Mesozoic and Cenozoic vertical movements in the Atlas system (Algeria, Morocco, Tunisia): An overview

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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

aUniv Cergy-Pontoise, Département des Sciences de la Terre, F-95 000 Cergy, France

bLaboratoire Géoressources, INRST, Bordj Cédria, 57 rue 7301, par av. Tahar-Ben-Ammar-Menzach 9B, 1013 Tunis, Tunisia

cUniv Neuchâtel, Geological Institute Emile Argand 11/CP 158 CH-2009 Neuchâtel, Switzerland

dLaboratoire de Géodynamique et Thermochronologie, Faculté des Sciences Aïn Chock, BP 5366, Casablanca Maarif, Morocco

eUniv Rennes 1 Geosciences-Rennes (UMR 6118); Campus de Beaulieu, 35042 Rennes Cedex, France

f10, rue des Jeûneurs, 75002 Paris, France

a b s t r a c t a r t i c l e i n f o

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 andﬁnally, 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.

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 orﬂexuration, (3) large scale upwelling or downwelling mantleﬂows. Their origin still remains a matter of

⁎Corresponding author.

E-mail address:[email protected](D.F. de Lamotte).

1Now at: Univ Paris-Sud, Département des Sciences de la Terre, Bat. 504, 91 405 Orsay Cedex, France.

doi:10.1016/j.tecto.2008.10.024

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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 inCloetingh 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 inFrizon 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 inFrizon 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 domain²

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 inRosenbaum 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 contrasting 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). Thefirst 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 extensivefield studies

2 The location of the cited geological structures is indicated onFig. 1A or B.

10 D.F. de Lamotte et al. / Tectonophysics 475 (2009) 9–28

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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 (afterMichard 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 the Anti-Atlas to the Tunisian Sahel area).

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(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)andMichard (1976), is classically interpreted as an emergent land corresponding to the shoulder of the proto-Atlantic basin. However, recent apatitefission 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 basalticflows pertaining to the Central Atlantic Magmatic Province (CAMP), which covered virtually the whole area, 200 Ma ago (Verati et al., 2007). Dikes and lavaflows 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 lavaflows have also been recognised (Meddah et al., 2007). On the other hand, dolerites associated 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 riftﬁlled 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 offluvial 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, afluvial system following the High and Middle Atlas was a transit zone for a sedimentaryflux, 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 continuous 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 inHaddoumi et al., 2008). The classical interpretation byLaville and Piqué (1992)is that these magmas emplaced during a major phase of folding and subsequent 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 onFig. 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).

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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 toCharrière (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 fromEl Arabi, 2007). B: Paleogeography during the Early and Middle Jurassic (modiﬁed fromJabour et al., 2004, inFrizon de Lamotte et al., 2008).

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interpretation appears also consistent with the petrologic and geochemical features of these magmatic rocks (Hailwood and Mitchell, 1971; Harmand and Laville, 1983; Beraâouz et al., 1994;

Amrhar et al., 1997; Lachkar et al., 2000; Lhachmi et al., 2001; Zayane et al., 2002; Charrière et al., 2005; Haddoumi et al., 2008).

Trying to link the WMA widening and uplift, and the High Atlas gabbroic magmatism, we put on the same map (Fig. 6) the Toarcian and Bajocian shorelines (Elmi, 1999) and the boundary of the Late Jurassic magmatic province. From this map, it can be suggested that both the eastward displacement and widening of the WMA and the basalt and gabbro emplacement are related to a Middle–Late Jurassic thermal doming. The reason why the magmatic activity lasted in this region is unknown. A particular conﬁguration of the lithosphere favouring the collection of magmaticﬂuids at depth could be suggested.

From the Valanginian to Aptian, the enlarged West Moroccan Arch was progressively divided into two distinct emergent lands, due to the formation of two narrow, elongated gulfs (Fig. 7A) (Charrière, in Frizon de Lamotte et al., 2008). The northern gulf followed the Middle

Atlas trend and was likely connected with the Peri-Tethyan seas. The southern gulf developed northeastward starting from the Atlantic margin (Essaouira basin) up to the Central Atlas border. These con- verging gulfs did not connect one to each other, being separated by continental deposits in the south Middle Atlas area. At the same time, the West Moroccan Arch was also disrupted by the Doukkala Gulf extending from the Atlantic toward the Meseta axis (Fig. 7A).

During the Late Jurassic and Early Cretaceous, Eastern Algeria and Tunisia received mostly pelagic sediments reaching a thickness of 2500 m for the Lower Cretaceous alone in the Tunisian Atlas south- west of Tunis (Jauzein, 1967; Turki, 1985; Ben Ferjani et al., 1990;

Herkat and Delfaud, 2000). This evolution does not result only from a post-rift thermal subsidence: north of the Jeffara fault systems, widespread thickness and facies variation indicate that the inherited E–W fault system continued to be active deﬁning paleo-highs charac- terized by stratigraphic omissions or condensed sequences, and elongated basins receiving several kilometres of sediments (Soussi and Ben Ismail, 2000; Bouaziz et al., 2002) (Fig. 7B). This extensional Fig. 4.Map of the main Triassic-Lower Jurassic faults at the scale of the Maghrebian Atlas system (modiﬁed afterFrizon de Lamotte et al., 2000). CAMP = Central Atlantic Magmatic Province (200 Ma).

Fig. 5.Maghreb paleogeography during the Late Jurassic (modiﬁed after Feddan inZizi, 2002). Note the widening of the WMA.

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system extends eastward in the gulfs of Gabès and Hammamet toward the East Mediterranean Basin and westward up to the Saharan Atlas (Dercourt et al., 1993, 2000).

In Tunisian basins, the reported Tethyan rifting of the Jurassic– Lower Cretaceous was controlled by major faults, presumably linked to basement discontinuities (Khomsi et al., 2004; Abbes, 2004). They have been interpreted as Jurassic faults, with a strike-slip component in a transform margin (Bédir, 1995), allowing the development of an instable carbonate platform (see reviews inTurki, 1985; Soussi, 2000) with tilted panels, hemi-grabens and horst zones as the Sidi Toui High, which represents a poorly subsident area since the Triassic (Busson, 1971; Ben Ferjani et al., 1990). Many of the Jurassic fault corridors are inherited from Triassic times as indicated by interstratiﬁed basaltic lavas to Triassic deposits. This syn-sedimentary extensional tectonics lagged until the Cenomanian. Thus the general conﬁguration of eastern Maghreb during the Early Cretaceous was typically a faulted, north deepening and subsiding platform (Fig. 7B) with large amounts of deep shales and turbiditic series north of the Kasserine–Aurès area (the Kasserine area is situated in the middle of the Tunisian Atlas). At the same time and south of Gafsa and Chotts areas, sedimentation was dominated by inner platforms and coastal plain deposits with large extent of evaporites, dolomites and proximal carbonates (Busson, 1971; Ben Ferjani et al., 1990).

4. Late Cretaceous–Paleocene: theﬁrst effects of plate convergence and Sirt rifting

By the early Late Cretaceous, the Africa–Eurasia relative movement changed drastically as a consequence of the opening of the South- Atlantic Ocean (see a review in Rosenbaum et al., 2002). The movement of Africa relative toﬁxed Europe, which was an eastward left-lateral displacement since 175 Ma, changed progressively into a N–S convergence between ca. 92 and 46 Ma, leading to the development of a set of structures at different scales. The asymmetry of the Maghreb remains marked due to the effects of the Sirt rifting in the eastern regions.

The Cenomanian–Turonian is a turning point of the geological history of the Maghreb for another reason. At that time the whole Maghreb has been covered by an importanttransgression leading to a very large marine incursion covering the northern platforms of the Sahara Domain with a shallow water sea marked by the deposition of a homogeneous carbonate platform, Late Cenomanian–Turonian in age, which forms a universal bench-mark for the subsequent vertical

movements. At the same time a subsident and deep marine basin developed along the northern side of the Aurès and in the Tunisian Atlas allowing the deposition of deep marine black shale, which is one of the most important hydrocarbon source rock of the Atlas system.

Finally, by the early Late Cretaceous, we note also the cessation of the magmatic activity in the High Atlas. It will take up again perhaps as soon as the Eocene but more massively by the Middle Miocene (see below).

4.1. Post-Turonian lithospheric folding in the western Maghreb In Morocco large areas, such as the Anti-Atlas or the northern part of the Middle Atlas are devoid of Cretaceous to Middle Eocene deposits.

This well-known observation leads us to revisit a paleogeographical concept developed as early as 1948, when Georges Choubert proposed a paleogeographical map of Morocco for the Cenomanian–Turonian period. On this map, he drew a 200 km-wide E–W seaway at the place of the present High Atlas, Southern Middle Atlas and Western Meseta between two emerged lands: the Anti-Atlas to the south and the“Terre émergée du Maroc Septentrional” (Emergent Land of Northern Morocco) to the north (Choubert, 1948). The latter re-named“Terre des Idrissides”byChoubert and Faure-Muret (1960–62, 1971) who claimed that it existed from the Late Jurassic to the Paleogene. This paleogeographic unit remains present in the subsequent paleogeographic maps (e.g.Vila, 1980; Dercourt et al., 2000; Guiraud et al., 2005). However, this interpretation, based on the lack of Cretaceous outcrops in the alleged land is disputable. In fact, sedimentological data from the Upper Cenomanian–Turonian series cropping out in the Western Meseta, Middle and High Atlas and South Rif domains do not indicate any coastal facies suggesting the proximity of a shoreline northward or southward (Charrière et al., in press; Frizon de Lamotte et al., 2008). Thus, it is likely that the entire north Moroccan territory wasﬂooded during the late Cenomanian–early Turonian high stand.

In this context, the lack of Cretaceous outcrops in the“Terre des Idrissides”can be interpreted as the result of post-Turonian erosion, which is, indeed, well documented in the Guercif Basin where the Miocene molasses rest directly onto the Jurassic strata (Zizi, 2002).

Similarly, the Anti-Atlas domain was covered, at least partly, by shallow water, Cenomanian–Turonian carbonates subsequently eroded during the Senonian–Cenozoic times (Zouhri et al., 2008).

This regional erosion phase is documented, for example, in the Tindouf Basin where the Cretaceous series are truncated by the Mio- Pliocene molasses of the Draa Hamada (Fig. 8). If correct, the“Terre des Idrissides”, the Anti-Atlas as well as the northern part of the Fig. 6.Map showing the displacement of the shoreline from Toarcian to Bajocian (data fromElmi, 1999) and the location of the Moroccan Jurassic–Lower Cretaceous Magmatic Province. A link between the widening of the WMA and the emplacement of the magmatic bodies is suggested.

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Fig. 7.Paleogeography during the Barremian and Aptian. A: Paleogeography during the Aptian in the western Maghreb (from Charrière inFrizon de Lamotte et al., 2008). B: Restored cross-section through Tunisia at the end of Aptian times [modiﬁed fromBusson (1971)andSoussi (2000)].

16D.F.deLamotteetal./Tectonophysics475(2009)9–28

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Reguibate Shield look like wide anticlines with intervening synclines in the Western Meseta, High and Middle Atlas on the one hand and Tindouf Basin on the other hand (Fig. 9). Given the wavelength (about 500 km) of the depicted folds, only a lithosphere folding (“plis de fond” of ancient authors, e.gArgand, 1924) can be advocated conﬁrming that

“this process is a primary response to recently induced compressional stressﬁelds”(Cloetingh et al., 1999), i.e. the beginning of N–S convergence. Accordingly, we propose to abandon the concept of Late Jurassic–Paleogene “Terre des Idrissides” and replace it by that of North Moroccan Lithosphere Anticline, post-Turonian in age.

Such a lithospheric folding has been already proposed byTeixell et al.

(2003)but for a very long period since the Late Paleozoic until recent times. We do not believe that such mechanism can be active during so long a time because of the occurrence of major changes in the geodynamic context during this period (including the Variscan orogeny). From a geodynamic point of view, the direction of these large structures is consistent with what can be expected for a consequence of Africa– Eurasia convergence. They could be related to the initiation of subduction in the western Mediterranean (see a review inJolivet and Faccenna, 2000). The age of lithospheric folding is difﬁcult to appraise. Geological

Fig. 9.Sketch map of the Cretaceous–Eocene plateaus of western Maghreb (afterZouhri et al., 2008) with indication of the inferred lithosphere folds, i.e. from north to south: (1) between the Rif and Atlas Domains;(2) on the site of the Anti-Atlas and (3) on the site of the Reguibat Shield. Red dashed lines: contours of the lithosphere anticlines; green dashed lines: contours of the lithosphere synclines.

Fig. 8.Cross-section through the“Hamada du Draa”and Tindouf Basin [from Gevin (unpublished, 1974) inFabre, 2005] showing the Upper Eocene(?)-Miocene unconformity onto the Cenomanian–Turonian series. Location: onFig. 9.

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data allow a large time bracket (post-Turonian and ante-Oligocene).

Taking into account the path of the Europe–Africa convergence (Rosenbaum et al., 2002), we can observe that it became really N–S (at almost right angle to the postulated folds) by the Paleocene.

In the Moroccan Atlas and Meseta domains, a moderate subsidence continued during the Senonian and Paleocene with an alternation of continental (mainly evaporitic coastal plain) and marine (mainly carbonate platform) sedimentation depending on the global sea level changes. Locally, tectonic shortening during the Senonian led to minor folding due to syn-sedimentary inversion of faults inherited from the rifting, with development of breccia along the faults and an unconformity at the bottom of the overlying Eocene strata (Froitzheim, 1984; Herbig, 1988), or to syn-sedimentary synclines ﬁlled by evaporites as in the Tadla Basin. Paleocene red beds overlie unconformably some of the anticlinal ridges of the Central High Atlas (Charrière et al., in press), and a regional disconformity is observed at the bottom of the Early–Middle Eocene in the Middle Atlas synclines (Herbig and Trappe, 1994). However, the Late Cretaceous–Paleocene deformation did not result in signiﬁcant relief building, and the High Atlas domain was partly submerged by shallow water seas until the late Middle Eocene (Tabuce et al., 2005).

4.2. Interference between convergence effects and Sirt rifting in eastern Maghreb

In Eastern Maghreb, the geodynamic context is also complex because it combines two different effects, namely the Sirt rifting and the Eurasia–Africa convergence. The Sirt rifting (Fig. 10) was active during at least the Late Cretaceous–Paleocene (Rusk, 2001; Abadi et al., 2008), and it developed NW–SE faults in a wide area from the Libyan Desert to the Hodna Basin in Algeria (Bouaziz et al., 2002). It is worth

noting that this NW–SE rift system is oblique to the Tethyan rifting, which developed mainly E–W normal faults (Figs. 4 and 7B).

Convergence is expressed by NE–SW trending folds, which are well depicted on seismic proﬁles from the Sahel area and Hammamet Gulf in Eastern Tunisia (Bédir et al., 1992; Patriat et al., 2003). On these lines (Fig. 11), growth strata show that the folding-related uplift remained lower than subsidence during the Late Cretaceous and Paleogene.

In Eastern Algeria and Tunisia, the existence of lithospheric folds cannot be displayed due to the persistence of subsidence in this domain.

However, the existence of basement highs (namely Talemzane, Sidi Toui and Jeffara Highs) are well known by the petroleum geologists just south of the eastern South Atlas Front (Ben Ferjani et al., 1990).

5. The Middle Eocene to Present inversion and uplift episodes in the whole Atlas system

By the Middle Eocene began the orogenic relief building, which contributes to the present topography (Fig. 1A). From this age until now, the deformation is discontinuous, in spite of the continuous plate convergence, and occurred in two distincts steps separated by a period of subsidence and relative tectonic quiescence. It is worth noting that the episodes of shortening appear quite synchronous at the scale of the whole Maghreb. Contrasting with this common evolution, a thermal component is superimposed in the western Maghreb, explaining the present asymmetry of the relief (Fig. 1A).

5.1. Theﬁrst Atlastectonic event (Middle and Late Eocene)

The timing of the Cenozoic inversion events in the Atlas system remains a matter of study and debate. We have shown that until the Middle Eocene, the Atlas system was rather in a depression relative to

Fig. 10.Main structures linked to the Late Cretaceous Sirt rifting from Lybia to Eastern Tunisia. Data from the International Structural Map of Africa (in prep.) and fromCasero and Roure (1994),Rusk (2001),Chamot-Rooke et al. (2005),Abadi et al. (2008).

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the adjacent areas. It seems that thefirst general inversion of the Triassic–Jurassic normal faults and associated basins occurred during the Middle–Upper Eocene. Evidence for an Eocene tectonic event has been put forwardfirstly byLaffitte (1939)in the Aurès Mountains (Algeria). For this author it was a major event, and he called it the

“Atlas event”. The unconformity described by Lafﬁtte is unquestion- able and has been recognized by subsequent studies in the Aurès Mountains (Ghandriche, 1911; Frizon de Lamotte et al., 1998) as well as in the Hodna Basin (Guiraud, 1975; Merikeche et al., 1998; Bracène and Frizon de Lamotte, 2002).

In the Saharan Atlas (Western Algeria) as well as in the Moroccan Atlas, such an event is difﬁcult to identify because of the scarcity of well-dated Oligocene–Lower Miocene sediments. In fact, in Morocco, the so-called“Mio-Pliocene”continental molasses rest unconformably everywhere on already folded strata (seeFraissinet et al., 1988;

El Harfiet al., 2001; Missenard et al., 2007). More precisely, in the Ouarzazate basin the earliest record of the Atlas uplift corresponds to the onset of continental sedimentation, sourced in the uprising belt, during the Late Eocene (Hadida Fm) (Teson et al., in press) The onlap of Oligocene? Lower Miocene deposits (Ait Ouglif Fm) above the folded and eroded Mesozoic–Eocene beds allow thefirst significant folding event of the Sub-Atlas Zone to be dated as Late Eocene–Early Miocene. These fundamental new data validate the speculations by Frizon de Lamotte et al. (2000)emphasising the importance of a late Eocene event in the Atlas system of Morocco.

In Tunisia, a Middle–Late Eocene event is recognized offshore in the foreland of the Atlas system (Patriat et al., 2003; El Euchi et al., 2004) and has been extended to the whole Tunisian Atlas byKhomsi et al.

(2006a,b). Thus, in line withFrizon de Lamotte et al. (2000, 2006)and Khomsi et al. (2006a,b), we assume that a Middle–Late Eocene compressive event (Atlas event) is general at the scale of the whole Maghreb.

The crustal thickening and relief building related to this event are difﬁcult to appraise. However, they are high enough to furnish coarse conglomerates and to trigger the sinking of foreland basins in front of the Aurès and Tunisian Atlas as well as on both sides of the High Atlas.

5.2. Oligo-Miocene general subsidence

After thisﬁrst generalized tectonic event and related inversion, the whole Atlas system suffered a general subsidence phase recorded by

the deposition of thick molasses. In Tunisia and Eastern Algeria, these Neogene molasses are partly marine and well-dated (Van Houten, 1981; Courme-Rault, 1985; Yaich et al., 2000). They are preserved not only in the eastern foreland (Sahel and gulfs of Hammamet and Gabès) but also in the Chotts area as well as in different basins superimposed to the chain (seeBracène and Frizon de Lamotte, 2002;

Khomsi et al., 2006a,b, 2009; Frizon de Lamotte et al., 2006, and references therein). Among the basins, the Hodna (Fig. 1A) is very puzzling because it crosses the whole Atlas chain and, for an unknown reason, did not suffer important uplift since its formation during the Lower Miocene. In Western Algeria and Morocco, the Neogene molasses, which cover unconformably the Atlas system, are continental and their age remains poorly constrained. In the Saharan Atlas, these clastic deposits are preserved in wide synclines, covering about one half of the chain (Fig. 12). In Morocco, these molasses are preserved mainly in the residual foreland basins fringing the chain (namely the Souss, Ouarzazate, Tadla-Bahira and Missour Basins,Fig. 1A) but also in the core of the mountain belts as isolated outcrops (“Rocher de la Cathédrale”in the Central High Atlas;Morel et al., 1999; Teixell et al., 2003) or as uplifted basins (Skoura and Guercif Basins in the Middle Atlas; Haouz basin between the Jebilet and the High Atlas). In the Toundout Nappe, north of the Ouarzazate Basin, the Miocene deposits situated originally in the chain are up to 1200 m thick. (Görler et al., 1988). Equivalent deposits, but often thinner are known on the Mesetas and on the northern Sahara where they top the“Hamadas” (plateaus). Oligocene (?)-Miocene deposits are also known in the Anti-Atlas domain, mostly in the depression formed by some of the Precambrian inliers (Zenaga and Agadir Melloul) or preserved under the Miocene phonolites of the Saghro Massif (Joly, 1962; Berrahma, 1995).Fig. 12presents the domains where Oligocene–Neogene deposits are cropping out. From this map, it is very likely that the Atlas has been completely buried beneath a mass of dominantly clastic deposits during this period (4000 m in the Hodna basin;Kheidri et al., 2007). In the front of the Tunisian Atlas, the Oligo-Miocene series attain a cumulative thickness of more than 2200 m. The source of this clastic sedimentation may be found in the adjacent Sahara domain (Fabre, 2005). The Neogene sinking of the Atlas is locally accompanied by extensional tectonics (Bracène and Frizon de Lamotte, 2002; Khomsi et al., 2006a,b) and, at larger scale, it is likely related to the Mediter- ranean dynamics (see below).

Fig. 11.N–S interpreted seismic line through the Tunisian Atlas foreland. (Sahel coastal plain) showing the Paleogene syn-sedimentary folding (fromKhomsi et al. 2009).

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5.3. The second Atlas tectonic event

After the deposition of the Neogene molasses, a second generalized compressional tectonic event is responsible for a second crustal shortening and relief building in the Atlas system. Examples of this renewed compressive activity can be found everywhere in the Atlas system: along the front of the Tunisian Atlas (Fig.13A), in the Aurès Mountains (Fig.13B), in the Saharan Atlas as well as in the Moroccan Atlas (Fig. 13D). In detail, we observe a widening of the inverted zone, which, in Morocco, includes from now onwards the Siroua plateau and the Jebilet range bounded by the Anti Atlas Major Fault and by the North-Jebilet Fault, respectively (Fig. 14) (Haﬁd et al., 2006; Missenard et al., 2007). In eastern Algeria and southern Tunisia, a southward propagation of the South Atlas Front (SAF) is also observed (Frizon de Lamotte et al., 1998, 2000). By contrast, in Eastern Tunisia, the N–S segment of the SAF is located behind the Sahel zone where Eocene deformation is evident (Fig. 11). Everywhere, the relief building related to this second event is still active during the Quaternary up to now (Caire, 1971; Coiffait, 1974; Turki, 1985; Chihi, 1995;

Anderson, 1996; Sébrier et al., 2006) and superimposed to the Middle– Late Eocene structures leading to seismic activity along many inherited and weak zones of the Sahel and gulf of Hammamet (Chihi, 1995).

5.4. The thermal component of the relief in the Moroccan Atlas The Cenozoic tectonic evolution is more or less similar (same age, same intensity) for the entire Atlas system, and then cannot be responsible for the asymmetry of the relief (Fig. 1A).

In fact, a thermal component is superimposed in the western Maghreb explaining its higher elevation. The age of the initiation of the relief related to this thermal anomaly is not very well constrained.

Missenard (2006)and Missenard et al. (2006)argued for a Middle Miocene age, based on the age of the beginning of the associated volcanism.Babault et al. (2008)suggested a more recent age (post Miocene), based on a morphological analysis.

5.4.1. Why a thermal component is necessary to explain the relief of Morocco?

The relief of the Moroccan Atlas cannot be accounted for by crustal shortening alone. This question long remained a matter of debate for the following reasons: (1) seismic surveys show the lack of deep roots under the belts (Moho at 33 km under the Anti-Atlas, and at a maxi- mum depth of 39 km under the highest part of the High Atlas) (Makris et al., 1985; Tadili et al., 1986; Wigger et al., 1992; Ramdani, 1998) (2) gravity surveys suggest a low density body at 50 km depth (Seber et al., 1996). In addition, based on teleseismic P-wave travel time tomography, these authors show that an upper mantle low velocity anomaly exists beneath the High Atlas in relation with higher tem- peratures. They proposed that this anomaly contributes to the relief.

Such an uplift of the lithosphere/asthenosphere boundary has been subsequently modelled byFrizon de Lamotte et al. (2004),Teixell et al.

(2005),Zeyen et al. (2005),Fullea Urchulutegui et al. (2006), and Missenard et al. (2006).

According toMissenard et al. (2006), the domain with thinned lithosphere/uplifted asthenosphere forms an oblique NE–SW strip, Fig. 12.Map of the present outcrops of Oligo-Neogene deposits in the Maghreb. This map suggests that the whole Maghreb was at that time completely covered by detritic sediments.

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which we propose to call the“Morocco Hot Line”(MHL). The MHL crosses the Anti Atlas, cuts the South Atlas Front in the Siroua region, crosses the central High Atlas, follows the Middle Atlas andﬁnally cuts the Rif front in the Eastern Rif (Fig. 14). It is worth noting that the MHL transects not only the Liassic rift system but also the area of the High Atlas where the crust is thickest (Fullea Urchulutegui et al., 2006), suggesting a sort of crust mantle decoupling during lithosphere thinning. The MHL is underlined by a diffuse seismicity and by an

intraplate-type alkaline volcanism (Hoernle et al., 1995), which spans the Miocene to late Pleistocene with lack of well deﬁned age versus position trend. Assuming that the volcanic activity is directly linked to the lithosphere thinning, its emplacement probably occurred during the Middle–Upper Miocene. However, the occurrence of some older strongly alkaline volcanic rocks might suggest an earlier beginning during the Paleocene–Eocene (R. Maury and M. El Azzouzi inFrizon de Lamotte et al., 2008).

Fig. 13.Field examples showing the existence of important deformation before (ﬁrst Atlas event) and after (second Atlas event) the deposition of the Oligo-Miocene molasses.

The drawings underline the unconformity of the molasses and the subsequent deformation A—Tunisia, Tell Mountains [fromKhomsi et al. (2009)]; B—Algeria, Aurès Mountains (after Frizon de Lamotte et al., 1998); the Aurès is the location whereLafﬁtte (1939)deﬁned the“Atlas event”; C—Morocco, northern front of the High Atlas (fromMissenard et al., 2007).

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5.4.2. Estimating the effects on the relief of lithosphere thinning along the MHL

Using four modelled lithospheric cross-sections,Missenard et al.

(2006)made afirst attempt to quantify the effect of this lithosphere thinning on the topography of Morocco. Each cros-section is modelled on the basis of gravity (Bouguer and Free Air anomaly), geoid, topography and heatflow data. In order to evaluate the effect of this thermal anomaly, we removed the lithosphere thinning by defining a lithosphere/astenosphere boundary gently dipping from 125 km at the Atlantic margin to 145 km below the Sahara domain, thus main- taining the difference between oceanic and continental lithosphere of our model. All other parameters of the model remain the same. This assumption minimizes the effect of the lithosphere thinning, as we could consider a small thickening under the thrust belt. The difference between the modeled topography and the present one therefore corresponds to the minimum “thermal” topography. We have extra- polated the thermal topography along the four lithospheric profiles crossing the Atlas belt using a minimum curvature interpolator to obtain a 3D map. It is therefore possible to present a map of what should be the relief of Moroccan Atlas system without the influence of the lithosphere thinning (Fig. 15).

From this map (Fig. 15) it is now possible to discuss the consequences of the thermal topography removal. In the Atlas domain, the Central High Atlas and the Middle Atlas loose up to 1000 m of elevation. The areas above 2500 m in the inner part of the present belt almost disappear when removing the thermal part of the relief, and their altitudes become similar to the one of the Saharan Atlas

(Algeria) (Fig. 1A), whose lithosphere is not thinned. This drastic reduction of the topography in the belt is in good agreement with the low shortening value (less than 5% in the Middle Atlas, about 15% in the High Atlas;Gomez et al., 1998; Brede, 1992, respectively). The Anti-Atlas belt goes down to approximately the same level than the adjacent Saharan area. In other words, the topography of this belt is exclusively lithosphere-related, and crustal shortening does not play a signiﬁcant role in this area (see alsoBabault et al., 2008). Several regions dive below the sea level, such as the Souss and Bahira Basins.

The South Rif Corridor and the Guercif basin, which formed the marine channel connecting the Western Mediterranean with the Atlantic before the Messinian salinity crisis, also dives below sea level.

This result is important because it shows that, in this area, the thermal uplift post dated the Messinian, which is well-dated in the Guercif basin (Krijgsman et al., 1999). Moreover, the geomorphological analysis performed byBabault et al. (2008)leads to the same conclusion at the scale of the whole Middle Atlas. So, incidentally and according to these authors, we infer a possible causal relationship between the Mediterranean salinity crisis and the MHL.

5.5. Discussion (Fig. 16)

After large-scale lithospheric folding during the Paleocene (Fig. 16A), theﬁrst general inversion of the Atlas system occurred during the Middle–Late Eocene (Fig. 16B). It resulted in the localization of the deformation along the faults inherited from the Triassic–Jurassic rifting and development of foreland basins along both sides of the High Atlas in Fig. 14.Map of the“Morocco Hot Line”showing the relationships with the Triassic–Liassic rift systems and the Upper Jurassic–Lower Cretaceous West Moroccan Arch (modiﬁed after Missenard, 2006).

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Fig. 15.The effects of the Morocco Hot Line on the Moroccan topography (afterMissenard, 2006). A—topographic map of Morocco (the black lines locate the lithospheric transects used for the modelling). B—the same map after subtraction of the thermal component of the relief.

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Morocco as well as along the Atlas front in southern and eastern Tunisia.

In Morocco the inversion developed within the formerly formed lithospheric syncline north of the Anti-Atlas (Fig. 9).

By the Oligocene, the uplift ceased and the whole system suffered a general subsidence (Fig. 16C and D). The net consequence is that the Eocene chain and foreland basins have beenﬂooded below siliciclastic molasses, which are mainly marine in the eastern Maghreb but mainly continental in the western Maghreb. In Morocco, where the question is more controversial, we have shown that the existence of remnants of Oligo-Miocene molasses in the core of the chain militated in favour of a sinking of the whole chain in agreement with the young AFT ages (9–24 Ma) found byMissenard et al. (2008). How to explain such a

subsidence? We suspect a direct relationship with theﬂexuration of the North-African lithosphere in front of the AlKaPeCa domain (internal zones of the Maghrebides), which started its trans-Mediter- ranean travel at that time (see a review inJolivet and Faccenna, 2000).

The paleogeographic reconstitutions of Oligocene–Lower Miocene deposits in Tunisia (Van Houten, 1981; Ben Ferjani et al., 1990; Yaich et al., 2000) allow us to visualize the forebulge, a NE–SW high sepa- rating the northern Numidian sandstone (ﬂysch) from the southern Fortuna sandstone.

The Late Oligocene–Miocene period corresponds to the climax of Alpine shortening in the External Maghrebides (Tell-Rif) resulting from the development of an accretionary prism at the expense of the Fig. 16.Schematic kinematic scenario for the Maghreb and West Mediterranean since the Upper Cretaceous (modiﬁed fromMissenard, 2006). A: Initiation of the subduction of the Alpine Tethys and development of lithospheric folds in the Maghreb area; B: First general inversion of the Atlas basin (ﬁrst Atlas event) followed by the initiation of the slab roll-back in the Mediterranean area; C and D: Slab roll-back, development of the Tell-Rif accretionary prism, formation of the West Mediterranean basin (violet color) as an oceanic (or with thinned continental crust) basin and possibly initiation of the Morocco Hot Line (MHL) the southern termination of the European Cenozoic Rift System.(ECRIS) is indicated. E and F: docking of the Kabylies (pertaining to the AlKaPeCa domain,Fig. 1) on the African margin, tearing of the lithosphere along the North-African margin and development of the MHL.

G: second inversion of the Atlas system (second Atlas Event). At that time, the slab is almost completely detached.

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6. Conclusion

The Maghreb forms an African salient situated east of the Atlantic Margin, west of the Central Mediterranean Basin and south of the West Mediterranean Basin (Fig. 1B). The Moroccan margin of the Atlantic, developed as soon as the Early Liassic, is one of the oldest passive margin preserved worldwide (Sahabi et al., 2004). The West Mediterranean Basin isﬂoored by a young (Miocene) thinned continental crust/oceanic basin born within the Alpine collisional domain (see a recent review in Cavazza et al., 2004). The Central Mediterranean basin is more complex and formed by thinned continental crust developed during the Tethyan (mainly Jurassic) and Sirt (Upper Cretaceous–Paleocene) rifting events (Rusk, 2001; Guiraud et al., 2005; Abadi et al., 2008) (Fig. 10). The geological evolution of the Maghrebian lithosphere is deeply marked by this situation and, in particular, by the Mediterranean geodynamics.

The present asymmetry of the Maghrebian topography (Fig. 1A) is due to the existence of the Cenozoic“Moroccan Hot Line”(MHL) and related relief building (Teixell et al., 2005; Zeyen et al., 2005; Missenard et al., 2006; Babault et al., 2008). However, the difference in the behaviour of the western Maghreb compared to the eastern one is inherited from a long and complex history since at least the Paleozoic.

First of all, Morocco is the only country where the Variscan orogeny is widely developed (see a review inMichard et al., 2008) (Fig. 1B). Then, during the Jurassic and Lower Cretaceous, this country suffered the up and down movement of the West Moroccan Arch (WMA) (Ghorbal et al., 2008; Saddiqi et al., 2009-this issue). Interestingly we note that the MHL and associated volcanoes transects the WMA (Fig. 5) and in particular, the zone (central High Atlas) where is concentrated a wide range of transitional to alkalic intrusions and lava ﬂows during the Middle-(Late?) Jurassic and Early Cretaceous. Such a discontinuous suc- cession of magmatic events since the Middle Jurassic until the Quaternary shows that this domain remained through time a zone of particular weakness at lithosphere scale. For such a pattern the“hot spot”expla- nation, which is sometime mentioned (seeMissenard et al., 2006), is not convincing because the magmatic events are recurrent along the same strip without any evidence of apparent migration related to plate motion.

At larger scale, the MHL belongs to the “Circum-Mediterranean Anorogenic Cenozoic Igneous”Province (“CiMACI”Province,Lustrino and Wilson, 2007) and a general question is to understand how the MHL is connected to the Mediterranean geodynamics. Along the Mediterra- nean coast of the Maghreb, the transition from calc-alcaline to alcaline igneous activity occurred during the Miocene (Maury et al., 2000;

Coulon et al., 2002; Duggen et al. 2005). At that time, the West Medi- terranean geodynamics was dominated by the lateral roll-back of the Mediterranean subduction toward the Gibraltar Arc and the Tyrrhenian Arc, respectively (Spakman and Wortel, 2004; Faccenna et al., 2004).

Lithosphere tearing along the Maghrebian margin could have triggered anorogenic magmatism related to adiabatic decompression of asthenosphere replacing detached lithosphere. In other words, it is conceivable that the space located in the rear of the retreating slab has beenﬁlled up by asthenospheric material coming laterally from the MHL and causing sublithospheric mantle upwelling along this trend. However,Missenard et al. (2006), emphasising the cross-cutting relationships with the main structural elements, in particular the Rif and South Atlas Fronts, suggest an independence between the mantle upwelling responsible for the

adjacent central Mediterranean basin have been the site of recurrent (quasi-permanent) rifting: (1) the Tethyan rifting, which began during the Upper Triassic and lasted up to the Lower Cretaceous in the area, developed E–W faults; (2) the Sirt rifting during the Upper Cretaceous–Paleocene and ﬁnally (3) a Plio-Quaternary rifting, already active at the moment (Casero and Roure, 1994). This last rifting as well as the Sirt rifting developed NW–SE trending faults, which are at right angle to the convergence direction between the Africa and Europe plate. The permanent subsidence in the Central Mediterranean basin is responsible for the huge thickness of the sedimentary cover (more than 18 km in some places). A magmatic activity is associated to these rifts during the Mesozoic (Laridhi Ouazaa and Bédir, 2004) the Cenozoic (Lustrino and Wilson, 2007).

However, the model explaining the origin of this magmatism requires lithospheric extension to induce decompression melting and passive upraise of asthenospheric and lithospheric melts. This model is very different from the one proposed in Morocco, which requires an active upraise of asthenospheric mantle. Moreover few relief is associated with the Cenozoic volcanoes present in Western Lybia (Fig. 1A).

A last and puzzling question is to understand why is the Africa– Europe plate convergence differently accommodated through time, either by lithospheric buckling (Upper Cretaceous (?) and Paleocene) or by inversion along the inherited rifts (Fig. 16)? A discussion on the mechanical reasons of this alternation is out of the purpose of our paper.Frizon de Lamotte et al. (2000)proposed that the periods of inversion in the Atlas system should correspond to periods of strong coupling between the Africa and Eurasia plates whereas the periods of relative tectonic quiescence, in fact periods of buckling, should be signiﬁcant of a low coupling between the two plates. Low coupling means that the convergence is mainly accommodated by subduction and we can expect in the adjacent continents quite constant horizontal tectonic forces triggering the development of large-scale compressive instabilities. Strong coupling means that the convergence is partly accommodated in the subduction zone, but also within the continents themselves. During such periods, typically the Middle–Late Eocene and the Plio-Quaternary, we can expect that the amount of deformation is directly under the control of plate kinematics. Lithospheric buckling then becomes insufﬁcient to accommodate shortening, and faulting (inversion) develops in the weaker zones (i.e. in the former rifts).

Acknowledgements

The reviews by two anonymous reviewers are gratefully acknowledged. We greatly benefited from the discussion in thefield or in the lab with L. Baidder and E.H. El Arabi (Univ Casablanca Aïn Chock, G. Bertotti and B. Ghorbal (Vrije Universiteit, Amsterdam), A. Teixell, M.L. Arboleya and J. Babault (Univ. Autonoma de Barcelona), M. Bedir (CRTE, Tunis), M. Hafid (Univ Kenitra).

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