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

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) similar to2 Ga Ebu rnian segment situated to the southwest. On the basis of observations in th e Zenaga and Saghro inliers and of a recent literature review, we suggest t hat this subdivision is inappropriate in that Eburnian and Pan-African mate rials occur throughout the Anti-Atlas region: the entire Anti-Atlas is unde rlain by Eburnian crust, unconformably overlain by a lower Neoproterozoic p assive margin, allochthonous Pan-African ocean crustal slices were thrust o nto the West African craton (WAC) passive margin sequence similar to 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 interve ning 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 t o the east in the Tuareg shield and to the north in Avalonian terranes. Thi s 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 p ropose that the AAMF corresponds to the southwestern boundary of an aulacog en that formed along the northern margin of the WAC during early Neoprotero zoic times. This is consistent with the development of the Gourma aulacogen on the eastern side of the WAC. This model further suggests that the north eastern boundary of the WAC occurs north, and not south, of Ougarta (Algeri a). Support for this model is provided by geological and geophysical eviden ce. (C) 2001 Elsevier Science BN. All rights reserved.