doi: 10.1111/j.1365-3121.2011.01033.x Can Moroccan Atlas lithospheric thinning and volcanism be induced by Edge-Driven Convection?

Yves Missenard1 and Anita Cadoux2 1UMR IDES 8148, De´partement des Sciences de la Terre, Universite´ Paris Sud-11, Baˆtiment 504, 91405 Orsay Cedex, France; 2Institut des Sciences de la Terre dÕOrle´ans (ISTO) UMR 6113 - CNRS ⁄ Universite´ dÕOrle´ans, 1A rue de la Fe´rollerie, 45071 Orle´ans Cedex 2, France

ABSTRACT The Moroccan lithosphere is characterized by an anomalously episodes during the last 80 Ma points out that volcanism occurs thinned area, located beneath the Atlas domains, which forms a when plate moves at velocities c.<1 cm a)1, a velocity suffi- singular narrow NE–SW directed strip overlain by Cenozoic ciently low to trigger EDC. This is the first process that could alkaline volcanism. The origin of this thinning and volcanism is explain the c. 20 Ma volcanism shutdown separating the two still a matter of debate. The proposed models invoke processes volcanic episodes of the Atlas. In addition, it may successfully either related to the Mediterranean slab or mantle plumes. account for the lithosphere thinning location and geometry and Herein, we propose an alternative Edge-Driven Convection volcanism geochemistry. (EDC) model involving small-scale convection at the boundary between the West-African craton and the Atlas lithosphere. Our Terra Nova, 00, 1–8, 2011 comparison of the Atlas lithosphere velocity and volcanism

spheres and thinner (oceanic or young Introduction Lithosphere structure and continental) lithospheres. The convec- volcanism of Morocco The high topography (>4000 m) of tion is induced by the temperature the Moroccan Atlas intraplate moun- contrast at the vertical wall separating The Moroccan Atlas lithosphere is tains (NW Africa, Fig. 1) is due to the the cold craton from the warmer anomalously thin (Seber et al., 1996; combination of significant lithospheric asthenosphere (King and Anderson, Teixell et al., 2005; Zeyen et al., 2005; thinning and crustal shortening during 1998). Decompression in the upwell- Fullea Urchulutegui et al., 2006, 2010; Cenozoic times (Missenard et al., ing part of the convection cell is Missenard et al., 2006): the Litho- 2006). The thinned lithosphere forms thought to be sufficient to trigger sphere–Asthenosphere Boundary a NE–SW directed narrow strip cross- partial melting (King and Anderson, (LAB) is 60–70 km deep below the cutting the E–W main structures of 1995; Farrington et al., 2010). How- Middle Atlas, Central High Atlas and the Atlas belt, and is overlain by ever, as EDC is a relatively weak Anti-Atlas, while it reaches depths of alkaline volcanism. The origin of this instability, fast relative motion be- 120–140 km beneath the Meseta thinning and the associated volcanism tween the lithosphere (craton and thin (Fig. 1B). This thinning is restricted still remains poorly understood. The lithosphere) and the underlying within a NE–SW strip (150 km wide vicinity of the northern Alboran slab asthenospheric mantle may produce by 1000 km long) cross-cutting the and the western Canary Hotspot a shear-coupling that completely over- Atlas belts and the main N–S hercy- (Fig. 1A) led to contrasting models, whelms EDC (King and Anderson, nian or E–W cenozoic crustal sutures invoking subduction-related and ⁄or 1998; Shahnas and Pysklywec, 2004; (Missenard et al., 2006; Fullea Urchu- intraplate mantle processes, which do Farrington et al., 2010). lutegui et al., 2010). not fully account for the geological In this article, we discuss the feasi- Volcanic activity took place in the features of Morocco. bility of EDC in the context of the three main geological domains of Herein, we propose an alternative Moroccan Atlas domains (Fig. 1B). Morocco (Rif, Atlas, Sahara) during Edge-Driven Convection model To estimate the Moroccan litho- Cenozoic (Fig. 1). The northernmost (EDC; Elder, 1976), which consider sphere–asthenosphere relative motion, magmatism, related to the Rif sub- the neighbour West-African Craton we calculate the absolute Atlas litho- duction system, includes Gourougou, rim (WAC; Fig. 1A). sphere velocities in a fixed Hotspot Guilliz volcanic centres and Oujda Indeed, EDC is a small-scale con- reference frame during the last 80 Ma. (Fig. 1B; Chalouan et al., 2008). It is vective instability forming at any step We evidence for the first time that composed of calcalkaline or transi- or discontinuous change in thickness volcanism occurs when plate moves at tional to alkaline lavas (e.g. Maury ) of a thermal boundary layer such as low velocities c. <1 cm a 1, whereas et al., 2000; Coulon et al., 2002). the limit between thick cratonic litho- it stops at higher velocities. This In the southern Atlas and Sahara constitutes an argument in favour of domains (Fig. 1A), volcanism exclu- Correspondence: Yves Missenard, UMR EDC at the northern boundary of the sively displays an alkaline intraplate IDES 8148, De´partement des Sciences de la WAC. Finally, we detail how this chemical affinity (e.g. Mokhtari and Terre, Universite´Paris Sud-11, Baˆ timent model could successfully account for Velde, 1988; Rachdi, 1995; El Azzouzi 504, 91405 Orsay Cedex, France. e-mail: the geological characteristics of the et al., 1999, 2010; Wagner et al., [email protected] Atlas. 2003). It comprises the Taourirt

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CU_SDT1.0 dVs/Vs (%) 150 km, Reference velocity 4.473 km s–1 district (south of the Rif Front), the Rekkame field, the large Middle-Atlas –7.0 –4.2 –3.5 –2.8 –2.1 –1.4 –0.7 0.0 0.9 1.8 2.7 3.6 4.5 5.4 9.0 volcanic field, the Tamazert complex (A) Spain Main tertiary alkaline (High Atlas belt) and the Sahro vol- volcanic provinces Alboran sea canic field (or ÔSaghroÕ) and Siroua Rif volcanic edifice (or ÔSirwaÕ), both

Algeria located in the Anti-Atlas belt (Fig. 1B, Madeira Atlas domain Table 1). Morocco

Targets and methods 30˚N Saharan domain (B) Canary Islands We focused our study to the anoma- lously thin lithosphere strip of Mor-

WEST AFRICAN CRATON occo (Fig. 1B) which includes the volcanic provinces of the Atlas and Sahara domains (Fig. 1B and 0˚ Table 1). –10° –8° –6° –4° –2° The thicknesses change between the MEDITERRANEAN SEA (B) Rif domain Area of thinned lithosphere T. Oujda, Guelliz, WAC (>200 km) and the Atlas lith- Mesetas (<100 km thick) Gourougou (7.6-0.6 Ma) Atlas domain osphere (120 km) constitutes an Atlas belts Tertiary alkaline volcanic provinces ideal configuration for EDC. How- 35° Saharan domain Rif belt R if ever, a slow relative motion between

fro nt the lithosphere and the asthenosphere

Taourirt (67-37 Ma) is also required. As the degree of Middle Atlas (1.5-0.6 Ma) Rekkame (50-35 Ma) Eastern coupling between the lithosphere and Me. Meseta R. Folded and ATLANTIC If. Middle High underlying mantle remains unknown Atlas Az. Plateaus OCEAN and, according to King and Anderson (1998), it is unlikely that the entire 33° Western Meseta upper mantle is moving together with Mi. the lithospheric plate, we made the Tamazert - Eastern High Atlas Bou Agrao (45-35 Ma) assumption (as Farrington et al., S outh Atlas Front 2010) that absolute plate motion Central High Atlas Ma. velocities can reflect the relative mo- tion between the continental litho- Sarrho (9.6-2.9 Ma) sphere (craton and thinner 31° A. Agadir Western High Atlas Az. Azrou lithosphere) and the asthenosphere. Ou. If. Ifrane las Ma. Marrakech Thus, we calculated the Atlas litho- Anti-At Mi. Midelt Me. Meknes sphere velocities, as well as the volume A. Siroua (11-2 Ma) Ou. Ouarzazate R. Rabat T. Tangier of volcanic products emitted, for the last 80 Ma. Plate velocities were calculated by Fig. 1 (A) North-West Africa map showing the 3D shear-wave velocity tomographic spherical trigonometry using palaeo- model based on surface wave diffraction tomography. Map generated from the CUB poles and rotation data from four model of N. Shapiro (http://ciei.colorado.edu/~nshapiro/MODEL/). See Ritzwoller independent studies of plate motions et al. (2002) and Shapiro and Ritzwoller (2002) for surface wave diffraction reconstructions (Morgan, 1983; Dun- tomography and data processing. Colour scale shows the shear velocity as percentage can and Richards, 1991; Garfunkel, perturbation relative to the reference velocity values of 4.473 km s)1 at a depth of 1992; Mu¨ller et al., 1993). Our results 150 km. The most important geological structures and domains around the WAC are correspond to the absolute motion reported: (i) to the North, the Rif-Tell domain including the Maghreb margin (North Algeria) that belongs to the Mediterranean subduction system, (ii) to the South, the velocities, in a hotspot absolute mo- intraplate Atlas domains. The southern part of the covered area corresponds to the tion frame, of a point located at 32N West-African Craton, close to the Saharian domain of Morocco. Red line: cross- 5W (i.e. in the Moroccan Central section, Fig. 3. (B) Details of the Moroccan Atlas systems (Anti-atlas, High Atlas, High Atlas, Fig. 1B) since 80 Ma. We Middle Atlas) and location of the Cenozoic alkaline volcanism outcrops. Periods of also included velocities computed volcanic activity are compiled from various sources (see Table 1 for references). It is from Madeira Islands and seamounts noteworthy that volcanism developed above the anomalously thinned lithosphere ages (Fig. 1A; Geldmacher et al., zone evidenced by Missenard et al. (2006) and Fullea Urchulutegui et al. (2010) and 2005). also called the Moroccan Hot Line (Frizon de Lamotte et al., 2009). In this elongated The computation of the volcanic zone (light grey), the lithosphere is <100 km thick, inducing at least 1000 m of products volumes first required a pre- surface doming. As underlined by Lie´geois et al. (2005), such a regional structure is cise digitization of each volcanic edi- too narrow and shallow to be seen in tomographic models (A), which only show the fice or field cited above (from large scale structure of the lithosphere–asthenosphere. Morocco 1 ⁄ 1 000 000 to 1 ⁄50 000 geological maps). Then, we extracted

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the altitudes from the SRTM30 eleva- tion dataset (USGS EROS data cen- ) ´ tre). We digitized the contour of the basement ⁄volcano contacts for each Sirwa ` edifice. The contours altitudes were then extrapolated using a minimum curvature method with Surfer soft- alkaline–peralkaline trachyte and phonolite with some mugearite and benmoreite and rare mafic rocks (1989) (11–2 Ma) Lavas and tuffs, mainly Berrahma and Delaloye Miocene to Pliocene Siroua (or volcano Anti-Atlas ware to reconstitute the basal surfaces. Volumes were obtained by subtract- ing the computed volcano basal sur- face from the present topography )

´ (Table 2). Finally, we calculated the

(1993), De volcanic production rates using the Sahro (1952) ` published timing of volcanic activities et al. (Table 1). et al. phonolitic lavas and tuffs lying on lacustrine Pliocene sediments, as well as necks and diatremes Sitter (9.6–2.9 Ma) Series of thin (<100 m) Berrahma Saghro (or field Anti-Atlas Results Computed ages, areas, volumes and rates of emission for each volcanic province are summarized in Table 2. et al. et al. Cumulative volcanic emission rates and Atlas lithosphere absolute veloc- ities since the last 80 Ma are presented

(1988), in Fig. 2. Although some discrepan- cies do exist between the five velocity pyroxenite, nepheline syenite and carbonatite, associated with dykes including lamprophyres Bernard-Griffiths et al. (1991), Mourtada (1997) Elongated intrusion with Agard (1973), Bouabdli Tamazert complex High Atlas models (in particular those of Mu¨ller et al., 1993 relative to the others), the general pattern is close. Two periods of relatively high velocity are identi- fied: one before 60 Ma (up to 3 cm )1 (1999, a ; Duncan and Richards, 1991), and the other between 35 and 15 Ma (up et al. to c. 2cma)1; Geldmacher et al., s field: strongly ` 2005). The data are particularly con- sistent at two specific times where all alkaline lavas (basanite, nephelinite and phonolite) and volcanic centres Oulme 2010) (1984), Rachdi (1995), El Azzouzi )1 Azrou field: basaltic flows Quaternary (1.5–0.6 Ma) Eocene (45–35 Ma) Miocene to Pliocene Middle-Atlas volcanic field velocities decrease below 1 cm a (Fig. 2): one between 80 and 60 Ma (speeds divided at least by two for each models), the second since 15 Ma (also confirmed by Atlantic seamount . 1.4 Ma) c

(1997) Harmand and Cantagrel ages study; OÕConnor et al., 1999). The remarkable feature for the first

et al. time evidenced in this study is that most of the volcanism occurs when the edifices (50-35 Ma) rarely Pleistocene ( Numerous basaltic Rachdi Palaeocene–Eocene absolute motion velocity is the lowest (1 cm a)1).

(2003) Discussion

et al. Previous models for Morocco lithosphere thinning and associated Wagner (camptonite and monchiquite including carbonatite enclaves), dykes, sills and breccia pipes, plus rarer olivine nephelinite lava flows and nepheline syenite intrusions (67–37 Ma) volcanism

Subduction process Main characteristics (locations, ages and lithologies) of the Cenozoic alkaline volcanic provinces of Morocco. Teixell et al. (2005) speculate that the Moroccan alkaline volcanism and References Mokhtari and Velde (1988), Lithologies Alkaline lamprophyre Table 1 Area (location Fig. 1B) Taourirt Rekkame field Ages Palaeocene–Eocene lithospheric thinning could originate

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Table 2 Area, volume and volcanic emission rates computed for Morocco alkaline-type volcanism of the Atlas and Sahara domains. See Table 1 for age ranges references and explanations in the text for calculation.

Min. Max Area Volume Emission rates Province Location Age (Ma) Age (Ma) (km2) (km3) (km3 Ma)1)

Taourirt North Morocco 37 67 121 2.42 0.08 Rekkame Eastern Meseta 35 50 21 0.84 0.06 Tamazert – Bou Agrao High Atlas 35 45 32 1.60 0.16 Siroua Anti-Atlas 2 11 600 65 7.22 Sagrho Anti-Atlas 2.9 9.6 28 5.60 0.84 Middle-Atlas field Atlas domain 0.6 1.5 874 2.62 2.91

) 9 –1 Mediterranean slab 8 3 roll-back African plate absolute velocities (left scale bar): First atlas 7 by Duncan and Richard (1991)

compressive phase ) by Garfunkel (1992) 6 –1 by Morgan (1983) 2 5 Ma

by Müller et al. (1993) ³ Deduced from Madeira Hot Spot 4 (Geldmacher et al., 2005) (km Average of all the above velocities curves 3 1

Alkaline volcanism production .5 Volcanic production (right scale bar, km3 Ma–1)

African plate absolute motion (cm a 0 0 80 70 60 50 40 30 20 10 0

Time (Ma) Second atlas compressive phase

Fig. 2 Atlas lithosphere absolute motion (cm a)1) in a fixed hotspot frame compared with volcanic production since the last 80 Ma (km3 Ma)1). The red curve represents the average velocity of the five models used in this study. The current speed of 4.5 mm a)1 is from Nuvel1 model. The two main periods of volcanic activity in the Atlas and Sahara domains coincide with absolute plate velocities lower than c. 1 cm a)1. The important difference of volcanic production between the two main phases of activity could be at least partly due to a major erosion event affecting the oldest volcanics during Middle to Late Miocene (Barbero et al., 2007; Missenard et al., 2008; Balestrieri et al., 2009). from lateral flow of asthenospheric Delamination of the Atlas lithosphere with the delamination model, such mantle at the tip of the Mediterranean could be supported by the occurrence process being expected to be con- (Eo-Alpine or Alboran) slabs, as it has of intermediate depth seismicity trolled by crustal structures; been proposed for Sicily alkaline vol- (Ramdani, 1998) and might explain 4 the age inferred by Ramdani (1998) canism (Gvirtzman and Nur, 1999; the volcanism geochemistry without for the delamination event, between Cadoux et al., 2007). requiring mantle plume material (e.g. 25 and 15 Ma, can only account for Thermal erosion by upwelling hot Pearce et al., 1990; Platt and England, the second main volcanism episode mantle is a suitable mechanism to thin 1993). However: (<13 Ma; Fig. 2). the lithosphere, but the relationship 1 such seismicity is not evidenced in The Atlas corridor invoked by Dug- between slab-induced flows and the the Anti-Atlas, where lithosphere is gen et al. (2009), necessary for Canary particular location and geometry of thin (<70 km thick); plume material to flow north-east- the Moroccan thinning and associated 2 the estimated shortening across the ward, was thus not present during volcanism remains unclear. Atlas domain is <20 km (Teixell the first volcanism period (67–35 Ma). et al., 2005; Frizon de Lamotte Consequently, this latter cannot be et al., 2009), which is insufficient issued from Canary mantle plume Delamination model with or without to generate lithospheric thickening material. Besides, as underlined by Canary mantle plume material flow and subsequent delamination (e.g. Berger et al. (2009), the ÔCanary-likeÕ Mantle plume material flowing Fullea Urchulutegui et al., 2010); geochemical compositions are not through a lithospheric corridor (cre- 3 the thinning developed indepen- restricted to areas above the litho- ated by delamination), from the Can- dently of the geological main do- spheric thinning in Morocco (e.g. in ary to the Atlas (Duggen et al., 2009) mains and E–W structures of the Algeria, Soudan, Lybia and Egypt; could account for the location and Atlas (Missenard et al., 2006). This Lie´geois et al., 2005; Lustrino and geochemistry of the volcanism. particularity seems inconsistent Wilson, 2007; Lucassen et al., 2008).

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ÔBaby-plumeÕ or convection cells IIvII < 1 cm a–1 Atlas belts Fullea Urchulutegui et al. (2010) sug- SE NW gest the involvement of a deep mantle 0 reservoir extending from the Canary Moroccan lithosphere Islands to the Western Mediterranean 50 100 West African Craton (Hoernle et al., 1995; Goes et al.,

1999): the hot material would flows ) 200 upward through a Ôbaby-plumesÕ or m 1300 °C isotherm k ( via convection cells. Nevertheless, the 300 h

existence of such a widespread deep t reservoir is far from being unani- p e 400 mously approved (e.g. Foulger et al., D 2005). Then, neither the convection 500 Asthenosphere cells nor the Ôbaby-plumeÕ provide satisfying explanation for the elon- 600 gated geometry of the lithospheric 0 600 1200 thinning and the volcanism periodic- Distance (km) ity. In addition, the lack of extensional structures in the thinned lithosphere Fig. 3 Edge-driven convection model applied to Morocco: schematic NW–SE cross- area is inconsistent with the Ôbaby- section (see red line, Fig. 1) illustrating the development of an edge-driven convection plumeÕ hypothesis, as extension is cell at the boundary between the West-African Craton and the Moroccan Litho- frequently associated to plume ⁄litho- sphere. The temperature contrast between the cold WAC wall and the hot sphere interactions (DÕAcremont et al., asthenosphere, combined with slow Africa plate motion (<1 cm a)1), constitute 2003; Burov et al., 2007). ideal conditions for EDC to initiate. The sublithopsheric convection cell triggers This review shows that none of the thermal erosion of the overlying lithosphere as well as partial melting of the mantle models currently proposed in the allowing volcanic activity. literature can account for all the geological particularities of the Atlas. vection can develop. We show that A possible scenario We argue hereafter why the EDC periods of faster plate motion could be a good alternative model. ) The onset of the Africa–Europe colli- (>1 cm a 1) coincides with absence sion at beginning of the Palaeocene of magmatic activity. The EDC could (Jolivet and Faccenna, 2000) is prob- be thus the first model explaining the Edge-Driven Convection: an ably at the origin of the drastic veloc- striking 20 Ma volcanic shutdown alternative model ity decrease of the African plate observed between the two main mag- (Fig. 2), which allowed EDC to devel- The boundary between the thick cold matic episodes (Fig. 2). op at the northern edge of the WAC WAC and the thinner Moroccan litho- An asthenospheric convection roll from Palaeocene to Eocene. Decom- sphere is an ideal location for small- acting parallel to the craton rim pression in the upper part of the scale convection development. This (Fig. 3) would perfectly explain the convection cell triggered sufficient hypothesis is supported by tomo- location and elongated geometry of partial melting of the subcontinental graphic images showing fast seismic the Atlas lithosphere thinning (Fig. 1 mantle to fed volcanic activity during velocities anomalies beneath the B). Indeed, this convection could c. 30 Ma. WAC, extending to depth 300– induce thermal erosion of the base At 35 Ma, volcanic activity ceased 400 km in narrow linear bands which of the continental lithosphere. The with the acceleration of the African King and Ritsema (2000) interpret as incorporation of thermally eroded plate that was most probably driven downwelling limbs of a small-scale metasomatized lithosphere into the by the northernmost Alboran slab roll convection cell associated with cra- asthenosphere (Raffone et al., 2009) back (Jolivet and Faccenna, 2000). tonic roots under Africa, consistent might account for the geochemically We interpret the successive velocity with EDC. enriched OIB-like character of the decrease as a consequence of the end The correlation between plate volcanic rocks. of slab roll back process in the Alb- velocities and volcanism activity dem- Importantly, EDC can explain the oran Sea, and the return to a strong onstrated in Fig. 2 represents an addi- lack of extension in the Atlas. Indeed, Africa–Europe coupling. tional argument supporting the in the EDC model, the asthenospheric As the Atlas lithosphere slowed occurrence of EDC processes. We flow is directed towards the craton in ) down again below 1 cm a 1, EDC show that during the last 80 Ma, the upper part of the cell (Elder, 1976; restarted and induced the second magmatism occurred in Morocco King and Anderson, 1998; Fig. 3). We Mio-Quaternary magmatism episode. when the Atlas lithosphere absolute might thus expect slight compression The initiation of this new period of motion slowed down below a speed of strains in the upper plate that could ) EDC is marked by a significant denu- about 1 cm a 1. Interestingly, this generate the crustal diffuse seismicity dation event affecting the relief above corresponds to the critical speed pro- observed above the lithospheric thin- the thin lithosphere (Barbero et al., posed by King and Anderson (1998) ning (see fig. 8 in Missenard et al., 2007; Missenard et al., 2008; Balestri- below which thermal buoyancy con- 2006).

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eri et al., 2009). This important ero- mantle source for Southern Italy volca- Acknowledgements sion phase is likely to have partly nics. Earth Planet. Sci. Lett., 259, 227– removed the products of the first We thank Prof. P. Sarda (Department of 238. magmatic episode; this could explain Earth Sciences, Orsay University, France) Chalouan, A., Michard, A., El Kadiri, K., the small preserved volumes com- for his careful reading of the manuscript. Negro, F., Frizon de Lamotte, D., Soto, We are also grateful to C. Missenard, Pr. P. J.I. and Saddiqi, O., 2008. The Rif Belt. pared with the youngest (<13 Ma) In: Continental Evolution: The Geology of volcanic edifices. Pansu (Department of Mathematics, Orsay University, France) for their help to Morocco (A. Michard, O. Saddiqi, A. Thermal erosion of the Moroccan compute velocities. The comments of Chalouan and D. Frizon de Lamotte, lithosphere above the EDC roll led to anonymous reviewers helped to improve eds), pp. 203–302. Lecture Notes in Earth the present channel-like LAB topo- the original manuscript. Finally, we Sciences, Springer-Verlag, Berlin. graphy that contributes to the uplift of acknowledge Pr. J. Phipps Morgan for Coulon, C., Megartsi, M., Fourcade, S., the Atlas (Missenard et al., 2006). editorial handling of the manuscript. Maury, R.C., Bellon, H., Louni-Hachi, Taking into account the distance A., Cotten, J. and Hermitte, D., 2002. between the WAC rim and the extent Post-collision transition from calc-alka- References line to alkaline volcanism during the of the Cenozoic alkaline volcanic Neogene in Oranie (Algeria): magmatic provinces of Morocco, we estimate Agard, J., 1973. Carte ge´ologique du complexe de roches alcalines a` carbona- expression of a slab breakoff. Lithos, 62, that this convection roll could be 87–110. about 600 km wide (Figs 1 and 3), a tites du Tamazeght (Haut Atlas de Mi- delt). Notes et Me´moires du Service DÕAcremont, E., Leroy, S. and Burov, width similar to the one observed by Ge´ologique du Maroc, 248, 53. E.B., 2003. Numerical modelling of a King (2007) in North Atlantic or Balestrieri, M.L., Moratti, G., Bigazzi, G. mantle plume: the plume head-litho- Reusch et al. (2010) in Central Africa. and Algouti, A., 2009. Neogene exhu- sphere interaction in the formation of an mation of the Marrakech High Atlas oceanic large igneous province. Earth (Morocco) recorded by apatite fission- Planet. Sci. Lett., 206, 379–396. Conclusions track analysis. Terra Nova, 21, 75–82. De Sitter, L.U., De Sitter-Koomans, C.M. and Heetveld, H., 1952. Les phonolites We show that EDC is an alternative Barbero, L., Teixell, A., Arboleya, M.-L., del Rio, P., Reiners, P.W. and Bougadir, du Jebel Saghro (Maroc occidental). model that cannot be excluded at Ge´ologie en Mijnbow, 14, 267–276. present time. The relationship be- B., 2007. 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