Piecewise delamination of Moroccan lithosphere from beneath the Atlas Mountains

The elevation of the intracontinental Atlas Mountains of Morocco and surrounding regions requires a mantle component of buoyancy, and there is consensus that this buoyancy results from an abnormally thin lithosphere. Lithospheric delamination under the Atlas Mountains and thermal erosion caused by upwelling mantle have each been suggested as thinning mechanisms. We use seismic tomography to image the upper mantle of Morocco. Our imaging resolves the location and shape of lithospheric cavities and of delaminated lithosphere 400 km beneath the Middle Atlas. We propose discontinuous delamination of an intrinsically unstable Atlas lithosphere, enabled by the presence of anomalously hot mantle, as a mechanism for producing the imaged structures. The Atlas lithosphere was made unstable by a combination of tectonic shortening and eclogite loading during Mesozoic rifting and Cenozoic magmatism. The presence of hot mantle sourced from regional upwellings in northern Africa or the Canary Islands enhanced the instability of this lithosphere. Flow around the retreating Alboran slab focused upwelling mantle under the Middle Atlas, which we infer to be the site of the most recent delamination. The Atlas Mountains of Morocco stand as an example of large-scale lithospheric loss in a mildly contractional orogen. Introduction The Atlas Mountains form an intracontinental mountain chain stretching 2000 km from the Atlantic coast in Morocco to the Mediterranean coast in Tunisia (Figure 1b). These mountains are a structural inversion of Triassic-Jurassic grabens created during the opening of the Atlantic and the Alpine Tethys [e.g., Frizon de Lamotte et al., 2008, and references therein]. Contraction occurred in two distinct episodes during the MidEocene and Pliocene-Quaternary [Frizon de Lamotte et al., 2000] and only produced modest amounts of shortening. Total shortening estimates are of 15–30% for the High Atlas [Beauchamp et al., 1999; Teixell et al., 2003] and 10% or lower for the Middle Atlas [Gomez et al., 1998; Frizon de Lamotte et al., 2009; Teixell et al., 2009]. Shortening estimates are consistent with the modest elevations found in the eastern Atlas (Saharan and Tunisian Atlas, Aures Mountains, Figure 1b), whereas the elevations in the Moroccan Atlas (High and Middle Atlas) are much higher (Figure 1b). These elevation differences occur in spite of the similar crustal thicknesses of the Moroccan and eastern Atlas, implying that the crust alone is not responsible for the high elevations of the Moroccan Atlas [Ayarza et al., 2005]. Additionally, the Moroccan Atlas are flanked by two young upwarps that locally reach elevations of 2000 m without significant Cenozoic deformation: the Moroccan Meseta, and the Anti-Atlas, located on the northern edge of the West African Craton [Hefferan et al., 2000; Ennih and Li egeois, 2001]. The eastern Atlas have no upwarps of similar magnitude. Given that the Moroccan and eastern Atlas share a common geologic history and crustal thickness [Frizon de Lamotte et al., 2009], and that much of the Moroccan Atlas elevation cannot be attributed to crustal tectonics, a mantle component of buoyancy is required to account for the high Moroccan Atlas elevations [Ayarza et al., 2005; Teixell et al., 2005; Missenard et al., 2006; Frizon de Lamotte et al., 2009]. Mantle buoyancy beneath the Moroccan Atlas requires a thinned lithosphere. This is consistent with the occurrence of Cenozoic magmatism with an alkaline, intraplate chemical affinity in Morocco that is absent from the eastern Atlas. A first pulse of magmatism occurred in the early Tertiary in eastern Morocco. After a 30 m.y. volcanic pause, this was followed by a significantly larger episode starting in the mid-Miocene (Figure 1a). This second episode was significantly more voluminous and erupted in the Middle Atlas and Key Points: We image lithospheric cavities beneath the Middle Atlas and central HighAtlas We image the delaminated lithosphere of the Middle Atlas at 400 km depth We propose piecewise delamination of Atlas lithosphere Supporting Information: Supplementary Information Text Correspondence to: M. J. Bezada, [email protected] Citation: Bezada, M. J., E. D. Humphreys, J.M. Davila, R. Carbonell, M. Harnafi, I. Palomeras, and A. Levander (2014), Piecewise delamination of Moroccan lithosphere from beneath the Atlas Mountains, Geochem. Geophys. Geosyst., 15, 975–985, doi:10.1002/ 2013GC005059. Received 23 SEP 2013 Accepted 7 FEB 2014 Accepted article online 14 FEB 2014 Published online 9 APR 2014 BEZADA ET AL. VC 2014. American Geophysical Union. All Rights Reserved. 975 Geochemistry, Geophysics, Geosystems PUBLICATIONS