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doi: 10.1515/geoca-2016-0024














University of Gabes, Faculty of Sciences, Research Unit of Geomatics, Structural Geology and Application, Erriadh City,  

6072 Gabes, Tunisia; 


University of Tunis El Manar, Research Unit of Geomatics, Structural Geology and Application, 2092 El Manar, Tunisia

(Manuscript received September 6, 2015; accepted in revised form June 7, 2016)

The quanti cation of deformation is one of the main objectives studied by geologists in order to control the 

evolution of tectonic structures and their kinematics during different tectonic phases. One of the most reliable methods 
of this theme is the direct calculation of quantity of deformation based on  eld data, while respecting several parame-
ters such as the notion of tectonic inheritance and reactivation of pre-existing faults, or the relationship between the 
elongation and shortening axis with major faults. Thus, such a quanti cation of deformation in an area may explain the 
relations of thin- and thick-skinned tectonics during this deformation. The study of structural evolution of the Jebel 
Elkebar domain in the southern-central Tunisian Atlas permits us to quantify the deformation during the extensional 
phase by a direct calculation of the vertical throw along normal faults. This approach is veri ed by calculation of 
thickness of eroded strata in the uplifted compartment and of resedimented series, named the Kebar Formation, in the 
downthrown compartment. The obtained results con rm the importance of the Aptian-Albian extensional tectonic 
regime. The extent of deformation during the compressional phase, related to reactivation of pre-existing faults, is less 
than that of extensional phases; indeed the compressive reactivation did not compensate the vertical throw of normal 
faults. The geometry of the Elkebar fold is interpreted in terms of the “fault-related fold” model with a décollement 
level in the Triassic series. This permitted the partition of deformation between the basement and cover, so that the 
basement was allowed for a limited transport only, and the maximum of observed deformation was concentrated in the 
thin-skinned tectonics.

 fault inversion; quanti cation of deformation; tectonic inheritance; tectonic phases; thin- and  

thick-skinned tectonics.

The early Mesozoic break-up of Pangaea was accompanied 
by opening of the Alpine and Mediterranean Tethys during 
the Middle Triassic (Bortolotti et al. 2007). From the late Oli-
gocene to early Miocene, the Ligurian-Provençal and Tyrrhe-
nian basins were established during the continuing N–S 
convergence of Africa and Eurasia and the deformation front 
advanced towards the Alpine foreland (Casado et al. 2001). 
From the Late Miocene onward, the African-Eurasian con-
vergence acquired a NW direction. The African-Eurasian 
convergence continued with the post-Villafranchian phase 
and has persisted with the N–S oriented shortening axis until 
recent times, which is expressed by the incipient closure of 
the Ligurian Basin (Larroque et al. 2001). Based on sparse 
focal mechanism solutions, the  rst-order seismo-tectonics 
framework for the Eastern Mediterranean was presented by 
McKenzie (1972). Geodetic measurements by 

Satellite Laser 


 (SLR) and then GPS led to a remarkable improve-

ment of the plate kinematic reconstructions. Nocquet (2012), 
referring to the available GPS data from southern part of the 
Tellian Atlas to the southern margin of the Ligurian– 

Provençal Basin, estimated the convergence rate at about 
4–5 mm/year. However, more precise estimates are ham-
pered by lack of GPS data from Tunisia and Libya. In general, 
the plate kinematic boundary conditions indicate the conver-
gence rate at 5.5–6 mm/year. 

Most of these studies were based on quantification of plate 

movements in the internal oro genic zones, without conside 
ration of deformation parameters in external zones. In addi-
tion, the choice and localization of the study area are important 
constraints for deciphering evolution of tectonic structures 
developing under different tectonic regimes. Tunisia is 
located at the northern limits of the African plate and, particu-
larly, at the eastern margin of the Atlas Range, which position 
enables us to register the maximum information about the 
deformation processes during opening of the Atlantic and 
closing of the Tethys oceans.

The aim of this contribution is to quantify the deforma-

tion during the rifting-related extensional phases of tec-
tonic evolution, recorded by movements along the normal 
faults, and then during their compressional reactivation in 
the external zones of the Atlas Mts in south-central 

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, 2016, 67, 4, 391–401

The methodology in this work is guided by the three main 

1. Quantification of deformation based on fault displacement 

during the extensional period and the geometry of inver-
sion-related folds during the compressional reactivation of 
pre-existing normal faults. For this purpose, we propose 
model solutions presented in the chapter “Parameters of 
deformation” below.

2. Determination of the chronology of tectonic events and 

kinematics of deformation structures resulting in recon-
struction of the structural evolution of the studied area.

3. Finally, the correlation between the thin- and thick-skinned 

deformation processes.
The proper choice of the study area is one of the most 

important conditions to verify the tectonic complication in 
order to interpret the chronology of tectonics events and the 
evolution of geological structures. For this aim we have cho-
sen the southern-central Tunisian Atlas (Fig. 1). The choice 
of this area is not arbitrary, because it records a maximum of 
registered tectonic phases; unlike the northern Atlas that 
occurs at the front of the Africa and Eurasia convergence and 
is strongly tectonized. In contrast, the central-southern Atlas 
represents the wide external zone in which the different 
stages of the structural evolution can be clearly 

In order to gather some fundamental parameters, it is 

important to quantify deformation in the external zone of the 
Atlas Mountains. These parameters should then be verified in 
any study of deformation during successive tectonic phases. 

The inheritance tectonics

The tectonic inheritance concept is based on the interpreta-

tion of the evolution of tectonic structures and their kinematics 
during different tectonics regimes. The Atlasic structures in 
the North African Craton are guided by particular dynamics 
over the geological time, especially since the opening of the 
Tethys in the Late Triassic, associated with extensional struc-
tures, until the current N-S convergence of Africa to Eurasia 
accompanied by a compressive tectonic regime (Caire 1971; 
Dercourt et al. 1985; Zargouni et al. 1985; Frizon de Lamotte 
et al. 2009).

One of the main parameters that have to be considered in 

this concept is the relationship between the direction of the 
elongation axis during the distensive phase and the shortening 
axis during the compressive reactivation. The Tunisian Atlas 
is distinguished almost by the same NW–SE direction of both 
events. The Tunisian Atlas, particularly the Southern-Central 
Atlas, shows normal faults trending NE–SW perpendicular to 

 Position of the Tunisian Atlas in the Mediterranean Alpine chains. The Tunisian Atlas occupies the eastern limit of the Atlas struc-

tures that were guided by opening of the Tethys in the Middle–Late Triassic and its subsequent closure continuing until the recent times.

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, 2016, 67, 4, 391–401

the NW–SE main elongation axis. The normal faults are 
reactivated during the Alpine compressional phase along 
a shortening axis trending NW–SE to NNW–SSE (Ouali et 
al. 1986; Ben Ayed 1993; Hlaiem 1999; Belguith et al. 2011), 
whereby the Atlasic folds with NE–SW to E–W direction 
developed parallel to the pre-existing normal faults (Frizon 
de Lamotte et al. 2009).

Partitioning of shortening-related deformation in pre- 
existing fault systems

Generally, the faults with a strike-slip component have the 

principal compressional and extensional stress axes (



) in a horizontal position. In a collision zone with the 

gene rally transpressive stress regime, where the pre-existing 
faults are oblique to the convergence vector, deformation is 
concentrated in the vertical strike-slip faults instead to 
involve an oblique-slip movement along an inclined fault. 
Hence, applying the principle of fundamental physics to 
mini mize the dissipated energy, movement along an inclined 
fault, which has a larger projected surface, is hampered by 
a more important friction than the strike-slip movement 
along a vertical fault. Consequently, it is more favourable to 
accommodate the strike-slip component along a vertical fault 
than along an inclined fault, while the compressive compo-
nent is accommodated by thrusting along the inclined fault 
(see Fig. 2). Therefore, it is important to verify the angle of 
convergence in the deformed area on the one hand and the 

strike and inclination of the pre-existing fault’s plane on the 
other hand.

Thin and thick-skinned notion

The problem of the thin- and thick-skinned concept is one 

of the fundamental parameters to quantify the orogenic 
deformation. Two concepts are generally available: (a) the 
basement is involved in deformation of the cover (Belguith et 
al. 2011, confirmed this style in the central Tunisian Atlas), 
or (b) it has a passive role and it is only the sedimentary cover 
which is tectonized (detachment style; e.g. Amamria et al. 
2013). In this latter case the thin- and thick-skinned segments 
are separated by a surface known as the décollement level.

If the basement is involved in shortening, deformation in 

the cover will be similar in two adjacent areas. On the con-
trary, when the basement is not integrated in the deformation 
of the cover, the resulting structures show great variations 
from one area to another. This is an effect related to inheri-
tance tectonics controlled by pre-existing fault systems in the 
cover complexes.

The typical example to demonstrate the deformation style 

in an African Alpine collisional orogen is the eastern limit of 
the Atlas chain, such as the Tunisian Atlas (Fig. 3A). This 

area records firstly the response to ope-
ning of the Atlantic Ocean and secondly 
the rotation of Africa and its approach to 
Eurasia. For our analysis, we have cho-
sen the central Tunisian Atlas in particu-
lar, which forms the boundary between 
two different structural domains 
(Fig. 3B): the northern, strongly tecto-
nized area and the southern, only mode-
rately deformed domain that gradually 
passes into the stable Saharan Platform.

The study sector occupies the central 

position of the central Tunisian Atlas, 
named the Elkebar chains (Fig. 3C). It is 
a large-scale anticline with 60–80° axial 
direction (Fig. 3D) bounded in the east 
by the north-south trending axial plunge, 
which represents a structural limit 
between two different palaeogeographic 
domains at the same time (Oriental Plat-
form; Fig. 3C). 

The Jebel Elkebar chain is geographi-

cally located on the southern border of 
the town of Sidi Bouzid, bounded by the 
following geographic coordinates: at lati-
tudes from 34°56’42”N to 34°59’42”N 
and at longitudes from 9°27’54”E to 
9°33’18”E. We will consider especially 

 Partitioning of deformation based on relationships between the horizontal shorten-

ing axis and the dip direction of a pre-existing fault ( ) and strike of the fault’s plane (B): 

 — in case the angle of a fault dip is less than 45° and approaches 0°, thrust faulting is 

generated, while if this angle exceeds 45° and increases toward 90°, the strike-slip faulting 
will be dominating in non-orthogonal shortening; B — if the angle between the shortening 
axis and the fault trend is more than 0 and less than 45° towards an inclined fault, than the 
strike-slip fault activity will be initiated, whereas oblique-slip to thrusting will dominate 
for angles more than 45°. The particular case with zero angle between the shortening axis 
and subvertical fault direction means no reactivation of the pre-existing fault. Further 
explanations in the text.

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 Localization of studied area: 

 — position of the 



Atlas at the eastern termination of the 

Atlas chains; 


 — delimitation of the southern central 



Atlas with the northern 



Atlas to the north, the Saharan Platform to the south and the North-South axis to the East; 


 — the position of the Jebel Elkebar domain in the central 





 — the NE–SW


trending anticlinal structure of Jebel Elkebar


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, 2016, 67, 4, 391–401

the eastern boundary of the Jebel Elkebar domain, in which 
the geometry and evolution of structures provide valuable 
data that facilitate interpretation of the kinematics and quan-
tification of deformation.

Stratigraphic data 

The sedimentary complexes outcropping in the study area 

range from the Late Jurassic to Quaternary, with dominance 
of Cretaceous formations. The succession begins with the 
Sidi Khalif Formation of Tithonian age that is composed of 
alternation of calcareous clay and marly deposits dated by 
ammonites. This formation is overlain by alternating sand-
stones and reddish dolomites of the Meloussi Formation of 
Berriasian to Valanginian age, with the thickness of 
300 metres. The Boudinar Formation of basal Hauterivian 
age is characterized by dominance of coarse-grained sand-
stones with fossil wood remnants; its thickness is 150 metres. 
It is overlain by limestone and dolomite of the Bouhedma 
Formation, which is of Hauterivian to Barremian age deter-
mined by ostracods, gastropods and echinoderms. It has 
a variegated composition with alternation of carbonate beds, 
clay, sand and gypsum with local intercalations of palaeo-
sols. The total thickness of this formation is about 200 metres. 
The Sidi Aich Formation, Barremian in age, consists of white 
sands with oblique stratification and thickness not exceeding 
10 metres. The Orbata Formation of Aptian age, determined 
by orbitolinids, is formed by a dolomitic layer at the base, 
followed by calcareous marl alternations; it is 20 metres 
thick and constitutes a competent sequence representing 
posi tive relief forms. The Cretaceous series finishes with the 
Zebbag Formation (Albian–Cenomanian up to earliest Turo-
nian age); it is a marl-carbonate alternation with rare interca-
lations of gypsum. The formation is capped by a massive 
dolomitic layer of the Guettar Member with a thickness of 10  to 
15 metres. The total thickness of the Zebbag Formation is 
200 metres. 

It should be noted that there is another typical formation 

occurring in the Jebel Elkebar area. It is named the Kebar 
Formation — these are redeposited conglomerates and varia-
ble deposits like sands, clays, gypsum and carbonates. These 
deposits are the lateral equivalents of the Orbata, Sidi Aich, 
Bouhedma formations and of a large part of the Boudinar 
Formation for a period ranging from the Hauterivian to 
Aptian. The Kebar Formation is attributed to the basal Albian 
(Trabelsi et al. 2010). The temporal and spatial distribution 
of the Kebar Formation is an important parameter which will 
be used in the interpretation of the kinematics and quantifica-
tion of deformation.

Geodynamics study 

The tectonic interpretation of the investigated area is based 

mainly on precise mapping associated with construction of 
detailed geological cross sections. The geological map of the 
study zone shows an asymmetric anticline of ENE–WSW 

direction; its northern flank is subhorizontal, dipping less 
than 15°, while the southern flank is subvertical with dips of 
about 70° (Fig. 4). The asymmetry between the two limbs is 
also observed in the dissimilar distribution of sedimentary 
formations in both. The core of the anticline is occupied by 
the Sidi Khalif Formation, the limbs are dominated by the 
Meloussi and Boudinar formations (Fig. 4). This anticline is 
cut out in the axial part by a pre-existent fault of N80 direc-
tion, which was subsequently reactivated during the com-
pressional tectonic regime. We utilize it as the principal phe-
nomenon controlling the evolution of tectonic structures in 
the area.

The A–A’ cross-section is with NW–SE direction parallel 

to the main shortening axis and perpendicular to the anticline 
axis and direction of the pre-existing fault. This cross-section 
confirms firstly the asymmetry of the anticline and on the 
other hand it shows the asymmetric distribution of sedimen-
tary series in two flanks of the anticline. The southern flank 
is formed by the Sidi Khalif, Meloussi and Boudinar forma-
tions unconformably overlain by the Zebbag Formation, 
while the north side is formed by an almost complete succes-
sion from the Sidi Khalif Formation at the base to the Zebbag 
Formation on the top (Fig. 5). Note that the northern flank of 
the Jebel Elkebar anticline also includes the specific Kebar 
Formation (Fig. 6) that is inserted between the Orbata For-
mation of the Aptian age and the Albian–Cenomanian Zeb-
bag Formation. The problems to be analysed are: 1) the sedi-
mentary gap of the missing Bouhedma, Sidi Aich, and Orbata 
formations in the southern flank and 2) the presence of the 
Kebar Formation in the northern flank of the anticline, knowing 
that this formation is characterized by a variable shape of con-
glomerate pebbles (angular, subangular and rounded) and 
also variabi lity of origin and facies (sand, clay, sandstone and 
carbonate) which are lithologically recall the Boudinar, 
Bouhedma, Sidi Aich and Orbata formations (Fig. 5). These 
problems cannot be resolved without deciphering the 
chronology of tectonic events and kinematics of develop-
ment of this structure.

Consequently, it is important to consider that the absence 

of several sedimentary units in the southern flank, indicated 
by a direct contact of the Boudinar Formation with the Zeb-
bag Formation, is justified by the deposition of the Kebar 
Formation only in the north limb and between the Albian and 
Aptian strata. The inferred erosion and redeposition should 
not have occurred over a long period, since the resedimented 
Kebar Formation occurs between two successive concordant 
series — the Orbata and Zebbag formations. This situation 
requires the formation of a palaeo-slope following some 
extensional tectonic activity (synsedimentary fault) allowing 
a rapid erosion of the southern uplifted block on one hand, 
and down-throw of the northern compartment to create 
a depocentre area for the Kebar Formation. 

The detailed examination of slickensides in the Garet at Tir 

area (fault F2 trending N80 in Fig. 4) revealed normal activ-
ity by contacting the Meloussi Formation with that of 
Bouhedma. In the southern compartment of this fault, the 

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formations of Orbata, Sidi Aich, Bouhedma and part of the 
Boudinar Formation are not exposed, whereas in the northern 
compartment the whole succession with intercalation of the 
Kebar Formation is cropping out.

Based on the tectonic development 

of the Jebel Elkebar structure and 
erosion and redeposition of sedimen-
tary formations during changed geo-
logical events, we have tried to pro-
pose a kinematic scenario and to 
quantify the deformation processes.

The quantification of deformation 

is one of the most important parame-
ters for resolving the structural evo-
lution. The calculation of the amount 
of deformation in the Jebel Elkebar 
site is closely related to the tectonic 
evolution and chronology of succes-
sive extensional and compressional 
phases. In principle, this zone was 
affected by two principal tectonic 
regimes: the first one extensional, 
which was later replaced by com-
pression. Methodologically, we will 
reconstruct the evolution of the main 
pre-existing fault during the exten-
sional phase and its reactivation 
during the ensuing compression.


In this case, the deformation con-

trolling the evolution of the structure 
is the linear deformation that can be 
determined by the formula:

l      l f – 0

 =       =             ,            

      l 0        0

where  is the linear strain.

To verify this notion it is important 

to determine the range of fault-slip 
movement, whatever lateral or verti-
cal. The structural study of the Jebel 
Elkebar structure revealed an asym-
metric anticline affected by a pre- 
existing fault in its axial part. The 
problem in this study is the determi-
nation of separation of the strati-
graphic markers that were displaced 
on both sides of the fault, since the 
markers can be completely hidden in 

the downthrown fault block, or eroded in the uplifted block. 
To resolve this problem, we have determined the vertical off-
set along the synsedimentary fault by calculations of the 

mes of eroded strata in the uplifted block and the 

 The geological map and cross-section A–A’ of the Jebel Elkebar structure (localization 

according UTM coordinates), showing an asymmetric anticline of the ENE–WSW direction, 
where its axial part is affected by a pre-existing ENE–WSW trending fault (F1). The cross 
section is oriented parallel to main Alpine shortening axis.

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resedimented formation in the downthrown block. The facies 
asymmetry of the Jebel Elkebar structure resulted from ero-
sion-related extensional activity along the fault, subsequently 
partly recovered by compression. This fault, designated as 

the Garet at Tir Fault (F2 in Figs. 4 and 7), is a north-dipping 
normal fault of Aptian age with prevailing dip-slip move-
ment confirmed by the slickenside measurements: N80, 
70NNW, 50W (Fig. 8).

The synsedimentary activity of the Garet at Tir Fault is 

documented by erosion of the Orbata, Sidi Aich and 
Bouhedma formations, as well as a great part of the Boudinar 
Formation in the southern uplifted compartment. The eroded 
material was redeposited in the subsided northern compart-
ment as the Kebar Formation intercalated between the Aptian 
Orbata and Albian Zebbag Formation (Fig. 5; cf. Trabelsi et 
al. 2010). Based on the constant thickness of the Zebbag For-
mation on both sides of the fault, the proposed model requires 
that the top of Orbata Formation collapsed and was put in 
contact with the Sidi Khalif Formation.

It is inferred that erosion of the uplifted block was mainly 

caused by gravity-driven and possibly earthquake-induced, 
down-slope mass wasting along the developing fault scarp. 
River erosion is less probable, since it reached the Boudinar 
Formation composed mostly of sandy lithologies that are 
only partly eroded. In case of the river erosion, a complete 
removal of this formation would be expected. The proposed 
model is based on two known principal parameters: the dip 
angle of the fault ( ) and its vertical throw (Rv). The vertical 
displacement brought about the contact between the Orbata 
Formation in the northern compartment with the upper level 
of the Sidi Khalif Formation, so the vertical offset (Rv) is 

 The geometry of the Elkebar anticline shows an asymmetry of geometry and facies distribution between two limbs — the steeper 

southern  ank is characterized by subvertical bedding and omission of some formations (Orbata, Sidi Aich, Bouhedma, and a part of Bou-
dinar), the slightly tilted northern  ank is characterized by a complete succession with intercalation of the Kebar Formation.

 Stratigraphic correlation between the two  anks of the Jebel 

Elkebar anticline. Note the dissymmetry of distribution of sedimen-
tary formations expressed by omission of the Bouhedma, Sidi Aich, 
Orbata and a part of the Boudinar formations in the southern  ank 
associated with occurrence of the resedimented Kebar Formation in 
the northern  ank.

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equivalent to the collective thickness of the formations 
Orbata (20 metres), Sidi Aich (10 metres), Bouhedma 
(200 metres), Boudinar (150 metres), Meloussi (300 metres) 
and the upper level of the Sidi Khalif Formation (20 metres). 
Hence the cumulative vertical throw of the Garet at Tir Fault 
is above 700 metres (Fig. 9). 

The lateral separation (Rl) of a fault can be derived from 

the relation: tg =Rv/Rl. The total horizontal component of 
movement along a fault then corresponds to the displacement 

l. To determine the initial length (l


) of the scaled model, it 

is enough to remove firstly the initial horizontal displace-
ment caused by the fault and secondly the strata dip produced 
by the superimposed compressional reactivation. Assuming 
that the dip of fault ( ) is 70° (Fig. 9B), the horizontal dis-
placement would be:

        Rv      700

Rl =         =         = 255.47 m

The resulting deformation then amounts:

l      255.47

=        =              = 0.127

       l0        2000

During the superposed compression and inversion of the 

Jebel Elkebar structure, the Garet at Tir normal fault was 
reactivated as a thrust fault. It is important to note that the 
shortening axis is parallel to the preceding elongation axis, so 
the principal compressive stress axis 

is perpendicular to 

the strike of the pre-existing fault and parallel to its dip. This 
relation between shortening axis and pre-existing normal 
fault is indicated by development of folds of the “fault propa-
gation fold” or “fault bend fold” type verified by the geome-
try of the Elkebar anticline. As a result, the Jebel Elkebar 
anticline is formed by two asymmetric flanks; the southern 
one shows dipping of layers to the south with angle of  
ca. 65 degrees, whereas the dip of in the northern flank is 
north-dipping at angles not exceeding 15 degrees (Fig. 7). 
The Elkebar anticline is moulded about the old normal fault 
inverted to a reverse fault with thrusting of the northern com-
partment over the southern one. 

The quantification of the compressive deformation is 

determined by the vertical offset along the inverted fault 
during thrusting that corresponds to the thickness of the 
Meloussi Formation (Fig. 9D):

        Rv      300

Rl =         =         = 109.48 m

l      109.48

=        =              = 0.054

       l0        2000
In this study, the quantity of deformation during exten-

sional and compressional phases with the parallel lengthe-
ning and shortening axes enabled reactivation of the pre- 
existing normal fault as a reverse fault. Although these quan-
tities of deformation are variable indeed, the amount of 
extensional deformation (0,127) exceeds that of the compres-
sional deformation (0,054) approximately twice. Accor-
dingly, the compressive phase did not compensate the total 
offset of the pre-existing Garet at Tir normal fault (Fig. 8), 
which still shows the normal offset in spite of its compres-
sional reactivation.

 The normal activity of the ElKebar fault during the Aptian 

that explains the erosion of Boudinar, Bouhedma, Sidi Aich, and 
Orbata Formation in the southern compartment and redeposition of 
Kebar Formation in the northern compartment.

 The slickenside plane related to the Garet at Tir fault shows 

striations and grooves still indicating the normal kinematics, despite 
its reactivation during the compressive phase.

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quantification of extensional and 

compressional deformation in orogenic 
structures requires the verification of some 
parameters that have an important role in the 
interpretation of the geometries of faults and 
folds. It is important to choose the suitable 
area for applying the main parameters of the 
evolution of deformation. Consequently, the 
Jebel Elkebar anticline may serve as a repre-
sentative example to control the develop-
ment of tectonic structures in the central 
Tunisian Atlas.

One of the most important parameters to 

verify the deformation is the inheritance tec-
tonics and the compressional reactivation of 
pre-existing normal faults. The Tunisian 
Atlas was subjected to two principal tec-
tonic phases, the first one was extensional 
and began in the Late Triassic with opening 
of the Tethys Ocean (Raulin et al. 2011). In 
the central Tunisian Atlas, this extensional 
phase is well distinguished in the Creta-
ceous sedimentary successions by the 
synsedimentary control of ENE–WSW 
orien ted normal fault F2, whose elongation 
axes are NW–SE directed with subsidence 
of north-western compartments. During 
extension, the volume of eroded formations 
in the uplifted compartment was approxi-
mately the same as that resedimented in the 
downthrown compartment (Kebar Forma-
tion), so there is a conservation of volumes 
and there was no significant lateral sedimen-
tary discharge.

Several problems of the compressional 

tectonics in the Tunisian Atlas, and particu-
larly the period of its activity, have been 
controversially discussed. Some studies 
showed that the compressive phase started 
during the Eocene period with the E–W 
shortening axis (El Ghali et al. 2003; 

 The proposed evolutionary model of the 

Jebel Elkebar anticline:   — the original (pre-Ap-
tian) disposition of sedimentary layers before 
deformation; B — the extensional activity of the 
Elkebar fault associated to the Aptian erosion of 
the southern compartment and redeposition of the 
Kebar Formation; C — after cessation of normal 
faulting, erosion and peneplanation, the area was 
sealed by the Albian Zebbag Formation; D — 
reactivation of pre-existing normal fault during 
the compressive phase following the model of 
“fault bend folding”.

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, 2016, 67, 4, 391–401

Khomsi et al. 2006; Mzali and Zouari 2006), which is cor-
related with the Eocene compressional phase in the Algerian 
Atlas. However, other authors confirmed that the Eocene 
shortening phase was not widespread throughout Tunisia as it 
was followed by periods of stress relaxation and extension. 
Therefore the main compressive phase rather corresponds to 
the Late Miocene Alpine phase with the NW-SE shortening 
axis followed later by post-Villafranchian phase with subme-
ridional shortening axis (Boukadi et al. 1998; Frizon De 
Lamonte et al. 2000; Piqué et al. 2002).

Field data in this study area do not show signs of Eocene 

compression. The late Alpine compressional phase with 
NW–SE shortening axis is well documented as perpendicular 
to the old normal faults trending ENE–WSW, hence allowing 
their reactivation as reverse faults and development of anti-
clinal folds according to the “fault bend fold” model (Fig. 9). 
On the other hand, the model of “fault propagation fold” can-
not be verified in this case, because its application requires 
conservation of thicknesses between two flanks of an anti-
cline, whereas the Elkebar anticline shows a big variation of 
thicknesses and facies between its two flanks.

Another parameter that is important to verify is related to 

the concept of inheritance tectonics and reactivation of old 
normal faults and especially the direction of the shortening 
axis with respect to the pre-existing faults. In the presented 
case study, the elongation axis during extensional periods 
was NW–SE directed, which controlled development of nor-
mal faults trending ENE–WSW. The reactivation of these 
faults by a compression with shortening axis parallel to the 
preceding elongation axis is indicated by the inversion of 
normal faults and their association with thrusting structures. 
If the axis of shortening was oblique to the old fault, the main 
component would generate a strike-slip along the pre-exis-
ting normal fault and the quantity of orthogonal deformation 
will be much less important. According to our observations, 
this is not the case.

The main driver of the compressive stresses in the Atlas 

Mountains and northern African Craton, particularly in Tuni-
sia, was the convergence of Africa relative to Eurasia. The 
resulting deformation gradually decreases away from the Afri-
can-Eurasian convergence zone to the external Atlasic areas, 
remarkably from north to south of Tunisia, which accounts for 
the tectonic stability of the Saharan Africa Platform. In this 
aspect, it is worth checking whether the basement is integrated 
into the deformation in the central Tunisian Atlas.

In the northern Tunisian Atlas, the basement is incorpo-

rated in deformation which allows in many cases its outcrop 
to the surface. In contrast, the basement complexes in sout-
hern-central Tunisia are not involved in shortening. Previous 
studies in some neighbouring structures, like Jebel Orbata 
(Bensalem et al. 2011) and Gafsa basin (Amamria et al. 
2013), have shown that the amount of deformation is very 
variable also between two very near areas, giving rise even to 
duplex structures, as for instance the comparatively thin 
 Triassic deposits stacked above the inverted pre-existing 
Gafsa Fault (Amamria et al. 2013).

Investigation of these structures indicates that the primary 

reason for various degrees of deformation is related to the 
tectonic inheritance and reactivation of pre-existing faults 
during the compressive phase. In fact, if shortening affected 
the basement areas with significant sediment load (Triassic 
and Jurassic that can reach the total thickness of 3000 to 4000 
metres), the style and amount of deformation is moderated 
between the two neighbouring areas. Consequently, we sup-
pose that the basement played a passive role in the transport 
of deformation along the décollement level in the Upper 
Trias sic to Lower Jurassic strata (Ahmadi et al. 2006) and the 
overall deformation and shortening style can be characte-
rized as thin-skinned tectonics. 

This contribution concerns the modes of extensional ver-

sus compressional deformation in external zones of southern- 
central Tunisia. The quantification of deformation in the 
Jebel Elkebar zone was based on the direct calculation of the 
amount of synsedimentary activity of normal faults. The 
observed sediment redeposition permitted to determine their 
vertical offset during extension, and consequently the quan-
tity of deformation could be determined. On the other hand, 
the compressive reactivation of pre-existing faults caused the 
overlap and re-contacting of older formations with younger 
ones. Consequently, a partial compensation of the vertical 
offset of original normal faults was reached.

In general, it was shown that the Aptian–Albian exten-

sional deformation resulting in development of synsedimen-
tary normal faults was not fully compensated later by the 
later compressional regime. This observation can be genera-
lized across the entire Tunisian Central Atlas, where nearly 
all old normal faults still predominantly record the exten-
sional activity, in spite of their subsequent inversion due to 
compressional reactivation.  

This work is supported by the Tunisian 

Ministry of Higher Education and Scienti c Research, unit 
of research UR 11 ES 13. The comments and suggestions of 
anonymous reviewers, as well as of Geologica Carpathica 
editors Dr. Dušan Plašienka and Dr. 

Igor Broska,


improved the quality of this manuscript. The authors want to 
dedicate this contribution to the memory of Managing Editor 

 who participated in the initial stages of this 


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