parts of Morocco, Algeria, and Tunisia.

Structural styles derived from regional transects and seismic interpretations

Several tectonic phases have impacted the geological history of the Tell, leading to a complex structural configuration. The presented structural cross-sections and transects (Fig. 5) will be discussed to document the structural styles observed within the Tell and Northern Atlas, as well as the geological processes responsible for their set up, and finally the tectonic agenda. These interpretative structural transects were compiled as including set of data field observations collected through several field-work campaigns in the Tell of Tunisia. The Jebel Rebia, Fernana, the jebels Adissa and Bessouagui, and ultimately the Kasseb region, constitute keyoutcrops structures for documenting the overall structural style in the Tell. The foreland of the latter (Mejerda Basin-Northern Atlas; Figs 2 and5) is mostly constrained by seismic interpretations in addition to field observations.

Tell fold-and-thrust belt

3.1.1-Fernana cross-sections

The Fernana area is documented here by a set of cross-sections cutting the structures in two orthogonal directions (two NW-SE cross-sections as well as one NE-SW cross-section; positions given in Fig. 5).

3.1.1.1-Fernana cross-section (CS1)

Along the first cross-section, two key outcrops have been investigated in details in order to better understand the tectonic position of the Kasseb unit (in Jebel Rebia; Fig. 5 and6) and of the Numidian unit (in Jebel Adissa; Fig. 5 and6). The Jebel Rebia corresponds to an thrust anticline structure (Fig. 7) trending N035, limited to the south by the Mejerda Valley (Fig. 5), which is considered as a wedge-top basin (Khelil et al. 2019[START_REF] Khelil | Reply to comment on "how to build an extensional basin in a contractional setting? Numerical and physical modeling applied to the Mejerda Basin at the front of the eastern Tell of Tunisia[END_REF](Khelil et al. , 2021)). Locally to the south, blue Jurassic dolomites crop out, forming a part of the Hairech inherited paleo-high (see [START_REF] Martinez | Évolution structurale et paléogéographie de la Tunisie[END_REF]. In the Jebel Rebia, the Kasseb unit overlies tectonically the Oligo-Miocene series (Fig. 7 and Fig. 8A, B andC) and the underlying Campanian carbonates which form the northern limb of the para-autochthonous anticline (Figs 7 and8C). This structural unit is affected by a normal fault delimiting to the north the Fernana wedge-top basin (Figs 6 and7), which is filled by late Miocene deposits derived from the erosion of different units. The Infra-Kasseb and Kasseb structural units, covered by the Fernana wedge-top basin, show different structural styles (Fig. 6). The first one is characterized by a duplex geometry with a set of sub-parallel ramps (Fig. 6).

The sole-thrust corresponds to the Upper Triassic salt representing the main regional décollement. The ramps are merged upward into the upper roof thrust, which is located in the Paleocene marls. The stacked imbricated horses form an antiformal stack (Fig. 6). Then, the Kasseb unit, derived from the unroofing of the underlying duplexes, has been detached from its initial substratum and moved forelandward to the south, defining thereby an active roof duplex.

The forward propagation of the roof-thrust induced the formation of ramps (Fig. 6). The roofthrust is emerging at Jebel Rebia (Figs 6 and7) while individual ramps were covered later by post-orogenic molasse deposits, after the set-up of the Fernana wedge-top basin. Because they are located beneath the thick and widely distributed Numidian Flysch unit, Infra-Numidian units are poorly exposed at the surface in northern Tunisia, except in the Nefza and Jebel Ben Amara tectonic windows [START_REF] Khomsi | Surface and subsurface architecture of the Kasseb structures: implications for petroleum exploration beneath the Tellian allochthon, the easternmost portion of the Maghrebides[END_REF]. Casually, these sparsely exposed series crop out around the Jebel Adissa which forms a Numidian Klippe. Therefore, we considered the Jebel Adissa as an important sector to study the relations and styles of these various structural units.

The Numidian sandstones of Jebel Adissa are truncated at a north-vergent contact and overlie tectonically the Kasseb unit. The basal thrust of the Numidian allochthon is well exposed and cuts down-section in the transport direction (Figs 6 and9). According to [START_REF] Morley | Out-of-sequence thrusts[END_REF] an outof-sequence thrusting model could explain such thrust geometry, which is mostly characterized by simple relationships, younger rocks over older ones, in contrast to a low-angle normal fault model proposed in former publications [START_REF] Jong | Gravity tectonics or plate tectonics: example of the Numidian Flysch, Tunisia[END_REF][START_REF] Rouvier | Géologie de l'extrême-nord tunisien: tectonique et paléogéographies superposées à l'extrêmité orientale de la chaîne nord-maghrébine[END_REF]. Nevertheless, we agree that this fault is not a thrust but instead corresponds to the frontal part of a low-angle listric fault (spoon-shaped) which resulted from slope instabilities and then to topographic readjustments.

Originally, the contact is a sedimentary contact marked by downlaps/lateral onlapping (Fig. 9).

The upper Paleogene unit shows a duplex configuration with the Paleocene marls form the basal décollement (Figs 6 and9A). Most of the outcropping series are dipping towards the north, forming duplicated slices. However, we have noticed at Jebel El Tebaga the occurrence of numerous folds (Fig. 9A). This is related to the impediment of thrust propagation caused by the Cap Serrat-Ghardimaou master fault. This is confirmed by the development of back-thrusting at Kef Ach El Agab (Figs 6 and9A).

3.1.1.2-Fernana cross-section (CS2)

The second cross-section (CS2; position given in Fig. 5 This diapiric structure is limited by two faults: the western fault on the western limb and eastern fault on the eastern limb. This diapir seems to be controlled by a major and well-know tectonic lineament of the Tunisian Tell , i.e., the Cap Serrat Ghardimaou Fault (CSGF; Fig. 5). On its western edge, the diapir of Jebel Debagh overhangs the eastern limb of the Jebel Ed Diss anticline, thus corresponding to a major pop-up structure controlled by two opposite dipping thrust faults. The Triassic salt overhangs eroded Late Cretaceous strata (Fig. 10). To the east of the CSGF and Jebel Debagh, the transect crosses the Jebel Bent Ahmed structure, which corresponds to an outcropping syncline structure. There, a low-angle out-of-the-syncline thrust has unrooted and transported horizontally the Triassic salt, overhanging a deeper duplex anticlinorium (Fig. 10). The later is made by duplications of Jurassic-Cretaceous strata, admitting the Triassic salt as the lower décollement level and the Late Cretaceous-Paleocene shales as roof décollement. To the east, the Triassic salt of the Jebel Bent Ahmed overhangs tectonically, at its most eastern edge, the late Miocene Fernana wedge-top basin, which is filled by more than 1200 m of late Miocene to Quaternary continental sands, shales and conglomerates. These wedge-top basin deposits overlap unconformably the underlying duplex structure made up by the duplicated Kasseb unit

3.1.1.3-Fernana cross-section (CS3)

This transect has a strike direction (CS3; position given in Fig. 5), being parallel to the dip of strata. It was compiled to show a three dimensional view of the structure and better characterize two distinct sets of faults, i.e. NE-trending thrust faults and orthogonal NW-trending transcurrent polyphased faults. In fact, we can distinguish 4 major subvertical and transcurrent faults separating 5 strucrural compartments, i.e., from SW to NE: Rejel El Mergeb, Jebel Bent Ahmed, Ghezala River, Jebel El Ach and Jebel Es Solah (Fig. 10).

These different panels are resulting from the tectonic activity of the inferred strike-slip faults which seem to control block tilting and differential infilling for the late Miocene-Quaternary wedge-top deposits. The recent reactivation of this transcurrent fault system seems post-date the emplacement of the nappes duplexes, because the nappes contacts are cross cut by the system orthogonally.

A 3-D view with map allows to visualize the junction of the different units spatially (Fig. 11). It is worth notice that the Triassic salt diapir south of Jebel Ed Diss is crosscut by the southernmost strike -slip fault, with a left-lateral horizontal offset of more than 2 km (Fig. 11).

3.1.2-Jebels Bessouagui-Hairech cross-section (CS4)

North of Jebel Bessouagui (Fig. 12A), the Numidian, Kasseb and the infra-Kasseb units are involved in the same overall structure, which is best described as an antiformal stack of duplexes (Fig. 12A).

Fig. 12 NW-SE cross-section through Jebels Hairech, Bou Khezara and Bessouagui (A),

showing the main structures and structural styles (the location is given in Fig. 5) and looking

E interpreted field photo (B), showing a duplex structure affecting Upper Cretaceous series at Jebel Bessouagui (the location of the photo is located in A).

This entire structure is thrusted forelandward over the infra-Kasseb series; the sole thrust corresponds to the Upper Triassic evaporite, which represents the main regional décollement.

On the one hand, the Kasseb unit issued from the unroofing of the underlying unit is detached and moved forward farther to the south (Fig. 12A). The Santonian shale forms an intermediate décollement layer with the development of duplex or horse structures made up of Campanian-Maastrichtian limestones and upper Miocene deposits (Fig. 12A andB). Viewing towards the north, a field photo (Fig. 12B) shows a syncline formed by the same series, lying over the Triassic salt. This syncline is interpreted as resulting from an out-of-the-syncline thrust detached and transported towards the south over the salt. On the other hand, south of CSGF, the Jebel Bou Khezara is essentially formed by the Numidian Flysch. The latter overlies tectonically the upper Miocene to Quaternary deposits which are essentially made up by shales, gypsum and conglomerates (Figs 12A and 13). The Triassic salt and tectonic breccia constitute the tectonic contact supporting the Numidian Flysch (Fig. 12A and13). These evaporites are likely to correspond to a former salt glacier or salt canopy developed along the Cap Serrat-Ghardimaou Fault (CSGF) during the Cretaceous.

The Jebel Chouichia (Fig. 12) corresponds to a monocline structure trending N035, limited to the north by the Fernana wedge-top basin and to the south by the Triassic and Jurassic rocks belonging to Jebel Hairech. In the same area, the Kasseb unit series are cropping out at Jebel Chouichia. We have adopted here the same structural interpretation as for the Jebel Rebia crosssection, with the Infra-Kasseb and Kasseb structural units covered by the Fernana wedge-top basin (Fig. 12A), thus characterizing again an antiformal stack. The thrust faults join together upward into an upper roof thrust made up of Paleocene marls. In a similar way as in the previous cross-section, the Kasseb unit issued from the unroofing of the underlying duplexes is detached and moved forward towards the south, thereby accounting for an active roof-duplex.

3.1.3-Kasseb cross-sections

The Kasseb area (Fig. 5), where the Kasseb unit was first described by [START_REF] Rouvier | Géologie de l'extrême-nord tunisien: tectonique et paléogéographies superposées à l'extrêmité orientale de la chaîne nord-maghrébine[END_REF], contains important outcrops outlining the different structural relations between the Kasseb unit, the Numidian unit and overlying Miocene series, characterized by the Bejaoua facies (crosssections, Fig. 14). Along their southern and eastern limits, the Ypresian limestones, made up of pelagic and organic-rich Globigerina facies [START_REF] Khomsi | Thrust belt systems of northern Africa. Petroleum implications. Field trip guide[END_REF][START_REF] Khomsi | Surface and subsurface architecture of the Kasseb structures: implications for petroleum exploration beneath the Tellian allochthon, the easternmost portion of the Maghrebides[END_REF], overhang tectonically the Aquitanian-Langhian marine sandstones of the Bejaoua Group (Fig. 15A). These Miocene deposits are unconformable over the Mesozoic to Paleogene series, including the Ypresian Numulitic limestones which belong to the para-autochthonous sedimentary package equivalent in time of the Kasseb deep marine sequence.

Fig. 14 NNW-SSE cross-sections through Kef Sidi Youssef (CS5) and Kef Es Salah (CS6)

showing the structures at the front of the Kasseb unit; location given in Fig. 5.

The para-autochthonous series were presumably folded during the middle to late Eocene (the so-called "Atlas Event" (e.g. Frizon de [START_REF] De Lamotte | The two main steps of the Atlas building and geodynamics of the western Mediterranean[END_REF]Lamotte et al., , 2009;;[START_REF] Khomsi | Mise en évidence d'un nouveau front de chevauchement dans l'Atlas tunisien oriental de Tunisie par sismique réflexion. Contexte structural régional et rôle du Trias salifère[END_REF]Khomsi et al., , 2009a;;[START_REF] De Lamotte | About the Cenozoic inversion of the Atlas domain in the North Africa[END_REF] while the overthrusting of the Kasseb unit over underlying Miocene deposits occurred during the Neogene Alpine orogeny.

Fig. 15 Interpreted field photos showing the structural relations between the different unit; (A) Looking NE photo of the Ypresian limestones thrusts lower-Miocene sandstones, (B) and Looking to the North panorama showing duplications of the Ypresian limestones belonging to the Kasseb unit. Notice the apparent stratigraphic continuity of the Numidian over the Kasseb

Unit.

In the Kasseb area, to the northeast of Jebel Bou Khezara (Figs. 12 and 13), the Numidian Flysch unit occupies different structural positions. In fact, the Numidian sandstone bars show an apparent continuous stratigraphic contact [START_REF] Khomsi | Surface and subsurface architecture of the Kasseb structures: implications for petroleum exploration beneath the Tellian allochthon, the easternmost portion of the Maghrebides[END_REF] with underlying deep marine deposits of the Kasseb unit (i.e. Kef Es Salah; CS6 of Fig. 14, and Figs. 16A andB).

Alternatively, some contacts between the two units correspond to a synsedimentary normal fault (i.e. Kef Sidi Youssef; CS5 of Figs. 14 and16C) which can be attributed to the Oligocene quiescence period (Khomsi et al. 2009b[START_REF] Khomsi | The Late Eocene and Late Miocene fronts of the Atlas Belt in eastern Maghreb: integration in the geodynamic evolution of the Mediterranean Domain[END_REF]. Because the Numidian Flysch in this region presents locally an apparent stratigraphic continuity with the Kasseb unit, it was thus transported and thrusted towards the south jointly with the Kasseb unit, this unique Kasseb-Numidian assemblage overhanging the already structured para-autochthonous Mesozoic to Paleogene series (Fig. 14).

Fig. 16 Different relations between the Kasseb and Numidian units. (A and B) Looking respectively E and N photos showing conformal serie formed by Ypresian limestones of Kasseb unit and the Numidian sandstones and (C) looking W photo of a normal fault separating the Kasseb and Numidian units. Location is given in Fig. 5 WOULD YOU LIKE TO SEND ME THE FULL INTERPRETATION OF THE FIG. 16A??

At Kef Es Salah (CS6 of Fig. 14), the duplicated Eocene limestones belonging to the Kasseb unit form a duplex (Fig. 14 and15B) with a sole thrust located in the Paleocene shales.

On the one hand, the Mesozoic series (namely Zaflana section, [START_REF] Rouvier | Géologie de l'extrême-nord tunisien: tectonique et paléogéographies superposées à l'extrêmité orientale de la chaîne nord-maghrébine[END_REF] in front of the Kasseb unit pertains to the para-autochthonous series. It is affected by a set of thrust faults duplicating the Upper Cretaceous series which crop out around Kef Guaria and to the north of Kef Rhezal (Fig. 14).

On the other hand, and as a whole, the structural configuration of the Kasseb area is complicated by the N160 Balta sinistral strike-slip fault which affects the whole structure and contributes to its tectonic transport towards the south. In fact, the horizontal lateral displacement of the Numidian front amounts to 6 km along this transcurrent fault (Fig. 5).

South-Tellian-Front: the Mejerda Basin

The upper Miocene Mejerda Basin is a major graben lying at the boundary between two distinct fold-and-thrust belts: The allochthonous Tell to the north and the inverted foreland domain of the Northern Atlas to south [START_REF] Khelil | How to build an extensional basin in a contractional setting? Numerical and physical modelling applied to the Mejerda Basin at the front of the eastern Tell of Tunisia[END_REF]2020). It is filled by upper Miocene to Quaternary deposits. Several diapirs are documented in the Mejerda zone [START_REF] El Euchi | Deformation, fluid flow and reservoir appraisal in foreland fold-and-thrust belts[END_REF][START_REF] Khomsi | An overview of the Late Cretaceous-Eocene positive inversions and Oligo-Miocene subsidence events in the foreland of the Tunisian Atlas: Structural style and implications for the tectonic agenda of the Maghrebian Atlas system[END_REF][START_REF] Amiri | Role of preexisting faults in the geodynamic evolution of northern Tunisia, insights from gravity data from the Medjerda Valley[END_REF][START_REF] Ayed-Khaled | New evidence on the geologic setting of Medjerda Valley plain (northern Tunisia) from integrated geophysical study of Triassic evaporite bodies[END_REF]. Although developed in an overall contractional context with the thrust emplacement of allochthonous nappes and mountain-building processes of the Tell, the Mejerda Basin is characterized by dominantly extensional deformation affecting Neogene-Quaternary deposits [START_REF] Khelil | How to build an extensional basin in a contractional setting? Numerical and physical modelling applied to the Mejerda Basin at the front of the eastern Tell of Tunisia[END_REF]2020).

The structural interpretation of the seismic section (Line 1; Fig. 17) documents the occurrence of a major extensional tectonic event as outlined by a set of normal and faults (Fig. 17), which controlled local thickness variations within the Miocene to Quaternary series. The innermost normal fault represents the northern tectonic boundary separating the Mejerda Basin from the Tell. This extensional fault boundary does not match with the overall regional Miocene to Present-day tectonic framework which corresponds to a contractional episode. Therefore, the Mejerda Basin build up is a result of special conditions [START_REF] Khelil | How to build an extensional basin in a contractional setting? Numerical and physical modelling applied to the Mejerda Basin at the front of the eastern Tell of Tunisia[END_REF]) related to salt diapirism that created several deep minibasins (Fig. 17).

Fig. 17 Seismic line interpretation (line 1) through the Mejerda basin outlining the extensional

faults and important salt activities; location is given in Fig. 5 The Jebel Hairech (Figs 5 and12) is made up of two NE-trending anticlines structures, separated by a syncline showing highly brecciation (Ghorabi and Henry, 1991;[START_REF] Martinez | Évolution structurale et paléogéographie de la Tunisie[END_REF]. It presents two distinct stratigraphic series; the first one is Triassic [START_REF] Perthuisot | Dynamique et pétrogenèse des extrusions triasiques en Tunisie septentrionale[END_REF][START_REF] Rouvier | Géologie de l'extrême-nord tunisien: tectonique et paléogéographies superposées à l'extrêmité orientale de la chaîne nord-maghrébine[END_REF]) and the second one dates from Liassic times [START_REF] Rouvier | Géologie de l'extrême-nord tunisien: tectonique et paléogéographies superposées à l'extrêmité orientale de la chaîne nord-maghrébine[END_REF]Alouani 1991). These rocks were affected according to (Ghorabi and Henry 1991) by burial metamorphism, which caused crystallization of strongly oriented phyllosilicates. This metamorphism was developed before the main Alpine folding and after a local tilting episode (Ghorabi and Henry 1991). The exhumation of these metamorphic rocks is related to the local extension characterizing the Mejerda Basin in front of the Tellian wedge during the Alpine orogeny.

Discussion

Revised tectonic styles

The analysis of field data allowed us to distinguish several superimposed structural units, resulting from thrust tectonics with horizontal displacements of different styles and magnitudes.

For instance, we have differentiated three main structural units, i.e., from bottom to top, .the Infra-Kasseb, Kasseb and Numidian units, and the two first units being also known as Infra-Numidian units.

Through the observations made in this paper, several deformation styles (Fig. 18) are described:

(II)

The roof thrust (Incipient Kasseb décollement, Fig. 18A) was activated to displace shallower and undeformed Paleogene units towards the south (Fig. 18C). The forward propagation of this roof thrust was hampered, inducing the propagation of thrusts affecting the Kasseb unit, thus resulting in local duplex structures Fig. 18C and D). The Kasseb unit is interpreted as made up of a succession of active roofthrust structures (Figs 6,10 and 12;i.e.,Jebel Rebia).

(III) The Numidian unit overlies these tectonic duplex/horse units and shows different deformation styles:

(III-1) it constitutes a neo-autochthon series (Fig. 18D), transported jointly towards the south with the Kasseb unit (i.e., in the Kasseb area) or (III-2) The Numidian unit is also locally and slightly displaced towards the South near the morphologic top of the Tell over north-verging low-angle normal faults remobilizing mostly Triassic salt (i.e., Jebel Bou Khezara; Fig. 18C), due to topographic instability induced by the stacking of the underlying structural units.

The salt was cropping out during pre-inversion times (Fig. 18B; [START_REF] Khelil | Reply to comment on "how to build an extensional basin in a contractional setting? Numerical and physical modeling applied to the Mejerda Basin at the front of the eastern Tell of Tunisia[END_REF].

(IV) Localized wedge-top basins (Fig. 18E) are filled with Mio-Plio-Quaternary deposits and developed since the late-Miocene in a similar way as in the model proposed by [START_REF] Platt | Stratal extension in thrust footwalls, Makran accretionary prism: implications for thrust tectonics[END_REF]. These minibasins are mostly related to the wedge topographic readjustment or mostly in relation with salt collapse (Figs 18E and F).

Regional tectonic events

In this final part, we propose a tectonic calendar that encompasses the regional events affecting the Tellian zone. The timing of this tectonic evolution is based on our observations and interpretations, coupled to those of our antecessors (e.g. [START_REF] Carr | Overthrust emplacement of Numidian Flysch complex in the westernmost Mogod Mountains, Tunisia[END_REF][START_REF] Rouvier | Géologie de l'extrême-nord tunisien: tectonique et paléogéographies superposées à l'extrêmité orientale de la chaîne nord-maghrébine[END_REF][START_REF] Morley | Out-of-sequence thrusts[END_REF][START_REF] Khomsi | Mise en évidence d'un nouveau front de chevauchement dans l'Atlas tunisien oriental de Tunisie par sismique réflexion. Contexte structural régional et rôle du Trias salifère[END_REF]Khomsi et al., , 2009bKhomsi et al., , 2009a) ) as well as on the tectonic scenario documented in Algeria (e.g. Frizon de [START_REF] De Lamotte | The two main steps of the Atlas building and geodynamics of the western Mediterranean[END_REF][START_REF] Bracène | The origin of intraplate deformation in the Atlas system of western and central Algeria: From Jurassic rifting to Cenozoic-Quaternary inversion[END_REF][START_REF] Benaouali-Mebarek | Post-Cretaceous kinematics of the Atlas and Tell systems in central Algeria : Early foreland folding and subduction-related deformation[END_REF][START_REF] Roure | Alpine inversion of the North African margin and delamination of its continental lithosphere[END_REF] and Morocco (e.g. Frizon de Lamotte et al., 2000;[START_REF] Missenard | Tectonic styles in the Marrakesh High Atlas (Morocco): The role of heritage and mechanical stratigraphy[END_REF].

The former Tellian basin, along the African plate margin, registered multiple rifting and inversion episodes:

(1) The main rifting stage that highlighted the Northern African Tethyan margin started in late Jurassic and continued during the Early Cretaceous. The NE-trending master faults cropping out in northern Tunisia (i.e., Cap Serrat-Ghardimaou), are involving the infra-Mesozoic substratum, being, inherited from this rifting episode:

(2) The Mesozoic basins installed along the African margin are inverted repeatedly. The first compressional events clearly expressed in the Tell occurred during the middle to late Eocene in relation with the onset of active convergence between Africa and Europe. We highlight the existence [i.e. in Jebel Ben Amara (Khomsi et al. 2009b), Jebel Rebia, etc.] of several angular unconformities separating the Oligo-Miocene from the Pre-Oligocene series. The Tellian foreland domain records far-field effects of this tectonic event inducing sedimentary hiatuses and erosion near the forebulge and in the vicinity of active thrusts [START_REF] Khomsi | The Late Eocene and Late Miocene fronts of the Atlas Belt in eastern Maghreb: integration in the geodynamic evolution of the Mediterranean Domain[END_REF]). The Infra-Kasseb unit is characterized by a stack of imbricated slices involving Mesozoic layers, result also from this compressional event.

(3) The resumption of shortening was set in middle Miocene times as a response of the major stage of the Africa-Europe collision. Since then, the Kasseb unit has moved forelandward, inducing the unroofing its Mesozoic duplexes which overlap early Miocene series, as described at Jebel Rebia. This event is characterized, also by wedge-top basins development (i.e. Mejerda Basin; [START_REF] Khelil | How to build an extensional basin in a contractional setting? Numerical and physical modelling applied to the Mejerda Basin at the front of the eastern Tell of Tunisia[END_REF][START_REF] Khelil | Reply to comment on "how to build an extensional basin in a contractional setting? Numerical and physical modeling applied to the Mejerda Basin at the front of the eastern Tell of Tunisia[END_REF][START_REF] Khelil | Late Miocene-Quaternary thrusting in the Utique-Kechabta foreland basin of the Tell, northern Tunisia[END_REF] and the reactivation of major inherited faults (i.e. Cap-Serrat-Ghardimaou Fault), which are associated to Neogene volcanism and/or Triassic movements. Shortening was still active during the Pliocene and Pleistocene, and is still active today. These late contractional events have amplified the south-to southeast-ward translation of structural units and halokinetic rise of Triassic salt bodies. Therefore, we show the structural superposition of the Numidian unit over younger series such as the upper Miocene deformed molasses as observed at Jebel Bou Khezara and Jebel Tegma.

4.3. Oil and gas exploration potentialities: The Paleozoic system/ pre-salt traps

As emphasized earlier by [START_REF] Tari | Is the Pre-salt sequence underexplored regionally in North Africa?[END_REF] for the Atlas FABT and its foreland basin and by [START_REF] Khomsi | A review of the crustal architecture and related pre-salt oil/gas objectives of the eastern Maghreb Atlas and Tell: Need for deep seismic reflection profiling[END_REF] for the Tell and Atlas belt, the Paleozoic system is obviously a very interesting system to explore and drill: in this system the main source rocks are represented by the Devonian and Silurian hot shales (see [START_REF] Aissaoui | Petroleum assessment of Berkine-Ghadames Basin, southern Tunisia[END_REF][START_REF] Tari | Is the Pre-salt sequence underexplored regionally in North Africa?[END_REF] and [START_REF] Khomsi | A review of the crustal architecture and related pre-salt oil/gas objectives of the eastern Maghreb Atlas and Tell: Need for deep seismic reflection profiling[END_REF]. The structural transects presented in this work underline that the major regional seal of the Paleozoic system is represented by the Triassic salt levels (Figs. 19 and20).

In the Fernana structures (Fig. 20), the Meso-Cenozoic sequences do not exceed 4500 m in thickness. The Triassic salt level represents a regional seal above the possible Paleozoic traps seated in the subsurface of the Tell. As emphasized by [START_REF] Tari | Is the Pre-salt sequence underexplored regionally in North Africa?[END_REF] for the Atlas belt and by [START_REF] Khomsi | A review of the crustal architecture and related pre-salt oil/gas objectives of the eastern Maghreb Atlas and Tell: Need for deep seismic reflection profiling[END_REF], the underlying Paleozoic inherited highs and anticlines should be sourced by the Silurian hot shales [START_REF] Aissaoui | Petroleum assessment of Berkine-Ghadames Basin, southern Tunisia[END_REF][START_REF] Khomsi | A review of the crustal architecture and related pre-salt oil/gas objectives of the eastern Maghreb Atlas and Tell: Need for deep seismic reflection profiling[END_REF]. Unfortunately, the lack of deep seismic soundings and investigations still prevents characterizing the sub-salt traps certainly seated in the Paleozoic sequences and subsurface structures. As recently argued by [START_REF] Khomsi | A review of the crustal architecture and related pre-salt oil/gas objectives of the eastern Maghreb Atlas and Tell: Need for deep seismic reflection profiling[END_REF] and [START_REF] Roure | Tethyan and Alpine controls on the crustal architecture of the Tunisian and Algerian Atlas and Tell: needs for coupled deep seismic soundings and tomography[END_REF] a deep seismic sounding project would be very important to unlock the Paleozoic system allowing to image the overall crustal architecture and potential traps in -10 S TWT (Two-way travel time). From this point of view it seems clear that the existing industrial seismic sections are limited to the shallow Mesozoic cover and of very low to moderate resolution and with a widely spaced grid in the Tell belt. In Fernana structures there is no seismic section yet available. Thus, we hope that in the next years an international oil/gas project with deep seismic sections records will allow to visualize properly the structures underneath the Tellian allochthon and the Triassic seal.

Conclusions

The present structural cross-sections throughout some representative structures and domains of the Tell of Tunisia allow us to better describe the structural style and tectonic events in the Tell

Fig. 1

 1 Fig. 1 Structural scheme of North Africa (after Frizon de Lamotte et al. 2000) highlighting the main structural domains: Atlas and Rif-Tell systems (a); geological map of Northern Tunisia (b) with the location of the studied area (Fig. 5)

Fig. 2

 2 Fig. 2 Lithostratigraphic columns of the different units of the Tunisian Tell and well data (W1) from the Mejerda basin. The well characterizes a thick possibly allochthonous Triassic evaporites body which is overlying Cretaceous series and capped directly with upper Miocene rocks. The different discontinuities, décollements and major unit thrusts are presented. Notice that the contact of the Numidian flysch over the Kasseb Units is a stratigraphic contact which was later on reactivated as a tectonic contact during the Alpine compressions.

Fig. 4

 4 Fig. 4 Northern Tunisia Tectono-lithostratigraphic units mapping (based and compiled from the existing 1/50.000 geologic maps, published by ONM). Notice that the Numidian flysch occupies the NW corner of the map and is dipping towards the NW with an onlapping lateral configuration above the Meso-Cenozoic structures.

Fig. 5

 5 Fig. 5 Geological map of the studied area indicating the emplacement of the different

Fig. 6

 6 Fig. 6 NW-SE cross section (CS1) through Jebels Rebia, Rhedir and Adissa, showing the main structures and structural styles; the location is given in Fig. 5. Notice on CS1 transect the late Miocene-Quaternary synkinematic thrusting/folding deposits.

Fig. 7

 7 Fig. 7 NW-SE Jebel Rebia cross section showing mainly the tectonic superposition of the Ypresian limestones (Globigerina facies) over the folded Meso-Cenozoic autochthon (Numulitic facies); the location is given in Fig. 6.

Fig. 8

 8 Fig. 8 Interpreted field photos from Jebel Rebia outlining (A+B) the thrusting of Kasseb unit over the lower-Miocene series and, (C) over the folded Campanian limestones and (D) tectonic breccia from the lower-Miocene deposits; the location of the photos is shown in Fig. 5.

Fig. 9

 9 Fig. 9 (A) WNW-ESE Jebel Adissa cross section (CS1) showing the structures below and around Jebel Adissa Numidian Klippe, (B+C) respectively looking N and W interpreted photos of the outcrop; the location of A is given in Fig. 6 and of B in A. Notice that the basal contact separating the Numidian from the underlying units corresponds chiefly to a downlap erosional surface which was reactivated later on during the Alpine late Miocene-Quaternary thrusting events.

  ) goes from Jebel Tegma in the NW to Jebel Halloufa in the SE (Figs. 10). From NW to SE , it shows the Jebel Tegma and Kef Kribissa duplex structures, with repeated sections of the Kasseb unit supporting the detached Numidian Unit above (Fig. 10). The major detachement level is situated within the Paleocene shales. The Jebel-Tegma anticline and the Jebel Kribissa NW-dipping monocline are separated by a vertical NW-trending transcurrent fault cutting through the entire subsurface structures and presumably related to inherited deep faulting rejuvenated during the Late Miocene-Quaternary compressions..

Fig. 10

 10 Fig. 10 Structural transects CS2 to CS3 in the Fernana area. Notice the importance of vertical Triassic salt movements. The structural styles presented on the dip section CS 2 is predominantly thin skinned with an episode of thick skinned tectonic materialized by NW-SE transcurrent strike slip subvertical faults occurring during late Miocene-Recent.

Fig. 11

 11 Fig. 11 Structural 3-D view looking North showing the major structures and structural styles. Notice that the transcurrent NW-SE fault crosscut the structures as Jebel Rhedir Diapir and rooting deeper underneath the allochthon cover. Location is given on Fig. 5.

Fig. 13

 13 Fig. 13 Interpreted looking NW field photos from Jebel Bou Khezara outlining the thrusting of the Numidian unit over the upper-Miocene wedge-top deposits where the Triassic salt decorates the contact.

  Covered by the thick Neogene wedge-top basins and/or the Numidian deposits, the Infra-Kasseb unit is characterized by several thrust slices (Fig.18A and C); the sole décollement is represented by the Triassic salt while the roof thrust is embedded mostly in Santonian or Paleocene marls (as demonstrated to the north of Jebel Bessouagui). This unit forms active antiformal stacks(Figs 6, 10 and 12).

Fig. 18 (

 18 Fig. 18 (A) Schematic-idealized cross-section through the Tell in a pre-inversion stage (Pre-Langhian, representing the tectonic units, salt activity and incipient décollements, (B)A zoom from A showing the onlapping of the Numidian sandstones on the pre-existing salt structure which is used lately as décollement, (C) Schematic-idealized cross-section highlighting the inversion structures such as the thrusting and duplexing of the Paleogene unit (Kasseb), (D) Schematic-idealized cross-section showing the duplexing of the Mesozoic unit (Infra-Kasseb), active-roof thrust duplexes affecting the Kasseb unit and the overthrusting of the Numidian and Triassic salt over the late Miocene wedge-top basin deposits, (E and F) show the important salt activity and mini-basins set-up

Fig. 19

 19 Fig. 19 Conceptual models for the play types seated in the Paleozoic subsurface system. (a) Trap categories in the Paleozoic productive petroleum system of the Sahara Platform. The map on the left indicates the position of the cross-section. (From[START_REF] Acheche | Devonian play attributes in southern Tunisia, Ghadames basin-North Africa[END_REF][START_REF] Tari | Is the Pre-salt sequence underexplored regionally in North Africa?[END_REF]. Notice that the main trap types are represented by wrench faults, horts, and deep seated anticlines underneath the Triassic salt level. (b) Two possible structural configurations underneath the Triassic salt levels and walls[START_REF] Tari | Is the Pre-salt sequence underexplored regionally in North Africa?[END_REF]. On the

Fig. 20

 20 Fig. 20 Conceptual models for the play types seated in the Paleozoic subsurface system along the Rebia-Adissa transect empathizing the occurrence of oil and gas traps in the Paleozoic structures. The sealing is ensured by the Triassic salt level. The presented transect allows to predict that the Top Paleozoic is around -3000 m beneath the surface. Structure A: large anticlinorium structure controlled by the Alpine rejuvenation of the Paleozoic inherited faults. B and C: pop-up anticlines affecting the Paleozoic structures with the occurrence of possible multiple oil/gas reservoirs in the axial part of the Paleozoic structures. D: Tilted blocks along inherited normal faults affecting the Paleozoic and possible traps related to fault trapping mechanisms. Notice: red color: potential gas accumulation, green color: potential oil accumulation.

  of Tunisia. The structural style of the investigated structures corresponds chiefly to thin-skinned tectonic evolution materialized by tectonic duplexing. The infra-Kasseb unit is characterized by a sole décollement level localized within the Triassic evaporites, resulting in the duplexes formation. The Kasseb unit is displaced towards the SE above the roof décollement at the top of the underlying infra-Kasseb duplexes. The most dominant style of the Tell is represented by active roof-thrust and duplex structures.In some areas as in Fernana, the Numidian Flysch is detached above the infra-Numidian units reworking an originally downlap contact. In some other areas, no clear detachment is visualized, and the Numidian rests in an apparent stratigraphic contact with underlying units, or is separated from the infra-Numidian units by a north-dipping normal fault, defining a neo-autochthon displaced together with the Kasseb nappe. Important NW-trending faults controlled the sedimentation during the Tethyan rifting, contributed then to the morpho-structural configuration of the Tell and have been reactivated as strike slip faults during the recent upper Miocene-Quaternary events. The Fernana structures underline the importance of the Triassic salt in the overall architecture of the belt and indicate high halokinetic movements in accordance with a Triassic salt-rich province. The Triassic salt has played a primordial role in the thrusting events forming the main décollement level. It is also implicated in the recent thrust rejuvenation with a good example given in Jebel bent Ahmed where the Triassic sole thrusts over overhangs the upper Miocene-Quaternary Fernana wedge-top basin. This sole thrust is interpreted as the apical part of the former diapiric structure. An important Aquitanian-Langhian angular unconformity occurs in the foreland domain of the Tell throughout the northern Atlas, materialized by Neogene proximal deposits onlapping already folded and partially eroded structures. The tectonic units of the Tellian wedge thrust above these Aquitanian-Langhian series. After the emplacement of the nappes in post-Langhian times, the area underwent tectonic compression and thrusting, allowing the folding of older nappes contacts as well as the rejuvenation of ancient inherited faults and thrust folds. The major tectonic event seems to be upper Miocene to Quaternary to Recent in age because the post-nappes upper Miocene continental series of the are folded and tectonically displaced along some thrusts. The upper Miocene-Quaternary deposits were affected by thrusting and folding and incorporated mechanically in the footwall of some thrust. The base of the post-nappes upper Miocene-Quaternary syntectonic deposits are lying with an angular and erosional unconformity above former duplex structures as in the Fernana wedge-top basin. Along the Tunisian external Tell as shown for Fernana structures, inherited Paleozoic highs and anticlines, which are sealed by Triassic salt and sourced by the Silurian hot shales characterize an interesting pre-salt trap system.