Piecewise delamination of Moroccan lithosphere from beneath the Atlas Mountains

Abstract

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.

Short wavelength lateral variability of lithospheric mantle beneath the Middle Atlas (Morocco) as recorded by mantle xenoliths

“…Moreover, the Anti-Atlas and the eastern Moroccan Meseta, including the 'tabular' Middle Atlas, although virtually unaffected by Cenozoic deformation, were uplifted to above 3000 and nearly 2000 m, respectively. A significant component of mantle buoyancy is therefore required to explain the Atlas reliefs, as also supported by evidence for abnormally thin lithospheric mantle based on gravity, heatflux, receiver functions, topography and crustal thickness data Fullea et al, 2006Fullea et al, , 2010Miller and Becker, 2014;Missenard et al, 2006;Teixell et al, 2005), as well as seismological tomography (Bezada et al, 2014;Palomeras et al, 2014). Further evidence for asthenospheric upwelling arises from recent volcanic activity involving generation of sublithospheric melts at 60-120 km depth (e.g.…”

“…Processes such as eclogite loading and thermal erosion can be assessed, and the xenolith studies also provide clues to understand the role of deformation and fluid-rock processes in the convective erosion of the lithosphere. The most conspicuous xenolith localities in Morocco are concentrated in the 'tabular' Middle Atlas, where intra-plate Cenozoic volcanism coincides with nearly 2000 m uplifting of the undeformed Mesozoic cover and -roughly -with the maximum of lithospheric thinning beneath continental Morocco (Bezada et al, 2014;Fullea et al, 2010;Miller and Becker, 2014). However, previous xenolith studies focused on a single locality -the Bou Ibalghatene maar, in the central part of the volcanic district (Lenaz et al, 2014;Natali et al, 2013;Raffone et al, 2009;Wittig et al, 2010a,b).…”

“…In detail, geophysical studies converge on the existence of a roughly SW-NE trending corridor of thinned lithosphere, 200-500 km wide, where the asthenosphere-lithosphere boundary raises to 90-50 km (Bezada et al, 2014;Fullea et al, 2010;Jiménez-Munt et al, 2011;Missenard et al, 2006;Teixell et al, 2005;Zeyen and Fernandez, 1994). The corridor (the 'Moroccan Hot Line' of Frizon de Lamotte et al, 2009) extends from the Atlantic margin to the Eastern Rif through the central high-Atlas and the eastern Meseta, and concentrates most of the Moroccan Cenozoic volcanism.…”

“…The proposed mechanisms of lithospheric thinning vary between two extremes: (1) relational-scale delamination of lithospheric mantle as a result of compressive lithosphere thickening (Duggen et al, 2009) and (2) thermal erosion of lower lithosphere by upwelling upper mantle, possibly reusing Mesozoic rift structures (Berger et al, 2009;Raffone et al, 2009). Between these extremes, to account for the complex topography of the lithosphere-asthenosphere boundary, Bezada et al (2014) suggested a model involving piecewise lithosphere delamination due to tectonic shortening and loading by high-pressure magmatic segregates, combined with local upwelling of hot mantle and thermal erosion.…”

On the use of sensitivity tests in seismic tomography

“…Downdip smearing can, however, be detected correctly by deriving from the observational model a special 'layer-cake' model in which complete layers in the observational model have been set to zero (Bijwaard et al 1998). An alternative suggested by Bezada et al (2014) is the so-called 'squeezing test', in which the inverse problem is preconditioned to find the best-fitting model that excludes a certain feature of interest. The residuals that result from the subtraction of the data predictions through this model from the observational data are then inverted to see if the data requires this feature to be present.…”