The Influence of pre-existing crust in the evolution of EARS and its impact on the geothermal exploration strategy of the Western Rift

Abstract

The East African Rift System (EARS) is a well developed continental rifting that splitted the Somalian Plate from Africa (Nubian). Volcanism initiated in the north in Mid Oligocene, in Kenya at 25 Ma followed by episodic rifting. The western rift is young, in which volcanism began at about 12 Ma in the north and at about 7 Ma in the south. Volcanism in the south is only confined to four volcanic provinces, the Tore-Ankole, Virunga, South Kivu and Rungwe. The lithospheric and crustal structure is addressed through P and S wave anisotropy. The East African Rift and related plume extending from Malawi to the Red Sea illustrate the geodynamics of the mantle below Africa and the underlying mantle convection. Studies of lithospheric thickness along the northern rift, Main Ethiopian Rift indicate that, under the very northern most sections of the rift near Afar, where the extension factors are highest, and rifting has progressed furthest, the lithosphere has been thinned appreciably from its pre-rift, steady-state thickness. It becomes thicker towards Tanzania and around Rukwa and Lake Tanganyika with crustal thickness of 40-44 km.

Pre-existing structures have controlled the location and rifting in the East African Rift System. The Paleoproterozoic Ubendian orogeny occurred between 2100 and 1800 Ma is prominent in the south is a high grade metamorphism and interpreted as a product of collisional orogeny. Whereas the MesoProterozoic Kibaran belt is a short lived but prominent at about 1375 Ma tectonomagmatic event. The Western Branch of EARS follows preferentially these two belts and avoids Archean. The EARS easily penetrated through the Juvenile NeoProtorozoic East African Orogeny. The less evolved western rift is characterized by thick crust, less volcanism and thick sedimentation makes it to define a different approach than the Eastern Rift. The region from Ruwenzori through Kivu to Rusizi and finally to south should be handled distinctly separable and rifting related to transfer zone should be considered to outline strategy for geothermal exploration.