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The Adrar Souttouf belt (Western Sahara) lies in the northern part of the Mauritanide belt and was considered as a part of the Hercynian Mauritanian-Appalachian belt. Recently the presence of a Panafrican protolith (595 Ma) was evidenced by the age of inherited zircons from an eclogitic unit and led to consider complex Panafrican events included in the Hercynian belt, as in the central and southern Mauritanide belt. Our new field survey completed by ten K Ar dating (whole rocks and separate minerals) confirmed that multiphased Panafrican formations outcrop within the center of the Adrar Souttouf massif. Diverse events ranged from ca. 1000 to 500 Ma, some of them being indicative of an oceanic type environment and metamorphism. The 1000 Ma metamorphic event is recorded for the first time in the West African craton. A large Hercynian remobilisation is also evidenced. To cite this article: M.

Résumé -A la suite de sa déformation en régime compressif sous des conditions métamorphiques barro�ennes culminant dans le faciès amphibolite supérieur, le massif des Rehamna est affecté par une tectonique extmsive. Celle-ci s'est manifestée dans trois zones de détachement (Kef El Mouneb, El Broumi et Ouled Zedness occid.ental) qui ont assuré le retour à l'équilibre de la croûte épaissie. L'amincissement crustal provoqué par la tectonique extensive est enregistré par les isogrades qui ont subi une forte condensation. La cinématique des mouv�ments extensifs est variable avec la profondeur; le déplacement tectonique dirigé du sudouest vers le nordest dans les parties profondes de la chaîne, s'opère par contre du nordouest vers le sudest dans les niveaux supérieurs de l'édifice orogénique.Abstract -In the Moroccan Hercynian helt, the Rehamna massif has been affected by an important extensional tectonism, developed inmediately after a compressive regime with associated Barrovian metamorphism up' to the lower-T part of the amphibolite facies. Three main detachments were developed during the extensional regime (Kef El Mouneb, El Broumi and western Ouled Zednes), which caused an important thinnin g and can he considered to he the origin of the extreme condensation outlined by the metamorphic zones. The extel!lsional movements were oriented from southwest to northeast in the lower parts of the massif, whereas in the upper parts a main sense of movement from northwest to southeast can be detected.

conditions. The structural and metamorphic study of the syn-peak mineral assemblages apparently suggests a similar tectonothermal evolution for the Skhour and Devonian-Visean formations. Nevertheless, mineral compositions in the developed metamorphic assemblages are indicative of higher metamorphic conditions in the first formation. Thermobarometric P-T conditions of the regional thermal peak were calculated at 560 Q C and 9 kbar in the Skhour Nappe, and 532 QC and 7 kbar in eastern Rehamna. These P-T conditions are also mainly coincident with those of the pressure peak. This suggests a very limited influence of decompressive thermal relaxation in the final tectonothermal evolution of the Rehamna Massif, interpreted as due to rapid exhumation of the belt as a result of syn-orogenic crustal extension. The syn-extensional part of the P-T path shows slightly retrogressive decompression, which finally reached low pressure conditions in the andalusite stability field .