Morphotectonic and Morphodynamic investigations

Earth Sci Inform DOI 10.1007/s12145-015-0208-4

RESEARCH ARTICLE

Morphotectonic and Morphodynamic investigations revealed by isobase surfaces analysis and derived differential mapping using GIS, Teboursouk area, northern Tunisia Tarek Slama & Benoit Deffontaines & Mohamed Moncef Turki

Received: 3 April 2014 / Accepted: 14 January 2015 # Springer-Verlag Berlin Heidelberg 2015

Abstract Teboursouk area within the northern Tunisian Atlas presents a complex and heterogeneous geological framework leading to various geomorphic landforms among them there are numerous Triassic salt extrusions having an evident impact on the topographic configuration, shaping and dynamics. Landforms characterization of the study area, by morphostructural and morphodynamic mapping, was carried out using the isobase approach which is mainly based on the construction of topographic base-level surface from each channel order. GIS environment and spatial data analysis tools have been used to perform this procedure using 10 m-grid DEM and digitized drainage network at a semi-detailed scale (1: 50 000). The analysis included the creation of isobase maps of 2nd, 3rd and 4th-order channels. Grid calculations allowed the construction of (1) a combined surface from 2nd and 3rd order and (2) a differential map which represents the topographic residual between present day surface (original DEM) and isobase of 4th order. In fact, qualitative analysis of different isobase surfaces shows a good delimitation of most morphostructures and highlights a model of an atypical morphostructure concerning the Triassic extrusion of Jebel ech Cheid (JEC). Isoline anomalies and disturbances have indicated that major morphotectonic lineaments, interpreted as fractures, trend NE-SW to NNE-SSW adapting to the regional strike-slip fault system in response to NW-SE late Alpine transpressive tectonics. Several isoline inflexions were, however, well correlated with observed tectonic fractures and some of them are considered to be closely related T. Slama (*) : M. M. Turki Département de Géologie, Faculté des Sciences de Tunis, Université de Tunis El Manar, 2092 El Manar, Tunis, Tunisia e-mail: [email protected] B. Deffontaines Laboratoire de Géomatique Appliquée ENSG-IGN, Université Paris-Est Marne-la-vallée, 77454, Marne-la-Vallée, Cedex 2, France

to basement as the 4th-order analysis demonstrates. In contrast, neotectonic features, active landforms and lineaments were quantitatively described and localized based on the differential mapping approach. Keywords Morphodynamics . Active tectonics . Isobase . GIS . Salt diapir . Northern Tunisia

Introduction The drainage network is one of the most significant components of the landform having an important impact on the topographic shaping. It reflects the continuous and complex interaction between exogenous processes and endogenous tectonics that led to the formation of an orogen as well as its folds and associated faults (Deffontaines 1990; Deffontaines and Chorowicz 1991; Jackson and Leeder 1994; Jackson et al. 1996; Burbank and Anderson 2001; Chen et al. 2003; Ahmadi et al. 2006; Delcaillau et al. 2006; Ribolini and Spagnolo 2008). These structural features are often associated with a morphostructural growth and/or a lateral propagation into a tectonically active region (Jackson et al. 1996; Delcaillau et al. 1998; Champel et al. 2002), with an obvious impact on the drainage network geometry which is highly influenced by the recent tectonics that the landfrom has experienced (Burbank and Anderson 2001; Delcaillau et al. 2006; Silva et al. 2008 and cited references). Isobase surface or base-level map (Filosofov 1960) is extracted from the drainage network and closely related to the configuration and spatial hierarchization of the considered streams into the studied area (Raczkowski et al. 1984; Grohmann et al. 2011). It states the composite relationship

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between drainage pattern and landform topography with an emphasizing on the tectonic-erosion influence on the relief development and its historical evolution during Quaternary times (Raczkowski et al. 1984; Golts and Rosenthal 1993). Moreover, the isobase approach has been used as a source of paleo-topographic information applied for characterization of past environments and their climatic processes (Leverington et al. 2002). It was also a very effective method for morphostructural mapping and analysis applied in many region of the world and at different topographic scales (e.g., Golts and Rosenthal 1993; Grohmann et al. 2007, 2011; Jaboyedoff et al. 2009). Filosofov (1960) proposed the use of 1:50 000 topographic data or less for local investigations within a region showing a low topography, 1: 100 000 scale for flat landform, and 1 : 1 000 000 topographic maps to examine the regional-scale morphostructural features and the general tectonic composition. However, it has been assumed that the base-level analysis could have more significant results and interpretations within a context of uniform lithologies (Grohmann et al. 2011). Technically, the isobase surfaces are intimately defined according to the drainage ordering and geometry which is, commonly, based on the Starhler (1952) rules of spatial numbering. Drainage segments without tributaries are assigned first order or number one and defined the headwater streams that their junctions creates confluence points. A second-order stream is the segment downstream the confluence of any two first-order streams and so on for the entire drainage network. Thus, for each stream order an isobase surface could be extracted and plotted depending on its intersection with topographic data using adequate digital elevation models (DEMs). Furthermore, many basic arithmetic operations are easily achievable based on the generated set of isobase surfaces (Grohmann et al. 2011). The most significant is the construction of maps of the differences between surfaces of various orders that, as cited by Raczkowski et al. (1984), could give information about the amplitude of erosional dissection during the period that elapsed between the formation of considered surfaces. This is the basic of the concept of “differential analysis” which derived from the isobase method. The differential analysis approach is always carried out between two isobase surfaces or between the present-day topography and a selected isobase map, and automatically generates a “differential map” which the spatial pattern is very sensitive to amplitudes of neotectonic movements. Therefore, the Early Quaternary geomorphic elements (predominated by the implemented Triassic landforms and surrounding plains) as well as the prevailing morphodynamic processes, mainly the tectonic uplifting, could be revealed and evidenced by analyzing the fourth order surface (Raczkowski et al. 1984). This latter, should be closely sensitive to the local diapiric mechanism and its obvious active neo-halokinesis (Perthuisot 1981; Chikhaoui 2002; Jallouli

et al. 2002; Benassi 2011) which has controlled, consequently, the evolution and the geometry of the drainage network, predominately the 4th-order streams. In this paper, we present a detailed quantitative geomorphic analysis based on isobase method and the derived differential surface to characterize the landforms within Teboursouk area which has undergone various tectonic stress of different ages. In fact, these phases have mainly controlled, until recent times (that is our postulated hypothesis), the evolution and the current implementation of Triassic diapirs (or extrusions) as well as the overall structures of the investigated region (Bouaziz et al. 2002; Chikhaoui 2002). Consequently, isobase surfaces is examined and analyzed as an efficient indicator of morphostructural pattern which were being formed during Plio-Quaternary times under neotectonic stress: this is the specific objective of this contribution. In addition, produced differential map is considered in this work to investigate the amplitude of neotectonic movements and to construct the morpho-neotectonic (Quaternary morphodynamics) sketch of the study area. This geodynamic context is the main target of our study that the isobase technique may allow new insights in depiction of morphostructural configuration and morphodynamic evolution of various landforms .

Study area The northern Tunisian Atlas is geologically described by many apparent NE-SW-trending Triassic salt outcrops characterized by an unusual sedimentary contacts with younger rocks aged from Cretaceous until Upper Cainozoic. These particular landforms have been folded and deformed by the Cainozoic Alpine orogeny and forming a region recognized as the “diapiric zone” (Burollet 1956; Perthuisot 1972; Zargouni 1975; Turki 1985; Ben Ayed 1986). This orogeny was originated from convergent tectonic activity between the Eurasian and African plates which started in the Late Cretaceous and continues until present days (Dercourt et al. 1986), causing the formation of the Atlas Mountains during the Late Miocene (Tortonian phase). This atlasic structure has been subjected to multiple tectonic events that produced the folded and faulted sedimentary rocks and various morphostructural entities (Chikhaoui 2002). Furthermore, major prominent accidents were likely to have impact on the generated orogenic framework (Chikhaoui and Turki 1995) which the structural and tectonic interpretations were quite diverse (Perthuisot and Rouvier 1992; Vila et al. 1994; Jallouli et al. 2002). The study area, Teboursouk region, is located into the central part of the diapiric zone (dz) which is, from a geomorphological point of view, shaped by several Triassic extrusions (or diapirs) with different dimensions and forms (Fig. 1). These atypical landforms have been generated and controlled by the halokinesis of salt rocks, known as salt tectonics, began in the

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Fig. 1 Geological map of the study area (1: 50 000, adapted from Perthuisot 1978) and location of the investigated region (the inner black square in the map of Tunisia)

Jurassic and pierced through the overlying layers in the Late Aptian-Albian, which has also concerned the entire diapiric zone of the northern Tunisian Atlas (Perthuisot 1981; Ben Ayed 1986; Boukadi and Bédir 1996; El Ouardi 1996). Within the investigated region, the Tortonian compressional deformation has created the most of folded structures outcropped today by the inversion of older normal faults, reactivation of uplifted structures, and regeneration of diapirs of Triassic evaporites by an active halokinesis (Perthuisot 1981; El Ouardi 1996, 2002; Chikhaoui 2002). However, the original mechanism of salt halokinesis and model of salt rising are considered, until today, as the most controversial subjects of debate regarding the morphostructural evolution of the northern Tunisian Atlas. In fact, most of Triassic extrusions have been explained by a diapiric mechanism which is considered as the generator of salt rising inside the folded Triassic structure (Perthuisot 1978; Hammami 1999; Perthuisot et al. 1999; Jallouli et al. 2002; Benassi et al. 2006; Benassi 2011), whereas Vila et al. (1994, 2002) and Ben Chalbi et al. (2006) have interpreted the same structures as being formed by large-scale allochtonous salt (glacier salt) movements interbedded within the Cretaceous series. Main geologic features The Téboursouk area is characterized by various Triassic outcrops bounded by thick sequences of Cretaceous and Cenozoic rocks whereas Jurassic rocks are absent on the surface (Fig. 1). However, the Jebel ech Cheid structure is the most prominent diapiric extrusion elongated along a NE-SW

direction and highly fractured as other Triassic bodies of lesser dimensions (Jebel Aïn Jemala and the Teboursouk part of Jebel Thibar). The Cretaceous and Tertiary sedimentary series are truncated by NE-rending faults and lie unconformably over the Triassic rocks. As was observed on site, against the flanks of the diapirs the post-Triassic beds are generally overturned and strongly crushed with a manifest reshuffle of Triassic rocks (Dali 1979; Perthuisot 1981; El Ouardi 2002). The Triassic series mostly appears as a strongly deformed, chaotic mass of gypsum, anhydrite and sandstones with thin layers of clay, limestone and dolostones (Perthuisot 1978; El Ouardi 2002) and which the total thickness has been estimated to be more than 1000 m (Perthuisot 1981). The Cretaceous sediments are mainly marly with interbedded limestone members having a considerable thickness (more than 3000 m) within the interdiapiric areas. In contrast, the Cretaceous peridiapiric series is recognized by a marked thickness reduction. The Cainozoic sequence consists of clays, marls and sands affected, as the Cretaceous series, by a complex faults system having NW-SE- and NE-SW-trending directions which have been reactivated during the Late Miocene and Quaternary compressional tectonics. In fact, this structural phase has controlled, until recent times, the evolution and the current implementation of Triassic diapirs (or extrusions) as well as the overall structures of the region (Bouaziz et al. 2002; Chikhaoui 2002). In this geodynamic context, the morphostructural configuration of the study area seems to be closely related to the morphodynamic evolution of Triassic diapirs.

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Local physiography and main morphostructures

General approach & GIS procedures

The main morphostructural configuration of the Teboursouk province is shaped by a multi-scale synclinal system with two principal directions: N-S (e.g., Aïn Tounga-Khalled) and NESW (e.g., Oued Hermoucha and Jebel Chetlou), and also showing two major synclinal zones having a rather complicated structure (Tabet ech Cherif and Aïn el Hamra-Dougga) associated with Triassic diapirs (Fig. 2). To the Southwest part of the study area, the morphology is dominated by the folded structure of Jebel ech Cheid which considered as one of the most outstanding Triassic landforms within the northern Tunisian Atlas. In fact, this mega-antiform with a symmetric morphology has an average altitude of ~550 m with elevations up to 700–760 m locally, whereas the mean elevation of the study area does not exceeds ~410 m and knowing that its lithology (Triassic rocks) has a higher erodibility among the surrounding rocks. From west to east, the Jebel ech Cheid massif is bordered by two relatively flat plateaus (plains of Khalled and El Aroussa) with a NE-SW orientation and a low altitude (Fig. 2). In contrast, the landform of the western part of the study area is dominated by two prominent structures (Jebel Goraa and Jebel Fej el Adoum-el Alia) with an elevation that exceeds ~830 m and are mainly Eocene sedimentary rocks: hard limestones with a high fracturation. In this context, the imposed morphology which is closely related to the lithology contrasts seems to be obvious. In fact, the Eocene carbonates, compared to Triassic rocks (mainly evaporitic), are very resistant and have a lower erosional index.

Data source and drainage hierarchization In the present work, the OTC (Office de la Topographie et de la Cartographie) topographic map at the scale of 1:50 000 with 10 m contour interval was used as source of elevation and hydrographic data. This semi detailed scale, with the given planimetric resolution, will allow an appropriate geomorphometric analysis and a relevant investigation of presented geomorphic features (Golts and Rosenthal 1993; Modenesi-Gauttieri et al. 2002; Jaboyedoff et al. 2009). Hence, a refined examination of the studied landforms could be possible. The scanned topographic map, with 300 dpi resolution, was georeferenced and manually vectorized using ESRI software package (ArcGIS) and specific extensions. GIS layers containing contour lines, elevation points, and the hydrographic stream network were stored in ESRI shape file format. From altimetry data, the digital elevation model (DEM) in GRID format was produced by an advanced geostatistical interpolation techniques such as kriging (Burrough and McDonnell 1998) with a resolution of 10 m (Fig. 2). This numerical model of landform is increasingly considered as a wealth of morphometric information from a quantitative or qualitative point of view (Iwahashi et al. 2001; Li et al. 2005; Iwahashi and Pike 2007; Ferraris et al. 2012; Karátson et al. 2012). Notwithstanding the time consuming and tedious digitizing, the results provide an acceptable model of local landform and a more detailed

Fig. 2 Digital elevation model (10-m-grid DEM) of the study area showing the main hypsometry and major landforms and massifs. Derived shaded relief map was used to create the final DEM map

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expression of morphotectonic features of the study area and associated topographic configuration. Thereby, a 10-m-grid DEM of Teboursouk region was produced and a model of shaded relief map was extracted using GIS spatial analysis approach and the hill (or oblique) shading method (Li et al. 2005; Kennelly and Stewart 2006). The generated illumination model is important in geomorphology and morphostructural mapping to create realistic surface renderings (Fig. 2). DEM-based algorithms of automatic drainage network extraction were not used for the definition of valley segments. Therefore, the vectorization process (by handdigitization) was carried out on the basis of a raster file acquired by scanning and rectifying the digitized drainage network (Fig. 3). Basin stream segments are ordered according to Strahler’s classification (1952) using the classical approach of numbering: each single valley segment, represented by a polyline, was manually digitized and attributed a given hierarchical order depending on its geometrical connection into the stream network. This “fractal” connection is based on the spatial arrangement of tributaries and confluence points in a hierarchy of first, second, third and higher orders. Moreover, the topological framework of the entire network has been considered and approved into the used GIS environment to ensure vector snapping and removal of digitizing errors. Finally, the generated vector GIS layer includes hierarchical order information and geometrical characteristics for each drain.

GIS construction of isobases and differential map Extracted DEM (Fig. 2) and digitized stream network (Fig. 3) were the basic data for the isobase mapping and differential analysis into GIS environment using typical spatial analysis procedures and approaches. In this case of study, the gridded topographic data was used as a source of elevation information that will be attributed to the drainage network segments. These latter, as a first step, have been rearranged into new shapefiles geometrically distinct according to the assigned hierarchical drainage order. Thereby, six vector layers were automatically generated containing drainage tributaries of the correspondent order and, thereafter, converted to point data. However, Grohmann et al. (2011) have suggested the use of contours derived from the DEM as a source of elevation instead of using all the elevation values along the stream lines and that might provide better results with the interpolation procedure. We are in agreement with their finding provided that the used elevation model is the SRTM30 with a spatial resolution of 0°0’30″ (~900 m); nonetheless, our assessment tests conducted over SRTM03 (~90 m) and the used DEM (~10 m) show no interpolation artifacts (Yue et al. 2007) into the produced isobase surfaces and consequently we have considered that the generated isobase surfaces are suitable to the morphotectonic/morphodynamic investigation at a local scale.

Fig. 3 Digitized drainage network of the study area draped over shaded relief map. 1: Oued hermoucha syncline; 2: Ain Jemala diapir; 3: Synclinal zone of Tabet ech Chereif; b Jebel Goraa, Ch Jebel ech Cheid, f Jebel el Alia

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The generation of isobase surfaces was based on created vector point layers, as shown above, and gridding technique by spatial interpolation. Continuous surfaces were created with Regularized Splines with Tension approach (Mitasova and Hofierka 1993; Mitasova and Mitas 1993; Hofierka et al. 2002) with a distinctive consideration of intermediate hierarchical orders. Thus, isobases of 2nd, 3rd and 4th order were constructed and the combination between 2nd and 3rd valley orders was conducted. It should be noted that the 1storder streams is, by default, disregarded during the base-level mapping and considered as a source of “noise” that could prevent the identification of a scarp or other significant topographic (or morphostructural) features (Grohmann et al. 2011). Also, higher drainage orders are most often represented by a low number of valleys; 9 and 4 segments for the 5th and 6th-order channels respectively in our case of study. Continuous surfaces created from these orders were spatially insignificant and without effective morphostructural configuration and, subsequently, they have been neglected in this study. Moreover, tributaries with major levels of geometric hierarchization seem to have a restricted representation of geomorphological events that couldn’t be generalized to the entire studied region. The construction of different isobase surfaces and differential analysis have been performed into a geographic information system (GIS) environment. The extracted geomorphometric indices (isobase surfaces and differential parameter) are a grid-based approach using digital elevation model (DEM), GIS tools, and spatial analysis procedures to perform arithmetic operations with multiple spatial datasets. In fact, the designed workflow for the shown approaches has been easily executed in a GIS platform (Leverington et al. 2002; Grohmann et al. 2011), and morphometric terrain parameters can be derived directly from the DEM using some local operations (Arrowsmith 2006). Our particular focusing on these intermediate orders was clearly argued by their substantial interpretations provided in terms of sedimentological investigations (Raczkowski et al. 1984), morphotectonic mapping (Golts and Rosenthal 1993), morphostructural analysis (Grohmann et al. 2011) and morphodynamic assessment. Differential mapping, used as a quantifying technique of morphodynamic events, was carried out by a fairly simple method in the GIS environment. In fact, the resulting map was constructed by the difference calculated between the two generated rasters: (1) the presentday topographic surface (original DEM), and (2) the isobase surface of the fourth order. The map algebra, which is a set-based algebra for manipulating geospatial data, was the used GIS tool ensuring the differential data extraction and spatial mapping.

Results & Discussion Two analytical approaches have been suggested in this work: (1) qualitative method that seems to be efficient in determining the morphotectonic and morphostructural configuration, while (2) quantitative method was used to reveal active morphostructures and lineaments that are likely to be of neotectonic origin (Quaternary tectonics). Consequently, the morphodynamic framework of the study area may therefore be designed and generalized. Isobase analysis and morphostructural configuration (qualitative interpretation) The proposed qualitative approach is based on the analysis and interpretation of the isoline pattern of isobase surfaces extracted from 2nd, 3rd and 4th-order channels. The only considered combination was made between the 2nd and 3rdorder valleys which allow the best results as has been concluded from different case studies of various geological and geomorphological contexts at diverse spatial scales (Raczkowski et al. 1984; Grohmann et al. 2011). The Fig. 4 shows 2D and 3D models of isobase surfaces in order to achieve a combined analysis taking into account the isobase spatial distribution and values trend. Isoline configurations and main trends As initial map for the interpretation of isobases within the Téboursouk region, the 2nd and 3rd-order map was involved (Fig. 4). It shows several areas indicating high isobase concentration, distinguished by narrow isolines with values falling between 450 and 700 m approximately (Fig. 4a). Almost all important massifs and structures (JEC, JG, JFH, JES, JEA and 3 in Fig. 4a) belong to such regions allowing a good morphostructural delimitation, and also may presents some typical features of areas that went through tectonic activities. While maximum isobase values are reached by the structures located in the western part of the study area (JEA, JFH and JG) with values between 750 and 850 m, the highest mean value (~650 m) is observed within the Jebel ech Cheid massif (JEC in the Fig. 4a) which dominates the eastern part and recognized as a large diapiric structure with exposed evaporites (Fig. 1). Also, most isobase lines within these prominent morphostructures show a trend towards NE-SW direction excluding the Jebel es Sfah massif (JES in the Fig. 4b) which indicates rather a NNW-SSE direction. It is then possible to suggest an obvious control of the late Miocene and Quaternary compressional tectonics, which has a NE-SW direction, on the 2nd and 3rd-order drainage network organization and on their segment’s orientation.

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Fig. 4 Extracted isobase models with constructed 2D and 3D surfaces. a 2nd and 3rd order; b 3rd order; c 4th order. Numbering is the same as in Fig. 2

It should be noted that the Jebel ech Cheid (JEC) structure shows a concentric model of irregular isolines (Fig. 5), whereas a concentric model of regular isolines characterizes the other important structures mentioned above. This specific isobase configuration of JEC massif with isolines of high sinuosity could be explained by a recent and/or active tectonics. However, various regions of contrasting isobase pattern can be seen particularly within synclinal areas (structures 2, 4, 5, 6 and 8 in Fig. 4a) and plains (PK and EP). They are in fact characterized by a low isobase density with lower values falling between 150 and 400 m. Relatively low isobase values are noticed into the easternmost part of the study area (100–

150 m) revealing a region that undergo topographic depression and a high drainage density as clearly shown in Figs. 3 and 5. It is therefore obvious that erosional mechanism was largely prevailing within this region. The isoline pattern is more complicated within the synclinal structures showing no evident trend for the most of them, while the widely spaced isobase pattern with an oversimplified model is noticed into plains, particularly the Plain of Khalled (PK). We suspect that it could reflect in some way a mutual relationship between lithological variation and present-day morphology. The third order map (Fig. 4b) shows a slightly different picture with a decrease of isobase values and a widely spaced

Earth Sci Inform Fig. 5 3D isoline model of isobase surface of 2nd and 3rdorder valleys. JEC Jebel ech Cheid, PK Plain of Khalled, EP El Aroussa Plain

isolines pattern. Nevertheless, most morphostructures of the study area are delimited and identified indicating a moderate concentration of isolines over the major massifs (JEC and JG) with values between 400 and 600 m. Unlike the synclinal structures the plains are still identified (PK and EP) and showing the most widely spaced pattern of isolines. This configuration seems to reflect the low impact of the 3rd-order streams on the morphology of plains. On the other hand, the analysis of isolines pattern of the fourth order map (Fig. 4c) revealed a substantial correlation between high values of isobase with an increase of concentration and some prominent massifs. The JEC, JES and JG structures are well delimited and show the highest values (up to 680 m) with a concentric model of regular isolines. Although JEA and JFH massifs are among the major structures into the western part of the study area, they are not identified and no specific trend into their isolines. However, the plain of Khalled (PK) has a good delimitation with the highest concentration of isolines compared to the other isobase maps (Fig. 4). A strong NE-SW and NNWSSE orientations of the 4th-order isolines is observable over the recognized structures (Fig. 4c). Morphostructural and morphotectonic model A joint analysis involving the isobase surfaces (Figs. 4 and 5) and the geological map of the study area (Fig. 1) shows an apparent spatial correlation between the high concentration of isolines and Paleogene sedimentary rocks (mainly Eocene limestone), except of the NE-SW trending body of Triassic evaporites (mainly salt and gypsum) of the Jebel ech Cheid (JEC) structure. The Cretaceous (mainly marly) and Neogene series (mainly sequences of sands and sandstone), observed into synclinal zones (2, 4, 5, 6 and 8 in Fig. 2), are broadly correlated with a widely spaced isolines pattern with lower isobase values showing rather a different picture implying

the lithological changes and their erosional properties (Fig. 4). As previously analyzed, an explanation of this correlation is possible assuming that isobase lines are very sensitive to erosional surfaces and are related to tectonic-erosional events, mainly the most recent one (Raczkowski et al. 1984; Golts and Rosenthal 1993; Grohmann et al. 2011). In fact, it is remarkable that high isobases are typically associated with structures having hard rocks with a lower erodibility and then considered as “rugged morphostructures”, whereas low isobases are associated with structures having soft rocks with a higher erodibility and thereafter considered as “brittle morphostructures”. However, it should be noted that the diapiric massif of JEC, which should be considered as a rugged morphostructure, is an atypical morphostructure characterized by an anomalous association between isobases with high values (Figs. 4 and 5) and rocks with high erodibility (Triassic evaporites) as observed in the geologic map (Fig. 1). It can be inferred that the diapiric massif is clearly influenced by recent and active halokinesis with an uplifting movement of Triassic evaporites which control the topographic growth of the Triassic landform of the JEC and, subsequently, explaining the high values of isobases and also the model of isolines concentration over the massif. This evidence is extremely important to help understand the studied salt extrusion morphodynamics during the Quaternary period and until recent times. This finding is argued, geologically, by a significant thinning of the post-Triassic series which are overturned and strongly crushed against the flanks of the JEC extrusion (Perthuisot 1981; Chikhaoui 2002; El Ouardi 2002). Furthermore, the well-preserved isolines delimitation of some morphostructures (particularly JEC, JES and JG in Fig. 4c) could possibly reflect a mutual relationship between deep basement fractures and reactivated relict alpine morphology, occurring during the late Quaternary shortening and even at the present time. Based on these

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analyses a morphostructural zoning of the study area could be proposed showing brittle, rugged and atypical rugged morphostructures (Fig. 6d). From a morphotectonic point of view, Golts and Rosenthal (1993) have outlined that abrupt deviations in isobase lines directions may reflect of tectonic dislocation or severe lithological changes. Nevertheless, most of the observed significant lithological variations were of tectonic origin; thus, it is quite adequate to correlate the apparent isolines anomalies with various level of tectonic fractures and faults systems. The identification of base-level lineaments from the interpretation of unusual deviations into isobase maps is largely qualitative and taking into account the geological context of the study area (Grohmann et al. 2011). However, significant change in isoline orientation is systematically induced from anomalous deviations in fluvial drainage system (Howard 1967) which are a sensitive indicator of tectonic lineaments (e.g., Mayer et al. 2003; Mrinalinee Devi and Singh 2006; Ribolini and Spagnolo 2008). Also, it is obvious that the

apparent orientations of identified morphotectonic fractures were largely controlled by the pre-existing tectonic constraints and by the orientation of the stress field. The identification of approximate faultline area within an isobase surface is based on the recognition of isoline inflexions (Grohmann et al. 2011) which are, for most of them, specific to tectonic fractures that the present-day topography couldn’t show (impact of an active erosional process). Nevertheless, some particular features should be recognized and regarded as helpful elements for morphotectonic interpretations: aligned and elongated isolines, sharp deviations in contours and unusual compression or spreading within the isoline pattern. A qualitative mapping of morphotectonic lineaments (interpreted as fractures) is then quite possible using the considered isobase maps, as shown in the Fig. 6, draped over their shaded surfaces which might be very useful to locate linear features. We would point that the extracted morphotectonic information, which is of approximate

Fig. 6 Morphotectonic interpretation of isoline configurations (a to c) and generalized morphostructural mapping (d). a 2nd and 3rd order; b 3rd order; c 4th order. BM Brittle Morphostructure, RB Rugged

Morphostructure, ARM Atypical Rugged Morphostructure, JES Jebel es Sfah, JEC Jebel ech Cheid. For structures 1 to 8: see Fig. 2

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nature, could be used in conjunction with on site investigations to depict the tectonic framework of the study area. Major identified morphotectonic lineaments (Fig. 6a, b and c) trend NE-SW to NNE-SSW adapting to the regional strike-slip fault system in response to NW-SE late Alpine transpressive tectonics which control the structural heritage of the entire region (Dercourt et al. 1986; Martinez et al. 1991). Furthermore, these directions have an important impact on the orientation of fracture zones and on the alignment of salt extrusions within the study area (Perthuisot 1981). The interpreted isobase surfaces indicate a morphotectonic configuration primarily based on NE-SW direction which demarcates most observed morphostructures (massifs in Fig. 6a and b). The assumed important fractures are investigated within the 4th order map (Fig. 6c) and likely regarded to be related to the deep basement. In fact, the isobase pattern generated from the 4th-order valleys provides a better distinction of isoline anomalies with identifiable tendency and distinguished orientation. We suppose that streams of higher order are much sensitive to fractures of more importance that might be closely related to basement. Moreover, Raczkowski et al. (1984) have demonstrated that large and distinctive disturbances in isoline pattern frequently seem to be caused by the impact of deep fractures. Therefore, three prominent morphostructures could be identified and tectonically characterized by major NE-SW basement lineaments (Fig. 6c), namely the massifs of JG, JES and the controversial structure of JEC (Fig. 5); This Triassic extrusion has been considered as an atypical rugged morphostructure with a rather complex geometry. According to the result of our isobase analysis, the inferred morphotectonic map (Fig. 6d) shows two categories of lineaments, interpreted as tectonic fractures: (1) lineaments with low extensions and probably of shallow origin extracted mostly from 2nd and 3rd-order map (Fig. 6a and b), and (2) lineaments with great extensions and might be of deep origin deduced from 4th-order map (Fig. 6c). It is remarkable that some of these lineaments are particularly represented by several aligned lineaments with low extensions into the 2nd and 3rd-order map (and also into the 3rd-order map); we suspect that it could be a faultline system affecting the superficial sedimentary cover and connected to the (deep) basement. In short, the study area and, quite possibly, the developed drainage network have undergone an obvious control by NE-trending faults, which were responsible for structural zoning and sedimentological thickness variations within formed basins. Isobase model and sedimentological configuration The isoline pattern of the 2nd and 3rd-order valleys has been also investigated in this work in term of

sedimentological thickness variation within the study area basins. Raczkowski et al. (1984) have indeed observed, within the Polish Carpathians isobases, that all zones of closely spaced isobases are related to a diminished thickness, while areas displaying a widely spaced isoline pattern correspond with regions of great thickness. Furthermore, Filosofov (1960) has clearly stressed that subsided areas have been demonstrated, nearly in most examined cases, by long distances between isolines within the isobase model. As previously analyzed, the inspected isobase map reveals two distinctive patterns of isolines according to the spatial concentration, the spacing distance and the mean isobase value. Thus, a pattern of isolines with long distances, low concentration and mean isobase values of 150 to 350 m characterizes distinctly the interdiapiric areas (Plain of Khalled, El Aroussa Plain, structures 2, 5 and 6 in Fig. 7) in which the post-Triassic series show a considerable thickness and considered as subsiding sedimentary basins (Perthuisot 1981; Chikhaoui 2002). Nevertheless, almost the same pattern but with higher mean isobase values (between 550 and 700) is also observed on top of several massifs including JES, JG, structure 7 and the Triassic diapir of Jebel ech Cheid (Fig. 7) suggesting an increase of thickness of their corresponding sedimentary series (Fig. 1). Indeed, in situ investigation carried out by Perthuisot et al. (1999) and Chikhaoui (2002) reveals an important thickening of the Paleogene series (Eocene limestones) and of the Triassic rocks (mainly salt and gypsum) within their morphostructures (Fig. 1), confirming therefore the interpreted isobase model. In the other hand, a pattern of isolines with higher concentration and a very narrow spacing (as second pattern of isolines) is noticed in peripheral zones of some structures, implying a particular thinning of sediments (Fig. 7). However, a pattern with an abrupt decrease of isolines spacing seems to be more significant and leads to helpful interpretations. Therefore, several outlying areas were identified and mainly localized at the flanks of the investigated morphostructures, including the eastern side of JES, JG, massifs 3 and 7 (Fig. 7). These isobase interpretations which reveal the diminished of sedimentary thickness were confirmed by local geological observations (Perthuisot 1981; Perthuisot et al. 1999), particularly in the peridiapiric areas of the Triassic extrusion of JEC. In fact, toward the tow flanks of this massif, thinning of the postTriassic layers (mainly Cretaceous and Paleogene) were observed, described (Dali 1979; Chikhaoui 2002) and recently confirmed from seismic reflection profile and complete Bouguer gravity analysis (Benassi 2011); suggesting therefore a model of deep-rooted diapiric structure (Jackson et al. 1998; Vendeville 2002) which the morphostructural evolution was reflected by local sedimentological variations displayed around the diapiric structure.

Earth Sci Inform Fig. 7 Sedimentological thickness interpretation of isobase map of 2nd and 3rd-order valleys. The interpretation is closely related to the isoline pattern and isobase values (see text for details). Numbering is the same as in Fig. 2

Differential mapping and morphodynamic investigation (quantitative interpretation) Extracted isoline patterns are logically related to tectonic-erosional events, mainly the most recent ones (Grohmann et al. 2011), whose the physical interactions control the tectonic growth of topography (e.g., Burbank and Anderson 2001; Kühni and Pfiffner 2001; Willett et al. 2006 and references therein). Thus, active tectonics may be evaluated quantitatively on the basis of differential computation between isobase surfaces produced from different stream orders. However, some surfaces are thought to be more sensitive to neotectonic vertical movements than others. In fact, it has been concluded from the work of Raczkowski et al. (1984) that calculation of the differences in height between the presentday topographic surface and the fourth order base-level surface can produce an estimate of relative tectonic uplift during the Quaternary. The created surface is then qualified as “differential map” showing a distribution model of differential values within the study area. This quantitative approach leads to interpretations in terms of Quaternary tectonics, erosional processes, landform dissections an d neotecton ic uplifting mov ements. Combined with morphostructural and morphotectonic mapping (Fig. 6) an evaluation of active faults and/or landforms could be quite possible. Moreover, a tentative

assessment of the amount of uplift rate is potentially achievable (Raczkowski et al. 1984). The computed differential map shows a positive differential values ranging from 0 to 215 m approximately with a localized distribution revealing particular morphostructures and topographic lineaments (Fig. 8). These values are between 100–150 m and ~200 m in the JES, JG, JFH, JEA, structure 3 and Jebel Zalia (JZ) indicating a limited spatial distribution enveloping the prominent parts of the investigated structures; in the Triassic massif of JEC they range from 60–80 m to ~215 m while in the plains of Khalled (PK) and El Aroussa (EP) they only reach 10–15 m (Fig. 8). There, the important decrease in differential values lead us to suspect the impact of a neotectonic subsidence on the morphostructural configuration of these geomorphic depressions likely related to the Quaternary tectonics imposed by the outstanding structure of JEC. However, within the eastern part of the El Aroussa plain (EP), shown by a white square in Fig. 8, a significant increase of differential values is noticed (between 45 and ~70 m), particularly at the meander area of Siliana river (Oued Siliana) and its associated landforms (Fig. 2). It is therefore obvious to suggest a region of active meandering with high sinuosity index that might be caused by tectonically active framework with a local and limited control on river geometry and configuration. Also, it is

Earth Sci Inform Fig. 8 Produced differential map obtained from the residual calculation between present-day surface and 4th-order isobase surface. JES Jebel es Sfah, JG Jebel Goraa, JZ Jebel Zalia, JFH Jebel Fej el Adoum, JEA Jebel el Alia, JEC Jebel ech Cheid, PK Plain of Khalled, EP El Aroussa Plain. The inner white square (lower right corner) shows the meander area of Oued Siliana. White arrows indicate morphotectonic lineaments. For massifs 1 to 8, see Fig. 2

important to point that along Siliana river, this meandering only occurs at the eastern plain against the Triassic structure of JEC, confirming its recent morphodynamics. The model of spatial spreading of differential values in the JEC morphostructure shows clearly several particular areas with high values exceeding 200 m (Fig. 8), while the mean value is about 70 m. Within this massif, the level of differential values diminishes, abruptly in some localities, to 10–30 m and even reaches 0 m showing thereby a morphostructure with a specific differential pattern. These areas demonstrate an important lowering of landform caused by active erosional process that could be associated with the impact of the neotectonic subsidence. In fact, this erosion mainly based on fluvial process has an important action on evaporitic rocks causing, thereby, a considerable dissolution of the Triassic halite in the southeastern flank of JEC, as clearly and recently observed in situ by Ben Slama et al. (2012). In this case, we propose thus a model with an “intra-landform dynamics” showing a spatial association between areas revealing Quaternary uplift movements (high differential values) and those indicating low neotectonic movements associated with a diminished scale of topographic dissection. Within this morphostructure, a steady state dynamics has took place allowing in some way a mutual relationship between topographic growth and destruction (or lowering). According to the results of our analysis (Figs. 5, 6d, 7 and 8), the Triassic landform of JEC has accommodated, but not into a localized region, recent topographic growth initiated and conducted by active neohalokinesis and salt rock uplifting, consistent with the observations made by Coque and Jauzein (1965) that revealed PlioQuaternary deposits unusually located in the summit areas of several diapirs. Moreover, recent formations (with superficial calcareous crust) have an important increase of topographic slopes within areas surrounding the JEC landform, and they are even overturned in some localities against the flanks of the

diapir (Dali 1979; Perthuisot 1981; Perthuisot et al. 1999; Chikhaoui 2002). The constructed differential map of 4th order has also shown several alignments of high values especially oriented NE-SW (indicated by white arrows in Fig. 8). Several of these topographic lineaments are associated with mapped faults (Fig. 1) and revealing the same directions, which lead us to suspect the influence of neotectonic movements. Lineaments located within Jebel ech Cheid (JEC) diapir, Jebel es Sfah (JES), structure 3, Jebel Zalia (JZ), and Jebel Fej el Adoum (JFH) are among the most significant in the study area (Fig. 8). It is therefore possible to suggest, as our quantitative analysis allows, that the considered morphostructural lineaments have undergone an active tectonics during Quaternary time. In fact, the computed differential values are proportional to the level of Quaternary tectonics (Raczkowski et al. 1984) and probably to the uplifting movements. It should be noted, however, that there is no significant geological studies regarding neotectonic activities of faults and structures that have been carried out within the study area. We therefore have a lack of later observations that would lead to confirm or to contradict our findings. We hope that our mapped results will stimulate such future work. Most importantly, the morphodynamic mapping allowed by this isobase approach shows various tectonic landforms associated with active morphostructures and recent development of geomorphic features such as diapiric extrusion (which we suspect a geometry of Triassic salt dome), folds and plains, as well as fractures and topographic lineaments. The related differential values are according to the magnitude of Quaternary tectonics within the study area that is, indeed, a complex function of uplift and exhumation cycles. Therefore, the origin of neotectonic movements seems to be closely linked to reactivation of deep faults (Fig. 6d) and, most particularly, to the active morphodynamics of Triassic diapirs

Earth Sci Inform

within the study area (e.g., JEC salt diapir, Fig. 5) stimulated by additional Atlasic shortening. In addition, a shallow reactivation of the deep overthrust of Teboursouk known as the “accident of Teboursouk”, which is a consequence of an important regional west-dipping reverse fault (Perthuisot 1972, 1978), may therefore be evidenced locally (Fig. 6) at its superficial known tectonic segments such as Ain Jemala diapir (structure 3), Jebel Zalia (JZ) and Jebel Faj el Adoum (JFH, Fig. 8). Although we could quantify the spatial distribution of the Quaternary tectonics intensity within the study area, it is not possible at the moment to precisely evaluate the amount of Quaternary uplift of the studied morphostructures.

Conclusion In the present work, the detailed isobase method was applied into an area that presents a complex and heterogeneous geological framework leading to various geomorphic landforms. This method is a digital geomorphic approach based on hydrographic and topographic components and attributes which were automatically extracted from 10 m-grid DEM and performed within GIS environment using spatial analysis tools and procedures. Isobase mapping, applied at a semi detailed scale (1: 50 000), was then easily carried out allowing the surfaces extraction from various stream orders. Performed qualitative isobase analysis of isolines pattern configuration, in examining the model of spatial distribution and anomalous trends, has allowed three levels of interpretation: (1) delimitation of morphostructural units, (2) approximate localization of morphotectonic lineaments and discontinuities, and (3) assessment of sedimentary thickness and its lateral variations. We confirm, considering previous studies, that isobase surface constructed with 2nd and 3rd-order valleys could yield, also at semi detailed scale, to valuable and significant morphostructural and morphotectonic investigations. It allows, furthermore, a helpful sedimentological interpretations closely related to deposits thickness variations, which were clearly evidenced into our study area and largely confirmed by geophysical data and in situ geological observations. Moreover, importance of 4th order surface has been demonstrated in the interpretation of regional-scale morphostructures and morphotectonic depiction. In fact, 4th order disturbances appear to be more sensitive to deep structures and basement fractures. As a general qualitative approach, we suggest an analysis involving various isobase surfaces, including 2nd and 3rd order, only 3rd order, and 4th order. The first two surfaces allow, in some way, a shallow morphostructural and morphotectonic investigation, while the 4th order surface, if visual correlation allows, could reflect the impact of basement structures. Several massifs have therefore been identified as brittle and

rugged morphostructures excepting the diapiric extrusion of Jebel ech Cheid (JEC) which is considered as an atypical rugged morphostructure. Most of these structures are associated with basement faults that could possibly be reactivated during Quaternary times. It has been shown that drainage network organization and neotectonic adjustment and adaptation, which reflect Quaternary tectonics, could be quantitatively examined using differential mapping. This promising approach is primarily based on isobase pattern and isolines sinuosity and disturbances of 4th-order channels. The spatial distribution of these areas seems to indicate a strong differentiation of erosion rates and, likely, of recent tectonic movements. It is also considered to represent the scale of neotectonic movements during PlioQuaternary times. The differential analysis allow us to conclude that the atypical and outstanding morphostructure of JEC, especially, appears to be an important component of the exhumation and uplift of the study area, supporting a model based on an “intra-landform dynamics” which affects the surrounding sediments. There, the Triassic landform of JEC has accommodated, but not into a localized region, recent topographic growth initiated and conducted by active neohalokinesis and salt rock uplifting stimulated by additional Atlasic shortening. We would point that our findings combined with geological evidences lead us to support this hypothesis and, thereafter, to consider the studied diapir as a salt dome with complex geometry. In addition, the main topographic configuration of the study area may therefore be closely influenced by the Triassic active halokinesis (halotectonics), that we suspect the occurrence in our study area and even within several areas of the “diapiric zone” in northern Tunisia. This tectonic geomorphic method, that usefulness was confirmed as a geomorphometric parameter of active tectonics, could help understanding active fold growth and associated landforms. Although the isobase analysis method presented herein has been little exploited in tectonic geomorphology, it provides new tools and helpful hypothesis that can be applied to areas where evidences regarding uplift rates is not available or hardly distinguished. Acknowledgments The authors thank anonymous reviewers for detailed and constructive comments and valuable suggestions that have greatly improved the final version of the content of the manuscript.

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