The Moroccan Anti-Atlas: the West African craton passive margin with limited Pan-African activity. Implications for the northern limit of the craton

Abstract

The Moroccan Anti-Atlas region, located south of the South Atlas Fault, has been viewed traditionally as containing two segments separated by the Anti-Atlas Major Fault. These two segments are said to consist of: (a) 600–700 Ma Pan-African segment located in the northeast; and (b) ∼2 Ga Eburnian segment situated to the southwest. On the basis of observations in the Zenaga and Saghro inliers and of a recent literature review, we suggest that this subdivision is inappropriate in that Eburnian and Pan-African materials occur throughout the Anti-Atlas region: the entire Anti-Atlas is underlain by Eburnian crust, unconformably overlain by a lower Neoproterozoic passive margin; allochthonous Pan-African ocean crustal slices were thrust onto the West African craton (WAC) passive margin sequence ∼685 Ma ago as a result of Pan-African accretion tectonics; high-level high-K calc-alkaline and alkaline granitoids locally intruded the Anti-Atlas sequence as a whole at the end of the Pan-African orogeny at 585–560 Ma; the intervening 100 m.y. interval was marked by quiescence. This succession of events can be related to the behaviour of one single rigid cratonic passive margin during an orogeny and correlated to the Pan-African events that occurred to the east in the Tuareg shield and to the north in Avalonian terranes. This model implies that the actual northern limit of the WAC is located at the South Atlas Fault (SAF) and not at the Anti-Atlas Major Fault (AAMF). We propose that the AAMF corresponds to the southwestern boundary of an aulacogen that formed along the northern margin of the WAC during early Neoproterozoic times. This is consistent with the development of the Gourma aulacogen on the eastern side of the WAC. This model further suggests that the northeastern boundary of the WAC occurs north, and not south, of Ougarta (Algeria). Support for this model is provided by geological and geophysical evidence.