“Pan African Orogeny” in the Coastal Region of Ghana

Abstract

SEVERAL authors^1–4 have advocated that in West Africa the 500–600 m.y. “Pan African Orogeny”⁵ was a complete orogenic cycle and not merely a thermal event that caused isotopic changes. It has been postulated that in Ghana^3,4 this cycle began when the Birrimian rocks, which had been involved in a previous orogeny between 1,700 and 2,000 m.y. ago, became the basement to a sedimentary cover sequence comprising the Togo (Akwapimian, Atacorian) and Buem Formations and lower parts of the Voltaian Group (Fig. 1) which are in part laterally equivalent to one another and in which there is a southeastward transition from miogeosynclinal to eugeosynclinal facies. The cover and basement to the south-east of a line running approximately between Accra and Niamey (inset of Fig. 1) were then involved in the thermotectonic phase of the “Pan African Orogeny” while that to the north-west remained essentially undisturbed as part of the West African craton. Within the orogenic zone the cover was strongly folded and thrust on southeasterly dipping planes, was lightly metamorphosed, and perhaps locally subjected to granitization^6,7 and to granite intrusion, while the Birrimian basement was reactivated to give the unit known as the Dahomeyan.