GeolCarp_Vol67_No4_391

GEOLOGICA CARPATHICA, AUGUST 2016, 67, 4, 391–401

doi: 10.1515/geoca-2016-0024

www.geologicacarpathica.com

MOHAMED SADOK BENSALEM

, SOULEF AMAMRIA

, MOHAMED GHANMI

and FOUAD ZARGOUNI

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

6072 Gabes, Tunisia; bensalemsadk@gmail.com

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

Ranging

(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
Tunisia.

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The methodology in this work is guided by the three main

objectives:
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
distinguished.

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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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 (

and

) 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

unisian

Atlas at the eastern termination of the

Atlas chains;

— delimitation of the southern central

unisian

Atlas with the northern

unisian

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

unisian

Atlas;

— the NE–SW

trending anticlinal structure of Jebel Elkebar

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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.

deformation

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 – l 0

= = ,

l 0 l 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
volu

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
tg

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
tg

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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The

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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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,

greatly

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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