The presence of ancient continental crust in the Afif terrane of the Arabian Shield has been indicated by radiometric data. Detailed mapping in the central Shield reveals a volcano-sedimentary succession older than one of the final cratonizing events of the Shield and unconformable upon high grade paragneisses, providing direct evidence for a western continental margin to the Afif terrane in late Proterozoic times. The basement gneisses were metamorphosed to almandine-sillimanite amphibolite facies and preserve evidence of a deformation not seen in any other rocks of the area. The Siham group, unconformable upon this basement, was metamorphosed to upper greenschist facies. It has been divided into six formations which provide an E–W section through the group. The lower members are volcanic and are absent in the easternmost formation. Their clastic content increases eastward. A carbonaceous shale with exhalative chert and marble, tuffs and associated volcanogenic massive sulphides is associated with the volcanics. It is absent in the east where shallow marine marbles and quartzites lie unconformably upon basement. Rhyolite and conglomerate complete the succession. In the west, the base of the group is not seen but the shale is associated with numerous sheet-like bodies of mafic and ultramafic rock emplaced in an extensional tectonic setting. Other plutonic rocks are calc-alkaline and are emplaced mainly in the west. Deep water oceanic conditions prevailed in the west and a shallow marine continental shelf environment existed in the east, with deposition of the group across this continental margin. The stratigraphy of the Siham group is particularly akin to the Mesozoic of Peru. Magmatic arc and marginal basin were coincident in space and time, and the marginal basin was probably extensional. There is some limited evldence that it was ensialic. The Siham group is bounded to the west by a N-S suture zone formed in one of the final collisional events in the cratonization of the Shield. This event caused the deformation of the Siham group and marked the termination of the magmatic are.