1-s2.0-S0040195108005222-main

Mesozoic and Cenozoic vertical movements in the Atlas system
(Algeria, Morocco, Tunisia): An overview
Dominique Frizon de Lamotte
a,
⁎, Pascale Leturmy
a
, Yves Missenard
a,1
, Sami Khomsi
b
, Geoffrey Ruiz
c
,
Omar Saddiqi
d
, Francois Guillocheau
e
, André Michard
f
a
Univ Cergy-Pontoise, Département des Sciences de la Terre, F-95 000 Cergy, France
b
Laboratoire Géoressources, INRST, Bordj Cédria, 57 rue 7301, par av. Tahar-Ben-Ammar-Menzach 9B, 1013 Tunis, Tunisia
c
Univ Neuchâtel, Geological Institute Emile Argand 11/CP 158 CH-2009 Neuchâtel, Switzerland
d
Laboratoire de Géodynamique et Thermochronologie, Faculté des Sciences Aïn Chock, BP 5366, Casablanca Maarif, Morocco
e
Univ Rennes 1 Geosciences-Rennes (UMR 6118); Campus de Beaulieu, 35042 Rennes Cedex, France
f
10, rue des Jeûneurs, 75002 Paris, France
abstractarticle info
Article history:
Received 25 March 2008
Received in revised form 18 September 2008
Accepted 16 October 2008
Available online 29 October 2008
Keywords:
Maghreb (Algeria, Morocco, Tunisia)
Vertical movements
Mesozoic
Cenozoic
The E–W trending Atlas System of Maghreb consists of weakly shortened, intra-continental fold belts
associated with plateau areas (“Mesetas”), extending between the south-westernmost branch of the
Mediterranean Alpine Belt (Rif-Tell) and the Sahara Platform. Although the Atlas system has been erected
contemporaneously from Morocco to Algeria and Tunisia during the Middle Eocene to Recent, it displays a
conspicuous longitudinal asymmetry, with i) Paleozoic outcrops restricted to its western part; ii) highest
elevation occurring in the west, both in the Atlas System and its foreland (Anti-Atlas); iii) low elevation
corridors (e.g. Hodna) and depressed foreland (Tunisian Chotts and Sahel area) in the east. We analyse the
origin of these striking contrasts in relation with i) the Variscan heritage; ii) crustal vertical movements
during the Mesozoic; iii) crustal shortening during the Cenozoic and finally, iv) the occurrence of a Miocene–
Quaternary hot mantle anomaly in the west. The Maghreb lithosphere was affected by the Variscan orogeny,
and thus thickened only in its western part. During the Late Permian–Triassic, a paleo-high formed in the
west between the Central Atlantic and Alpine Tethys rift systems, giving birth to the emergent/poorly
subsident West Moroccan Arch. During the late Middle Jurassic–Early Cretaceous, Morocco and western
Algeria were dominantly emergent whereas rifting lasted on in eastern Algeria and Tunisia. We ascribe the
uplift of the western regions to thermal doming, consistent with the Late Jurassic and Barremian gabbroic
magmatism observed there. After the widespread transgression of the high stand Cenomanian–Turonian
seas, the inversion of the Atlas System began during the Senonian as a consequence of the Africa–Eurasia
convergence. Erosion affected three ENE-trending uplifted areas of NW Africa, which we consider as
lithospheric anticlines related to the incipient Africa–Europe convergence. In contrast, in eastern Algeria and
Tunisia a NW-trending rift system developed contemporaneously (Sirt rifting), normal to the general trend
of the Atlas System. The general inversion and orogenesis of the Atlas System occurred during two distinct
episodes, Middle–Late Eocene–Oligocene and Late Miocene–Pliocene, respectively, whereas during the
intervening period, the Africa–Europe convergence was mainly accommodated in the Rif-Tell system.
Inversion tectonics and crustal thickening may account for the moderate uplift of the eastern Atlas System,
not for the high elevation of the western mountain ranges (Middle Atlas, High Atlas, Anti-Atlas). In line with
previous authors, we ascribe part of the recent uplift of the latter regions to the occurrence of a NE-trending,
high-temperature mantle anomaly, here labelled the Moroccan Hot Line (MHL), which is also marked by a
strip of late Miocene–Quaternary alkaline magmatism and significant seismicity.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Vertical movements of the continental crust and related changes of
topography are the consequence of a wide variety of processes,
occurring at different time and spatial scales. Among these processes,
the more important are: (1) crustal/lithospheric thinning or thickening;
(2) crustal/lithospheric folding or flexuration, (3) large scale upwelling
or downwelling mantle flows. Their origin still remains a matter of
Tectonophysics 475 (2009) 9–28
⁎Corresponding author.
E-mail address: [email protected] (D.F. de Lamotte).
1
Now at: Univ Paris-Sud, Département des Sciences de la Terre, Bat. 504, 91 405
Orsay Cedex, France.
0040-1951/$ –see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.tecto.2008.10.024
Contents lists available at ScienceDirect
Tectonophysics
journal homepage: www.elsevier.com/locate/tecto

debate and the quantification of their consequences on the topography
is poorly constrained due to the complicated interactions between them.
The topography of the diffuse, Mesozoic–Cenozoic Africa/Eurasia
plate boundary zone (i.e. the Alpine system) is currently extensively
studied but most of the previous work concentrates on the Alps, the
Pyrenees and their European foreland (see a review in Cloetingh et al.,
2007). By contrast, the topography of the North African mountain
belts and foreland has been poorly studied yet. In this paper, we focus
on this area (Fig. 1). More precisely, the study area is formed by the
Atlas system of Algeria, Morocco and Tunisia and subsidiary by both its
hinterland (the Tell-Rif) and foreland (the Sahara domain). This
system consists of weakly shortened, intracontinental fold-and-thrust
belts including plateau areas (the so-called “Mesetas”)(Fig. 1) (review
in Frizon de Lamotte et al., 2000). It is an excellent natural laboratory
to illustrate the different processes controlling the temporal evolution
of topography for the following reasons: (1) except in Tunisia, there is
no Cenozoic extension interfering with the overall compressional
regime as the European Cenozoic Rift System in Europe; (2) the
paleostress direction remained broadly stable (NNW) since the Late
Cretaceous (Aït Brahim et al., 2002; Bracène and Frizon de Lamotte,
2002; Bouaziz et al., 2002); (3) the relative independence between
the operating processes allows us to distinguish and describe each of
them separately (Missenard, 2006); and (4) the Atlas domain
underwent a protracted but relatively simple and well documented
post-Paleozoic geological history (see a review in Frizon de Lamotte et
al., 2000; Frizon de Lamotte et al., 2008).
Our main concern is to understand the present asymmetry of the
Maghreb topography from west to east (Fig. 1). Why do the highest
mountains develop in Morocco, in the Marrakech High Atlas, and not in
Algeria or Tunisia? Why the only mountain range south of the South
Atlas Front (SAF) is in the Moroccan Anti-Atlas? Why, by contrast, the
front of the Tunisian Atlas is an actively subsiding area? In this paper, we
propose a review of the main events which affected the Atlas system
since the Triassic with emphasis on the periods when the lithosphere
behaviour was not uniform at the scale of the Maghreb. Then we
examine the different processes responsible for the vertical movements:
thermal uplift or subsidence, lithospheric folding, tectonic inversion and
crustal shortening. Finally we discuss how these processes alternate
through time and how they can explain the present topography.
2. Geological setting and present topography of the Maghrebian
orogenic domain
2
The Maghrebian orogenic domain comprises two different systems:
the Alboran–Kabylias–Peloritan–Calabria (AlKaPeCa; Bouillin, 1986)
and Tell-Rif (shortly Tell-Rif, also referred to as Maghrebide Belt) to
the north and the Atlas to the south (Fig. 1B). AlKaPeCa domain is of
European origin and corresponds to the former northern margin of the
Alpine Tethys now included in the Tell-Rif, whose it forms the internal
domain. So, the Tell-Rif pertains to the Western Mediterranean Alpine
belts and results from the closure of the Maghrebian branch of the
Alpine Tethys (Durand-Delga and Fontboté, 1980; Bouillin,1986; Favre et
al., 1991). By contrast, the Atlas is an intra-continental asymmetric
system, which comprises both mountain belts (High and Middle Atlas in
Morocco, Saharan Atlas and Aurès Mountains in Algeria, and Tunisian
Atlas in Tunisia) but also poorly deformed, broadly tabular domains (the
so-called Western, or Moroccan, and Eastern or Oran Mesetas) only
present in its western part. The asymmetry of the system is also obvious
in the repartition of rock material with older rocks (Paleozoic and lower
Mesozoic) cropping out widely in the western Maghreb whereas
Cenozoic rocks dominate in the eastern part of the system.
South of the South Atlas Front is the Sahara foreland with only few
Meso-Cenozoic deformation. This domain was affected by the Variscan
orogeny in its western part as shown by Late Paleozoic folding in the
Anti-Atlas and Ougarta ranges (Fig. 1B). The Variscan Front cut
obliquely the Atlas system (Fig. 1B) introducing an initial asymmetry
in the Maghreb with a thickened lithosphere only in its western part.
The geometry of the Atlas system is directly inherited from the Early
Mesozoic rifting of both Central Atlantic and Alpine Tethys riftings
(Favre et al.,1991). By the Upper Cretaceous, the convergence between
the Africa and Eurasia plates (see review in Rosenbaum et al., 2002)
resulted in its progressive inversion, which reached a climax by the
Middle Eocene.
It is generally acknowledged that the Maghrebian orogenic domain
results from the Cenozoic Eurasia–Africa convergence and that the
present-day relief is a direct consequence of the resulting collision.
Interestingly, the highest peaks as well as highest mean altitude are
situated in the intra-continental Atlas and not in the Tell-Rif, and a
strong E–W asymmetry of the topography can be observed:
- the Tell-Rif system exhibits a mean altitude of only 500 m with
highest altitude of c.a. 2500 m in the Central Rif (Morocco) and
Kabylias (Algeria). In Western Algeria, the Cheliff Miocene basin
represents an area of low elevation superimposed onto the Tell
system;
- the Atlas system presents a strong longitudinal asymmetry with a
mean elevation of 1500 m (top: 4167 m) in the High Atlas
(Morocco) against 1050 m (top: 2120 m) in the Saharan Atlas and
only 600 m in the Aurès and Tunisian Atlas (top: 2225 m in the
Aurès and 1542 m in the Tunisian Atlas). Between the Saharan
Atlas and the Aurès, the Hodna Miocene basin crosses the Atlas
system and represents a puzzling cross-element sealing the
earliest Atlas tectonic events.
South and east of the South Atlas Front, the Sahara foreland
domain presents similar longitudinal asymmetry with decreasing
altitude from west to east (Figs. 1A, 2). In Morocco, the Anti-Atlas
domain is an uplifted area (mean altitude: 800 m; top: 2500 m)
separated from the High Atlas by narrow foreland basins, namely the
Souss and Ouarzazate basins. Between the two basins, the Pan-African
basement of the Marrakech High Atlas is in direct contact with the
basement of the Anti-Atlas (Siroua massif). In Eastern Morocco and
Western Algeria, the Sahara domain exhibits a regular southward
slope of 0.2°. This slope suffers an active incision by transverse rivers,
suggesting a present uplift of the foreland domain. By contrast, in
Eastern Algeria and Southern Tunisia, the Atlas foreland corresponds
to the “Chotts”(=sebkhas) domain, which is an active subsiding area.
To the east, the South Atlas Front suffers a 90° swing and becomes N–S.
The Tunisian Atlas foreland is partly exposed in the Sahel coastal plain
and in the interior part of the Gulf of Gabès, which displays very low
elevation, close to the sea level or even below, and is partly occupied
by salt lakes recording high rate Quaternary subsidence.
3. Uplifted areas of Late Permian–Early Cretaceous age
After the Variscan orogeny, the beginning of the Mesozoic Era is
dominated by the rifting, which led to the break up of Pangea and
resulted in the formation of both Atlantic and Alpine-Tethys margins
of the Maghreb. At that time the asymmetry of the Maghreb was
marked with development of uplifted areas in its western part con-
trasting with continuous subsidence in its eastern part.
3.1. The Triassic–Liassic West Moroccan Arch (WMA) revisited
In Morocco, the break up of Pangea is expressed by successive
extensional episodes (Laville et al., 2004). The first episode is Late
Permian–Late Triassic, but particularly active during the Middle–Late
Triassic being related to the Central Atlantic rifting. Thanks to seismic
profiles (Hafid et al., 2006; Hafid, 2006) or to extensive field studies
2
The location of the cited geological structures is indicated on Fig. 1AorB.
10 D.F. de Lamotte et al. / Tectonophysics 475 (2009) 9–28

Fig. 1. Topography and main structural domains of NW Africa. A: Topographic map (GTOPO30 data); the red doted lines indicate the position of the topographic profiles (Fig. 2).
The numbers refer to the geological structures cited in the text: Anti-Atlas: 1 —Aurès Massif: 2 —Bahira Basin: 3 —Cheliff Basin: 4 —Chotts area: 5 —Doukkala Gulf: 6 —Essaouira
Basin: 7 —Gabès Gulf: 8 —Guercif Basin: 9 —Hammamet Gulf: 10 —High Atlas: 11 —Hodna Basin: 12 —Jebilet Massif; 13 —Jeffara High: 14 —Middle Atlas: 15 —Missour Basin:
16 —Ouarzazate Basin: 17 —Ougarta Range: 18 —Saghro Massif: 19 —Saharan Atlas: 20 —Sahel plain: 21 —Sidi Toui High: 22 —Siroua Massif: 23 —Skoura Basin: 24 —Souss
Basin: 25 —Tadla Basin: 26 —Talemzane High: 27 —Tunisian Atlas: 28. B: Main structural domains (after Michard et al., 2008). The geological “Maghreb”essentially corresponds
to the Rif-Tell and Atlas orogenic domains. Also of interest in this paper are the close southern and eastern foreland areas of the Atlas system (from theAnti-AtlastotheTunisian
Sahel area).
11D.F. de Lamotte et al. / Tectonophysics 475 (2009) 9–28

(Medina, 1995; El Arabi, 2007), this event is well studied along the
Atlantic coast and in the Marrakech High Atlas, west and east of the
West Moroccan Arch (WMA), respectively (Fig. 3A). The WMA,
previously called “Terre des Almohades”by Choubert and Faure
Muret (1960–62) and “Dorsale du Massif Hercynien Central”by
Du Dresnay (1971) and Michard (1976), is classically interpreted as
an emergent land corresponding to the shoulder of the proto-Atlantic
basin. However, recent apatite fission track studies (Ghorbal et al.,
2008; Saddiqi et al., 2009-this issue) suggest that it was rather a
subsident (Ghorbal et al., 2008) or poorly subsident (Saddiqi et al.,
2009-this issue) domain during the Late Triassic–Early Jurassic, whose
cover was subsequently eroded (Fig. 3B). West of the WMA, the normal
faults are mainly NNE-trending and developed before the outpour of
the basaltic flows pertaining to the Central Atlantic Magmatic Province
(CAMP), which covered virtually the whole area, 200 Ma ago (Verati
et al., 2007). Dikes and lava flows of the CAMP extended east and north
of the WMA in a very large area covering the western half of the
Maghreb (Fig. 4). In the eastern domain, Triassic rocks are buried
beneath a thick Meso-Cenozoic cover (more than 10 km thick in
some places). However, seismic data show that the Triassic rifting is
expressed as far as in the Ksour basin in the middle of the Saharan Atlas
(Yelles-Chaouche et al., 2001) where CAMP lava flows have also been
recognised (Meddah et al., 2007). On the other hand, dolerites asso-
ciated with Upper Triassic evaporites are well known in the Rif-Tell
domain of the whole Maghreb indicating that rifting was also active
along the northern margin of Africa (Wildi, 1983).
A second rifting episode is related to the development of
the Maghrebian part of the Alpine Tethys (Fig. 4), resulting in an
outstanding example of a rift filled dominantly by carbonate rocks
(Du Dresnay, 1987; Warme, 1988). It was initiated after the
Hettangian, which corresponds to the development of a widespread
shallow platform, and propagated westward along the Maghreb
during the Lower to Middle Liassic and lasting up to the Bajocian in the
Middle Atlas (Charrière, 1990; Zizi, 2002). From a geometric point of
view, the Tethyan rifting developed mainly ENE to NE normal faults
(Stampfli and Borel, 2002)(Fig. 4).
3.2. Late Middle Jurassic–Early Cretaceous: widening of the West
Moroccan Arch versus Tunisian rifting
By the Bathonian, sedimentation became siliciclastic in Morocco
with the deposition of fluvial red beds supplied by the neighbouring
lands of the WMA, which emerged synchronously, and by the Sahara
domain (Jenny et al., 1981; Haddoumi et al., 2008)(Fig. 5). At that
time, a fluvial system following the High and Middle Atlas was a
transit zone for a sedimentary flux, which fed north- and northeast-
orientated wide deltas and deep sea fan sedimentation along the foot
of the Tethyan margin (in the present Rif domain) and in the Saharan
Atlas (Wildi, 1983). Farther to the east, in the Central an Eastern Tell as
well as in the Aurès and Tunisian Atlas, the sedimentation remained
dominated by carbonate deposition (Vila, 1980) in a context of conti-
nuous subsidence (Bracène et al., 2003). Hence, we observe an uplift
of the western half of the Maghreb contrasting with the continuous
subsidence of the eastern half. Another specific aspect of the western
domain and of the Central High Atlas in particular is the occurrence of
transitional/alkaline intraplate magmatism responsible for a number
of plutonic and volcanic bodies emplaced during the Bathonian and
later during the Barremian (see a recent review in Haddoumi et al.,
2008). The classical interpretation by Laville and Piqué (1992) is that
these magmas emplaced during a major phase of folding and subse-
quent erosion leading to the exposure of the plutonic rocks. For
these authors, the plutonic rocks and associated folds should be
Fig. 2. Topographic profiles crossing the Maghreb (see location on Fig. 1A). A: Longitudinal profiles in the Atlas system (black profile) and foreland (red profile). B: Transversal profiles
through Morocco, Algeria and Tunisia (from top to bottom).
12 D.F. de Lamotte et al. / Tectonophysics 475 (2009) 9–28

unconformably covered by Jurassic–Cretaceous red beds. However,
recent thermochronologic data suggest that the plutonic rocks were
still situated at depth (TN60°) 90–80 Ma ago (Barbero et al., 2007) and
not close to the surface as proposed by Laville and Piqué (1992).
Finally, the red beds overlying some of the gabbroic ridges have been
ultimately dated from the Late Paleocene (Charrière et al., in press). So
according to Charrière (1990) and Zizi (2002), it seems more rea-
sonable to link the Jurassic magmatism of the High Atlas to the
continuation of an extensional tectonic activity (alternation of rift-sag
sequences in an overall extensional context; Warme, 1988). This
Fig. 3. The Triassic to Middle Jurassic West Moroccan Arch and surrounding areas. A: Paleogeography during the Late Triassic (modified from El Arabi, 2007). B: Paleogeography
during the Early and Middle Jurassic (modified from Jabour et al., 2004,inFrizon de Lamotte et al., 2008).
13D.F. de Lamotte et al. / Tectonophysics 475 (2009) 9–28