Archaean Suture In Sierra-Leone

Considerable evidence has appeared supporting plate-tectonic mechanisms for the production of Archaean metamorphic terrains1–4. Windley1 has proposed models to explain both the relationship between the high grade ‘gneiss’ and the low grade ‘granite–greenstone’ terrains and the building of continental crust by collision of small continental masses. His model uses the same geological mechanisms as those typically exhibited by Phanerozoic orogenic belts, except that they were more rapid during the Archaean as a result of the fast seafloor spreading rate at that time5,6. I propose here that an extensive zone of mylonites, up to 10 km thick, on the coastal margin of the West African Craton7,8 is a suture developed during the late Archaean at the site of collision of two granitic continental masses, now preserved in a partially reworked state as the Guyana Shield and the West African Craton (Fig. 1). The suture is evidence for continental coalescence in the late Archaean.