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 Ó 2011 Blackwell Publishing Ltd 1 Edge-Driven Convection in Morocco? • Y. Missenard and A. Cadoux Terra Nova, Vol 00, No. 0, 1–7 ............................................................................................................................................................. 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 32°N West-African Craton, close to the Saharian domain of Morocco. Red line: cross- 5°W (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).